ip-sysctl.rst

.. SPDX-License-Identifier: GPL-2.0

=========
IP Sysctl
=========

/proc/sys/net/ipv4/* Variables
==============================

ip_forward - BOOLEAN
	- 0 - disabled (default)
	- not 0 - enabled

	Forward Packets between interfaces.

	This variable is special, its change resets all configuration
	parameters to their default state (RFC1122 for hosts, RFC1812
	for routers)

ip_default_ttl - INTEGER
	Default value of TTL field (Time To Live) for outgoing (but not
	forwarded) IP packets. Should be between 1 and 255 inclusive.
	Default: 64 (as recommended by RFC1700)

ip_no_pmtu_disc - INTEGER
	Disable Path MTU Discovery. If enabled in mode 1 and a
	fragmentation-required ICMP is received, the PMTU to this
	destination will be set to min_pmtu (see below). You will need
	to raise min_pmtu to the smallest interface MTU on your system
	manually if you want to avoid locally generated fragments.

	In mode 2 incoming Path MTU Discovery messages will be
	discarded. Outgoing frames are handled the same as in mode 1,
	implicitly setting IP_PMTUDISC_DONT on every created socket.

	Mode 3 is a hardened pmtu discover mode. The kernel will only
	accept fragmentation-needed errors if the underlying protocol
	can verify them besides a plain socket lookup. Current
	protocols for which pmtu events will be honored are TCP, SCTP
	and DCCP as they verify e.g. the sequence number or the
	association. This mode should not be enabled globally but is
	only intended to secure e.g. name servers in namespaces where
	TCP path mtu must still work but path MTU information of other
	protocols should be discarded. If enabled globally this mode
	could break other protocols.

	Possible values: 0-3

	Default: FALSE

min_pmtu - INTEGER
	default 552 - minimum discovered Path MTU

ip_forward_use_pmtu - BOOLEAN
	By default we don't trust protocol path MTUs while forwarding
	because they could be easily forged and can lead to unwanted
	fragmentation by the router.
	You only need to enable this if you have user-space software
	which tries to discover path mtus by itself and depends on the
	kernel honoring this information. This is normally not the
	case.

	Default: 0 (disabled)

	Possible values:

	- 0 - disabled
	- 1 - enabled

fwmark_reflect - BOOLEAN
	Controls the fwmark of kernel-generated IPv4 reply packets that are not
	associated with a socket for example, TCP RSTs or ICMP echo replies).
	If unset, these packets have a fwmark of zero. If set, they have the
	fwmark of the packet they are replying to.

	Default: 0

fib_multipath_use_neigh - BOOLEAN
	Use status of existing neighbor entry when determining nexthop for
	multipath routes. If disabled, neighbor information is not used and
	packets could be directed to a failed nexthop. Only valid for kernels
	built with CONFIG_IP_ROUTE_MULTIPATH enabled.

	Default: 0 (disabled)

	Possible values:

	- 0 - disabled
	- 1 - enabled

fib_multipath_hash_policy - INTEGER
	Controls which hash policy to use for multipath routes. Only valid
	for kernels built with CONFIG_IP_ROUTE_MULTIPATH enabled.

	Default: 0 (Layer 3)

	Possible values:

	- 0 - Layer 3
	- 1 - Layer 4
	- 2 - Layer 3 or inner Layer 3 if present

fib_sync_mem - UNSIGNED INTEGER
	Amount of dirty memory from fib entries that can be backlogged before
	synchronize_rcu is forced.

	Default: 512kB   Minimum: 64kB   Maximum: 64MB

ip_forward_update_priority - INTEGER
	Whether to update SKB priority from "TOS" field in IPv4 header after it
	is forwarded. The new SKB priority is mapped from TOS field value
	according to an rt_tos2priority table (see e.g. man tc-prio).

	Default: 1 (Update priority.)

	Possible values:

	- 0 - Do not update priority.
	- 1 - Update priority.

route/max_size - INTEGER
	Maximum number of routes allowed in the kernel.  Increase
	this when using large numbers of interfaces and/or routes.

	From linux kernel 3.6 onwards, this is deprecated for ipv4
	as route cache is no longer used.

neigh/default/gc_thresh1 - INTEGER
	Minimum number of entries to keep.  Garbage collector will not
	purge entries if there are fewer than this number.

	Default: 128

neigh/default/gc_thresh2 - INTEGER
	Threshold when garbage collector becomes more aggressive about
	purging entries. Entries older than 5 seconds will be cleared
	when over this number.

	Default: 512

neigh/default/gc_thresh3 - INTEGER
	Maximum number of non-PERMANENT neighbor entries allowed.  Increase
	this when using large numbers of interfaces and when communicating
	with large numbers of directly-connected peers.

	Default: 1024

neigh/default/unres_qlen_bytes - INTEGER
	The maximum number of bytes which may be used by packets
	queued for each	unresolved address by other network layers.
	(added in linux 3.3)

	Setting negative value is meaningless and will return error.

	Default: SK_WMEM_MAX, (same as net.core.wmem_default).

		Exact value depends on architecture and kernel options,
		but should be enough to allow queuing 256 packets
		of medium size.

neigh/default/unres_qlen - INTEGER
	The maximum number of packets which may be queued for each
	unresolved address by other network layers.

	(deprecated in linux 3.3) : use unres_qlen_bytes instead.

	Prior to linux 3.3, the default value is 3 which may cause
	unexpected packet loss. The current default value is calculated
	according to default value of unres_qlen_bytes and true size of
	packet.

	Default: 101

mtu_expires - INTEGER
	Time, in seconds, that cached PMTU information is kept.

min_adv_mss - INTEGER
	The advertised MSS depends on the first hop route MTU, but will
	never be lower than this setting.

IP Fragmentation:

ipfrag_high_thresh - LONG INTEGER
	Maximum memory used to reassemble IP fragments.

ipfrag_low_thresh - LONG INTEGER
	(Obsolete since linux-4.17)
	Maximum memory used to reassemble IP fragments before the kernel
	begins to remove incomplete fragment queues to free up resources.
	The kernel still accepts new fragments for defragmentation.

ipfrag_time - INTEGER
	Time in seconds to keep an IP fragment in memory.

ipfrag_max_dist - INTEGER
	ipfrag_max_dist is a non-negative integer value which defines the
	maximum "disorder" which is allowed among fragments which share a
	common IP source address. Note that reordering of packets is
	not unusual, but if a large number of fragments arrive from a source
	IP address while a particular fragment queue remains incomplete, it
	probably indicates that one or more fragments belonging to that queue
	have been lost. When ipfrag_max_dist is positive, an additional check
	is done on fragments before they are added to a reassembly queue - if
	ipfrag_max_dist (or more) fragments have arrived from a particular IP
	address between additions to any IP fragment queue using that source
	address, it's presumed that one or more fragments in the queue are
	lost. The existing fragment queue will be dropped, and a new one
	started. An ipfrag_max_dist value of zero disables this check.

	Using a very small value, e.g. 1 or 2, for ipfrag_max_dist can
	result in unnecessarily dropping fragment queues when normal
	reordering of packets occurs, which could lead to poor application
	performance. Using a very large value, e.g. 50000, increases the
	likelihood of incorrectly reassembling IP fragments that originate
	from different IP datagrams, which could result in data corruption.
	Default: 64

INET peer storage
=================

inet_peer_threshold - INTEGER
	The approximate size of the storage.  Starting from this threshold
	entries will be thrown aggressively.  This threshold also determines
	entries' time-to-live and time intervals between garbage collection
	passes.  More entries, less time-to-live, less GC interval.

inet_peer_minttl - INTEGER
	Minimum time-to-live of entries.  Should be enough to cover fragment
	time-to-live on the reassembling side.  This minimum time-to-live  is
	guaranteed if the pool size is less than inet_peer_threshold.
	Measured in seconds.

inet_peer_maxttl - INTEGER
	Maximum time-to-live of entries.  Unused entries will expire after
	this period of time if there is no memory pressure on the pool (i.e.
	when the number of entries in the pool is very small).
	Measured in seconds.

TCP variables
=============

somaxconn - INTEGER
	Limit of socket listen() backlog, known in userspace as SOMAXCONN.
	Defaults to 4096. (Was 128 before linux-5.4)
	See also tcp_max_syn_backlog for additional tuning for TCP sockets.

tcp_abort_on_overflow - BOOLEAN
	If listening service is too slow to accept new connections,
	reset them. Default state is FALSE. It means that if overflow
	occurred due to a burst, connection will recover. Enable this
	option _only_ if you are really sure that listening daemon
	cannot be tuned to accept connections faster. Enabling this
	option can harm clients of your server.

tcp_adv_win_scale - INTEGER
	Count buffering overhead as bytes/2^tcp_adv_win_scale
	(if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale),
	if it is <= 0.

	Possible values are [-31, 31], inclusive.

	Default: 1

tcp_allowed_congestion_control - STRING
	Show/set the congestion control choices available to non-privileged
	processes. The list is a subset of those listed in
	tcp_available_congestion_control.

	Default is "reno" and the default setting (tcp_congestion_control).

tcp_app_win - INTEGER
	Reserve max(window/2^tcp_app_win, mss) of window for application
	buffer. Value 0 is special, it means that nothing is reserved.

	Default: 31

tcp_autocorking - BOOLEAN
	Enable TCP auto corking :
	When applications do consecutive small write()/sendmsg() system calls,
	we try to coalesce these small writes as much as possible, to lower
	total amount of sent packets. This is done if at least one prior
	packet for the flow is waiting in Qdisc queues or device transmit
	queue. Applications can still use TCP_CORK for optimal behavior
	when they know how/when to uncork their sockets.

	Default : 1

tcp_available_congestion_control - STRING
	Shows the available congestion control choices that are registered.
	More congestion control algorithms may be available as modules,
	but not loaded.

tcp_base_mss - INTEGER
	The initial value of search_low to be used by the packetization layer
	Path MTU discovery (MTU probing).  If MTU probing is enabled,
	this is the initial MSS used by the connection.

tcp_mtu_probe_floor - INTEGER
	If MTU probing is enabled this caps the minimum MSS used for search_low
	for the connection.

	Default : 48

tcp_min_snd_mss - INTEGER
	TCP SYN and SYNACK messages usually advertise an ADVMSS option,
	as described in RFC 1122 and RFC 6691.

	If this ADVMSS option is smaller than tcp_min_snd_mss,
	it is silently capped to tcp_min_snd_mss.

	Default : 48 (at least 8 bytes of payload per segment)

tcp_congestion_control - STRING
	Set the congestion control algorithm to be used for new
	connections. The algorithm "reno" is always available, but
	additional choices may be available based on kernel configuration.
	Default is set as part of kernel configuration.
	For passive connections, the listener congestion control choice
	is inherited.

	[see setsockopt(listenfd, SOL_TCP, TCP_CONGESTION, "name" ...) ]

tcp_dsack - BOOLEAN
	Allows TCP to send "duplicate" SACKs.

tcp_early_retrans - INTEGER
	Tail loss probe (TLP) converts RTOs occurring due to tail
	losses into fast recovery (draft-ietf-tcpm-rack). Note that
	TLP requires RACK to function properly (see tcp_recovery below)

	Possible values:

		- 0 disables TLP
		- 3 or 4 enables TLP

	Default: 3

tcp_ecn - INTEGER
	Control use of Explicit Congestion Notification (ECN) by TCP.
	ECN is used only when both ends of the TCP connection indicate
	support for it.  This feature is useful in avoiding losses due
	to congestion by allowing supporting routers to signal
	congestion before having to drop packets.

	Possible values are:

		=  =====================================================
		0  Disable ECN.  Neither initiate nor accept ECN.
		1  Enable ECN when requested by incoming connections and
		   also request ECN on outgoing connection attempts.
		2  Enable ECN when requested by incoming connections
		   but do not request ECN on outgoing connections.
		=  =====================================================

	Default: 2

tcp_ecn_fallback - BOOLEAN
	If the kernel detects that ECN connection misbehaves, enable fall
	back to non-ECN. Currently, this knob implements the fallback
	from RFC3168, section 6.1.1.1., but we reserve that in future,
	additional detection mechanisms could be implemented under this
	knob. The value	is not used, if tcp_ecn or per route (or congestion
	control) ECN settings are disabled.

	Default: 1 (fallback enabled)

tcp_fack - BOOLEAN
	This is a legacy option, it has no effect anymore.

tcp_fin_timeout - INTEGER
	The length of time an orphaned (no longer referenced by any
	application) connection will remain in the FIN_WAIT_2 state
	before it is aborted at the local end.  While a perfectly
	valid "receive only" state for an un-orphaned connection, an
	orphaned connection in FIN_WAIT_2 state could otherwise wait
	forever for the remote to close its end of the connection.

	Cf. tcp_max_orphans

	Default: 60 seconds

tcp_frto - INTEGER
	Enables Forward RTO-Recovery (F-RTO) defined in RFC5682.
	F-RTO is an enhanced recovery algorithm for TCP retransmission
	timeouts.  It is particularly beneficial in networks where the
	RTT fluctuates (e.g., wireless). F-RTO is sender-side only
	modification. It does not require any support from the peer.

	By default it's enabled with a non-zero value. 0 disables F-RTO.

tcp_fwmark_accept - BOOLEAN
	If set, incoming connections to listening sockets that do not have a
	socket mark will set the mark of the accepting socket to the fwmark of
	the incoming SYN packet. This will cause all packets on that connection
	(starting from the first SYNACK) to be sent with that fwmark. The
	listening socket's mark is unchanged. Listening sockets that already
	have a fwmark set via setsockopt(SOL_SOCKET, SO_MARK, ...) are
	unaffected.

	Default: 0

tcp_invalid_ratelimit - INTEGER
	Limit the maximal rate for sending duplicate acknowledgments
	in response to incoming TCP packets that are for an existing
	connection but that are invalid due to any of these reasons:

	  (a) out-of-window sequence number,
	  (b) out-of-window acknowledgment number, or
	  (c) PAWS (Protection Against Wrapped Sequence numbers) check failure

	This can help mitigate simple "ack loop" DoS attacks, wherein
	a buggy or malicious middlebox or man-in-the-middle can
	rewrite TCP header fields in manner that causes each endpoint
	to think that the other is sending invalid TCP segments, thus
	causing each side to send an unterminating stream of duplicate
	acknowledgments for invalid segments.

	Using 0 disables rate-limiting of dupacks in response to
	invalid segments; otherwise this value specifies the minimal
	space between sending such dupacks, in milliseconds.

	Default: 500 (milliseconds).

tcp_keepalive_time - INTEGER
	How often TCP sends out keepalive messages when keepalive is enabled.
	Default: 2hours.

tcp_keepalive_probes - INTEGER
	How many keepalive probes TCP sends out, until it decides that the
	connection is broken. Default value: 9.

tcp_keepalive_intvl - INTEGER
	How frequently the probes are send out. Multiplied by
	tcp_keepalive_probes it is time to kill not responding connection,
	after probes started. Default value: 75sec i.e. connection
	will be aborted after ~11 minutes of retries.

tcp_l3mdev_accept - BOOLEAN
	Enables child sockets to inherit the L3 master device index.
	Enabling this option allows a "global" listen socket to work
	across L3 master domains (e.g., VRFs) with connected sockets
	derived from the listen socket to be bound to the L3 domain in
	which the packets originated. Only valid when the kernel was
	compiled with CONFIG_NET_L3_MASTER_DEV.

	Default: 0 (disabled)

tcp_low_latency - BOOLEAN
	This is a legacy option, it has no effect anymore.

tcp_max_orphans - INTEGER
	Maximal number of TCP sockets not attached to any user file handle,
	held by system.	If this number is exceeded orphaned connections are
	reset immediately and warning is printed. This limit exists
	only to prevent simple DoS attacks, you _must_ not rely on this
	or lower the limit artificially, but rather increase it
	(probably, after increasing installed memory),
	if network conditions require more than default value,
	and tune network services to linger and kill such states
	more aggressively. Let me to remind again: each orphan eats
	up to ~64K of unswappable memory.

tcp_max_syn_backlog - INTEGER
	Maximal number of remembered connection requests (SYN_RECV),
	which have not received an acknowledgment from connecting client.

	This is a per-listener limit.

	The minimal value is 128 for low memory machines, and it will
	increase in proportion to the memory of machine.

	If server suffers from overload, try increasing this number.

	Remember to also check /proc/sys/net/core/somaxconn
	A SYN_RECV request socket consumes about 304 bytes of memory.

tcp_max_tw_buckets - INTEGER
	Maximal number of timewait sockets held by system simultaneously.
	If this number is exceeded time-wait socket is immediately destroyed
	and warning is printed. This limit exists only to prevent
	simple DoS attacks, you _must_ not lower the limit artificially,
	but rather increase it (probably, after increasing installed memory),
	if network conditions require more than default value.

tcp_mem - vector of 3 INTEGERs: min, pressure, max
	min: below this number of pages TCP is not bothered about its
	memory appetite.

	pressure: when amount of memory allocated by TCP exceeds this number
	of pages, TCP moderates its memory consumption and enters memory
	pressure mode, which is exited when memory consumption falls
	under "min".

	max: number of pages allowed for queueing by all TCP sockets.

	Defaults are calculated at boot time from amount of available
	memory.

tcp_min_rtt_wlen - INTEGER
	The window length of the windowed min filter to track the minimum RTT.
	A shorter window lets a flow more quickly pick up new (higher)
	minimum RTT when it is moved to a longer path (e.g., due to traffic
	engineering). A longer window makes the filter more resistant to RTT
	inflations such as transient congestion. The unit is seconds.

	Possible values: 0 - 86400 (1 day)

	Default: 300

tcp_moderate_rcvbuf - BOOLEAN
	If set, TCP performs receive buffer auto-tuning, attempting to
	automatically size the buffer (no greater than tcp_rmem[2]) to
	match the size required by the path for full throughput.  Enabled by
	default.

tcp_mtu_probing - INTEGER
	Controls TCP Packetization-Layer Path MTU Discovery.  Takes three
	values:

	- 0 - Disabled
	- 1 - Disabled by default, enabled when an ICMP black hole detected
	- 2 - Always enabled, use initial MSS of tcp_base_mss.

tcp_probe_interval - UNSIGNED INTEGER
	Controls how often to start TCP Packetization-Layer Path MTU
	Discovery reprobe. The default is reprobing every 10 minutes as
	per RFC4821.

tcp_probe_threshold - INTEGER
	Controls when TCP Packetization-Layer Path MTU Discovery probing
	will stop in respect to the width of search range in bytes. Default
	is 8 bytes.

tcp_no_metrics_save - BOOLEAN
	By default, TCP saves various connection metrics in the route cache
	when the connection closes, so that connections established in the
	near future can use these to set initial conditions.  Usually, this
	increases overall performance, but may sometimes cause performance
	degradation.  If set, TCP will not cache metrics on closing
	connections.

tcp_no_ssthresh_metrics_save - BOOLEAN
	Controls whether TCP saves ssthresh metrics in the route cache.

	Default is 1, which disables ssthresh metrics.

tcp_orphan_retries - INTEGER
	This value influences the timeout of a locally closed TCP connection,
	when RTO retransmissions remain unacknowledged.
	See tcp_retries2 for more details.

	The default value is 8.

	If your machine is a loaded WEB server,
	you should think about lowering this value, such sockets
	may consume significant resources. Cf. tcp_max_orphans.

tcp_recovery - INTEGER
	This value is a bitmap to enable various experimental loss recovery
	features.

	=========   =============================================================
	RACK: 0x1   enables the RACK loss detection for fast detection of lost
		    retransmissions and tail drops. It also subsumes and disables
		    RFC6675 recovery for SACK connections.

	RACK: 0x2   makes RACK's reordering window static (min_rtt/4).

	RACK: 0x4   disables RACK's DUPACK threshold heuristic
	=========   =============================================================

	Default: 0x1

tcp_reordering - INTEGER
	Initial reordering level of packets in a TCP stream.
	TCP stack can then dynamically adjust flow reordering level
	between this initial value and tcp_max_reordering

	Default: 3

tcp_max_reordering - INTEGER
	Maximal reordering level of packets in a TCP stream.
	300 is a fairly conservative value, but you might increase it
	if paths are using per packet load balancing (like bonding rr mode)

	Default: 300

tcp_retrans_collapse - BOOLEAN
	Bug-to-bug compatibility with some broken printers.
	On retransmit try to send bigger packets to work around bugs in
	certain TCP stacks.

tcp_retries1 - INTEGER
	This value influences the time, after which TCP decides, that
	something is wrong due to unacknowledged RTO retransmissions,
	and reports this suspicion to the network layer.
	See tcp_retries2 for more details.

	RFC 1122 recommends at least 3 retransmissions, which is the
	default.

tcp_retries2 - INTEGER
	This value influences the timeout of an alive TCP connection,
	when RTO retransmissions remain unacknowledged.
	Given a value of N, a hypothetical TCP connection following
	exponential backoff with an initial RTO of TCP_RTO_MIN would
	retransmit N times before killing the connection at the (N+1)th RTO.

	The default value of 15 yields a hypothetical timeout of 924.6
	seconds and is a lower bound for the effective timeout.
	TCP will effectively time out at the first RTO which exceeds the
	hypothetical timeout.

	RFC 1122 recommends at least 100 seconds for the timeout,
	which corresponds to a value of at least 8.

tcp_rfc1337 - BOOLEAN
	If set, the TCP stack behaves conforming to RFC1337. If unset,
	we are not conforming to RFC, but prevent TCP TIME_WAIT
	assassination.

	Default: 0

tcp_rmem - vector of 3 INTEGERs: min, default, max
	min: Minimal size of receive buffer used by TCP sockets.
	It is guaranteed to each TCP socket, even under moderate memory
	pressure.

	Default: 4K

	default: initial size of receive buffer used by TCP sockets.
	This value overrides net.core.rmem_default used by other protocols.
	Default: 87380 bytes. This value results in window of 65535 with
	default setting of tcp_adv_win_scale and tcp_app_win:0 and a bit
	less for default tcp_app_win. See below about these variables.

	max: maximal size of receive buffer allowed for automatically
	selected receiver buffers for TCP socket. This value does not override
	net.core.rmem_max.  Calling setsockopt() with SO_RCVBUF disables
	automatic tuning of that socket's receive buffer size, in which
	case this value is ignored.
	Default: between 87380B and 6MB, depending on RAM size.

tcp_sack - BOOLEAN
	Enable select acknowledgments (SACKS).

tcp_comp_sack_delay_ns - LONG INTEGER
	TCP tries to reduce number of SACK sent, using a timer
	based on 5% of SRTT, capped by this sysctl, in nano seconds.
	The default is 1ms, based on TSO autosizing period.

	Default : 1,000,000 ns (1 ms)

tcp_comp_sack_nr - INTEGER
	Max number of SACK that can be compressed.
	Using 0 disables SACK compression.

	Default : 44

tcp_slow_start_after_idle - BOOLEAN
	If set, provide RFC2861 behavior and time out the congestion
	window after an idle period.  An idle period is defined at
	the current RTO.  If unset, the congestion window will not
	be timed out after an idle period.

	Default: 1

tcp_stdurg - BOOLEAN
	Use the Host requirements interpretation of the TCP urgent pointer field.
	Most hosts use the older BSD interpretation, so if you turn this on
	Linux might not communicate correctly with them.

	Default: FALSE

tcp_synack_retries - INTEGER
	Number of times SYNACKs for a passive TCP connection attempt will
	be retransmitted. Should not be higher than 255. Default value
	is 5, which corresponds to 31seconds till the last retransmission
	with the current initial RTO of 1second. With this the final timeout
	for a passive TCP connection will happen after 63seconds.

tcp_syncookies - INTEGER
	Only valid when the kernel was compiled with CONFIG_SYN_COOKIES
	Send out syncookies when the syn backlog queue of a socket
	overflows. This is to prevent against the common 'SYN flood attack'
	Default: 1

	Note, that syncookies is fallback facility.
	It MUST NOT be used to help highly loaded servers to stand
	against legal connection rate. If you see SYN flood warnings
	in your logs, but investigation	shows that they occur
	because of overload with legal connections, you should tune
	another parameters until this warning disappear.
	See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow.

	syncookies seriously violate TCP protocol, do not allow
	to use TCP extensions, can result in serious degradation
	of some services (f.e. SMTP relaying), visible not by you,
	but your clients and relays, contacting you. While you see
	SYN flood warnings in logs not being really flooded, your server
	is seriously misconfigured.

	If you want to test which effects syncookies have to your
	network connections you can set this knob to 2 to enable
	unconditionally generation of syncookies.

tcp_fastopen - INTEGER
	Enable TCP Fast Open (RFC7413) to send and accept data in the opening
	SYN packet.

	The client support is enabled by flag 0x1 (on by default). The client
	then must use sendmsg() or sendto() with the MSG_FASTOPEN flag,
	rather than connect() to send data in SYN.

	The server support is enabled by flag 0x2 (off by default). Then
	either enable for all listeners with another flag (0x400) or
	enable individual listeners via TCP_FASTOPEN socket option with
	the option value being the length of the syn-data backlog.

	The values (bitmap) are

	=====  ======== ======================================================
	  0x1  (client) enables sending data in the opening SYN on the client.
	  0x2  (server) enables the server support, i.e., allowing data in
			a SYN packet to be accepted and passed to the
			application before 3-way handshake finishes.
	  0x4  (client) send data in the opening SYN regardless of cookie
			availability and without a cookie option.
	0x200  (server) accept data-in-SYN w/o any cookie option present.
	0x400  (server) enable all listeners to support Fast Open by
			default without explicit TCP_FASTOPEN socket option.
	=====  ======== ======================================================

	Default: 0x1

	Note that that additional client or server features are only
	effective if the basic support (0x1 and 0x2) are enabled respectively.

tcp_fastopen_blackhole_timeout_sec - INTEGER
	Initial time period in second to disable Fastopen on active TCP sockets
	when a TFO firewall blackhole issue happens.
	This time period will grow exponentially when more blackhole issues
	get detected right after Fastopen is re-enabled and will reset to
	initial value when the blackhole issue goes away.
	0 to disable the blackhole detection.

	By default, it is set to 1hr.

tcp_fastopen_key - list of comma separated 32-digit hexadecimal INTEGERs
	The list consists of a primary key and an optional backup key. The
	primary key is used for both creating and validating cookies, while the
	optional backup key is only used for validating cookies. The purpose of
	the backup key is to maximize TFO validation when keys are rotated.

	A randomly chosen primary key may be configured by the kernel if
	the tcp_fastopen sysctl is set to 0x400 (see above), or if the
	TCP_FASTOPEN setsockopt() optname is set and a key has not been
	previously configured via sysctl. If keys are configured via
	setsockopt() by using the TCP_FASTOPEN_KEY optname, then those
	per-socket keys will be used instead of any keys that are specified via
	sysctl.

	A key is specified as 4 8-digit hexadecimal integers which are separated
	by a '-' as: xxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxx. Leading zeros may be
	omitted. A primary and a backup key may be specified by separating them
	by a comma. If only one key is specified, it becomes the primary key and
	any previously configured backup keys are removed.

tcp_syn_retries - INTEGER
	Number of times initial SYNs for an active TCP connection attempt
	will be retransmitted. Should not be higher than 127. Default value
	is 6, which corresponds to 63seconds till the last retransmission
	with the current initial RTO of 1second. With this the final timeout
	for an active TCP connection attempt will happen after 127seconds.

tcp_timestamps - INTEGER
	Enable timestamps as defined in RFC1323.

	- 0: Disabled.
	- 1: Enable timestamps as defined in RFC1323 and use random offset for
	  each connection rather than only using the current time.
	- 2: Like 1, but without random offsets.

	Default: 1

tcp_min_tso_segs - INTEGER
	Minimal number of segments per TSO frame.

	Since linux-3.12, TCP does an automatic sizing of TSO frames,
	depending on flow rate, instead of filling 64Kbytes packets.
	For specific usages, it's possible to force TCP to build big
	TSO frames. Note that TCP stack might split too big TSO packets
	if available window is too small.

	Default: 2

tcp_pacing_ss_ratio - INTEGER
	sk->sk_pacing_rate is set by TCP stack using a ratio applied
	to current rate. (current_rate = cwnd * mss / srtt)
	If TCP is in slow start, tcp_pacing_ss_ratio is applied
	to let TCP probe for bigger speeds, assuming cwnd can be
	doubled every other RTT.

	Default: 200

tcp_pacing_ca_ratio - INTEGER
	sk->sk_pacing_rate is set by TCP stack using a ratio applied
	to current rate. (current_rate = cwnd * mss / srtt)
	If TCP is in congestion avoidance phase, tcp_pacing_ca_ratio
	is applied to conservatively probe for bigger throughput.

	Default: 120

tcp_tso_win_divisor - INTEGER
	This allows control over what percentage of the congestion window
	can be consumed by a single TSO frame.
	The setting of this parameter is a choice between burstiness and
	building larger TSO frames.

	Default: 3

tcp_tw_reuse - INTEGER
	Enable reuse of TIME-WAIT sockets for new connections when it is
	safe from protocol viewpoint.

	- 0 - disable
	- 1 - global enable
	- 2 - enable for loopback traffic only

	It should not be changed without advice/request of technical
	experts.

	Default: 2

tcp_window_scaling - BOOLEAN
	Enable window scaling as defined in RFC1323.

tcp_wmem - vector of 3 INTEGERs: min, default, max
	min: Amount of memory reserved for send buffers for TCP sockets.
	Each TCP socket has rights to use it due to fact of its birth.

	Default: 4K

	default: initial size of send buffer used by TCP sockets.  This
	value overrides net.core.wmem_default used by other protocols.

	It is usually lower than net.core.wmem_default.

	Default: 16K

	max: Maximal amount of memory allowed for automatically tuned
	send buffers for TCP sockets. This value does not override
	net.core.wmem_max.  Calling setsockopt() with SO_SNDBUF disables
	automatic tuning of that socket's send buffer size, in which case
	this value is ignored.

	Default: between 64K and 4MB, depending on RAM size.

tcp_notsent_lowat - UNSIGNED INTEGER
	A TCP socket can control the amount of unsent bytes in its write queue,
	thanks to TCP_NOTSENT_LOWAT socket option. poll()/select()/epoll()
	reports POLLOUT events if the amount of unsent bytes is below a per
	socket value, and if the write queue is not full. sendmsg() will
	also not add new buffers if the limit is hit.

	This global variable controls the amount of unsent data for
	sockets not using TCP_NOTSENT_LOWAT. For these sockets, a change
	to the global variable has immediate effect.

	Default: UINT_MAX (0xFFFFFFFF)

tcp_workaround_signed_windows - BOOLEAN
	If set, assume no receipt of a window scaling option means the
	remote TCP is broken and treats the window as a signed quantity.
	If unset, assume the remote TCP is not broken even if we do
	not receive a window scaling option from them.

	Default: 0

tcp_thin_linear_timeouts - BOOLEAN
	Enable dynamic triggering of linear timeouts for thin streams.
	If set, a check is performed upon retransmission by timeout to
	determine if the stream is thin (less than 4 packets in flight).
	As long as the stream is found to be thin, up to 6 linear
	timeouts may be performed before exponential backoff mode is
	initiated. This improves retransmission latency for
	non-aggressive thin streams, often found to be time-dependent.
	For more information on thin streams, see
	Documentation/networking/tcp-thin.rst

	Default: 0

tcp_limit_output_bytes - INTEGER
	Controls TCP Small Queue limit per tcp socket.
	TCP bulk sender tends to increase packets in flight until it
	gets losses notifications. With SNDBUF autotuning, this can
	result in a large amount of packets queued on the local machine
	(e.g.: qdiscs, CPU backlog, or device) hurting latency of other
	flows, for typical pfifo_fast qdiscs.  tcp_limit_output_bytes
	limits the number of bytes on qdisc or device to reduce artificial
	RTT/cwnd and reduce bufferbloat.

	Default: 1048576 (16 * 65536)

tcp_challenge_ack_limit - INTEGER
	Limits number of Challenge ACK sent per second, as recommended
	in RFC 5961 (Improving TCP's Robustness to Blind In-Window Attacks)
	Default: 1000

tcp_rx_skb_cache - BOOLEAN
	Controls a per TCP socket cache of one skb, that might help
	performance of some workloads. This might be dangerous
	on systems with a lot of TCP sockets, since it increases
	memory usage.

	Default: 0 (disabled)

UDP variables
=============

udp_l3mdev_accept - BOOLEAN
	Enabling this option allows a "global" bound socket to work
	across L3 master domains (e.g., VRFs) with packets capable of
	being received regardless of the L3 domain in which they
	originated. Only valid when the kernel was compiled with
	CONFIG_NET_L3_MASTER_DEV.

	Default: 0 (disabled)

udp_mem - vector of 3 INTEGERs: min, pressure, max
	Number of pages allowed for queueing by all UDP sockets.

	min: Below this number of pages UDP is not bothered about its
	memory appetite. When amount of memory allocated by UDP exceeds
	this number, UDP starts to moderate memory usage.

	pressure: This value was introduced to follow format of tcp_mem.

	max: Number of pages allowed for queueing by all UDP sockets.

	Default is calculated at boot time from amount of available memory.

udp_rmem_min - INTEGER
	Minimal size of receive buffer used by UDP sockets in moderation.
	Each UDP socket is able to use the size for receiving data, even if
	total pages of UDP sockets exceed udp_mem pressure. The unit is byte.

	Default: 4K

udp_wmem_min - INTEGER
	Minimal size of send buffer used by UDP sockets in moderation.
	Each UDP socket is able to use the size for sending data, even if
	total pages of UDP sockets exceed udp_mem pressure. The unit is byte.

	Default: 4K

RAW variables
=============

raw_l3mdev_accept - BOOLEAN
	Enabling this option allows a "global" bound socket to work
	across L3 master domains (e.g., VRFs) with packets capable of
	being received regardless of the L3 domain in which they
	originated. Only valid when the kernel was compiled with
	CONFIG_NET_L3_MASTER_DEV.

	Default: 1 (enabled)

CIPSOv4 Variables
=================

cipso_cache_enable - BOOLEAN
	If set, enable additions to and lookups from the CIPSO label mapping
	cache.  If unset, additions are ignored and lookups always result in a
	miss.  However, regardless of the setting the cache is still
	invalidated when required when means you can safely toggle this on and
	off and the cache will always be "safe".

	Default: 1

cipso_cache_bucket_size - INTEGER
	The CIPSO label cache consists of a fixed size hash table with each
	hash bucket containing a number of cache entries.  This variable limits
	the number of entries in each hash bucket; the larger the value the
	more CIPSO label mappings that can be cached.  When the number of
	entries in a given hash bucket reaches this limit adding new entries
	causes the oldest entry in the bucket to be removed to make room.

	Default: 10

cipso_rbm_optfmt - BOOLEAN
	Enable the "Optimized Tag 1 Format" as defined in section 3.4.2.6 of
	the CIPSO draft specification (see Documentation/netlabel for details).
	This means that when set the CIPSO tag will be padded with empty
	categories in order to make the packet data 32-bit aligned.

	Default: 0

cipso_rbm_structvalid - BOOLEAN
	If set, do a very strict check of the CIPSO option when
	ip_options_compile() is called.  If unset, relax the checks done during
	ip_options_compile().  Either way is "safe" as errors are caught else
	where in the CIPSO processing code but setting this to 0 (False) should
	result in less work (i.e. it should be faster) but could cause problems
	with other implementations that require strict checking.

	Default: 0

IP Variables
============

ip_local_port_range - 2 INTEGERS
	Defines the local port range that is used by TCP and UDP to
	choose the local port. The first number is the first, the
	second the last local port number.
	If possible, it is better these numbers have different parity
	(one even and one odd value).
	Must be greater than or equal to ip_unprivileged_port_start.
	The default values are 32768 and 60999 respectively.

ip_local_reserved_ports - list of comma separated ranges
	Specify the ports which are reserved for known third-party
	applications. These ports will not be used by automatic port
	assignments (e.g. when calling connect() or bind() with port
	number 0). Explicit port allocation behavior is unchanged.

	The format used for both input and output is a comma separated
	list of ranges (e.g. "1,2-4,10-10" for ports 1, 2, 3, 4 and
	10). Writing to the file will clear all previously reserved
	ports and update the current list with the one given in the
	input.

	Note that ip_local_port_range and ip_local_reserved_ports
	settings are independent and both are considered by the kernel
	when determining which ports are available for automatic port
	assignments.

	You can reserve ports which are not in the current
	ip_local_port_range, e.g.::

	    $ cat /proc/sys/net/ipv4/ip_local_port_range
	    32000	60999
	    $ cat /proc/sys/net/ipv4/ip_local_reserved_ports
	    8080,9148

	although this is redundant. However such a setting is useful
	if later the port range is changed to a value that will
	include the reserved ports.

	Default: Empty

ip_unprivileged_port_start - INTEGER
	This is a per-namespace sysctl.  It defines the first
	unprivileged port in the network namespace.  Privileged ports
	require root or CAP_NET_BIND_SERVICE in order to bind to them.
	To disable all privileged ports, set this to 0.  They must not
	overlap with the ip_local_port_range.

	Default: 1024

ip_nonlocal_bind - BOOLEAN
	If set, allows processes to bind() to non-local IP addresses,
	which can be quite useful - but may break some applications.

	Default: 0

ip_autobind_reuse - BOOLEAN
	By default, bind() does not select the ports automatically even if
	the new socket and all sockets bound to the port have SO_REUSEADDR.
	ip_autobind_reuse allows bind() to reuse the port and this is useful
	when you use bind()+connect(), but may break some applications.
	The preferred solution is to use IP_BIND_ADDRESS_NO_PORT and this
	option should only be set by experts.
	Default: 0

ip_dynaddr - BOOLEAN
	If set non-zero, enables support for dynamic addresses.
	If set to a non-zero value larger than 1, a kernel log
	message will be printed when dynamic address rewriting
	occurs.

	Default: 0

ip_early_demux - BOOLEAN
	Optimize input packet processing down to one demux for
	certain kinds of local sockets.  Currently we only do this
	for established TCP and connected UDP sockets.

	It may add an additional cost for pure routing workloads that
	reduces overall throughput, in such case you should disable it.

	Default: 1

ping_group_range - 2 INTEGERS
	Restrict ICMP_PROTO datagram sockets to users in the group range.
	The default is "1 0", meaning, that nobody (not even root) may
	create ping sockets.  Setting it to "100 100" would grant permissions
	to the single group. "0 4294967295" would enable it for the world, "100
	4294967295" would enable it for the users, but not daemons.

tcp_early_demux - BOOLEAN
	Enable early demux for established TCP sockets.

	Default: 1

udp_early_demux - BOOLEAN
	Enable early demux for connected UDP sockets. Disable this if
	your system could experience more unconnected load.

	Default: 1

icmp_echo_ignore_all - BOOLEAN
	If set non-zero, then the kernel will ignore all ICMP ECHO
	requests sent to it.

	Default: 0

icmp_echo_ignore_broadcasts - BOOLEAN
	If set non-zero, then the kernel will ignore all ICMP ECHO and
	TIMESTAMP requests sent to it via broadcast/multicast.

	Default: 1

icmp_ratelimit - INTEGER
	Limit the maximal rates for sending ICMP packets whose type matches
	icmp_ratemask (see below) to specific targets.
	0 to disable any limiting,
	otherwise the minimal space between responses in milliseconds.
	Note that another sysctl, icmp_msgs_per_sec limits the number
	of ICMP packets	sent on all targets.

	Default: 1000

icmp_msgs_per_sec - INTEGER
	Limit maximal number of ICMP packets sent per second from this host.
	Only messages whose type matches icmp_ratemask (see below) are
	controlled by this limit.

	Default: 1000

icmp_msgs_burst - INTEGER
	icmp_msgs_per_sec controls number of ICMP packets sent per second,
	while icmp_msgs_burst controls the burst size of these packets.

	Default: 50

icmp_ratemask - INTEGER
	Mask made of ICMP types for which rates are being limited.

	Significant bits: IHGFEDCBA9876543210

	Default mask:     0000001100000011000 (6168)

	Bit definitions (see include/linux/icmp.h):

		= =========================
		0 Echo Reply
		3 Destination Unreachable [1]_
		4 Source Quench [1]_
		5 Redirect
		8 Echo Request
		B Time Exceeded [1]_
		C Parameter Problem [1]_
		D Timestamp Request
		E Timestamp Reply
		F Info Request
		G Info Reply
		H Address Mask Request
		I Address Mask Reply
		= =========================

	.. [1] These are rate limited by default (see default mask above)

icmp_ignore_bogus_error_responses - BOOLEAN
	Some routers violate RFC1122 by sending bogus responses to broadcast
	frames.  Such violations are normally logged via a kernel warning.
	If this is set to TRUE, the kernel will not give such warnings, which
	will avoid log file clutter.

	Default: 1

icmp_errors_use_inbound_ifaddr - BOOLEAN

	If zero, icmp error messages are sent with the primary address of
	the exiting interface.

	If non-zero, the message will be sent with the primary address of
	the interface that received the packet that caused the icmp error.
	This is the behaviour network many administrators will expect from
	a router. And it can make debugging complicated network layouts
	much easier.

	Note that if no primary address exists for the interface selected,
	then the primary address of the first non-loopback interface that
	has one will be used regardless of this setting.

	Default: 0

igmp_max_memberships - INTEGER
	Change the maximum number of multicast groups we can subscribe to.
	Default: 20

	Theoretical maximum value is bounded by having to send a membership
	report in a single datagram (i.e. the report can't span multiple
	datagrams, or risk confusing the switch and leaving groups you don't
	intend to).

	The number of supported groups 'M' is bounded by the number of group
	report entries you can fit into a single datagram of 65535 bytes.

	M = 65536-sizeof (ip header)/(sizeof(Group record))

	Group records are variable length, with a minimum of 12 bytes.
	So net.ipv4.igmp_max_memberships should not be set higher than:

	(65536-24) / 12 = 5459

	The value 5459 assumes no IP header options, so in practice
	this number may be lower.

igmp_max_msf - INTEGER
	Maximum number of addresses allowed in the source filter list for a
	multicast group.

	Default: 10

igmp_qrv - INTEGER
	Controls the IGMP query robustness variable (see RFC2236 8.1).

	Default: 2 (as specified by RFC2236 8.1)

	Minimum: 1 (as specified by RFC6636 4.5)

force_igmp_version - INTEGER
	- 0 - (default) No enforcement of a IGMP version, IGMPv1/v2 fallback
	  allowed. Will back to IGMPv3 mode again if all IGMPv1/v2 Querier
	  Present timer expires.
	- 1 - Enforce to use IGMP version 1. Will also reply IGMPv1 report if
	  receive IGMPv2/v3 query.
	- 2 - Enforce to use IGMP version 2. Will fallback to IGMPv1 if receive
	  IGMPv1 query message. Will reply report if receive IGMPv3 query.
	- 3 - Enforce to use IGMP version 3. The same react with default 0.

	.. note::

	   this is not the same with force_mld_version because IGMPv3 RFC3376
	   Security Considerations does not have clear description that we could
	   ignore other version messages completely as MLDv2 RFC3810. So make
	   this value as default 0 is recommended.

``conf/interface/*``
	changes special settings per interface (where
	interface" is the name of your network interface)

``conf/all/*``
	  is special, changes the settings for all interfaces

log_martians - BOOLEAN
	Log packets with impossible addresses to kernel log.
	log_martians for the interface will be enabled if at least one of
	conf/{all,interface}/log_martians is set to TRUE,
	it will be disabled otherwise

accept_redirects - BOOLEAN
	Accept ICMP redirect messages.
	accept_redirects for the interface will be enabled if:

	- both conf/{all,interface}/accept_redirects are TRUE in the case
	  forwarding for the interface is enabled

	or

	- at least one of conf/{all,interface}/accept_redirects is TRUE in the
	  case forwarding for the interface is disabled

	accept_redirects for the interface will be disabled otherwise

	default:

		- TRUE (host)
		- FALSE (router)

forwarding - BOOLEAN
	Enable IP forwarding on this interface.  This controls whether packets
	received _on_ this interface can be forwarded.

mc_forwarding - BOOLEAN
	Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE
	and a multicast routing daemon is required.
	conf/all/mc_forwarding must also be set to TRUE to enable multicast
	routing	for the interface

medium_id - INTEGER
	Integer value used to differentiate the devices by the medium they
	are attached to. Two devices can have different id values when
	the broadcast packets are received only on one of them.
	The default value 0 means that the device is the only interface
	to its medium, value of -1 means that medium is not known.

	Currently, it is used to change the proxy_arp behavior:
	the proxy_arp feature is enabled for packets forwarded between
	two devices attached to different media.

proxy_arp - BOOLEAN
	Do proxy arp.

	proxy_arp for the interface will be enabled if at least one of
	conf/{all,interface}/proxy_arp is set to TRUE,
	it will be disabled otherwise

proxy_arp_pvlan - BOOLEAN
	Private VLAN proxy arp.

	Basically allow proxy arp replies back to the same interface
	(from which the ARP request/solicitation was received).

	This is done to support (ethernet) switch features, like RFC
	3069, where the individual ports are NOT allowed to
	communicate with each other, but they are allowed to talk to
	the upstream router.  As described in RFC 3069, it is possible
	to allow these hosts to communicate through the upstream
	router by proxy_arp'ing. Don't need to be used together with
	proxy_arp.

	This technology is known by different names:

	  In RFC 3069 it is called VLAN Aggregation.
	  Cisco and Allied Telesyn call it Private VLAN.
	  Hewlett-Packard call it Source-Port filtering or port-isolation.
	  Ericsson call it MAC-Forced Forwarding (RFC Draft).

shared_media - BOOLEAN
	Send(router) or accept(host) RFC1620 shared media redirects.
	Overrides secure_redirects.

	shared_media for the interface will be enabled if at least one of
	conf/{all,interface}/shared_media is set to TRUE,
	it will be disabled otherwise

	default TRUE

secure_redirects - BOOLEAN
	Accept ICMP redirect messages only to gateways listed in the
	interface's current gateway list. Even if disabled, RFC1122 redirect
	rules still apply.

	Overridden by shared_media.

	secure_redirects for the interface will be enabled if at least one of
	conf/{all,interface}/secure_redirects is set to TRUE,
	it will be disabled otherwise

	default TRUE

send_redirects - BOOLEAN
	Send redirects, if router.

	send_redirects for the interface will be enabled if at least one of
	conf/{all,interface}/send_redirects is set to TRUE,
	it will be disabled otherwise

	Default: TRUE

bootp_relay - BOOLEAN
	Accept packets with source address 0.b.c.d destined
	not to this host as local ones. It is supposed, that
	BOOTP relay daemon will catch and forward such packets.
	conf/all/bootp_relay must also be set to TRUE to enable BOOTP relay
	for the interface

	default FALSE

	Not Implemented Yet.

accept_source_route - BOOLEAN
	Accept packets with SRR option.
	conf/all/accept_source_route must also be set to TRUE to accept packets
	with SRR option on the interface

	default

		- TRUE (router)
		- FALSE (host)

accept_local - BOOLEAN
	Accept packets with local source addresses. In combination with
	suitable routing, this can be used to direct packets between two
	local interfaces over the wire and have them accepted properly.
	default FALSE

route_localnet - BOOLEAN
	Do not consider loopback addresses as martian source or destination
	while routing. This enables the use of 127/8 for local routing purposes.

	default FALSE

rp_filter - INTEGER
	- 0 - No source validation.
	- 1 - Strict mode as defined in RFC3704 Strict Reverse Path
	  Each incoming packet is tested against the FIB and if the interface
	  is not the best reverse path the packet check will fail.
	  By default failed packets are discarded.
	- 2 - Loose mode as defined in RFC3704 Loose Reverse Path
	  Each incoming packet's source address is also tested against the FIB
	  and if the source address is not reachable via any interface
	  the packet check will fail.

	Current recommended practice in RFC3704 is to enable strict mode
	to prevent IP spoofing from DDos attacks. If using asymmetric routing
	or other complicated routing, then loose mode is recommended.

	The max value from conf/{all,interface}/rp_filter is used
	when doing source validation on the {interface}.

	Default value is 0. Note that some distributions enable it
	in startup scripts.

arp_filter - BOOLEAN
	- 1 - Allows you to have multiple network interfaces on the same
	  subnet, and have the ARPs for each interface be answered
	  based on whether or not the kernel would route a packet from
	  the ARP'd IP out that interface (therefore you must use source
	  based routing for this to work). In other words it allows control
	  of which cards (usually 1) will respond to an arp request.

	- 0 - (default) The kernel can respond to arp requests with addresses
	  from other interfaces. This may seem wrong but it usually makes
	  sense, because it increases the chance of successful communication.
	  IP addresses are owned by the complete host on Linux, not by
	  particular interfaces. Only for more complex setups like load-
	  balancing, does this behaviour cause problems.

	arp_filter for the interface will be enabled if at least one of
	conf/{all,interface}/arp_filter is set to TRUE,
	it will be disabled otherwise

arp_announce - INTEGER
	Define different restriction levels for announcing the local
	source IP address from IP packets in ARP requests sent on
	interface:

	- 0 - (default) Use any local address, configured on any interface
	- 1 - Try to avoid local addresses that are not in the target's
	  subnet for this interface. This mode is useful when target
	  hosts reachable via this interface require the source IP
	  address in ARP requests to be part of their logical network
	  configured on the receiving interface. When we generate the
	  request we will check all our subnets that include the
	  target IP and will preserve the source address if it is from
	  such subnet. If there is no such subnet we select source
	  address according to the rules for level 2.
	- 2 - Always use the best local address for this target.
	  In this mode we ignore the source address in the IP packet
	  and try to select local address that we prefer for talks with
	  the target host. Such local address is selected by looking
	  for primary IP addresses on all our subnets on the outgoing
	  interface that include the target IP address. If no suitable
	  local address is found we select the first local address
	  we have on the outgoing interface or on all other interfaces,
	  with the hope we will receive reply for our request and
	  even sometimes no matter the source IP address we announce.

	The max value from conf/{all,interface}/arp_announce is used.

	Increasing the restriction level gives more chance for
	receiving answer from the resolved target while decreasing
	the level announces more valid sender's information.

arp_ignore - INTEGER
	Define different modes for sending replies in response to
	received ARP requests that resolve local target IP addresses:

	- 0 - (default): reply for any local target IP address, configured
	  on any interface
	- 1 - reply only if the target IP address is local address
	  configured on the incoming interface
	- 2 - reply only if the target IP address is local address
	  configured on the incoming interface and both with the
	  sender's IP address are part from same subnet on this interface
	- 3 - do not reply for local addresses configured with scope host,
	  only resolutions for global and link addresses are replied
	- 4-7 - reserved
	- 8 - do not reply for all local addresses

	The max value from conf/{all,interface}/arp_ignore is used
	when ARP request is received on the {interface}

arp_notify - BOOLEAN
	Define mode for notification of address and device changes.

	 ==  ==========================================================
	  0  (default): do nothing
	  1  Generate gratuitous arp requests when device is brought up
	     or hardware address changes.
	 ==  ==========================================================

arp_accept - BOOLEAN
	Define behavior for gratuitous ARP frames who's IP is not
	already present in the ARP table:

	- 0 - don't create new entries in the ARP table
	- 1 - create new entries in the ARP table

	Both replies and requests type gratuitous arp will trigger the
	ARP table to be updated, if this setting is on.

	If the ARP table already contains the IP address of the
	gratuitous arp frame, the arp table will be updated regardless
	if this setting is on or off.

mcast_solicit - INTEGER
	The maximum number of multicast probes in INCOMPLETE state,
	when the associated hardware address is unknown.  Defaults
	to 3.

ucast_solicit - INTEGER
	The maximum number of unicast probes in PROBE state, when
	the hardware address is being reconfirmed.  Defaults to 3.

app_solicit - INTEGER
	The maximum number of probes to send to the user space ARP daemon
	via netlink before dropping back to multicast probes (see
	mcast_resolicit).  Defaults to 0.

mcast_resolicit - INTEGER
	The maximum number of multicast probes after unicast and
	app probes in PROBE state.  Defaults to 0.

disable_policy - BOOLEAN
	Disable IPSEC policy (SPD) for this interface

disable_xfrm - BOOLEAN
	Disable IPSEC encryption on this interface, whatever the policy

igmpv2_unsolicited_report_interval - INTEGER
	The interval in milliseconds in which the next unsolicited
	IGMPv1 or IGMPv2 report retransmit will take place.

	Default: 10000 (10 seconds)

igmpv3_unsolicited_report_interval - INTEGER
	The interval in milliseconds in which the next unsolicited
	IGMPv3 report retransmit will take place.

	Default: 1000 (1 seconds)

promote_secondaries - BOOLEAN
	When a primary IP address is removed from this interface
	promote a corresponding secondary IP address instead of
	removing all the corresponding secondary IP addresses.

drop_unicast_in_l2_multicast - BOOLEAN
	Drop any unicast IP packets that are received in link-layer
	multicast (or broadcast) frames.

	This behavior (for multicast) is actually a SHOULD in RFC
	1122, but is disabled by default for compatibility reasons.

	Default: off (0)

drop_gratuitous_arp - BOOLEAN
	Drop all gratuitous ARP frames, for example if there's a known
	good ARP proxy on the network and such frames need not be used
	(or in the case of 802.11, must not be used to prevent attacks.)

	Default: off (0)


tag - INTEGER
	Allows you to write a number, which can be used as required.

	Default value is 0.

xfrm4_gc_thresh - INTEGER
	(Obsolete since linux-4.14)
	The threshold at which we will start garbage collecting for IPv4
	destination cache entries.  At twice this value the system will
	refuse new allocations.

igmp_link_local_mcast_reports - BOOLEAN
	Enable IGMP reports for link local multicast groups in the
	224.0.0.X range.

	Default TRUE

Alexey Kuznetsov.
kuznet@ms2.inr.ac.ru

Updated by:

- Andi Kleen
  ak@muc.de
- Nicolas Delon
  delon.nicolas@wanadoo.fr




/proc/sys/net/ipv6/* Variables
==============================

IPv6 has no global variables such as tcp_*.  tcp_* settings under ipv4/ also
apply to IPv6 [XXX?].

bindv6only - BOOLEAN
	Default value for IPV6_V6ONLY socket option,
	which restricts use of the IPv6 socket to IPv6 communication
	only.

		- TRUE: disable IPv4-mapped address feature
		- FALSE: enable IPv4-mapped address feature

	Default: FALSE (as specified in RFC3493)

flowlabel_consistency - BOOLEAN
	Protect the consistency (and unicity) of flow label.
	You have to disable it to use IPV6_FL_F_REFLECT flag on the
	flow label manager.

	- TRUE: enabled
	- FALSE: disabled

	Default: TRUE

auto_flowlabels - INTEGER
	Automatically generate flow labels based on a flow hash of the
	packet. This allows intermediate devices, such as routers, to
	identify packet flows for mechanisms like Equal Cost Multipath
	Routing (see RFC 6438).

	=  ===========================================================
	0  automatic flow labels are completely disabled
	1  automatic flow labels are enabled by default, they can be
	   disabled on a per socket basis using the IPV6_AUTOFLOWLABEL
	   socket option
	2  automatic flow labels are allowed, they may be enabled on a
	   per socket basis using the IPV6_AUTOFLOWLABEL socket option
	3  automatic flow labels are enabled and enforced, they cannot
	   be disabled by the socket option
	=  ===========================================================

	Default: 1

flowlabel_state_ranges - BOOLEAN
	Split the flow label number space into two ranges. 0-0x7FFFF is
	reserved for the IPv6 flow manager facility, 0x80000-0xFFFFF
	is reserved for stateless flow labels as described in RFC6437.

	- TRUE: enabled
	- FALSE: disabled

	Default: true

flowlabel_reflect - INTEGER
	Control flow label reflection. Needed for Path MTU
	Discovery to work with Equal Cost Multipath Routing in anycast
	environments. See RFC 7690 and:
	https://tools.ietf.org/html/draft-wang-6man-flow-label-reflection-01

	This is a bitmask.

	- 1: enabled for established flows

	  Note that this prevents automatic flowlabel changes, as done
	  in "tcp: change IPv6 flow-label upon receiving spurious retransmission"
	  and "tcp: Change txhash on every SYN and RTO retransmit"

	- 2: enabled for TCP RESET packets (no active listener)
	  If set, a RST packet sent in response to a SYN packet on a closed
	  port will reflect the incoming flow label.

	- 4: enabled for ICMPv6 echo reply messages.

	Default: 0

fib_multipath_hash_policy - INTEGER
	Controls which hash policy to use for multipath routes.

	Default: 0 (Layer 3)

	Possible values:

	- 0 - Layer 3 (source and destination addresses plus flow label)
	- 1 - Layer 4 (standard 5-tuple)
	- 2 - Layer 3 or inner Layer 3 if present

anycast_src_echo_reply - BOOLEAN
	Controls the use of anycast addresses as source addresses for ICMPv6
	echo reply

	- TRUE:  enabled
	- FALSE: disabled

	Default: FALSE

idgen_delay - INTEGER
	Controls the delay in seconds after which time to retry
	privacy stable address generation if a DAD conflict is
	detected.

	Default: 1 (as specified in RFC7217)

idgen_retries - INTEGER
	Controls the number of retries to generate a stable privacy
	address if a DAD conflict is detected.

	Default: 3 (as specified in RFC7217)

mld_qrv - INTEGER
	Controls the MLD query robustness variable (see RFC3810 9.1).

	Default: 2 (as specified by RFC3810 9.1)

	Minimum: 1 (as specified by RFC6636 4.5)

max_dst_opts_number - INTEGER
	Maximum number of non-padding TLVs allowed in a Destination
	options extension header. If this value is less than zero
	then unknown options are disallowed and the number of known
	TLVs allowed is the absolute value of this number.

	Default: 8

max_hbh_opts_number - INTEGER
	Maximum number of non-padding TLVs allowed in a Hop-by-Hop
	options extension header. If this value is less than zero
	then unknown options are disallowed and the number of known
	TLVs allowed is the absolute value of this number.

	Default: 8

max_dst_opts_length - INTEGER
	Maximum length allowed for a Destination options extension
	header.

	Default: INT_MAX (unlimited)

max_hbh_length - INTEGER
	Maximum length allowed for a Hop-by-Hop options extension
	header.

	Default: INT_MAX (unlimited)

skip_notify_on_dev_down - BOOLEAN
	Controls whether an RTM_DELROUTE message is generated for routes
	removed when a device is taken down or deleted. IPv4 does not
	generate this message; IPv6 does by default. Setting this sysctl
	to true skips the message, making IPv4 and IPv6 on par in relying
	on userspace caches to track link events and evict routes.

	Default: false (generate message)

nexthop_compat_mode - BOOLEAN
	New nexthop API provides a means for managing nexthops independent of
	prefixes. Backwards compatibilty with old route format is enabled by
	default which means route dumps and notifications contain the new
	nexthop attribute but also the full, expanded nexthop definition.
	Further, updates or deletes of a nexthop configuration generate route
	notifications for each fib entry using the nexthop. Once a system
	understands the new API, this sysctl can be disabled to achieve full
	performance benefits of the new API by disabling the nexthop expansion
	and extraneous notifications.
	Default: true (backward compat mode)

IPv6 Fragmentation:

ip6frag_high_thresh - INTEGER
	Maximum memory used to reassemble IPv6 fragments. When
	ip6frag_high_thresh bytes of memory is allocated for this purpose,
	the fragment handler will toss packets until ip6frag_low_thresh
	is reached.

ip6frag_low_thresh - INTEGER
	See ip6frag_high_thresh

ip6frag_time - INTEGER
	Time in seconds to keep an IPv6 fragment in memory.

IPv6 Segment Routing:

seg6_flowlabel - INTEGER
	Controls the behaviour of computing the flowlabel of outer
	IPv6 header in case of SR T.encaps

	 == =======================================================
	 -1  set flowlabel to zero.
	  0  copy flowlabel from Inner packet in case of Inner IPv6
	     (Set flowlabel to 0 in case IPv4/L2)
	  1  Compute the flowlabel using seg6_make_flowlabel()
	 == =======================================================

	Default is 0.

``conf/default/*``:
	Change the interface-specific default settings.


``conf/all/*``:
	Change all the interface-specific settings.

	[XXX:  Other special features than forwarding?]

conf/all/forwarding - BOOLEAN
	Enable global IPv6 forwarding between all interfaces.

	IPv4 and IPv6 work differently here; e.g. netfilter must be used
	to control which interfaces may forward packets and which not.

	This also sets all interfaces' Host/Router setting
	'forwarding' to the specified value.  See below for details.

	This referred to as global forwarding.

proxy_ndp - BOOLEAN
	Do proxy ndp.

fwmark_reflect - BOOLEAN
	Controls the fwmark of kernel-generated IPv6 reply packets that are not
	associated with a socket for example, TCP RSTs or ICMPv6 echo replies).
	If unset, these packets have a fwmark of zero. If set, they have the
	fwmark of the packet they are replying to.

	Default: 0

``conf/interface/*``:
	Change special settings per interface.

	The functional behaviour for certain settings is different
	depending on whether local forwarding is enabled or not.

accept_ra - INTEGER
	Accept Router Advertisements; autoconfigure using them.

	It also determines whether or not to transmit Router
	Solicitations. If and only if the functional setting is to
	accept Router Advertisements, Router Solicitations will be
	transmitted.

	Possible values are:

		==  ===========================================================
		 0  Do not accept Router Advertisements.
		 1  Accept Router Advertisements if forwarding is disabled.
		 2  Overrule forwarding behaviour. Accept Router Advertisements
		    even if forwarding is enabled.
		==  ===========================================================

	Functional default:

		- enabled if local forwarding is disabled.
		- disabled if local forwarding is enabled.

accept_ra_defrtr - BOOLEAN
	Learn default router in Router Advertisement.

	Functional default:

		- enabled if accept_ra is enabled.
		- disabled if accept_ra is disabled.

accept_ra_from_local - BOOLEAN
	Accept RA with source-address that is found on local machine
	if the RA is otherwise proper and able to be accepted.

	Default is to NOT accept these as it may be an un-intended
	network loop.

	Functional default:

	   - enabled if accept_ra_from_local is enabled
	     on a specific interface.
	   - disabled if accept_ra_from_local is disabled
	     on a specific interface.

accept_ra_min_hop_limit - INTEGER
	Minimum hop limit Information in Router Advertisement.

	Hop limit Information in Router Advertisement less than this
	variable shall be ignored.

	Default: 1

accept_ra_pinfo - BOOLEAN
	Learn Prefix Information in Router Advertisement.

	Functional default:

		- enabled if accept_ra is enabled.
		- disabled if accept_ra is disabled.

accept_ra_rt_info_min_plen - INTEGER
	Minimum prefix length of Route Information in RA.

	Route Information w/ prefix smaller than this variable shall
	be ignored.

	Functional default:

		* 0 if accept_ra_rtr_pref is enabled.
		* -1 if accept_ra_rtr_pref is disabled.

accept_ra_rt_info_max_plen - INTEGER
	Maximum prefix length of Route Information in RA.

	Route Information w/ prefix larger than this variable shall
	be ignored.

	Functional default:

		* 0 if accept_ra_rtr_pref is enabled.
		* -1 if accept_ra_rtr_pref is disabled.

accept_ra_rtr_pref - BOOLEAN
	Accept Router Preference in RA.

	Functional default:

		- enabled if accept_ra is enabled.
		- disabled if accept_ra is disabled.

accept_ra_mtu - BOOLEAN
	Apply the MTU value specified in RA option 5 (RFC4861). If
	disabled, the MTU specified in the RA will be ignored.

	Functional default:

		- enabled if accept_ra is enabled.
		- disabled if accept_ra is disabled.

accept_redirects - BOOLEAN
	Accept Redirects.

	Functional default:

		- enabled if local forwarding is disabled.
		- disabled if local forwarding is enabled.

accept_source_route - INTEGER
	Accept source routing (routing extension header).

	- >= 0: Accept only routing header type 2.
	- < 0: Do not accept routing header.

	Default: 0

autoconf - BOOLEAN
	Autoconfigure addresses using Prefix Information in Router
	Advertisements.

	Functional default:

		- enabled if accept_ra_pinfo is enabled.
		- disabled if accept_ra_pinfo is disabled.

dad_transmits - INTEGER
	The amount of Duplicate Address Detection probes to send.

	Default: 1

forwarding - INTEGER
	Configure interface-specific Host/Router behaviour.

	.. note::

	   It is recommended to have the same setting on all
	   interfaces; mixed router/host scenarios are rather uncommon.

	Possible values are:

		- 0 Forwarding disabled
		- 1 Forwarding enabled

	**FALSE (0)**:

	By default, Host behaviour is assumed.  This means:

	1. IsRouter flag is not set in Neighbour Advertisements.
	2. If accept_ra is TRUE (default), transmit Router
	   Solicitations.
	3. If accept_ra is TRUE (default), accept Router
	   Advertisements (and do autoconfiguration).
	4. If accept_redirects is TRUE (default), accept Redirects.

	**TRUE (1)**:

	If local forwarding is enabled, Router behaviour is assumed.
	This means exactly the reverse from the above:

	1. IsRouter flag is set in Neighbour Advertisements.
	2. Router Solicitations are not sent unless accept_ra is 2.
	3. Router Advertisements are ignored unless accept_ra is 2.
	4. Redirects are ignored.

	Default: 0 (disabled) if global forwarding is disabled (default),
	otherwise 1 (enabled).

hop_limit - INTEGER
	Default Hop Limit to set.

	Default: 64

mtu - INTEGER
	Default Maximum Transfer Unit

	Default: 1280 (IPv6 required minimum)

ip_nonlocal_bind - BOOLEAN
	If set, allows processes to bind() to non-local IPv6 addresses,
	which can be quite useful - but may break some applications.

	Default: 0

router_probe_interval - INTEGER
	Minimum interval (in seconds) between Router Probing described
	in RFC4191.

	Default: 60

router_solicitation_delay - INTEGER
	Number of seconds to wait after interface is brought up
	before sending Router Solicitations.

	Default: 1

router_solicitation_interval - INTEGER
	Number of seconds to wait between Router Solicitations.

	Default: 4

router_solicitations - INTEGER
	Number of Router Solicitations to send until assuming no
	routers are present.

	Default: 3

use_oif_addrs_only - BOOLEAN
	When enabled, the candidate source addresses for destinations
	routed via this interface are restricted to the set of addresses
	configured on this interface (vis. RFC 6724, section 4).

	Default: false

use_tempaddr - INTEGER
	Preference for Privacy Extensions (RFC3041).

	  * <= 0 : disable Privacy Extensions
	  * == 1 : enable Privacy Extensions, but prefer public
	    addresses over temporary addresses.
	  * >  1 : enable Privacy Extensions and prefer temporary
	    addresses over public addresses.

	Default:

		* 0 (for most devices)
		* -1 (for point-to-point devices and loopback devices)

temp_valid_lft - INTEGER
	valid lifetime (in seconds) for temporary addresses.

	Default: 604800 (7 days)

temp_prefered_lft - INTEGER
	Preferred lifetime (in seconds) for temporary addresses.

	Default: 86400 (1 day)

keep_addr_on_down - INTEGER
	Keep all IPv6 addresses on an interface down event. If set static
	global addresses with no expiration time are not flushed.

	*   >0 : enabled
	*    0 : system default
	*   <0 : disabled

	Default: 0 (addresses are removed)

max_desync_factor - INTEGER
	Maximum value for DESYNC_FACTOR, which is a random value
	that ensures that clients don't synchronize with each
	other and generate new addresses at exactly the same time.
	value is in seconds.

	Default: 600

regen_max_retry - INTEGER
	Number of attempts before give up attempting to generate
	valid temporary addresses.

	Default: 5

max_addresses - INTEGER
	Maximum number of autoconfigured addresses per interface.  Setting
	to zero disables the limitation.  It is not recommended to set this
	value too large (or to zero) because it would be an easy way to
	crash the kernel by allowing too many addresses to be created.

	Default: 16

disable_ipv6 - BOOLEAN
	Disable IPv6 operation.  If accept_dad is set to 2, this value
	will be dynamically set to TRUE if DAD fails for the link-local
	address.

	Default: FALSE (enable IPv6 operation)

	When this value is changed from 1 to 0 (IPv6 is being enabled),
	it will dynamically create a link-local address on the given
	interface and start Duplicate Address Detection, if necessary.

	When this value is changed from 0 to 1 (IPv6 is being disabled),
	it will dynamically delete all addresses and routes on the given
	interface. From now on it will not possible to add addresses/routes
	to the selected interface.

accept_dad - INTEGER
	Whether to accept DAD (Duplicate Address Detection).

	 == ==============================================================
	  0  Disable DAD
	  1  Enable DAD (default)
	  2  Enable DAD, and disable IPv6 operation if MAC-based duplicate
	     link-local address has been found.
	 == ==============================================================

	DAD operation and mode on a given interface will be selected according
	to the maximum value of conf/{all,interface}/accept_dad.

force_tllao - BOOLEAN
	Enable sending the target link-layer address option even when
	responding to a unicast neighbor solicitation.

	Default: FALSE

	Quoting from RFC 2461, section 4.4, Target link-layer address:

	"The option MUST be included for multicast solicitations in order to
	avoid infinite Neighbor Solicitation "recursion" when the peer node
	does not have a cache entry to return a Neighbor Advertisements
	message.  When responding to unicast solicitations, the option can be
	omitted since the sender of the solicitation has the correct link-
	layer address; otherwise it would not have be able to send the unicast
	solicitation in the first place. However, including the link-layer
	address in this case adds little overhead and eliminates a potential
	race condition where the sender deletes the cached link-layer address
	prior to receiving a response to a previous solicitation."

ndisc_notify - BOOLEAN
	Define mode for notification of address and device changes.

	* 0 - (default): do nothing
	* 1 - Generate unsolicited neighbour advertisements when device is brought
	  up or hardware address changes.

ndisc_tclass - INTEGER
	The IPv6 Traffic Class to use by default when sending IPv6 Neighbor
	Discovery (Router Solicitation, Router Advertisement, Neighbor
	Solicitation, Neighbor Advertisement, Redirect) messages.
	These 8 bits can be interpreted as 6 high order bits holding the DSCP
	value and 2 low order bits representing ECN (which you probably want
	to leave cleared).

	* 0 - (default)

mldv1_unsolicited_report_interval - INTEGER
	The interval in milliseconds in which the next unsolicited
	MLDv1 report retransmit will take place.

	Default: 10000 (10 seconds)

mldv2_unsolicited_report_interval - INTEGER
	The interval in milliseconds in which the next unsolicited
	MLDv2 report retransmit will take place.

	Default: 1000 (1 second)

force_mld_version - INTEGER
	* 0 - (default) No enforcement of a MLD version, MLDv1 fallback allowed
	* 1 - Enforce to use MLD version 1
	* 2 - Enforce to use MLD version 2

suppress_frag_ndisc - INTEGER
	Control RFC 6980 (Security Implications of IPv6 Fragmentation
	with IPv6 Neighbor Discovery) behavior:

	* 1 - (default) discard fragmented neighbor discovery packets
	* 0 - allow fragmented neighbor discovery packets

optimistic_dad - BOOLEAN
	Whether to perform Optimistic Duplicate Address Detection (RFC 4429).

	* 0: disabled (default)
	* 1: enabled

	Optimistic Duplicate Address Detection for the interface will be enabled
	if at least one of conf/{all,interface}/optimistic_dad is set to 1,
	it will be disabled otherwise.

use_optimistic - BOOLEAN
	If enabled, do not classify optimistic addresses as deprecated during
	source address selection.  Preferred addresses will still be chosen
	before optimistic addresses, subject to other ranking in the source
	address selection algorithm.

	* 0: disabled (default)
	* 1: enabled

	This will be enabled if at least one of
	conf/{all,interface}/use_optimistic is set to 1, disabled otherwise.

stable_secret - IPv6 address
	This IPv6 address will be used as a secret to generate IPv6
	addresses for link-local addresses and autoconfigured
	ones. All addresses generated after setting this secret will
	be stable privacy ones by default. This can be changed via the
	addrgenmode ip-link. conf/default/stable_secret is used as the
	secret for the namespace, the interface specific ones can
	overwrite that. Writes to conf/all/stable_secret are refused.

	It is recommended to generate this secret during installation
	of a system and keep it stable after that.

	By default the stable secret is unset.

addr_gen_mode - INTEGER
	Defines how link-local and autoconf addresses are generated.

	=  =================================================================
	0  generate address based on EUI64 (default)
	1  do no generate a link-local address, use EUI64 for addresses
	   generated from autoconf
	2  generate stable privacy addresses, using the secret from
	   stable_secret (RFC7217)
	3  generate stable privacy addresses, using a random secret if unset
	=  =================================================================

drop_unicast_in_l2_multicast - BOOLEAN
	Drop any unicast IPv6 packets that are received in link-layer
	multicast (or broadcast) frames.

	By default this is turned off.

drop_unsolicited_na - BOOLEAN
	Drop all unsolicited neighbor advertisements, for example if there's
	a known good NA proxy on the network and such frames need not be used
	(or in the case of 802.11, must not be used to prevent attacks.)

	By default this is turned off.

enhanced_dad - BOOLEAN
	Include a nonce option in the IPv6 neighbor solicitation messages used for
	duplicate address detection per RFC7527. A received DAD NS will only signal
	a duplicate address if the nonce is different. This avoids any false
	detection of duplicates due to loopback of the NS messages that we send.
	The nonce option will be sent on an interface unless both of
	conf/{all,interface}/enhanced_dad are set to FALSE.

	Default: TRUE

``icmp/*``:
===========

ratelimit - INTEGER
	Limit the maximal rates for sending ICMPv6 messages.

	0 to disable any limiting,
	otherwise the minimal space between responses in milliseconds.

	Default: 1000

ratemask - list of comma separated ranges
	For ICMPv6 message types matching the ranges in the ratemask, limit
	the sending of the message according to ratelimit parameter.

	The format used for both input and output is a comma separated
	list of ranges (e.g. "0-127,129" for ICMPv6 message type 0 to 127 and
	129). Writing to the file will clear all previous ranges of ICMPv6
	message types and update the current list with the input.

	Refer to: https://www.iana.org/assignments/icmpv6-parameters/icmpv6-parameters.xhtml
	for numerical values of ICMPv6 message types, e.g. echo request is 128
	and echo reply is 129.

	Default: 0-1,3-127 (rate limit ICMPv6 errors except Packet Too Big)

echo_ignore_all - BOOLEAN
	If set non-zero, then the kernel will ignore all ICMP ECHO
	requests sent to it over the IPv6 protocol.

	Default: 0

echo_ignore_multicast - BOOLEAN
	If set non-zero, then the kernel will ignore all ICMP ECHO
	requests sent to it over the IPv6 protocol via multicast.

	Default: 0

echo_ignore_anycast - BOOLEAN
	If set non-zero, then the kernel will ignore all ICMP ECHO
	requests sent to it over the IPv6 protocol destined to anycast address.

	Default: 0

xfrm6_gc_thresh - INTEGER
	(Obsolete since linux-4.14)
	The threshold at which we will start garbage collecting for IPv6
	destination cache entries.  At twice this value the system will
	refuse new allocations.


IPv6 Update by:
Pekka Savola <pekkas@netcore.fi>
YOSHIFUJI Hideaki / USAGI Project <yoshfuji@linux-ipv6.org>


/proc/sys/net/bridge/* Variables:
=================================

bridge-nf-call-arptables - BOOLEAN
	- 1 : pass bridged ARP traffic to arptables' FORWARD chain.
	- 0 : disable this.

	Default: 1

bridge-nf-call-iptables - BOOLEAN
	- 1 : pass bridged IPv4 traffic to iptables' chains.
	- 0 : disable this.

	Default: 1

bridge-nf-call-ip6tables - BOOLEAN
	- 1 : pass bridged IPv6 traffic to ip6tables' chains.
	- 0 : disable this.

	Default: 1

bridge-nf-filter-vlan-tagged - BOOLEAN
	- 1 : pass bridged vlan-tagged ARP/IP/IPv6 traffic to {arp,ip,ip6}tables.
	- 0 : disable this.

	Default: 0

bridge-nf-filter-pppoe-tagged - BOOLEAN
	- 1 : pass bridged pppoe-tagged IP/IPv6 traffic to {ip,ip6}tables.
	- 0 : disable this.

	Default: 0

bridge-nf-pass-vlan-input-dev - BOOLEAN
	- 1: if bridge-nf-filter-vlan-tagged is enabled, try to find a vlan
	  interface on the bridge and set the netfilter input device to the
	  vlan. This allows use of e.g. "iptables -i br0.1" and makes the
	  REDIRECT target work with vlan-on-top-of-bridge interfaces.  When no
	  matching vlan interface is found, or this switch is off, the input
	  device is set to the bridge interface.

	- 0: disable bridge netfilter vlan interface lookup.

	Default: 0

``proc/sys/net/sctp/*`` Variables:
==================================

addip_enable - BOOLEAN
	Enable or disable extension of  Dynamic Address Reconfiguration
	(ADD-IP) functionality specified in RFC5061.  This extension provides
	the ability to dynamically add and remove new addresses for the SCTP
	associations.

	1: Enable extension.

	0: Disable extension.

	Default: 0

pf_enable - INTEGER
	Enable or disable pf (pf is short for potentially failed) state. A value
	of pf_retrans > path_max_retrans also disables pf state. That is, one of
	both pf_enable and pf_retrans > path_max_retrans can disable pf state.
	Since pf_retrans and path_max_retrans can be changed by userspace
	application, sometimes user expects to disable pf state by the value of
	pf_retrans > path_max_retrans, but occasionally the value of pf_retrans
	or path_max_retrans is changed by the user application, this pf state is
	enabled. As such, it is necessary to add this to dynamically enable
	and disable pf state. See:
	https://datatracker.ietf.org/doc/draft-ietf-tsvwg-sctp-failover for
	details.

	1: Enable pf.

	0: Disable pf.

	Default: 1

pf_expose - INTEGER
	Unset or enable/disable pf (pf is short for potentially failed) state
	exposure.  Applications can control the exposure of the PF path state
	in the SCTP_PEER_ADDR_CHANGE event and the SCTP_GET_PEER_ADDR_INFO
	sockopt.   When it's unset, no SCTP_PEER_ADDR_CHANGE event with
	SCTP_ADDR_PF state will be sent and a SCTP_PF-state transport info
	can be got via SCTP_GET_PEER_ADDR_INFO sockopt;  When it's enabled,
	a SCTP_PEER_ADDR_CHANGE event will be sent for a transport becoming
	SCTP_PF state and a SCTP_PF-state transport info can be got via
	SCTP_GET_PEER_ADDR_INFO sockopt;  When it's diabled, no
	SCTP_PEER_ADDR_CHANGE event will be sent and it returns -EACCES when
	trying to get a SCTP_PF-state transport info via SCTP_GET_PEER_ADDR_INFO
	sockopt.

	0: Unset pf state exposure, Compatible with old applications.

	1: Disable pf state exposure.

	2: Enable pf state exposure.

	Default: 0

addip_noauth_enable - BOOLEAN
	Dynamic Address Reconfiguration (ADD-IP) requires the use of
	authentication to protect the operations of adding or removing new
	addresses.  This requirement is mandated so that unauthorized hosts
	would not be able to hijack associations.  However, older
	implementations may not have implemented this requirement while
	allowing the ADD-IP extension.  For reasons of interoperability,
	we provide this variable to control the enforcement of the
	authentication requirement.

	== ===============================================================
	1  Allow ADD-IP extension to be used without authentication.  This
	   should only be set in a closed environment for interoperability
	   with older implementations.

	0  Enforce the authentication requirement
	== ===============================================================

	Default: 0

auth_enable - BOOLEAN
	Enable or disable Authenticated Chunks extension.  This extension
	provides the ability to send and receive authenticated chunks and is
	required for secure operation of Dynamic Address Reconfiguration
	(ADD-IP) extension.

	- 1: Enable this extension.
	- 0: Disable this extension.

	Default: 0

prsctp_enable - BOOLEAN
	Enable or disable the Partial Reliability extension (RFC3758) which
	is used to notify peers that a given DATA should no longer be expected.

	- 1: Enable extension
	- 0: Disable

	Default: 1

max_burst - INTEGER
	The limit of the number of new packets that can be initially sent.  It
	controls how bursty the generated traffic can be.

	Default: 4

association_max_retrans - INTEGER
	Set the maximum number for retransmissions that an association can
	attempt deciding that the remote end is unreachable.  If this value
	is exceeded, the association is terminated.

	Default: 10

max_init_retransmits - INTEGER
	The maximum number of retransmissions of INIT and COOKIE-ECHO chunks
	that an association will attempt before declaring the destination
	unreachable and terminating.

	Default: 8

path_max_retrans - INTEGER
	The maximum number of retransmissions that will be attempted on a given
	path.  Once this threshold is exceeded, the path is considered
	unreachable, and new traffic will use a different path when the
	association is multihomed.

	Default: 5

pf_retrans - INTEGER
	The number of retransmissions that will be attempted on a given path
	before traffic is redirected to an alternate transport (should one
	exist).  Note this is distinct from path_max_retrans, as a path that
	passes the pf_retrans threshold can still be used.  Its only
	deprioritized when a transmission path is selected by the stack.  This
	setting is primarily used to enable fast failover mechanisms without
	having to reduce path_max_retrans to a very low value.  See:
	http://www.ietf.org/id/draft-nishida-tsvwg-sctp-failover-05.txt
	for details.  Note also that a value of pf_retrans > path_max_retrans
	disables this feature. Since both pf_retrans and path_max_retrans can
	be changed by userspace application, a variable pf_enable is used to
	disable pf state.

	Default: 0

ps_retrans - INTEGER
	Primary.Switchover.Max.Retrans (PSMR), it's a tunable parameter coming
	from section-5 "Primary Path Switchover" in rfc7829.  The primary path
	will be changed to another active path when the path error counter on
	the old primary path exceeds PSMR, so that "the SCTP sender is allowed
	to continue data transmission on a new working path even when the old
	primary destination address becomes active again".   Note this feature
	is disabled by initializing 'ps_retrans' per netns as 0xffff by default,
	and its value can't be less than 'pf_retrans' when changing by sysctl.

	Default: 0xffff

rto_initial - INTEGER
	The initial round trip timeout value in milliseconds that will be used
	in calculating round trip times.  This is the initial time interval
	for retransmissions.

	Default: 3000

rto_max - INTEGER
	The maximum value (in milliseconds) of the round trip timeout.  This
	is the largest time interval that can elapse between retransmissions.

	Default: 60000

rto_min - INTEGER
	The minimum value (in milliseconds) of the round trip timeout.  This
	is the smallest time interval the can elapse between retransmissions.

	Default: 1000

hb_interval - INTEGER
	The interval (in milliseconds) between HEARTBEAT chunks.  These chunks
	are sent at the specified interval on idle paths to probe the state of
	a given path between 2 associations.

	Default: 30000

sack_timeout - INTEGER
	The amount of time (in milliseconds) that the implementation will wait
	to send a SACK.

	Default: 200

valid_cookie_life - INTEGER
	The default lifetime of the SCTP cookie (in milliseconds).  The cookie
	is used during association establishment.

	Default: 60000

cookie_preserve_enable - BOOLEAN
	Enable or disable the ability to extend the lifetime of the SCTP cookie
	that is used during the establishment phase of SCTP association

	- 1: Enable cookie lifetime extension.
	- 0: Disable

	Default: 1

cookie_hmac_alg - STRING
	Select the hmac algorithm used when generating the cookie value sent by
	a listening sctp socket to a connecting client in the INIT-ACK chunk.
	Valid values are:

	* md5
	* sha1
	* none

	Ability to assign md5 or sha1 as the selected alg is predicated on the
	configuration of those algorithms at build time (CONFIG_CRYPTO_MD5 and
	CONFIG_CRYPTO_SHA1).

	Default: Dependent on configuration.  MD5 if available, else SHA1 if
	available, else none.

rcvbuf_policy - INTEGER
	Determines if the receive buffer is attributed to the socket or to
	association.   SCTP supports the capability to create multiple
	associations on a single socket.  When using this capability, it is
	possible that a single stalled association that's buffering a lot
	of data may block other associations from delivering their data by
	consuming all of the receive buffer space.  To work around this,
	the rcvbuf_policy could be set to attribute the receiver buffer space
	to each association instead of the socket.  This prevents the described
	blocking.

	- 1: rcvbuf space is per association
	- 0: rcvbuf space is per socket

	Default: 0

sndbuf_policy - INTEGER
	Similar to rcvbuf_policy above, this applies to send buffer space.

	- 1: Send buffer is tracked per association
	- 0: Send buffer is tracked per socket.

	Default: 0

sctp_mem - vector of 3 INTEGERs: min, pressure, max
	Number of pages allowed for queueing by all SCTP sockets.

	min: Below this number of pages SCTP is not bothered about its
	memory appetite. When amount of memory allocated by SCTP exceeds
	this number, SCTP starts to moderate memory usage.

	pressure: This value was introduced to follow format of tcp_mem.

	max: Number of pages allowed for queueing by all SCTP sockets.

	Default is calculated at boot time from amount of available memory.

sctp_rmem - vector of 3 INTEGERs: min, default, max
	Only the first value ("min") is used, "default" and "max" are
	ignored.

	min: Minimal size of receive buffer used by SCTP socket.
	It is guaranteed to each SCTP socket (but not association) even
	under moderate memory pressure.

	Default: 4K

sctp_wmem  - vector of 3 INTEGERs: min, default, max
	Currently this tunable has no effect.

addr_scope_policy - INTEGER
	Control IPv4 address scoping - draft-stewart-tsvwg-sctp-ipv4-00

	- 0   - Disable IPv4 address scoping
	- 1   - Enable IPv4 address scoping
	- 2   - Follow draft but allow IPv4 private addresses
	- 3   - Follow draft but allow IPv4 link local addresses

	Default: 1


``/proc/sys/net/core/*``
========================

	Please see: Documentation/admin-guide/sysctl/net.rst for descriptions of these entries.


``/proc/sys/net/unix/*``
========================

max_dgram_qlen - INTEGER
	The maximum length of dgram socket receive queue

	Default: 10