netfilter: doc: add nf_conntrack sysctl api documentation

I grepped through the code and picked bits about nf_conntrack sysctl api
and put that into one documentation file.

[ I have mangled this patch including comments from several grammar
  improvements proposed by Neal Murphy <neal.p.murphy@alum.wpi.edu>,
  any new grammar error is my mistake --pablo ]
Signed-off-by: NJiri Pirko <jiri@resnulli.us>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

Documentation/networking/nf_conntrack-sysctl.txt

+/proc/sys/net/netfilter/nf_conntrack_* Variables:
+
+nf_conntrack_acct - BOOLEAN
+	0 - disabled (default)
+	not 0 - enabled
+
+	Enable connection tracking flow accounting. 64-bit byte and packet
+	counters per flow are added.
+
+nf_conntrack_buckets - INTEGER (read-only)
+	Size of hash table. If not specified as parameter during module
+	loading, the default size is calculated by dividing total memory
+	by 16384 to determine the number of buckets but the hash table will
+	never have fewer than 32 or more than 16384 buckets.
+
+nf_conntrack_checksum - BOOLEAN
+	0 - disabled
+	not 0 - enabled (default)
+
+	Verify checksum of incoming packets. Packets with bad checksums are
+	in INVALID state. If this is enabled, such packets will not be
+	considered for connection tracking.
+
+nf_conntrack_count - INTEGER (read-only)
+	Number of currently allocated flow entries.
+
+nf_conntrack_events - BOOLEAN
+	0 - disabled
+	not 0 - enabled (default)
+
+	If this option is enabled, the connection tracking code will
+	provide userspace with connection tracking events via ctnetlink.
+
+nf_conntrack_events_retry_timeout - INTEGER (seconds)
+	default 15
+
+	This option is only relevant when "reliable connection tracking
+	events" are used.  Normally, ctnetlink is "lossy", that is,
+	events are normally dropped when userspace listeners can't keep up.
+
+	Userspace can request "reliable event mode".  When this mode is
+	active, the conntrack will only be destroyed after the event was
+	delivered.  If event delivery fails, the kernel periodically
+	re-tries to send the event to userspace.
+
+	This is the maximum interval the kernel should use when re-trying
+	to deliver the destroy event.
+
+	A higher number means there will be fewer delivery retries and it
+	will take longer for a backlog to be processed.
+
+nf_conntrack_expect_max - INTEGER
+	Maximum size of expectation table.  Default value is
+	nf_conntrack_buckets / 256. Minimum is 1.
+
+nf_conntrack_frag6_high_thresh - INTEGER
+	default 262144
+
+	Maximum memory used to reassemble IPv6 fragments.  When
+	nf_conntrack_frag6_high_thresh bytes of memory is allocated for this
+	purpose, the fragment handler will toss packets until
+	nf_conntrack_frag6_low_thresh is reached.
+
+nf_conntrack_frag6_low_thresh - INTEGER
+	default 196608
+
+	See nf_conntrack_frag6_low_thresh
+
+nf_conntrack_frag6_timeout - INTEGER (seconds)
+	default 60
+
+	Time to keep an IPv6 fragment in memory.
+
+nf_conntrack_generic_timeout - INTEGER (seconds)
+	default 600
+
+	Default for generic timeout.  This refers to layer 4 unknown/unsupported
+	protocols.
+
+nf_conntrack_helper - BOOLEAN
+	0 - disabled
+	not 0 - enabled (default)
+
+	Enable automatic conntrack helper assignment.
+
+nf_conntrack_icmp_timeout - INTEGER (seconds)
+	default 30
+
+	Default for ICMP timeout.
+
+nf_conntrack_icmpv6_timeout - INTEGER (seconds)
+	default 30
+
+	Default for ICMP6 timeout.
+
+nf_conntrack_log_invalid - INTEGER
+	0   - disable (default)
+	1   - log ICMP packets
+	6   - log TCP packets
+	17  - log UDP packets
+	33  - log DCCP packets
+	41  - log ICMPv6 packets
+	136 - log UDPLITE packets
+	255 - log packets of any protocol
+
+	Log invalid packets of a type specified by value.
+
+nf_conntrack_max - INTEGER
+	Size of connection tracking table.  Default value is
+	nf_conntrack_buckets value * 4.
+
+nf_conntrack_tcp_be_liberal - BOOLEAN
+	0 - disabled (default)
+	not 0 - enabled
+
+	Be conservative in what you do, be liberal in what you accept from others.
+	If it's non-zero, we mark only out of window RST segments as INVALID.
+
+nf_conntrack_tcp_loose - BOOLEAN
+	0 - disabled
+	not 0 - enabled (default)
+
+	If it is set to zero, we disable picking up already established
+	connections.
+
+nf_conntrack_tcp_max_retrans - INTEGER
+	default 3
+
+	Maximum number of packets that can be retransmitted without
+	received an (acceptable) ACK from the destination. If this number
+	is reached, a shorter timer will be started.
+
+nf_conntrack_tcp_timeout_close - INTEGER (seconds)
+	default 10
+
+nf_conntrack_tcp_timeout_close_wait - INTEGER (seconds)
+	default 60
+
+nf_conntrack_tcp_timeout_established - INTEGER (seconds)
+	default 432000 (5 days)
+
+nf_conntrack_tcp_timeout_fin_wait - INTEGER (seconds)
+	default 120
+
+nf_conntrack_tcp_timeout_last_ack - INTEGER (seconds)
+	default 30
+
+nf_conntrack_tcp_timeout_max_retrans - INTEGER (seconds)
+	default 300
+
+nf_conntrack_tcp_timeout_syn_recv - INTEGER (seconds)
+	default 60
+
+nf_conntrack_tcp_timeout_syn_sent - INTEGER (seconds)
+	default 120
+
+nf_conntrack_tcp_timeout_time_wait - INTEGER (seconds)
+	default 120
+
+nf_conntrack_tcp_timeout_unacknowledged - INTEGER (seconds)
+	default 300
+
+nf_conntrack_timestamp - BOOLEAN
+	0 - disabled (default)
+	not 0 - enabled
+
+	Enable connection tracking flow timestamping.
+
+nf_conntrack_udp_timeout - INTEGER (seconds)
+	default 30
+
+nf_conntrack_udp_timeout_stream2 - INTEGER (seconds)
+	default 180
+
+	This extended timeout will be used in case there is an UDP stream
+	detected.