network: allow limiting a <forwarder> element to certain domains

For some unknown reason the original implementation of the <forwarder>
element only took advantage of part of the functionality in the
dnsmasq feature it exposes - it allowed specifying the ip address of a
DNS server which *all* DNS requests would be forwarded to, like this:

   <forwarder addr='192.168.123.25'/>

This is a frontend for dnsmasq's "server" option, which also allows
you to specify a domain that must be matched in order for a request to
be forwarded to a particular server. This patch adds support for
specifying the domain. For example:

   <forwarder domain='example.com' addr='192.168.1.1'/>
   <forwarder domain='www.example.com'/>
   <forwarder domain='travesty.org' addr='10.0.0.1'/>

would forward requests for bob.example.com, ftp.example.com and
joe.corp.example.com all to the DNS server at 192.168.1.1, but would
forward requests for travesty.org and www.travesty.org to
10.0.0.1. And due to the second line, requests for www.example.com,
and odd.www.example.com would be resolved by the libvirt network's own
DNS server (i.e. thery wouldn't be immediately forwarded) even though
they also match 'example.com' - the match is given to the entry with
the longest matching domain. DNS requests not matching any of the
entries would be resolved by the libvirt network's own DNS server.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1331796

docs/formatnetwork.html.in

@@ -847,7 +847,8 @@
         <dns>
           <txt name="example" value="example value" />
           <forwarder addr="8.8.8.8"/>
-          <forwarder addr="8.8.4.4"/>
+          <forwarder domain='example.com' addr="8.8.4.4"/>
+          <forwarder domain='www.example.com'/>
           <srv service='name' protocol='tcp' domain='test-domain-name' target='.' port='1024' priority='10' weight='10'/>
           <host ip='192.168.122.2'>
             <hostname>myhost</hostname>
@@ -915,12 +916,25 @@
         Currently supported sub-elements of <code>&lt;dns&gt;</code> are:
         <dl>
           <dt><code>forwarder</code></dt>
-          <dd>A <code>dns</code> element can have 0 or
-            more <code>forwarder</code> elements.  Each forwarder
-            element defines an IP address to be used as forwarder in
-            DNS server configuration. The addr attribute is required
-            and defines the IP address of every
-            forwarder. <span class="since">Since 1.1.3</span>
+          <dd>The dns element can have 0 or
+            more <code>&lt;forwarder&gt;</code> elements.  Each
+            forwarder element defines an alternate DNS server to use
+            for some, or all, DNS requests sent to this network's DNS
+            server. There are two attributes - <code>domain</code>,
+            and <code>addr</code>; at least one of these must be
+            specified in any <code>&lt;forwarder&gt;</code>
+            element. If both <code>domain</code> and <code>addr</code>
+            are specified, then all requests that match the given
+            domain will be forwarded to the DNS server at addr. If
+            only <code>domain</code> is specified, then all matching
+            domains will be resolved locally (or via the host's
+            standard DNS forwarding if they can't be resolved
+            locally). If an <code>addr</code> is specified by itself,
+            then all DNS requests to the network's DNS server will be
+            forwarded to the DNS server at that address with no
+            exceptions. <code>addr</code> <span class="since">Since
+            1.1.3</span>, <code>domain</code> <span class="since">Since
+            2.2.0</span>.
           </dd>
           <dt><code>txt</code></dt>
           <dd>A <code>dns</code> element can have 0 or more <code>txt</code> elements.

docs/schemas/network.rng

@@ -260,7 +260,13 @@
             <interleave>
               <zeroOrMore>
                 <element name="forwarder">
-                  <attribute name="addr"><ref name="ipAddr"/></attribute>
+                  <optional>
+                    <attribute name="addr"><ref name="ipAddr"/></attribute>
+                  </optional>
+                  <optional>
+                    <attribute name="domain"><ref name="dnsName"/></attribute>
+                  </optional>
+                  <empty/>
                 </element>
               </zeroOrMore>
               <zeroOrMore>

src/conf/network_conf.c

@@ -349,12 +349,20 @@ virNetworkDNSSrvDefClear(virNetworkDNSSrvDefPtr def)
     VIR_FREE(def->target);
 }
 
+
+static void
+virNetworkDNSForwarderClear(virNetworkDNSForwarderPtr def)
+{
+    VIR_FREE(def->domain);
+}
+
+
 static void
 virNetworkDNSDefClear(virNetworkDNSDefPtr def)
 {
     if (def->forwarders) {
         while (def->nfwds)
-            VIR_FREE(def->forwarders[--def->nfwds]);
+            virNetworkDNSForwarderClear(&def->forwarders[--def->nfwds]);
         VIR_FREE(def->forwarders);
     }
     if (def->txts) {
@@ -1379,14 +1387,25 @@ virNetworkDNSDefParseXML(const char *networkName,
             goto cleanup;
 
         for (i = 0; i < nfwds; i++) {
-            def->forwarders[i] = virXMLPropString(fwdNodes[i], "addr");
-            if (virSocketAddrParse(NULL, def->forwarders[i], AF_UNSPEC) < 0) {
+            char *addr = virXMLPropString(fwdNodes[i], "addr");
+
+            if (addr && virSocketAddrParse(&def->forwarders[i].addr,
+                                           addr, AF_UNSPEC) < 0) {
                 virReportError(VIR_ERR_XML_ERROR,
-                           _("Invalid forwarder IP address '%s' "
-                             "in network '%s'"),
-                           def->forwarders[i], networkName);
+                               _("Invalid forwarder IP address '%s' "
+                                 "in network '%s'"),
+                               addr, networkName);
+                VIR_FREE(addr);
+                goto cleanup;
+            }
+            def->forwarders[i].domain = virXMLPropString(fwdNodes[i], "domain");
+            if (!(addr || def->forwarders[i].domain)) {
+                virReportError(VIR_ERR_XML_ERROR, "%s",
+                               _("Invalid forwarder element, must contain "
+                                 "at least one of addr or domain"));
                 goto cleanup;
             }
+            VIR_FREE(addr);
             def->nfwds++;
         }
     }
@@ -2554,8 +2573,22 @@ virNetworkDNSDefFormat(virBufferPtr buf,
     virBufferAdjustIndent(buf, 2);
 
     for (i = 0; i < def->nfwds; i++) {
-        virBufferAsprintf(buf, "<forwarder addr='%s'/>\n",
-                          def->forwarders[i]);
+
+        virBufferAddLit(buf, "<forwarder");
+        if (def->forwarders[i].domain) {
+            virBufferEscapeString(buf, " domain='%s'",
+                                  def->forwarders[i].domain);
+        }
+        if (VIR_SOCKET_ADDR_VALID(&def->forwarders[i].addr)) {
+        char *addr = virSocketAddrFormat(&def->forwarders[i].addr);
+
+        if (!addr)
+            return -1;
+
+        virBufferAsprintf(buf, " addr='%s'", addr);
+        VIR_FREE(addr);
+        }
+        virBufferAddLit(buf, "/>\n");
     }
 
     for (i = 0; i < def->ntxts; i++) {

src/conf/network_conf.h

@@ -125,6 +125,12 @@ struct _virNetworkDNSHostDef {
     char **names;
 };
 
+
+typedef struct _virNetworkDNSForwarder {
+    virSocketAddr addr;
+    char *domain;
+} virNetworkDNSForwarder, *virNetworkDNSForwarderPtr;
+
 typedef struct _virNetworkDNSDef virNetworkDNSDef;
 typedef virNetworkDNSDef *virNetworkDNSDefPtr;
 struct _virNetworkDNSDef {
@@ -137,7 +143,7 @@ struct _virNetworkDNSDef {
     size_t nsrvs;
     virNetworkDNSSrvDefPtr srvs;
     size_t nfwds;
-    char **forwarders;
+    virNetworkDNSForwarderPtr forwarders;
 };
 
 typedef struct _virNetworkIPDef virNetworkIPDef;

src/network/bridge_driver.c

@@ -958,8 +958,21 @@ networkDnsmasqConfContents(virNetworkObjPtr network,
     if (wantDNS && network->def->dns.forwarders) {
         virBufferAddLit(&configbuf, "no-resolv\n");
         for (i = 0; i < network->def->dns.nfwds; i++) {
-            virBufferAsprintf(&configbuf, "server=%s\n",
-                              network->def->dns.forwarders[i]);
+            virNetworkDNSForwarderPtr fwd = &network->def->dns.forwarders[i];
+
+            virBufferAddLit(&configbuf, "server=");
+            if (fwd->domain)
+                virBufferAsprintf(&configbuf, "/%s/", fwd->domain);
+            if (VIR_SOCKET_ADDR_VALID(&fwd->addr)) {
+                char *addr = virSocketAddrFormat(&fwd->addr);
+
+                if (!addr)
+                    goto cleanup;
+                virBufferAsprintf(&configbuf, "%s\n", addr);
+            } else {
+                /* "don't forward requests for this domain" */
+                virBufferAddLit(&configbuf, "#\n");
+            }
         }
     }
 

tests/networkxml2confdata/nat-network-dns-forwarders.conf

@@ -8,6 +8,8 @@ strict-order
 no-resolv
 server=8.8.8.8
 server=8.8.4.4
+server=/example.com/192.168.1.1
+server=/www.example.com/#
 except-interface=lo
 bind-dynamic
 interface=virbr0

tests/networkxml2confdata/nat-network-dns-forwarders.xml

@@ -6,6 +6,8 @@
   <dns>
     <forwarder addr='8.8.8.8'/>
     <forwarder addr='8.8.4.4'/>
+    <forwarder domain='example.com' addr='192.168.1.1'/>
+    <forwarder domain='www.example.com'/>
   </dns>
   <ip address='192.168.122.1' netmask='255.255.255.0'>
   </ip>

tests/networkxml2xmlin/nat-network-dns-forwarders.xml

@@ -4,8 +4,10 @@
   <forward dev='eth0' mode='nat'/>
   <bridge name='virbr0' stp='on' delay='0' />
   <dns>
-    <forwarder addr='8.8.8.8' />
-    <forwarder addr='8.8.4.4' />
+    <forwarder addr='8.8.8.8'/>
+    <forwarder addr='8.8.4.4'/>
+    <forwarder domain='example.com' addr='192.168.1.1'/>
+    <forwarder domain='www.example.com'/>
   </dns>
   <ip address='192.168.122.1' netmask='255.255.255.0'>
   </ip>

tests/networkxml2xmlout/nat-network-dns-forwarders.xml

@@ -8,6 +8,8 @@
   <dns>
     <forwarder addr='8.8.8.8'/>
     <forwarder addr='8.8.4.4'/>
+    <forwarder domain='example.com' addr='192.168.1.1'/>
+    <forwarder domain='www.example.com'/>
   </dns>
   <ip address='192.168.122.1' netmask='255.255.255.0'>
   </ip>