[libvirt] [PATCH] handle DNS over IPv6

[Paweł Krześniak]

Firstly: Add ip6tables rules to allow DNS over IPv6 in network.

Secondly: start dnsmasq with --interface option instead of --listen-address.

Dnsmasq currently uses "--listen-address IPv4_address" option, which
restricts DNS service to one IPv4 address only.
We could append --listen-address for every IPv[46] address defined on
interface, but it's cleaner to use "--interface brname".

There were some problems in the past with --interface option. Dnsmasq
version 2.46 and earlier exited with error when tired to bind() to IPv6
addresses on just brought up interfaces, because DAD (Duplicate
Address Detection) takes some time to finish and bind() returns
EADDRNOTAVAIL which caused dnsmasq to exit.
Dnsmasq version 2.47 (released on 05-Feb-2009) fixed this issue by
retrying bind() after getting EADDRNOTAVAIL error (as described in
http://www.thekelleys.org.uk/dnsmasq/CHANGELOG;
loop itself is defined in dnsmasq-2.47/src/network.c:404)

* Using --interface option causes longer network startup:
$ time virsh -c qemu:///system net-start isolated1
Network isolated1 started

real	0m0.112s
user	0m0.013s
sys	0m0.009s

$ time virsh -c qemu:///system net-start isolated1
Network isolated1 started

real	0m2.101s
user	0m0.011s
sys	0m0.011s

* Dnsmasq exits after DAD complets which guarantees that radvd will no
more produces following warnings:
Dec 28 19:42:11 nemo radvd[14652]: sendmsg: Invalid argument
---
 src/network/bridge_driver.c |   32 +++++++++++++++++++++++++-------
 1 files changed, 25 insertions(+), 7 deletions(-)

diff --git a/src/network/bridge_driver.c b/src/network/bridge_driver.c
index 7d43ef5..a689c9f 100644
--- a/src/network/bridge_driver.c
+++ b/src/network/bridge_driver.c
@@ -469,16 +469,13 @@ networkBuildDnsmasqArgv(virNetworkObjPtr network,
     virCommandAddArgList(cmd, "--conf-file=", "", NULL);

     /*
-     * XXX does not actually work, due to some kind of
-     * race condition setting up ipv6 addresses on the
-     * interface. A sleep(10) makes it work, but that's
-     * clearly not practical
+     * It's safe to use --interface option for dnsmasq 2.47 and later.
+     * With earlier versions we had to use --listen-address option.
      *
-     * virCommandAddArg(cmd, "--interface");
-     * virCommandAddArg(cmd, ipdef->bridge);
+     * virCommandAddArgList(cmd, "--listen-address", bridgeaddr);
      */
     virCommandAddArgList(cmd,
-                         "--listen-address", bridgeaddr,
+                         "--interface", network->def->bridge,
                          "--except-interface", "lo",
                          NULL);

@@ -1157,9 +1154,30 @@ networkAddGeneralIptablesRules(struct
network_driver *driver,
         goto err9;
     }

+    /* allow DNS over IPv6 requests through to dnsmasq */
+    if (iptablesAddTcpInput(driver->iptables, AF_INET6,
+                            network->def->bridge, 53) < 0) {
+        networkReportError(VIR_ERR_SYSTEM_ERROR,
+                           _("failed to add ip6tables rule to allow
DNS requests from '%s'"),
+                           network->def->bridge);
+        goto err10;
+    }
+
+    if (iptablesAddUdpInput(driver->iptables, AF_INET6,
+                            network->def->bridge, 53) < 0) {
+        networkReportError(VIR_ERR_SYSTEM_ERROR,
+                           _("failed to add ip6tables rule to allow
DNS requests from '%s'"),
+                           network->def->bridge);
+        goto err11;
+    }
+
     return 0;

     /* unwind in reverse order from the point of failure */
+err11:
+    iptablesRemoveTcpInput(driver->iptables, AF_INET6,
network->def->bridge, 53);
+err10:
+    networkRemoveGeneralIp6tablesRules(driver, network);
 err9:
     iptablesRemoveForwardAllowCross(driver->iptables, AF_INET,
network->def->bridge);
 err8: