Re: [libvirt] [PATCH] Fix dnsmasq/radvd on bridges not being able to be bound to IPv6 address on "recent" kernels

Hi, I hit this problem recently when trying to create a bridge with an IPv6 address on a 3.2 kernel: dnsmasq (and, further, radvd) would not bind to the given address (resp. interface), waiting 20s and then giving up with -EADDRNOTAVAIL (resp. exiting immediately with "error parsing or activating the config file", without libvirt noticing it, BTW). This can be reproduced with (I think) any kernel >= 2.6.39 and the following XML (to be used with "virsh net-create"): <network> <name>test-bridge</name> <bridge name='testbr0' /> <ip family='ipv6' address='fd00::1' prefix='64'> </ip> </network> (it happens even when you have an IPv4, too) The problem is that since commit [1] (which, ironically, was made to “help IPv6 autoconfiguration”) the linux bridge code makes bridges behave like “real” devices regarding carrier detection. This makes the bridges created by libvirt, which are started without any up devices, stay with the NO-CARRIER flag set, and thus prevents DAD (Duplicate address detection) from happening, thus letting the IPv6 address flagged as “tentative”. Such addresses cannot be bound to (see RFC 2462), so dnsmasq fails binding to it (for radvd, it detects that "interface XXX is not RUNNING", thus that "interface XXX does not exist, ignoring the interface" (sic)). It seems that this behavior was enhanced somehow with commit [2] by avoiding setting NO-CARRIER on empty bridges, but I couldn't reproduce this behavior on my kernel. Anyway, with the “dummy tap to set MAC address” trick, this wouldn't work. To fix this, the idea is to get the bridge's attached device to be up so that DAD can happen (deactivating DAD altogether is not a good idea, I think). Currently, libvirt creates a dummy TAP device to set the MAC address of the bridge, keeping it down. But even if we set this device up, it is not RUNNING as soon as the tap file descriptor attached to it is closed, thus still preventing DAD. So, we must modify the API a bit, so that we can get the fd, keep the tap device persistent, run the daemons, and close it after DAD has taken place. After that, the bridge will be flagged NO-CARRIER again, but the daemons will be running, even if not happy about the device's state (but we don't really care about the bridge's daemons doing anything when no up interface is connected to it). Other solutions that I envisioned were: * Keeping the *-nic interface up: this would waste an fd for each bridge during all its life. May be acceptable, I don't really know.

* Stop using the dummy tap trick, and set the MAC address directly on the bridge: it is possible since quite some time it seems, even if then there is the problem of the bridge not being RUNNING when empty, contrary to what [2] says, so this will need fixing (and this fix only happened in 3.1, so it wouldn't work for 2.6.39)

* Using the --interface option of dnsmasq, but I saw somewhere that it's not used by libvirt for backward compatibility. I am not sure this would solve this problem, though, as I don't know how dnsmasq binds itself to it with this option.

This is why this patch does what's described earlier. I see radvd yelling quite often in the logs when the interface is NO-CARRIER, but it's ok, it keeps running. Still, this patch is not exactly correct, as radvd get daemonized “too soon” most of the time (i.e. when not debugging libvirtd…) and probes the bridge once it has been set down (even if started before setting it down), thus failing as before (and libvirt lets it be like that: this would need some more checking, maybe). One /may/ introduce some delay between networkStartRadvd() and setting the dummy tap down to solve it, but it seemed too hackish to me. But I couldn't come with a better solution. I would welcome suggestions here.