Re: [libvirt] [PATCH v3] nwfilter: probe for inverted ctdir

For reference of people new to this thread, here is the start of the thread: https://www.redhat.com/archives/libvir-list/2013-March/msg01403.html This concerns changes to libvirt to cope with the newly discovered (by us :-) difference in interpretation of ctdir by different versions of netfilter. On 03/28/2013 07:11 AM, Stefan Berger wrote: > On 03/27/2013 09:09 PM, Stefan Berger wrote: >> On 03/27/2013 02:01 PM, Eric Blake wrote: >>> On 03/27/2013 10:30 AM, Laine Stump wrote: >>>> My opinion is that the patch we should apply should be a simple patch >>>> that just removes use of --ctdir. According to the netfilter developer >>>> who responded to the thread on libvirt-users, it doesn't add any extra >>>> security not already provided by conntrack: >>>> >>>> https://www.redhat.com/archives/libvirt-users/2013-March/msg00121.html >>>> https://www.redhat.com/archives/libvirt-users/2013-March/msg00128.html >>>> >>>> Not being an expert on netfilter internals, I can't dispute his claim. >>>> >>>> Does anyone else have an opinion? >>> What filters specifically caused the use of --ctdir, and are they >>> broken >>> if we omit the use of --ctdir? >> It depends on how you write the filters that the --ctdir is being used. >> >> iirc: The effect of the --ctdir usage is that if one has an incoming >> rule and and outgoing rule with the same IP address on the 'other' >> side the check for an ESTABLISHED state is not enough to ACCEPT the >> traffic, if one was to remove one of the rules while communication in >> both directions was occurring and an immediate cut of the traffic in >> one way was expected. The effect so far was that if the rule for the >> incoming rule was removed it would cut the incoming traffic >> immediately while the traffic in outgoing direction was >> uninterrupted. I think that if we remove this now the traffic in both >> directions will continue. I will verify tomorrow. > Verified. I have a ping running from the VM to destination 'A' and > from 'A' to the VM. The --ctdir enforces the direction of the traffic > and if one of the following rules is removed, the ping is immediately > cut. > > <rule action='accept' direction='out' priority='500'> > <icmp/> > </rule> > <rule action='accept' direction='in' priority='500'> > <icmp/> > </rule> > > The ping is not cut anymore upon removal of one of the above rules if > --ctdir was to be removed entirely. Okay, as I understand from your description, the difference is that when a ping in one direction is already in action, and you remove the rule allowing that ping, that existing ping "session" will continue to be allowed *if* there is still a rule allowing pings in the other direction. Is that correct? I'm guessing that *new* attempts to ping in that direction will no longer be allowed though, is that also correct? For the benefit of Pablo and the other netfilter developers, can you paste the iptables commands that are generated for the two rules above? Possibly they can suggest alternative rules that have the desired effect.

If they have no alternatives, then I do now agree that we shouldn't just scrap --ctdir. Then we have two choices: 1) take your patch, which hopefully will successfully guess the polarity of --ctdir correctly in all cases.

2) switch unconditionally to the new "correct" polarity of --ctdir, release-note the heck out of it, and require that any distro with a kernel old enough to have the old style --ctdir backport at least the one patch to netfilter to change that.

My comments about (1): while I'll again say that the patch truly is poetic in its ability to overcome obstacles, it's really a workaround for a bug, and as time goes on will become less and less relevant (and more and more difficult to explain/rationalize). I really think I would prefer to be broken on very old distros rather than have that patch in the tree (although others may have a different opinion, and I would gracefully withdraw my objection in that case). About (2): Fedora at least as far back as F16 has a new enough kernel/iptables (iptables-1.4.12) that it uses the new polarity for --ctdir, and no older Fedora is supported. RHEL6 and CentOS6 still have iptables-1.4.7, so they have the old polarity. I'd be willing to bet that RHEL6 would take the netfilter patch to change --ctdir; of course that would leave us with temporary brokenness for anybody who upgraded their kernel without upgrading libvirt, or vice-versa.