Subject: Re: Question: should local address be expired when updating PMTU?

From: shengyong <[email-masked]>
Date: Tue, 3 Feb 2015 11:21:17 +0800

在 2015/2/3 10:10, Calvin Owens 写道:
> On Monday 02/02 at 16:52 -0800, Alex Gartrell wrote:
>> Hello Shengyong,
>>
>>> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
>>> index b2614b2..b80317a 100644
>>> --- a/net/ipv6/route.c
>>> +++ b/net/ipv6/route.c
>>> @@ -1136,6 +1136,9 @@ static void ip6_rt_update_pmtu(struct
>> dst_entry *dst, struct sock *sk,
>>>   {
>>>          struct rt6_info *rt6 = (struct rt6_info*)dst;
>>>
>>> +       if (rt6->rt6i_flags & RTF_LOCAL)
>>> +               return;
>>> +
>>>          dst_confirm(dst);
>>>          if (mtu < dst_mtu(dst) && rt6->rt6i_dst.plen == 128) {
>>>                  struct net *net = dev_net(dst->dev);
>>>
>>> So is this modification correct? Or how can we avoid such expiring? 
>>
>> FWIW, we encountered this problem with IPVS tunneling.  Here's a
>> patch done by Calvin (cc'ed) that fixes my attempted fix for this.
>> We're not particularly proud of this...
>>
>> At a high level, I don't think the RTF_LOCAL check was sufficient,
>> but I didn't investigate deeply enough and hopefully Calvin can say
>> why.
> 
> I honestly didn't spend much time at all finding the underlying cause
> because it appeared to be fixed upstream: on 3.19-rc5 you get all 3
> expected routes after the last step of my repro below.
Hi,
I do my test on 3.19.0-rc7 just now, it seems it still doesn't solve the
local-addr-expired problem.
 I just really
> needed to get this working at the time, and the gross disgusting
> horrible ugly awful [more negative adjectives] patch included below made
> it work.
> 
> FWIW, the explanation I wrote down in my notes was:
> 
> "The absence of RTF_NONEXTHOP is causing COWs to happen, which are
> always marked as RTF_CACHE. Somehow that's screwing things up in
> rt6_do_redirect()"
> 
> That could be BS though, I don't at all remember how I came to that
> conclusion. 
> 
> (/me resolves to write better notes in the future...)
> 
> Here's how to get the weird behavior on 3.10 (+stable):
> 
> $ sudo ip addr add local 4444::1 dev lo
> ### Now I have 2 routes in /proc/net/ipv6_route, a local and a non-local
> ### Both have the RTF_NONEXTHOP flag set (0x00200000)
> $ sudo ip route add local 4444::1 dev lo
> ### Now I have 3 routes in /proc/net/ipv6_route to 4444::1
> ### Notice the new route does NOT have the RTF_NONEXTHOP flag set
> $ sudo ip addr del local 4444::1 dev lo
> ### Now I just have the one route I created before
> $ sudo ip addr add local 4444::1 dev lo
> ### And now I have 3 routes again
> $ sudo ping6 4444::1
> [blah blah blah successful ping]
> $ sudo ip addr del local 4444::1 dev lo
> $ sudo ip addr add local 4444::1 dev lo
> ### Still have 3 routes
> $ sudo ip addr del local 4444::1 dev lo
> ### Now I just have my one route yet again
> ### Now, *without the address on lo*, talk to it (it works), then re-add it
> $ ping6 4444::1
> [blah blah blah successful ping]
> $ sudo ip addr add local 4444::1 dev lo
> ### Now I only have 2 routes... WAT!?
> ### Notice the LOCAL (0x80000000) route doesn't have the RTF_NONEXTHOP flag set
Looks like we meet different problems. Here is how I do my test (as well as on 3.10
+stable):
      Host only
PC <------------> Virtual Machine
create and send a packet using scapy:
-----------------------------------
| IPv6 (src=PC-addr, dst=VM-addr) |
|---------------------------------|
|     ICMPv6 (Packet Too Big)     |
|---------------------------------|
| IPv6 (src=VM-addr, dst=VM-addr) |
|---------------------------------|
| ICMPv6 (Neighbor Advertisement) |
-----------------------------------
Then the local-addr is set to expire. After expired, the VM is unreachable from
PC side.

thanks,
Sheng
> 
> Thanks,
> Calvin
> 
>> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
>> index f14d49b..c607a42 100644
>> --- a/net/ipv6/route.c
>> +++ b/net/ipv6/route.c
>> @@ -1159,18 +1159,18 @@ static void ip6_rt_update_pmtu(struct
>> dst_entry *dst, struct sock *sk,
>>                 }
>>                 dst_metric_set(dst, RTAX_MTU, mtu);
>>
>> -               /* FACEBOOK HACK: We need to not expire local non-expiring
>> -                * routes so that we don't accidentally start blackholing
>> -                * ipvs traffic when we happen to use it locally for
>> -                * healthchecking (see ip_vs_xmit.c --
>> -                * __ip_vs_get_out_rt_v6 invokes update_pmtu if the rt is
>> -                * associated with a socket)
>> -                * Alex Gartrell <[email-masked]>
>> +               /*
>> +                * FACEBOOK HACK: Only expire routes that aren't destined for
>> +                * the loopback interface.
>> +                *
>> +                * This prevents the strange route coalescing that happens when
>> +                * you add an address to the loopback that had a route that had
>> +                * been used when the address didn't exist from getting expired
>> +                * and causing packet loss in shiv.
>>                  */
>> -               if (!(rt6->rt6i_flags & RTF_LOCAL) ||
>> -                   (rt6->rt6i_flags & (RTF_EXPIRES | RTF_CACHE)))
>> -                       rt6_update_expires(
>> -                               rt6, net->ipv6.sysctl.ip6_rt_mtu_expires);
>> +               if (!(dst->dev->flags & IFF_LOOPBACK))
>> +                       rt6_update_expires(rt6,
>> + net->ipv6.sysctl.ip6_rt_mtu_expires);
>>         }
>>  }
>>
>>
>> Cheers,
>> -- 
>> Alex Gartrell <[email-masked]>
> 
> .
>