Bug#861115: Please consider increasing net.ipv6.route.max_size default value

From	Asbjørn Sloth Tønnesen <ast@fiberby.net>
Newsgroups	linux.debian.bugs.dist, linux.debian.kernel
Subject	Bug#861115: Please consider increasing net.ipv6.route.max_size default value
Date	2020-02-27 15:50 +0100
Message-ID	<zEsE9-5iq-9@gated-at.bofh.it> (permalink)
References	<tzR3X-26a-9@gated-at.bofh.it> <zr7e9-2A1-1@gated-at.bofh.it> <tzR3X-26a-9@gated-at.bofh.it> <zr7e9-2A1-1@gated-at.bofh.it>
Organization	linux.* mail to news gateway

Cross-posted to 2 groups.

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Hi Tim,

On 1/21/20 6:58 PM, Tim Bray wrote:
> I also agree.
> 
> I've just spent a month tracking down an issue where IPv6 networking connections hang.  Using Debian buster, 
> 4.19.0-6-amd64.

I also had fun finding a workaround for this bug, back in December, but I originally
noticed some weirdness in v4.9 (neighbor being gc'ed immediately, possibly due to route
falling out of cache).

The below messages seams related:

Basil Fillan <jack@basilfillan.uk> on the bird-users list[1]:
 > We've also experienced this after upgrading a few routers to Debian
 > Buster. With a kernel bisect we found that a bug was introduced in the
 > following commit:
 >
 > 3b6761d18bc11f2af2a6fc494e9026d39593f22c
 >
 > This bug was still present in master as of a few weeks ago.
 >
 > It appears entries are added to the IPv6 route cache which aren't
 > visible from "ip -6 route show cache", but are causing the route cache
 > garbage collection system to trigger extremely often (every packet?)
 > once it exceeds the value of net.ipv6.route.max_size. [...]

commit 3b6761d18bc11f2af2a6fc494e9026d39593f22c
Author: David Ahern <dsahern@gmail.com>
Date:   Tue Apr 17 17:33:20 2018 -0700

     net/ipv6: Move dst flags to booleans in fib entries

     Continuing to wean FIB paths off of dst_entry, use a bool to hold
     requests for certain dst settings. Add a helper to convert the
     flags to DST flags when a FIB entry is converted to a dst_entry.

     Signed-off-by: David Ahern <dsahern@gmail.com>
     Signed-off-by: David S. Miller <davem@davemloft.net>

Brian Rak <brak@gameservers.com> on the netdev list[2]:
> I have net.ipv6.route.max_size=4096, and the machine only has 376 routes 
> (calculated by `ip -6 route | wc -l`).  However, raising this sysctl to 
> 65k seems to instantly fix IPv6 (I'm not sure if this is a permanent fix 
> yet)

I also found an older thread[3] from 2014 on netdev, that revolves around
the same topic as this bug, but it's might only share the workaround.


[1] Re: IPv6 BGP & kernel 4.19
     https://bird.network.cz/pipermail/bird-users/2020-February/014270.html

[2] Re: IPv6 neighbor discovery issues on 4.18 (and now 4.19)
     https://lore.kernel.org/netdev/de54e925-9536-f2cc-7b89-7205b3fb2c18@gameservers.com/

[3] IPv6 routing table max_size badly dimensioned compared to IPv4
     https://lore.kernel.org/netdev/20140227192403.GA13472@xs.powerdns.com/

-- 
Best regards
Asbjørn Sloth Tønnesen
Network Engineer
Fiberby ApS - AS42541

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Thread

Bug#861115: Please consider increasing net.ipv6.route.max_size default value Tim Bray <tim@provu.co.uk> - 2020-01-21 20:20 +0100
  Bug#861115: Please consider increasing net.ipv6.route.max_size default value Vincent Bernat <bernat@debian.org> - 2020-01-21 20:50 +0100
    Bug#861115: Please consider increasing net.ipv6.route.max_size default value Tim Bray <tim@provu.co.uk> - 2020-01-22 01:00 +0100
      Bug#861115: Please consider increasing net.ipv6.route.max_size default value Vincent Bernat <bernat@debian.org> - 2020-01-22 07:00 +0100
  Bug#861115: Please consider increasing net.ipv6.route.max_size default value Asbjørn Sloth Tønnesen <ast@fiberby.net> - 2020-02-27 15:50 +0100

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