Linux 4.20 / incomplete ipv4 addr list / RTM_GETADDR

[Arthur Gautier]

Hello bird users,

Today I tried to upgrade one of platform to from linux 4.19 to linux 4.20.
I started noticing the bgp protocol could not start after this message:
  <TRACE> uplink: Waiting for 1.2.3.4 to become my neighbor

The neighbor (directly connected on an interface) would never appear to
bird (as in show interfaces) because the AF_INET/RTM_GETADDR reply from the
kernel looks incomplete. It works fine in v6.

The results from kernel are capped to 104 ip address (and we run servers
with >700 ip address) on loopback.

I confirm this behavour with nlmon / tcpdump / wireshark.
Tested with bird v2.0.2 and v1.6.3 (debian stable).

I haven't reproduced the behavior elsewhere than bird, but I believe this
could prove useful to someone else.

-- 
\o/ Arthur
 G  Gandi.net