BGP on /32 (/128) interfaces

[Arzhel Younsi]

Hi,

A message between bug report and feature request, unless it's a silly
oversight on my side.

On Linux it's possible to configure interfaces with /32 (or v6 /128) IPs. I
detailed our use-case and examples on
https://phabricator.wikimedia.org/phame/post/view/312/ganeti_on_modern_network_design/
if you're curious.

So focusing on v4 only for now I have this configuration on a VM:
testvm2006:~$ ip addr
inet 10.192.24.4/32 brd 10.192.24.4 scope global ens13
testvm2006:~$ ip route
default via 10.192.24.1 dev ens13 onlink

And this on the hypervisor (but it could also be 2 physical servers, or a
router and a server) :
ganeti2033:~$ ip addr
inet 10.192.24.1/32 scope link tap0
ganeti2033:~$ ip route
10.192.24.4 dev tap0 proto static scope link

Note that 10.192.24.1 is close to 10.192.24.4, but they could be totally
different IPs (see the v6 example bellow).

Overall connectivity works well, TCP handshake as well:
testvm2006:~$ nc -zv 10.192.24.1 179
Connection to 10.192.24.1 179 port [tcp/bgp] succeeded!

The issue is that with a basic configuration like:
protocol bgp {
    ipv4 {
        import none;
        export none;
    };
    local 10.192.24.4 as 64613;
    neighbor 10.192.24.1 external;
}
Bird 2.0.12 doesn't try to established the session, and waits with:
2024-04-12T08:42:29.497756+00:00 testvm2006 bird: bgp1: Waiting for
10.192.24.1 to become my neighbor

For IPv4 it's possible to workaround the issue by setting the session as
"multihop;" on both side
bgp1: Connecting to 10.192.24.1 from local address 10.192.24.4
bgp1: Connected
bgp1: Sending OPEN(ver=4,as=64613,hold=240,id=0ac01804)
bgp1: Got OPEN(as=64612,hold=240,id=10.192.21.6)
bgp1: Sending KEEPALIVE
bgp1: Got KEEPALIVE
bgp1: BGP session established

But for IPv6, it's cleaner to only require the router's link local address:
testvm2006:~$ ip -6 addr
inet6 2620:0:860:140:10:192:24:4/128 scope global
testvm2006:~$ ip -6 route
default via fe80::2022:22ff:fe22:2201 dev ens13 metric 1024 pref medium

In Bird:
neighbor fe80::2022:22ff:fe22:2201%ens13 external;

But then the link local address doesn't work with multihop (for obvious
reason).
bird: /etc/bird/bird.conf:22:1 Multihop BGP cannot be used with link-local
addresses

The question is then, would it be possible to support that usecase,
something along the lines of if the local IP is a /32 (or v6 /128), Treat
the neighbor as directly connected, especially if it's a link local IP
(unless nultihop is specified).
I haven't looked at BFD yet, so not sure if anything would need to change
there as well.

Don't hesitate to ask if something needs any clarifications.

Thank for that great tool, Bird is awesome,

-- 
Arzhel
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://trubka.network.cz/pipermail/bird-users/attachments/20240412/9e70022f/attachment.htm>