netdev - Maybe a bug with adding default routes?

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Date:   Wed, 19 Apr 2023 08:47:54 +0200
From:   Robert Landers <landers.robert@...il.com>
To:     netdev@...r.kernel.org
Subject: Maybe a bug with adding default routes?

Hello netdev,

I believe I either found a bug, or I'm doing something wrong (probably
the latter, or both!). I was experimenting with getting a "floating
IP" for my home lab, and eventually, I got it to work, but it requires
some voodoo, which intrigued me and I think I found some strange
behavior that smells like a bug. I'm on Ubuntu 22.04 (and Pop OS! on
my desktop), so it is also possible that this is fixed upstream (in
which case, I'll email that list next).

To reproduce is quite simple:

echo "1234 test" >> /etc/iproute2/rt_tables
ip route add default via 167.235.212.73 dev enp9s0 table test

This will fail with the error:

Error: Nexthop has invalid gateway.

Now, I think this makes sense, however, the routing table shouldn't
need to know about hops, Right? Maybe I'm wrong, but this voodoo
results in a correct routing table:

ip addr add 167.235.212.72/29 dev enp9s0
ip route add default via 167.235.212.73 dev enp9s0 table test
ip addr del 167.235.212.72/29 dev enp9s0

I'm not sure if this is a bug or working as designed. It smells like a
bug, but I could just as easily be doing something wrong. I grew up in
"simpler" times and am not nearly as familiar with iproute2 as I was
with the old stuff.

PS. I've removed other commands (like rules, etc) to reduce the
example down to it's most basic reproduction.

Robert Landers
Software Engineer
Utrecht NL