On a Debian 12 VM I have 2 network interfaces set up, namely eth0 and eth1. Both interfaces have access to the Internet and set up for SLAAC autoconfiguration.
Since both networks have a router, my routing table on this VM ends up looking like this:
2a01:xxxx:xxxx:xxxx::/64 dev eth1 proto kernel metric 256 expires 86388sec pref medium
fd45:xxxx:xxxx:xxxx::/64 dev eth0 proto kernel metric 256 expires 2591988sec pref medium
fe80::/64 dev eth0 proto kernel metric 256 pref medium
fe80::/64 dev eth1 proto kernel metric 256 pref medium
default via fe80::xxxx:xxxx:xxxx:xxxx dev eth1 proto ra metric 1024 expires 1788sec hoplimit 64 pref medium
default via fe80::be24:xxxx:xxxx:xxxx dev eth0 proto ra metric 1024 expires 1788sec pref medium
On the first few days everything was fine, and I can access internal content in fd45:: through eth0. However it suddenly broke now, with all outbound traffic routed through eth1, as described by traceroute.
I want to force outbound connections to use eth0. I can temporarily remove the default route on eth1 by using ip -6 route del default dev eth1 but it would come back in a few minutes. Is there any way I could achieve this?
My /etc/network/interfaces looks like this:
auto lo
iface lo inet loopback
auto eth0
auto eth1
iface eth1 inet6 auto
iface eth0 inet dhcp
iface eth0 inet6 auto