Rejects network with gateway outside subnet

[waldiTM]

Creating an IPv6 subnet using podman fails if the gateway is not inside the subnet itself. In IPv6 this is a rather common setup, where the gateway uses the link-local range fe80::/10.

With IPv6 in Linux, using such a link-local address will just work with a normal routing setup. With IPv4 in Linux this will in theory also work, by using the special "onlink" flag.

Error: gateway fe80::1 not in subnet 2001::db8::/48

[Luap99]

If ipv4 requires extra netlink option then this would need to be added in netavark. Although I don't think many would use an ipv4 gateway in another subnet?
For ipv6 I think you are right we likely should just drop that check? Does that only work for the link local range? In that case we should likely only allow the link local address and still reject others.

Interested in opening a PR to implement that?