I am currently trying to simulate a network using network namespaces under Linux. I have already set up the nodes and connected them, and they can ping each other, one hop at a time. But I am really struggling trying to enable IP forwarding.
I am using Ubuntu Server 21.04 and networking on my system is controlled by systemd-networkd. systemd's version is 247.3-3ubuntu3.4. net.ipv6.conf.all.forwarding and net.ipv4.ip_forward are already enabled.
Because networkd is used, forwarding has to be enabled in configuration files additionally.
For one of my namespaces, this looks as follows:
/etc/systemd/network/router1i.network:
[Match]
Name=router1i
[Network]
IPForward=yes
and /etc/systemd/network/router1i2.network:
[Match]
Name=router1i2
[Network]
IPForward=yes
Those (router1i and router1i2) are both veth interfaces and the only 2 interfaces in the namespace.
If I use the command ip -6 route get to fe80::1:0:200 iif router1i2 in the namespace, I get the correct answer fe80::1:0:200 from :: dev router1i2 proto kernel metric 256 iif router1i2 pref medium, because the route doesn't involve forwarding. If I use the similar command ip -6 route get to fe80::1:0:200 iif router1i that starts from the other interface, the answer suddenly is RNETLINK answers: Network is unreachable. So apparently, forwarding isn't enabled.
I already tried to get networkd to update by using networkctl reconfigure router1i from within the namespace, but it says Failed to reconfigure network interface router1i: No such device or address. This is strange, because when I use networkctl status router1i, it lists all the information correctly. A full reload using networkctl reload was also already tried and doesn't change anything.
I'm out of ideas. I don't even necessarily need to get it to work with networkd. What can I try next?
