I have an Ubuntu host with two ethernet interfaces, one connected to the internet, the other connected to a Raspberri Pi.

Goal: reach the Raspberri Pi from my container (ssh 169.254.1.25). I can not use Docker’s host network because it conflicts with other requirements I have.

These are the relevant IPs I am testing:

• 192.168.178.1 my router in the WLAN
• 192.168.178.155 my host in my WLAN
• 169.254.1.25 my Raspberri Pi connected to an ethernet interface in the linux host
• 169.254.1.100: the second eth interface in my host (the one connected to the Raspberri pi)
• 8.8.8.8: a known internet host
• 172.70.0.1: docker0 on the host (bridge created by docker)
• 172.70.0.2: veth on the container (created by docker)

The Ubuntu host can ping everything:

• my router (192.168.178.1)
• itself (both 169.254.1.100 and 192.168.178.155)
• the Raspberri Pi (169.254.1.25)
• the 8.8.8.8

My docker container, started in the default manner (Docker attaches the docker0 bridge), gets the following interfaces:

• eth0: 172.70.0.2
• lo: 127.0.0.1

The container can ping:

• itself (127.0.0.1 and 172.70.0.2)
• my router (192.168.178.1)
• the docker0 bridge (172.70.0.1)
• the host as seen on WLAN (192.168.178.155)
• the 8.8.8.8

But it has trouble with:

• the host on the second eth interface (192.168.178.155)
• the Raspberri Pi (169.254.1.25)

This is my routing table:

» ip route
default via 192.168.178.1 dev wlp2s0 proto dhcp metric 600 
169.254.1.0/24 dev enx000ec6a42183 proto kernel scope link src 169.254.1.100 metric 100 
172.70.0.0/24 dev docker0 proto kernel scope link src 172.70.0.1 
192.168.178.0/24 dev wlp2s0 proto kernel scope link src 192.168.178.155 metric 600 

IP forwarding is enabled on my host:

» sysctl net.ipv4.ip_forward
net.ipv4.ip_forward = 1

The host routing table clearly specifies what to do when it sees a packet with destination 169.254.1.x: send it on the interface connect it to Raspberri Pi.

Why is it not working then? Routing is enabled!