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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 (ping 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 host on the second eth interface (169.254.1.100)
  • the 8.8.8.8

But it has trouble with:

  • the Raspberri Pi (169.254.1.25)

This is the routing table in the host:

» 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 the host:

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

These are the docker networks:

» docker network ls 
NETWORK ID     NAME      DRIVER    SCOPE
8e2d62a7186d   bridge    bridge    local
4d041aedcd2a   host      host      local
71278e64d14b   none      null      local

The routing table in the docker container:

root@8f50776e53dc:~# ip route
default via 172.70.0.1 dev eth0 
172.70.0.0/24 dev eth0 proto kernel scope link src 172.70.0.2

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 connected to Raspberri Pi.

Why is it not working then? Routing is enabled!

blueFast
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