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dockeropenvpn

Docker exposed port stops working when connected to a VPN

I'm trying to create a Docker image which will forward a port through a VPN. I've created a simple image which exposes port 5144, and tested that it works properly:

sudo docker run -t -d -p 5144:5144 \
                --name le-bridge \
                --cap-add=NET_ADMIN \
                --device=/dev/net/tun \
                bridge
sudo docker exec -it le-bridge /bin/bash

I check that the port is exposed correctly like this:

[CONTAINER] root@6116787b1c1e:~# nc -lvvp 5144
[HOST] user$ nc -vv 127.0.0.1 5144

Then, whatever I type is correctly echoed in the container's terminal. However, as soon as I start the openvpn daemon, this doesn't work anymore:

[CONTAINER] root@6116787b1c1e:~# openvpn logger.ovpn &
[1] 33
Sun Apr  5 22:52:54 2020 OpenVPN 2.4.4 x86_64-pc-linux-gnu [SSL (OpenSSL)] [LZO] [LZ4] [EPOLL] [PKCS11] [MH/PKTINFO] [AEAD] built on May 14 2019
Sun Apr  5 22:52:54 2020 library versions: OpenSSL 1.1.1  11 Sep 2018, LZO 2.08
Sun Apr  5 22:52:54 2020 TCP/UDP: Preserving recently used remote address: [AF_INET]
Sun Apr  5 22:52:54 2020 UDPv4 link local (bound): [AF_INET][undef]:1194
Sun Apr  5 22:52:54 2020 UDPv4 link remote: 
Sun Apr  5 22:52:54 2020 WARNING: this configuration may cache passwords in memory -- use the auth-nocache option to prevent this
Sun Apr  5 22:52:55 2020 [] Peer Connection Initiated with [AF_INET]
Sun Apr  5 22:53:21 2020 TUN/TAP device tun0 opened
Sun Apr  5 22:53:21 2020 do_ifconfig, tt->did_ifconfig_ipv6_setup=0
Sun Apr  5 22:53:21 2020 /sbin/ip link set dev tun0 up mtu 1500
Sun Apr  5 22:53:21 2020 /sbin/ip addr add dev tun0 10.X.0.2/24 broadcast 10.X.0.255
Sun Apr  5 22:53:21 2020 Initialization Sequence Completed

root@6116787b1c1e:~#
root@6116787b1c1e:~# nc -lvvp 5144
listening on [any] 5144 ...

From here, using the exact same netcat command, I cannot reach the exposed port anymore from the host. What am I missing?

EDIT: It's maybe worth mentioning that after the VPN is started, the connexion still succeeds from the host ; it just never reaches the netcat process inside the container.

Solution

  • I'm not exactly sure why, but it turns out that routes need to be fixed inside the container. In my case, the following command solves the issue:

    ip route add 192.168.0.0/24 via 172.17.42.1 dev eth0

    ...where 172.17.42.1 is the IP of the docker0 interface on my host. Hopefully this is helpful to someone one day.