The solution has been found, see the "Solution" section for the full write up and config files.

Initial Question

What I'm looking to do is to route WAN traffic from my personal wireguard server through a gluetun container. So that I can connect a client my personal wireguard server and have my traffic still go through the gluetun VPN as follows:

client <--> wireguard container <--> gluetun container <--> WAN

I've managed to set both the wireguard and gluetun container up in a docker-compose file and made sure they both work independently (I can connect a client the the wireguard container and the gluetun container is successfully connecting to my paid VPN for WAN access). However, I cannot get route traffic from the wireguard container through the gluetun container.

Since I've managed to set both up independently I don't believe that there is an issue with the docker-compose file I used for setup. What I believe to be the issue is either the routing rules in my wireguard container, or the firewall rules on the gluetun container.

I tried following this linuxserver.io guide to get the following wg0.conf template for my wireguard container:

[Interface]
Address = ${INTERFACE}.1
ListenPort = 51820
PrivateKey = $(cat /config/server/privatekey-server)
PostUp = iptables -A FORWARD -i %i -j ACCEPT; iptables -A FORWARD -o %i -j ACCEPT; iptables -t nat -A POSTROUTING -o eth+ -j MASQUERADE
# Adds fwmark 51820 to any packet traveling through interface wg0
PostUp = wg set wg0 fwmark 51820
# If a packet is not marked with fwmark 51820 (not coming through the wg connection) it will be routed to the table "51820".
# PostUp = ip -4 rule add not fwmark 51820 table 51820
# Creates a table ("51820") which routes all traffic through the gluetun container
PostUp = ip -4 route add 0.0.0.0/0 via 172.22.0.100
# If the traffic is destined for the subnet 192.168.1.0/24 (internal) send it through the default gateway.
PostUp = ip -4 route add 192.168.1.0/24 via 172.22.0.1
PostDown = iptables -D FORWARD -i %i -j ACCEPT; iptables -D FORWARD -o %i -j ACCEPT; iptables -t nat -D POSTROUTING -o eth+ -j MASQUERADE

Along with the default firewall rules of the gluetun container

Chain INPUT (policy DROP 13 packets, 1062 bytes)
 pkts bytes target     prot opt in     out     source               destination
15170 1115K ACCEPT     0    --  lo     *       0.0.0.0/0            0.0.0.0/0
14403   12M ACCEPT     0    --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate RELATED,ESTABLISHED
    1    60 ACCEPT     0    --  eth0   *       0.0.0.0/0            172.22.0.0/24

Chain FORWARD (policy DROP 4880 packets, 396K bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain OUTPUT (policy DROP 360 packets, 25560 bytes)
 pkts bytes target     prot opt in     out     source               destination
15170 1115K ACCEPT     0    --  *      lo      0.0.0.0/0            0.0.0.0/0
12716 1320K ACCEPT     0    --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate RELATED,ESTABLISHED
    0     0 ACCEPT     0    --  *      eth0    172.22.0.100         172.22.0.0/24
    1   176 ACCEPT     17   --  *      eth0    0.0.0.0/0            68.235.48.107        udp dpt:1637
 1349 81068 ACCEPT     0    --  *      tun0    0.0.0.0/0            0.0.0.0/0

When I run the wireguard container with this configuration I can successfully connect my client however I cannot connect to any website, or ping any IP.

During my debugging process I ran tcpdump on the docker network both containers are in which showed me that my client is successfully sending packets to the wireguard container, but that no packets were sent from my wireguard container to the gluetun container. The closest I got to this was the following line:

17:27:38.871259 IP 10.13.13.1.domain > 10.13.13.2.41280: 42269 ServFail- 0/0/0 (28)

Which I believe is telling me that the wireguard server is trying, and failing, to send packets back to the client.

I also checked the firewall rules of the gluetun container and got the following results:

Chain INPUT (policy DROP 13 packets, 1062 bytes)
 pkts bytes target     prot opt in     out     source               destination
18732 1376K ACCEPT     0    --  lo     *       0.0.0.0/0            0.0.0.0/0
16056   12M ACCEPT     0    --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate RELATED,ESTABLISHED
    1    60 ACCEPT     0    --  eth0   *       0.0.0.0/0            172.22.0.0/24

Chain FORWARD (policy DROP 5386 packets, 458K bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain OUTPUT (policy DROP 360 packets, 25560 bytes)
 pkts bytes target     prot opt in     out     source               destination
18732 1376K ACCEPT     0    --  *      lo      0.0.0.0/0            0.0.0.0/0
14929 1527K ACCEPT     0    --  *      *       0.0.0.0/0            0.0.0.0/0            ctstate RELATED,ESTABLISHED
    0     0 ACCEPT     0    --  *      eth0    172.22.0.100         172.22.0.0/24
    1   176 ACCEPT     17   --  *      eth0    0.0.0.0/0            68.235.48.107        udp dpt:1637
 1660 99728 ACCEPT     0    --  *      tun0    0.0.0.0/0            0.0.0.0/0

Which shows that the firewall for the gluetun container is dropping all FORWARD traffic which (as I understand it) is the sort of traffic I'm trying to set up. What is odd is that I don't see any of those packets in the tcpdump of the docker network.

Has anyone successfully set this up or have any indication on what I should try next? At this point any ideas would be helpful, whether that be more debugging steps or recommendations for routing/firewall rules.

While there have been similar posts on this topic (Here and Here) the responses on both did not really help me.

---

Solution

Docker Compose Setup

My final working setup uses the following docker-compose file:

networks:
  default:
    ipam:
      config:
        - subnet: 172.22.0.0/24
services:
  gluetun_vpn:
    image: qmcgaw/gluetun:latest
    container_name: gluetun_vpn
    cap_add:
      - NET_ADMIN # Required
    environment:
      - VPN_TYPE=wireguard # I tested this with a wireguard setup
      # Setup Gluetun depending on your provider.
    volumes:
      - {docker config path}/gluetun_vpn/conf:/gluetun
      - {docker config path}/gluetun_vpn/firewall:/iptables
    sysctls:
      # Disables ipv6
      - net.ipv6.conf.all.disable_ipv6=1
    restart: unless-stopped
    networks:
      default:
        ipv4_address: 172.22.0.100
  wireguard_server:
    image: lscr.io/linuxserver/wireguard:latest
    container_name: wg_server
    cap_add:
      - NET_ADMIN
    environment:
      - TZ=America/Detroit
      - PEERS=1
      - SERVERPORT=3697 # Optional
      - PEERDNS=172.22.0.100 # Set this as the Docker network IP of the gluetun container to use your vpn's dns resolver
    ports:
      - 3697:51820/udp # Optional
    volumes:
      - {docker config path}/wg_server/conf:/config
    sysctls:
      - net.ipv4.conf.all.src_valid_mark=1
    networks:
      default:
        ipv4_address: 172.22.0.2
    restart: unless-stopped

Once you get both docker containers working you still need to edit some configuration files.

Wireguard Server Setup

After the wireguard container setup you need to edit {docker config path}/wg_server/conf/templates/server.conf to the following:

[Interface]
Address = ${INTERFACE}.1
ListenPort = 51820
PrivateKey = $(cat /config/server/privatekey-server)

# Default from the wg container
PostUp = iptables -A FORWARD -i %i -j ACCEPT; iptables -A FORWARD -o %i -j ACCEPT; iptables -t nat -A POSTROUTING -o eth+ -j MASQUERADE

## Add this section
# Adds fwmark 51820 to any packet traveling through interface wg0
PostUp = wg set wg0 fwmark 51820
# If a packet is not marked with fwmark 51820 (not coming through the wg connection) it will be routed to the table "51820".
PostUp = ip -4 rule add not fwmark 51820 table 51820
PostUp = ip -4 rule add table main suppress_prefixlength 0
# Creates a table ("51820") which routes all traffic through the vpn container
PostUp = ip -4 route add 0.0.0.0/0 via 172.22.0.100 table 51820
# If the traffic is destined for the subnet 192.168.1.0/24 (internal) send it through the default gateway.
PostUp = ip -4 route add 192.168.1.0/24 via 172.22.0.1

# Default from the wg container
PostDown = iptables -D FORWARD -i %i -j ACCEPT; iptables -D FORWARD -o %i -j ACCEPT; iptables -t nat -D POSTROUTING -o eth+ -j MASQUERADE

The above config is a slightly modified setup from this linuxserver.io tutorial

Gluetun Setup

If you've setup your gluetun container properly the only thing you have to do is create {docker config path}/gluetun_vpn/firewall/post-rules.txt containing the following:

iptables -t nat -A POSTROUTING -o tun+ -j MASQUERADE
iptables -t filter -A FORWARD -d 172.22.0.2 -j ACCEPT
iptables -t filter -A FORWARD -s 172.22.0.2 -j ACCEPT

These commands should be automatically run once you restart the gluetun container. You can test the setup by running iptables-legacy -vL -t filter from within the gluetun container. Your output should look like:

Chain INPUT (policy DROP 7 packets, 444 bytes)
 pkts bytes target     prot opt in     out     source               destination
27512 2021K ACCEPT     all  --  lo     any     anywhere             anywhere
43257   24M ACCEPT     all  --  any    any     anywhere             anywhere             ctstate RELATED,ESTABLISHED
  291 28191 ACCEPT     all  --  eth0   any     anywhere             172.22.0.0/24

# These are the important rules
Chain FORWARD (policy DROP 12276 packets, 2476K bytes)
 pkts bytes target     prot opt in     out     source               destination
17202 8839K ACCEPT     all  --  any    any     anywhere             172.22.0.2
26704 5270K ACCEPT     all  --  any    any     172.22.0.2           anywhere

Chain OUTPUT (policy DROP 42 packets, 2982 bytes)
 pkts bytes target     prot opt in     out     source               destination
27512 2021K ACCEPT     all  --  any    lo      anywhere             anywhere
53625 9796K ACCEPT     all  --  any    any     anywhere             anywhere             ctstate RELATED,ESTABLISHED
    0     0 ACCEPT     all  --  any    eth0    c6d5846467f3         172.22.0.0/24
    1   176 ACCEPT     udp  --  any    eth0    anywhere             64.42.179.50         udp dpt:1637
 2463  148K ACCEPT     all  --  any    tun0    anywhere             anywhere

And iptables-legacy -vL -t nat which should look like:

Chain PREROUTING (policy ACCEPT 18779 packets, 2957K bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain INPUT (policy ACCEPT 291 packets, 28191 bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain OUTPUT (policy ACCEPT 7212 packets, 460K bytes)
 pkts bytes target     prot opt in     out     source               destination

# This is the important rule
Chain POSTROUTING (policy ACCEPT 4718 packets, 310K bytes)
 pkts bytes target     prot opt in     out     source               destination
13677  916K MASQUERADE  all  --  any    tun+    anywhere             anywhere

The commands in post-rules.txt are a more precises version of @CumBroth@discuss.tchncs.de solution in the comments.