Routing between wg interfaces with WireGuard

Aha! This was the last piece I was really looking for with WireGuard. It gets a bit tricky when you want packets to route between WireGuard clients. But once you grok how it works, well, it works.

This also works for passing traffic between WireGuard clients on the same interface – the trick is in making certain that AllowedIPs in the client configs includes the entire IP subnet services by the server, not just the single IP address of the server itself (with a /32 subnet)… and that you not only set up the tunnel on each client, but initialize it with a bit of data as well.

Set up your server with two WireGuard interfaces:

root@server:~# touch /etc/wireguard/keys/server.wg0.key
root@server:~# chmod 600 /etc/wireguard/keys/server.wg0.key 
root@server:~# wg genkey > /etc/wireguard/keys/server.wg0.key root@server:~# wg pubkey < /etc/wireguard/keys/server.wg0.key > /etc/wireguard/keys/server.wg0.pub

root@server:~# touch /etc/wireguard/keys/server.wg1.key
root@server:~# chmod 600 /etc/wireguard/keys/server.wg1.key 
root@server:~# wg genkey > /etc/wireguard/keys/server.wg1.key root@server:~# wg pubkey < /etc/wireguard/keys/server.wg1.key > /etc/wireguard/keys/server.wg1.pub

Don’t forget to make sure ipv4 forwarding is enabled on your server:

root@server:~# sed -i 's/^#net\.ipv4\.ip_forward=1/net.ipv4.ip_forward=1/' /etc/sysctl.conf
root@server:~# sysctl -p

Now set up your wg0.conf:

# server.wg0.conf

[Interface] 
   Address = 10.0.0.1/24 
   ListenPort = 51820
   PrivateKey = WG0_SERVER_PRIVATE_KEY
   SaveConfig = false

[Peer] 
   # client1 
   PublicKey = PUBKEY_FROM_CLIENT_ONE
   AllowedIPs = 10.0.0.2/32

And your wg1.conf:

# server.wg1.conf

[Interface] 
 Address = 10.0.1.1/24 
 ListenPort = 51821
 PrivateKey = WG1_SERVER_PRIVATE_KEY
 SaveConfig = false

[Peer] 
 # client2 
 PublicKey = PUBKEY_FROM_CLIENT_TWO
 AllowedIPs = 10.0.1.2/32

Gravy. Now enable both interfaces, and bring them online.

root@server:~# systemctl enable wg-quick@wg0 ; systemctl enable wg-quick@wg1
root@server:~# systemctl start wg-quick@wg0 ; systemctl start wg-quick@wg1

Server’s done. Now, set up your clients.

Client install, multi-wg server:

Client one will connect to the server’s wg0, and client two will connect to the server’s wg1. After creating your keys, set them up as follows:

# /etc/wireguard/wg0.conf on Client1
#    connecting to server/wg0
 
[Interface]
   Address = 10.0.0.2/24
   PrivateKey = PRIVATE_KEY_FROM_CLIENT1
   # set up routing from server/wg0 to server/wg1
   PostUp = route add -net 10.0.1.0/24 gw 10.0.0.1 ; ping -c1 10.0.0.1
   PostDown = route delete -net 10.0.1.0/24 gw 10.0.0.1
   SaveConfig = false

[Peer]
   PublicKey = PUBKEY_FROM_SERVER
   AllowedIPs = 10.0.0.1/24, 10.0.1.1/24
   Endpoint = wireguard.yourdomain.tld:51820

Now set up wg0 on Client2:

# /etc/wireguard/wg0.conf on Client2
#   connecting to server/wg1 

[Interface]
   Address = 10.0.1.2/24
   PrivateKey = PRIVATE_KEY_FROM_CLIENT2
   # set up routing from server/wg1 to server/wg0
   PostUp = route add -net 10.0.0.0/24 gw 10.0.1.1 ; ping -c1 10.0.1.1
   PostDown = route delete -net 10.0.0.0/24 gw 10.0.1.1
   SaveConfig = false

[Peer]
   PublicKey = PUBKEY_FROM_SERVER
   AllowedIPs = 10.0.0.1/24, 10.0.1.1/24
   Endpoint = wireguard.yourdomain.tld:51821

Now start up the client interfaces. First, Client1:

root@client1:~# systemctl start wg-quick@wg0
root@client1:~# wg
interface: wg0
 public key: MY_PUBLIC_KEY
 private key: (hidden)
 listening port: some-random-port

peer: SERVER_PUBLIC_KEY
 endpoint: server-ip-address:51820
 allowed ips: 10.0.0.0/24, 10.0.1.0/24
 latest handshake: 3 seconds ago
 transfer: 2.62 KiB received, 3.05 KiB sent

Now Client2:

root@client2:~# systemctl start wg-quick@wg0
root@client2:~# wg
interface: wg0
 public key: MY_PUBLIC_KEY
 private key: (hidden)
 listening port: some-random-port

peer: SERVER_PUBLIC_KEY
 endpoint: server-ip-address:51821
 allowed ips: 10.0.0.0/24, 10.0.1.0/24
 latest handshake: 4 seconds ago
 transfer: 2.62 KiB received, 3.05 KiB sent

Now that both clients are connected, we can successfully send traffic back and forth between client1 and client2.

root@client1:~# ping -c3 10.0.1.2
PING 10.0.1.2 (10.0.1.2) 56(84) bytes of data.
64 bytes from 10.0.1.2: icmp_seq=1 ttl=63 time=80.4 ms
64 bytes from 10.0.1.2: icmp_seq=2 ttl=63 time=83.5 ms
64 bytes from 10.0.1.2: icmp_seq=3 ttl=63 time=83.2 ms

--- 10.0.1.2 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2003ms
rtt min/avg/max/mdev = 80.492/82.410/83.501/1.380 ms

Sweet.

The step some of you undoubtedly missed:

There’s a little trick that will frustrate you to no end if you glossed over the client config files without paying attention.

PostUp = route add -net 10.0.0.0/24 gw 10.0.1.1 ; ping -c1 10.0.1.1

If you left out that ping command in one or both of the clients, then you won’t be able to ping client1 from client2, or vice versa, until each client has sent some data down the tunnel. This frustrated the crap out of me initially – ping client2 from client1 would be broken, but then ping client1 from client2 would work and then ping client2 from client1 would work. The solution, as shown, is just to make sure we immediately chuck a packet down the tunnel whenever each client connects; then everything works as intended.

What if I don’t want multiple interfaces?

If all you want to do is pass traffic from one client to the next, you don’t need two interfaces on the server, and you don’t need PostUp and PostDown route commands, either. The trick is that you do need to make sure that AllowedIPs on each client is set to a range that includes the other client, and you do need the PostUp ping -c1 server-ip-addresscommand on each client as well. Without that PostUp ping, you’re going to get frustrated by connectivity that “sometimes works and sometimes doesn’t”.

If you want to give access to some clients but not all clients, you can do that by setting multiple AllowedIPs arguments on the clients, like so:

[Peer]
   PublicKey = PUBKEY_FROM_SERVER
   # this stanza allows access from the server (.1), client one (.2),
   # and client two (.3) - but not from any clients at .4-.254.
   AllowedIPs = 10.0.0.1/32, 10.0.0.2/32, 10.0.0.3/32
   Endpoint = wireguard.yourdomain.tld:51820

That is a sample [Peer] stanza of a client wg config, not a[Peer] stanza of the server wg config! The[Peer] stanzas of the server config should only allow connection to a single IP (using a /32 subnet) for each individual[Peer] definition.

If you try to set AllowedIPs 10.0.0.0/24 on both client1 and client2’s[Peer] stanzas in the server’s wg config, you’ll break one or the other client – they can’t BOTH be allowed the entire subnet.

This bit’s a little confusing, I know, but that’s how it works.

As always, caveat imperator.

Just like the last couple of posts, everything here is tested and working. Also just like the last couple of posts, this is day zero of my personal experience with Wireguard. While everything here works, I still have not delved into any of the actual crypto components’ configurability or personally evaluated/researched how sane the defaults are.

These configs absolutely will encrypt the data sent down the tunnel – I’ve verified that much! – but I cannot offer a well-established and researched opinion on how well they encrypt that data, or what weaknesses there might be. I have no particular reason to believe they’re not secure, but I haven’t done any donkey-work to thoroughly establish that they are secure either.

Caveat imperator.

I may come back to edit out these dire warnings in a few weeks or months as I hammer at this stuff more. Or I might forget to! Feel free to tweet @jrssnet if you want to ask how or if things have changed; I am definitely going to continue my evaluation and testing.

Published by

Jim Salter

Mercenary sysadmin, open source advocate, and frotzer of the jim-jam.

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