client uses wrong source ip for outgoing connections

Tue Nov 23 16:26:04 UTC 2021

Hi,

I could at least (temporary) fix this issue by adding the correct src IP to the routes like shown in the following example.
Now I just don't fully understand, what causes wireguard to select the IP from the wrong interface. Or based on what it selects the IP in the first place.
Even ping gives the warning: "ping: Warning: source address might be selected on device other than wg0."
That warning goes away when the routes have the correct IP set as src.
-> But I can definitely say that wireguard somehow selects the wrong IP for outgoing packets.
Just that I don't know why this happens only on 5 out of over 20 devices with same configuration..

ip route del <NET>
ip route add <NET> dev <ALIAS_DEV> src <SRC_IP>

information about interfaces:
root at zi1-router:~# ip a sh wg0
18: wg0: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1420 qdisc noqueue state UNKNOWN group default qlen 1000
     link/none
root at zi1-router:~# ip -4 a sh br0
11: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
     inet 78.41.x.y/32 scope global br0
        valid_lft forever preferred_lft forever
root at zi1-router:~# ip -4 a sh br1
12: br1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
     inet 10.34.0.100/24 brd 10.34.0.255 scope global br1
        valid_lft forever preferred_lft forever

root at zi1-router:~# ip r sh dev wg0
10.5.44.0/24 scope link
172.27.0.0/24 scope link
root at zi1-router:~# ip r d 172.27.0.0/24
root at zi1-router:~# ip r d 10.5.44.0/24
root at zi1-router:~# ip r a 172.27.0.0/24 dev wg0 src 10.34.0.100
root at zi1-router:~# ip r a 10.5.44.0/24 dev wg0 src 10.34.0.100
root at zi1-router:~# ip r sh dev wg0
10.5.44.0/24 scope link src 10.34.0.100
172.27.0.0/24 scope link src 10.34.0.100

root at zi1-router:~# ping 172.27.0.1
PING 172.27.0.1 (172.27.0.1) 56(84) bytes of data.
64 bytes from 172.27.0.1: icmp_seq=1 ttl=64 time=13.1 ms

Kind regards,
Christoph

Am 19.11.2021 um 01:11 schrieb Christoph Loesch:
> if relevant, some more details about interface and routes from good and bad example to compare:
>
> root at eng196-router:~# ip a sh wg0
> 46: wg0: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1420 qdisc noqueue state UNKNOWN group default qlen 1000
>     link/none
> root at eng196-router:~# ip r sh dev wg0
> 10.5.44.0/24 scope link
> 172.27.0.0/24 scope link
> root at eng196-router:~# ip a sh br1
> 11: br1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
>     link/ether 44:d9:e7:x:y:z brd ff:ff:ff:ff:ff:ff
>     inet 10.29.85.100/24 brd 10.29.85.255 scope global br1
>        valid_lft forever preferred_lft forever
>     inet6 fe80::7c4c:1dff:fe84:fece/64 scope link
>        valid_lft forever preferred_lft forever
>
> root at zi1-router:~# ip a sh wg0
> 18: wg0: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1420 qdisc noqueue state UNKNOWN group default qlen 1000
>     link/none
> root at zi1-router:~# ip r sh dev wg0
> 10.5.44.0/24 scope link
> 172.27.0.0/24 scope link
> root at zi1-router:~# ip a sh br1
> 12: br1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
>     link/ether 74:83:c2:x:y:z brd ff:ff:ff:ff:ff:ff
>     inet 10.34.0.100/24 brd 10.34.0.255 scope global br1
>        valid_lft forever preferred_lft forever
>     inet6 fe80::2c2e:76ff:fedc:d8e/64 scope link
>        valid_lft forever preferred_lft forever
>
> Am 19.11.2021 um 00:40 schrieb Christoph Loesch:
>> Hi,
>>
>> I am using wireguard on about 20 EdgeRouters (based on Debian stretch).
>> Each router has exact same configuration (apart from router ip addresses and wireguard keys/passphrases).
>> Works very well on most of them but on five routers wireguard uses the wrong ip address for outgoing connections over the tunnel.
>> All routers use kernel 4.14.54-UBNT and wireguard-tools v1.0.20210914
>> Wireguard debian package is from github/WireGuard/wireguard-vyatta-ubnt
>>
>> On the problematic routers the public ip address is used for the tunnel instead the private ip address.
>> Interestingly even in the bad example the wg tunnel is running and the server can reach the routers(=wg clients), but not the other way round.
>>
>> In the following examples 172.27.0.1 is the wireguard server internal ip address.
>> Routers use ip addresses in the 10.0.0.0/8 range for the wg tunnel which are allowed on the server.
>> I already even debugged this with tcpdump where I found out it uses the wrong ip.
>> But looking at a simple ping you also notice the wrong ip after the word "from".
>>
>> Good example:
>> eng196-router:~$ \ping -I wg0 -c1 172.27.0.1
>> ping: Warning: source address might be selected on device other than wg0.
>> PING 172.27.0.1 (172.27.0.1) from 10.29.85.100 wg0: 56(84) bytes of data.
>> 64 bytes from 172.27.0.1: icmp_seq=1 ttl=64 time=6.82 ms
>> --- 172.27.0.1 ping statistics ---
>> 1 packets transmitted, 1 received, 0% packet loss, time 0ms
>> rtt min/avg/max/mdev = 6.826/6.826/6.826/0.000 ms
>>
>> Bad example:
>> zi1-router:~$ \ping -I wg0 -c1 172.27.0.1
>> ping: Warning: source address might be selected on device other than wg0.
>> PING 172.27.0.1 (172.27.0.1) from 78.41.x.y wg0: 56(84) bytes of data.
>> --- 172.27.0.1 ping statistics ---
>> 1 packets transmitted, 0 received, 100% packet loss, time 0ms
>>
>> Configurations:
>> eng196-router:~# wg
>> interface: wg0
>>   public key: SoV2obcH0qWfCRY3gZbkLNeMa1QRcnhNDCeiI9weszA=
>>   private key: (hidden)
>>   listening port: 58205
>> peer: 1syRMYD1jIVFMUMm5hF/j0MzjMQmuC5mlcT1VVugIkU=
>>   preshared key: (hidden)
>>   endpoint: 86.59.x.y:1024
>>   allowed ips: 172.27.0.0/24, 10.5.44.0/24
>>   latest handshake: 53 seconds ago
>>   transfer: 24.57 MiB received, 26.48 MiB sent
>>   persistent keepalive: every 25 seconds
>>
>> zi1-router:~# wg
>> interface: wg0
>>   public key: aYtVhblpR0XSsAb/dXF3zM9Hu+LxlvrR5RWFU2psF3M=
>>   private key: (hidden)
>>   listening port: 45514
>> peer: 1syRMYD1jIVFMUMm5hF/j0MzjMQmuC5mlcT1VVugIkU=
>>   preshared key: (hidden)
>>   endpoint: 86.59.x.y:51820
>>   allowed ips: 172.27.0.0/24, 10.5.44.0/24
>>   latest handshake: 13 seconds ago
>>   transfer: 1.79 MiB received, 6.26 MiB sent
>>   persistent keepalive: every 25 seconds
>>
>> What could cause the wrong selection?
>> Why does that work for most routers but for some not? There must be some difference or something gets confused up by specific ip addresses I guess?
>> How could I debug this further to find the difference and/or cause for this problem?
>>
>> Thanks for any hints and kind regards,
>> Christoph
>>