Overlapping AllowedIPs Configuration

[Aleksa Sarai]

Hi all,

I just found out that WireGuard apparently allows you to configure an
interface that has peers with overlapping AllowedIPs ranges -- which
obviously won't work with cryptokey routing -- but additionally is
strange since I feel this should cause an error when configuring the
interface.

In my case, I accidentally used /32 when generating the IPv6 addresses
of my clients and ended up with a config like:

  [Interface]
  Address = 10.13.37.1/32,fd00:dead:beef:cafe::1/64
  ListenPort = 51820
  PrivateKey = [key]

  # Peer A.
  [Peer]
  PublicKey = [pub]
  PreSharedKey = [psk]
  AllowedIPs = 10.13.40.1/32,fd00:dead:beef:1000::/32

  # Peer B.
  [Peer]
  PublicKey = [pub]
  PreSharedKey = [psk]
  AllowedIPs = 10.13.41.1/32,fd00:dead:beef:1001::/32

This config is wrong (because both peers have overlapping addresses
specified for AllowedIPs), but wireguard will happily accept it:

  % wg-quick up wg-foo
  [#] ip link add wg-yavin type wireguard
  [#] wg setconf wg-yavin /dev/fd/63
  [#] ip address add 10.13.37.1/32 dev wg-yavin
  [#] ip address add fd00:dead:beef:cafe::1/64 dev wg-yavin
  [#] ip link set mtu 1420 up dev wg-yavin
  [#] ip route add fd42:dead::/32 dev wg-yavin
  [#] ip route add 10.13.41.1/32 dev wg-yavin
  [#] ip route add 10.13.40.1/32 dev wg-yavin

This configuration results in only one of the peers actually being given
the IPv6 range, but I feel like "wg setconf" should've rejected this
configuration.

  % wg
  interface: wg-foo
    public key: [pub]
    private key: (hidden)
    listening port: 51820

  peer: [peer A]
    preshared key: (hidden)
    allowed ips: 10.13.40.1/32

  peer: [peer B]
    preshared key: (hidden)
    allowed ips: 10.13.41.1/32, fd42:dead::/32

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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