Table=off behavior (not adding any route *at all*)

[Alex Davies <alex@davz.net>]

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Hi,

We have been trying to use WireGuard on FreeBSD (we are using the WG plugin
inside the open source opnsense.org software). These run FreeBSD
11.2-RELEASE-p9-HBSD (OPNsense 19.7.a_288-amd64).

We noticed that by default (i.e. no Table=) wireguard-go wg0 adds default
routes (as two /31's) as expected. However, if table=off, we get no route
at all - not even to the VPN peer.

The announcement for the Table= option[1] stated:

  In collaboration with Luis Ressel, wg-quick(8) grew an option! We
generally
  do not like to add things to wg-quick or allow feature-creep, but this was
  basic enough and mostly involves disabling functionality. Specifically,
  wg-quick now accepts a Table= parameter with these semantics:

    ~ Table=auto (default) selects the current behaviour
    ~ Table=off disables creation of routes from allowed ips altogether
    ~ All other values are passed through to "ip route add"'s table option

  This should enable people to do basic policy routing. It also matches the
  functionality provided by LEDE/OpenWRT's uci config as well as NixOS's
  networking configuration.

Ignoring the "creation of routes from allowed ips", it does not even add
the subnet defined in [Interface]. netstat -r | grep wg returns nothing.

As a concrete example, if I take the trivial config at
https://wiki.archlinux.org/index.php/WireGuard:

[Interface]
Address = 10.200.200.2/24
PrivateKey = [FOO's PRIVATE KEY]
DNS = 10.200.200.1

[Peer]
PublicKey = [SERVER PUBLICKEY]
PresharedKey = [PRE-SHARED KEY]
AllowedIPs = 0.0.0.0/0, ::/0
Endpoint = my.ddns.address.com:51820

I would (naively) expect this:
Table=auto: inject route for 10.200.200.2/24 *and* 0.0.0.0/0 via wg0
Table=off: inject route for 10.200.200.2/24 *only* via wg0

What actually happens is:
Table=auto: as above/expected
Table=off: no route out wg0

This mean with Table=off, you are in the extremely confusing situation that
you cant even ping the other peer.

Testing on Linux (Kernel 4.15.0-1032-aws inside a 18.04 AMI (public AMI
- ami-07dc734dc14746eab)) shows that the behavior is different - its as I
expect for both Table values. With this wg0.conf:

root@ip-172-31-39-185:~# cat /etc/wireguard/wg0.conf
[Interface]
Address = 192.168.2.1/24
PrivateKey = eEIwdXp8jKV9/2MEwxYBqQLu4TZqBv9YWvG9fbMuaG4=
Table = off

[Peer]
PublicKey = pHQfWzLAUM85vDO6+MZAneBYhapOHUkPAuxr0lJdZlY=
AllowedIPs = 0.0.0.0/0
Endpoint = 18.130.138.71:51820

I get this route:

root@ip-172-31-39-185:~# ip route show | grep wg0
192.168.2.0/24 dev wg0 proto kernel scope link src 192.168.2.1

Note the /24 route (as expected). With Table undefined or set to auto, I
get the 0.0.0.0 route (also as expected).

I dont know much about FreeBSD, but I launched a test EC2 instance (FreeBSD
12.0-RELEASE based on public ami-0d244633039d93966 with kernel reported as
12.0-RELEASE-p3) and I think I see the same thing (i.e. no /24 route):

root@freebsd:/etc/wireguard # netstat -rn | grep wg0
192.168.2.5        link#3             UH          wg0
fe80::%wg0/64                     link#3                        U
 wg0
fe80::1427:e888:767c:dce1%wg0     link#3                        UHS
 lo0
root@freebsd:/etc/wireguard # ping 192.168.2.5

Somebody more expert than me can comment on whether this is expected or
not. At the very least, hopefully this post is useful for somebody else.
For our specific problem, we have fixed this by putting a static route in
for the "Address" subnet across wg0.

-Alex

[1] https://lists.zx2c4.com/pipermail/wireguard/2017-December/002231.html

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