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wg: man: include kill-switch documentation using fwmark 

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
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Jason A. Donenfeld 2017-10-11 01:55:45 +02:00
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@ -129,6 +129,42 @@ indicates that a DNS server for the interface should be configured via
The peer's allowed IPs entry implies that this interface should be configured as the default gateway,
which this script does.
Building on the last example, one might attempt the so-called ``kill-switch'', in order
to prevent the flow of unencrypted packets through the non-WireGuard interfaces:
[Interface]
.br
Address = 10.200.100.8/24
.br
DNS = 10.200.100.1
.br
PrivateKey = oK56DE9Ue9zK76rAc8pBl6opph+1v36lm7cXXsQKrQM=
.br
\fBPostUp = iptables -I OUTPUT ! -o %i -m mark ! --mark $(wg show %i fwmark) -j REJECT\fP
.br
\fBPreDown = iptables -D OUTPUT ! -o %i -m mark ! --mark $(wg show %i fwmark) -j REJECT\fP
.br
.br
[Peer]
.br
PublicKey = GtL7fZc/bLnqZldpVofMCD6hDjrK28SsdLxevJ+qtKU=
.br
PresharedKey = /UwcSPg38hW/D9Y3tcS1FOV0K1wuURMbS0sesJEP5ak=
.br
AllowedIPs = 0.0.0.0/0
.br
Endpoint = demo.wireguard.com:51820
.br
The `PostUp' and `PreDown' fields have been added to specify an
.BR iptables (8)
command which, when used with interfaces that have a peer that specifies 0.0.0.0/0 as part of the
`AllowedIPs', works together with wg-quick's fwmark usage in order to drop all packets that
are either not coming out of the tunnel encrypted or not going through the tunnel itself. (Note
that this continues to allow most DHCP traffic through, since most DHCP clients make use of PF_PACKET
sockets, which bypass Netfilter.)
Here is a more complicated example, fit for usage on a server:
[Interface]