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21 Mar 2021

Mail Out

Here are some quick notes on how I used iptables and kernel IP routing to get my home server’s outgoing mail out over a wireguard VPN connection.

The idea is to mark packets coming from the docker subnet destined for port 25 and route them over the vpn interface.

This effectively bypasses the ISP blocking outgoing connections to port 25.

Notes

  • The mark chosen, 1, is arbitrary. You can use any hex value, i.e. 0x25
  • The iproute table name and priority are also arbitrary, you can use anything you like as long as it doesn’t conflict with existing tables.

You can check for existing route tables with cat /etc/iproute2/rt_tables

#
# reserved values
#
255	local
254	main
253	default
0	unspec
#
# local
#
#1	inr.ruhep
201 mail.out # only after the following steps would this line be present

Mark those mail packets

172.16.0.0/12 covers the entire range of 172.16.x.x which is the subnet my mail specific containers are in.

iptables -A PREROUTING -t mangle -s 172.16.0.0/12 -p tcp --dport 25 -j MARK --set-mark 1

Create table for outgoing mail

echo 201 mail.out >> /etc/iproute2/rt_tables

Add rule to filter packets with the firewall mark to our new table

ip rule add fwmark 1 table mail.out

Add default route for marked packets over the wireguard interface

10.0.1.1 is the upstream peer on the vpn and wg0 is my vpn interface, you can change these to match whatever vpn connection you have available.

ip route add default via 10.0.1.1 dev wg0 table mail.out