relaying messages from docker network

[Ondřej Žára]

Hi,

I am looking for tips about handling outgoing smtp traffic in a dockerized scenario. My setup has three containers in a bridge network:

- reverse proxy (to route incoming traffic): 172.19.0.1
- postfix: 172.19.0.2
- webapp (needs to send e-mails): 172.19.0.3

The goal here is to allow the webapp to send e-mails without any particular checks. On a single machine, this would be done via

mynetworks = 127.0.0.1
smtpd_relay_restrictions = permit_mynetworks

...because the webapp would connect from 127.0.0.1. However, in a dockerized world, the webapp connects from 172.19.0.3 - and I cannot really mention this ip in $mynetworks, because docker is free to change the address on container restarts.

An alternative solution would be to set

mynetworks = 172.19/16

...but that would be incorrect as well, because all incoming traffic (from the internet) comes from the proxy, i.e. from 172.19.0.1 - and we do not want to relay that.

So what would be the best practice in this scenario?

Thanks,
Ondrej Zara