Hello postfix experts:
So today I finished my initial setup of docker with different containers
running different email services. There's a container for postfix, one
for dovecot, one for fetchmail, one for postfixadmin, etc.
The networking is basically bridged from every container to the outside.
In fact, I didn't set this manually, as docker-compose automatically
creates a network for all its components. The binding is strictly with
localhost. The outside world accesses the containers through tcp
forwarding with HAProxy, which binds to the required ports in the
containers (143, 587, etc). The command `docker ps` shows the bindings
for postfix:
127.0.0.1:3725->25/tcp, 127.0.0.1:37465->465/tcp, 127.0.0.1:37587->587/tcp
In order to give other containers easy access to postfix (like
fetchmail, for example, to deliver fetched emails), I chose the setting:
mynetworks_style = subnet
The rationale behind this is that (in my head, and that's why I'm asking
here since I'm no expert), is that everything in the subnet, i.e., the
containers, should consider each other "mynetwork". Does that make sense?
Within the postfix container, this is what ifconfig returns:
```
# ifconfig -a
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 172.30.0.3 netmask 255.255.0.0 broadcast 172.30.255.255
ether 02:42:ac:1e:00:03 txqueuelen 0 (Ethernet)
RX packets 434 bytes 57106 (55.7 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 427 bytes 133987 (130.8 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
loop txqueuelen 1000 (Local Loopback)
RX packets 72 bytes 6173 (6.0 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 72 bytes 6173 (6.0 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
```
Other containers seem to share the same subnet with that subnet mask.
Please let me know whether I'm reasoning correctly about this and
whether it's right to choose that setting (and, also, whether I missed
something in my setup, if obvious).
Best regards,
Sam
