You have to be careful with that NAT rule; ANY packet leaving the
eth0 interface will have its source IP address and port changed to the IP address of the
eth0 interface (random port), even if the source of the connection is your server. This could affect services on your server that send traffic from
eth0 and expect to not have its ports changed.
For example, if you're running an IPSec VPN service bound to the
eth0 interface on your server, it will have the source port in its outbound packets changed and thus will cause the connection to fail.
I recommend you change that NAT rule to the following:
iptables --table nat --append POSTROUTING --source $lan_network --out-interface eth0 -j MASQUERADE
This will ensure that ONLY your LAN network's Internet connections will be masqueraded out of the WAN interface.
Hope this helps :)