Postfix SMTP Rotating IP using IPTables

I got 5 Public IPs. i’m Gonna configure them, so Postfix can use multiple interfaces/ips for outgoing smtp connections.

First we need creating Interface aliases for those 5 public IPs.

In my system, using centos:

# cd /etc/sysconfig/network-scripts/
# cp ifcfg-eth0 ifcfg-eth0:1

Edit ifcfg-eth0:1
# vi ifcfg-eth0\:1

DEVICE=eth0 <-- default device
HWADDR=XX:XX:XX:XX:XX:XX
ONBOOT=yes
TYPE=Ethernet
BOOTPROTO=none
IPADDR=202.XXX.XX.2 <-- default eth0 IP address
PREFIX=24
GATEWAY=202.XXX.XX.1
DNS1=202.XXX.XX.XX

Change DEVICE and IPADDR parameters

DEVICE=eth0:1 <-- device alias #1
HWADDR=XX:XX:XX:XX:XX:XX
ONBOOT=yes
TYPE=Ethernet
BOOTPROTO=none
IPADDR=202.XXX.XX.3 <-- IP alias #1
PREFIX=24
GATEWAY=202.XXX.XX.1
DNS1=202.XXX.XX.XX

We can continue with next interfaces for IP aliases same way as mention above. when we were done, bring those IP aliases up.

#ifup eth0:1
#ifup eth0:2
#ifup eth0:3
#ifup eth0:4
......
next interfaces

Check if interfaces is up

#ifconfig
eth0 Link encap:Ethernet HWaddr XX:XX:XX:XX:XX:XX 
inet addr:202.XXX.XX.2 Bcast:202.XXX.XX.255 Mask:255.255.255.0
inet6 addr: fe80::20c:29ff:feb0:e91/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:598678 errors:0 dropped:0 overruns:0 frame:0
TX packets:26348 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000 
RX bytes:49088016 (46.8 MiB) TX bytes:7707579 (7.3 MiB)

eth0:1 Link encap:Ethernet HWaddr XX:XX:XX:XX:XX:XX 
inet addr:202.XXX.XX.3 Bcast:202.XXX.XX.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1

eth0:2 Link encap:Ethernet HWaddr XX:XX:XX:XX:XX:XX 
inet addr:202.XXX.XX.4 Bcast:202.XXX.XX.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1

eth0:3 Link encap:Ethernet HWaddr XX:XX:XX:XX:XX:XX 
inet addr:202.XXX.XX.5 Bcast:202.XXX.XX.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1

……
and so on

There’s no particular configuration need to adjust in postfix. Now the iptables part.make sure your iptables support for statistic match module.

# iptables -m statistic -h
......
......
......
statistic match options:
--mode mode Match mode (random, nth)
random mode:
--probability p Probability
nth mode:
--every n Match every nth packet
--packet p Initial counter value (0 <= p <= n-1, default 0)

Next continue with iptables rule for rotating source IP addresses.

# iptables -t nat -I POSTROUTING -m state --state NEW -p tcp --dport 25 -o eth0 -m statistic --mode nth --every 5 -j SNAT --to-source 202.XXX.XX.2
# iptables -t nat -I POSTROUTING -m state --state NEW -p tcp --dport 25 -o eth0 -m statistic --mode nth --every 5 -j SNAT --to-source 202.XXX.XX.3
# iptables -t nat -I POSTROUTING -m state --state NEW -p tcp --dport 25 -o eth0 -m statistic --mode nth --every 5 -j SNAT --to-source 202.XXX.XX.4
# iptables -t nat -I POSTROUTING -m state --state NEW -p tcp --dport 25 -o eth0 -m statistic --mode nth --every 5 -j SNAT --to-source 202.XXX.XX.5
# iptables -t nat -I POSTROUTING -m state --state NEW -p tcp --dport 25 -o eth0 -m statistic --mode nth --every 5 -j SNAT --to-source 202.XXX.XX.6