IP ROTATION VIA IPTABLES

Our scenario with 5 public ips.

Create interface aliaces for your main IP

( How to https://www.cyberciti.biz/faq/linux-creating-or-adding-new-network-alias-to-a-network-card-nic/)

# ifup eth0:1
# ifup eth0:2
# ifup eth0:3
# ifup eth0:4

Be sure that 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)

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 213.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 213.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 213.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 213.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 213.XXX.XX.6