How to setup SMTP port redirect with IPTABLES and NAT

RedHat/Centos

Okay its really easy to do. You will need to add the following in /etc/sysctl.conf
Note: these are kernel parameter changes

1.vi /etc/sysctl.conf add the following lines

kernel.sysrq = 1

net.ipv4.tcp_syncookies=1

net/ipv4/ip_forward=1 (important)

net.ipv4.conf.all.route_localnet=1 (important)

net.ipv4.conf.default.send_redirects = 0

net.ipv4.conf.all.send_redirects = 0

.

2.Save the file and run
 Sysctl -p (this will load the new kernel parameters)
3.Now you if you already have iptables running you want to save the running config and add the new redirect rules
 Iptables-save > iptables.back
4.Now you want to edit the iptables.back file and add the redirect rules
 vi iptables.back

It will probably look something like the rules below.

EXAMPLE

# Generated by iptables-save v1.2.8 on Thu July 6 18:50:55 2020

*filter

:INPUT ACCEPT [0:0]

:FORWARD ACCEPT [0:0]

:OUTPUT ACCEPT [2211:2804881]

:RH-Firewall-1-INPUT – [0:0]

-A INPUT -j RH-Firewall-1-INPUT

-A FORWARD -j RH-Firewall-1-INPUT

-A RH-Firewall-1-INPUT -i lo -j ACCEPT

-A RH-Firewall-1-INPUT -p icmp -m icmp –icmp-type 255 -j ACCEPT

-A RH-Firewall-1-INPUT -p esp -j ACCEPT

-A RH-Firewall-1-INPUT -p ah -j ACCEPT

-A RH-Firewall-1-INPUT -m state –state RELATED,ESTABLISHED -j ACCEPT

-A RH-Firewall-1-INPUT -p tcp -m tcp –dport 1025-m state –state NEW -j ACCEPT (make sure to have open)

-A RH-Firewall-1-INPUT -p tcp -m tcp –dport 443 -m state –state NEW -j ACCEPT

-A RH-Firewall-1-INPUT -p tcp -m tcp –dport 8443 -m state –state NEW -j ACCEPT

-A RH-Firewall-1-INPUT -p tcp -m tcp –dport 25 -m state –state NEW -j ACCEPT (make sure to have open)

-A RH-Firewall-1-INPUT -p tcp -m tcp –dport 80 -m state –state NEW -j ACCEPT

-A RH-Firewall-1-INPUT -p tcp -m tcp –dport 21 -m state –state NEW -j ACCEPT

-A RH-Firewall-1-INPUT -p tcp -m tcp –dport 22 -m state –state NEW -j ACCEPT

-A RH-Firewall-1-INPUT -p tcp -m tcp –dport 106 -m state –state NEW -j ACCEPT

-A RH-Firewall-1-INPUT -p tcp -m tcp –dport 143 -m state –state NEW -j ACCEPT

-A RH-Firewall-1-INPUT -p tcp -m tcp –dport 465 -m state –state NEW -j ACCEPT

-A RH-Firewall-1-INPUT -p tcp -m tcp –dport 993 -m state –state NEW -j ACCEPT

-A RH-Firewall-1-INPUT -p tcp -m tcp –dport 995 -m state –state NEW -j ACCEPT

-A RH-Firewall-1-INPUT -p tcp -m tcp –dport 8222 -m state –state NEW -j ACCEPT

-A RH-Firewall-1-INPUT -j REJECT –reject-with icmp-host-prohibited

COMMIT

#ADD this section with another Commit like below

# Completed on Thu July 6 18:50:55 2020

# Generated by iptables-save v1.2.8 on Thu July 6 18:50:55 2020

*nat

:PREROUTING ACCEPT [388:45962]

:POSTROUTING ACCEPT [25:11595]

:OUTPUT ACCEPT [25:11595]

-A PREROUTING -p tcp -m tcp –dport 1025 -j REDIRECT –to-ports 25

COMMIT

# Completed on Thu July 6 18:50:55 2020

.

 Save the file

.

5.Next you want to reload the new config
 Iptables-restore < iptables.back
6.Now you should be able see the new rules and test
 Iptables -L -n -t nat (should show the rules)

.

[root@nick ~]# iptables -L -n | grep 1025

ACCEPT tcp — 0.0.0.0/0 0.0.0.0/0 tcp dpt:1025 state NEW

[root@nick ~]# iptables -L -n -t nat| grep 1025

REDIRECT tcp — 0.0.0.0/0 0.0.0.0/0 tcp dpt:1025 redir ports 25

.

Note:

You will need to run telnet from outside the host as you cant NAT to localhost locally. 🙂

.

[root@nick1 ~]# telnet 192.168.86.111 1025

Trying 192.168.86.111…

Connected to localhost.

Escape character is ‘^]’.

220 nick.ansible.com ESMTP Postfix

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