How to configure port forwarding under CentOS?

Enable IP forwarding

First enable the IP forwarding function, which is turned off by default.

Temporary modification:

# echo 1 >/proc/sys/net/ipv4/ip_forward

It will take effect immediately after modification, but if the system is restarted, it will return to the default value 0.

Permanent modification:

vi /etc/sysctl.conf

# Find the value below and change 0 to 1

net.ipv4.ip_forward = 1

# sysctl –p (make it take effect immediately)

The default value of 0 disables ip forwarding. Changing it to 1 enables the ip forwarding function.

Configure port forwarding

Assume that when a user accesses 172.16.4.247:728, I want it to be forwarded to 172.16.4.97:80:

# iptables -t nat -A PREROUTING -p tcp -d 113.108.110.61 --dport 728 -j DNAT --to-destination 172.16.4.97:80

# iptables -t nat -A POSTROUTING -p tcp -s 172.16.4.97 --sport 80 -j SNAT --to-source 172.16.4.247

# service iptables save (save the current rules to /etc/sysconfig/iptables)

Alternatively, you can modify the /etc/sysconfig/iptables file directly:

-A PREROUTING -d 172.16.4.247/32 -p tcp -m tcp --dport 728 -j DNAT --to-destination 172.16.4.97:80

-A POSTROUTING -s 172.16.4.97/32 -p tcp -m tcp --sport 80 -j SNAT --to-source 172.16.4.247

Finally don’t forget to open port 728

-A INPUT -p tcp -m state --state NEW -m tcp --dport 728 -j ACCEPT

After the configuration is completed, remember to restart the firewall:

# service iptables restart

Native port forwarding

If you only need to forward different ports between local machines, it will be easier. For example, when visiting http://ip:729 I want to return the content of http://ip:80. The configuration is as follows:

[root@localhost sbin]# iptables -t nat -A PREROUTING -p tcp --dport 729 -j REDIRECT --to-ports 80

[root@localhost sbin]# service iptables save

[root@localhost sbin]# service iptables restart

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