Knowledgebase

Forward (redirect/nat) traffic with iptables

If you want to redirect/nat some traffic to IP 2.2.2.2 via IP 1.1.1.1, it simply can be done with iptables on IP 1.1.1.1.
You can also redirect/nat traffic to specific port by specifying a port instead of range.

It's useful for example if you would like to configure "double openvpn": in this case you connect to 1st ip address which forward you to your open vpn server and you "exit under second ip".

First of all enable IP forwarding in /etc/sysctl.conf on 1.1.1.1 host:

net.ipv4.ip_forward=1

Execute following for the changes to take effect:

sysctl -p

NOTE we are using eth0 as interface name .. change it to your interface name it can be eth1 or venet0(in case you are using vps on openvz)

iptables rules example to forward&nat with another ip all tcp/udp traffic with port ranges 1000-65500

iptables -A FORWARD -d 2.2.2.2 -i eth0 -p tcp -m tcp --dport 1000:65500 -j ACCEPT #forward tcp port range
iptables -A FORWARD -d 2.2.2.2 -i eth0 -p udp -m udp --dport 1000:65500 -j ACCEPT #forward udp port range
iptables -t nat -A PREROUTING -d 1.1.1.1 -p tcp -m tcp --dport 1000:65500 -j DNAT --to-destination 2.2.2.2  #tcp port range
iptables -t nat -A PREROUTING -d 1.1.1.1 -p udp -m udp --dport 1000:65500 -j DNAT --to-destination 2.2.2.2  #udp port range
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

Example for single tcp port:

iptables -A FORWARD -d 2.2.2.2 -i eth0 -p tcp -m tcp --dport 3389 -j ACCEPT 
iptables -t nat -A PREROUTING -d 1.1.1.1 -p tcp -m tcp --dport 3389 -j DNAT --to-destination 2.2.2.2
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

Save and restart

iptables-save >/etc/sysconfig/iptables
service iptables restart

NOTE if you trying to forward openvpn traffic (for make smthng like double vpn) you should also add rule for internal ovpn network like this:

iptables -t nat -A POSTROUTING -s 10.17.0.0/255.255.255.0 -o eth0 -j SNAT --to-source 1.1.1.1

where 10.17.0.0 is the ypour openvpn internal network

Here is an example of working iptables:

# Generated by iptables-save v1.4.7 on Tue Apr  9 14:27:04 2013
*filter
:FORWARD ACCEPT [0:0]
-A FORWARD -d 2.2.2.2 -i eth0 -p tcp -m tcp --dport 23:65500 -j ACCEPT
-A FORWARD -d 2.2.2.2 -i eth0 -p udp -m udp --dport 23:65500 -j ACCEPT
COMMIT
# Completed on Tue Apr  9 14:27:04 2013
# Generated by iptables-save v1.4.7 on Tue Apr  9 14:27:04 2013
*nat
:PREROUTING ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A PREROUTING -d 1.1.1.1 -p tcp -m tcp --dport 23:65500 -j DNAT --to-destination 2.2.2.2
-A PREROUTING -d 1.1.1.1 -p udp -m udp --dport 23:65500 -j DNAT --to-destination 2.2.2.2
-A POSTROUTING -o eth0 -j MASQUERADE
COMMIT
# Completed on Tue Apr  9 14:27:04 2013

Warning: Make sure your ssh port is out of range 23:65500, otherwise you will lose ssh access to 1.1.1.1.

 

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example for openvpn "forwarder":---

in current example we will forward tcp/udp traffic with port range 1000:65500 also you can see there's a rule for openvpn internal NW (10.8.0.0/23)

#udp
iptables -A FORWARD -d <OVPNIP> -i venet0 -p udp -m udp --dport 1000:65500 -j ACCEPT 
iptables -t nat -A PREROUTING -d <currentSrvIP> -p udp -m udp --dport 1000:65500 -j DNAT --to-destination <OVPNIP>

 

#tcp
iptables -A FORWARD -d <OVPNIP> -i venet0 -p tcp -m tcp --dport 1000:65500 -j ACCEPT 
iptables -t nat -A PREROUTING -d <currentSrvIP> -p tcp -m tcp --dport 1000:65500 -j DNAT --to-destination <OVPNIP>

#masquarading ust be on
iptables -t nat -A POSTROUTING -o venet0 -j MASQUERADE

#postrouting for openvpn subnet
iptables -t nat -A POSTROUTING -s <10.8.0.0>/255.255.255.0 -o venet0 -j SNAT --to-source <OVPNIP>

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