Difference between revisions of "NAT Linux"

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= Linux IP Tables =
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{{TOCright}}
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IP Tables is used for packet filtering and NAT/PAT translation on several Linux distributions including
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[[iptables]] is used for packet filtering and NAT/PAT translation on several Linux distributions including
 
*Ubunto
 
*Ubunto
 
*Redhat
 
*Redhat
 
*Centos
 
*Centos
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== Enable IP Forwarding (Routing) between interfaces ==
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== Basic NAT example using iptables ==
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Before you can route packets from the Inside network Interface to the outside network Interface, you need to enable IP forwarding.
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[[iptables]] can be configured in two different ways when you boot your machine. The examples will assume you have two Network interface cards. '''eth0''' connected to the internal - private network - and '''eth1''' connected to the external - Internet.
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=== Enabling forwarding until next boot ===
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*Using a startup script. Example below.
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To enable IP forwarding you need to toggle a switch in the kernel, you can do that with the following command. But remember, next time you boot the machine, it will be switched of again.
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*Using [[iptables]] save and restore facelity. Example below.
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[[Image:Linux_nat_1.png‎|none|500px|thumb|Example network]]
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=== Using a startup script to configure iptables ===
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==== When you have a fixed address on the external Interface ====
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When you know your address on the external Interface - not using DHCP - you should use source NAT (SNAT), which is slightly more efficient than using masquerading.<br>
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Add the following lines to '''/etc/rc.local'''
 
<pre>
 
<pre>
 +
#  Enable IP Forwading between Interfaces (Routing)
 
echo "1" > /proc/sys/net/ipv4/ip_forward
 
echo "1" > /proc/sys/net/ipv4/ip_forward
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# Use iptables Source NAT (SNAT) to translate internal 192.168.1.0/24 IP addresses to the external IP Address 83.90.47.30
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# Allow the internal hosts to connect to any IP address on the outside 0.0.0.0/24
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iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -d 0.0.0.0/0 -j SNAT --to 83.90.47.30
 
</pre>
 
</pre>
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=== Enable IP forwarding permanently ===
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==== When you have a floating address on the external Interface ====
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To enable IP forwarding permanently, you either need to issue the command below in a boot-script. '''/etc/rc.local''' or similar<br/>
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When you don't know your external IP Address and are fetching it from a DHCP server, you can use masquerading<br/>
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or<br/>
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Add the following lines to '''/etc/rc.local'''
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Change the file '''/etc/sysctl.conf''' to include the following line. Perhaps you only need to uncomment a line. (Remove the # from beginning of the line)
 
 
<pre>
 
<pre>
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net.ipv4.ip_forward=1
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#  Enable IP Forwarding between Interfaces (Routing)
 +
echo "1" > /proc/sys/net/ipv4/ip_forward
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# Use iptables Source NAT (SNAT) to translate internal 192.168.1.0/24 IP addresses to the external IP Address that eth0 has
 +
# Allow the internal hosts to connect to any IP address on the outside 0.0.0.0/24
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iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -d 0.0.0.0/0 -o eth1 -j MASQUERADE
 
</pre>
 
</pre>
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Note that altering '''/etc/sysctl.conf''' will first be effective after reboot.
 
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== Basic NAT example using IP Tables ==
 
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In the example below the internal network 192.168.1.0/24 is Source Natted (SNAT) to the external IP Address 83.90.47.30. Source nat also makes port translations. So the example uses NAT/PAT, and would be sufficient as a NAT/PAT solution for a private network.<br/>
 
  
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<pre>
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=== Checking iptables NAT rules and traffic ===
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iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -d 0.0.0.0/0 -j SNAT --to 83.90.47.30
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The [[iptables]] list option can be used to see [[iptables]] rules and [[iptables]] traffic.
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</pre>
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use the command ''iptables -L -t nat'' to see the rule in the IPTABLES chains.
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use the command ''iptables -L -t nat'' to see the rules in the [[iptables]] NAT chains.
 
<pre>
 
<pre>
 
[root@bkshost etc]# iptables -L -t nat
 
[root@bkshost etc]# iptables -L -t nat
Line 34: Line 42:
 
Chain POSTROUTING (policy ACCEPT)
 
Chain POSTROUTING (policy ACCEPT)
 
target    prot opt source              destination
 
target    prot opt source              destination
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SNAT      all  --  192.168.1.0/24        anywhere           to:83.90.47.30
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MASQUERADE  all  --  192.168.1.0/24        anywhere
  
 
Chain OUTPUT (policy ACCEPT)
 
Chain OUTPUT (policy ACCEPT)
 
target    prot opt source              destination
 
target    prot opt source              destination
 
</pre>
 
</pre>
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=== Webserver located on internal network ===
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use the command ''iptables -L -t nat '''-v''''' to see the rules in the [[iptables]] chains and the traffic for each rule.
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To redirect WEB traffic originating from the outside to a WEB-server on the inside you would use a rule as showed below.
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<pre>
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[root@bkshost etc]# iptables -L -t nat -v
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Chain PREROUTING (policy ACCEPT 275K packets, 19M bytes)
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pkts bytes target    prot opt in    out    source              destination
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Chain POSTROUTING (policy ACCEPT 13846 packets, 1355K bytes)
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pkts bytes target    prot opt in    out    source              destination
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99447 6447K MASQUERADE  all  --  *      eth1    172.22.0.0/24        0.0.0.0/0
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Chain OUTPUT (policy ACCEPT 15307 packets, 1758K bytes)
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pkts bytes target    prot opt in    out    source              destination
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</pre>
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=== WEB server on the inside network ===
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If you have a WEB server on the inside network for example 192.168.1.30, which should be accessible from the outside. (eth1)
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[[Image:Linux_nat_2.png‎|none|500px|thumb|Example network]]
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<pre>
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iptables -A PREROUTING -t nat -i eth1 -p tcp --dport 80 -j DNAT --to 192.168.1.30:80
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iptables -A PREROUTING -t nat -i eth1 -p tcp --dport 443 -j DNAT --to 192.168.1.30:443
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</pre>
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==IP destination nat Forwarding with source nat==
 
<pre>
 
<pre>
 +
iptables -t nat -A PREROUTING -i eth1 -d 83.90.239.187  -j DNAT --to 192.168.139.103
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iptables -t nat -A POSTROUTING -o eth0 -d 192.168.139.103 -j SNAT --to-source 172.16.4.17
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</pre>
 
[[Category:Linux]]
 
[[Category:Linux]]

Latest revision as of 15:25, 11 December 2013

iptables is used for packet filtering and NAT/PAT translation on several Linux distributions including

  • Ubunto
  • Redhat
  • Centos

Basic NAT example using iptables

iptables can be configured in two different ways when you boot your machine. The examples will assume you have two Network interface cards. eth0 connected to the internal - private network - and eth1 connected to the external - Internet.

  • Using a startup script. Example below.
  • Using iptables save and restore facelity. Example below.
Example network

Using a startup script to configure iptables

When you have a fixed address on the external Interface

When you know your address on the external Interface - not using DHCP - you should use source NAT (SNAT), which is slightly more efficient than using masquerading.
Add the following lines to /etc/rc.local 

#  Enable IP Forwading between Interfaces (Routing)
echo "1" > /proc/sys/net/ipv4/ip_forward
# Use iptables Source NAT (SNAT) to translate internal 192.168.1.0/24 IP addresses to the external IP Address 83.90.47.30
# Allow the internal hosts to connect to any IP address on the outside 0.0.0.0/24
iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -d 0.0.0.0/0 -j SNAT --to 83.90.47.30

When you have a floating address on the external Interface

When you don't know your external IP Address and are fetching it from a DHCP server, you can use masquerading
Add the following lines to /etc/rc.local 

#  Enable IP Forwarding between Interfaces (Routing)
echo "1" > /proc/sys/net/ipv4/ip_forward
# Use iptables Source NAT (SNAT) to translate internal 192.168.1.0/24 IP addresses to the external IP Address that eth0 has
# Allow the internal hosts to connect to any IP address on the outside 0.0.0.0/24
iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -d 0.0.0.0/0 -o eth1 -j MASQUERADE

Checking iptables NAT rules and traffic

The iptables list option can be used to see iptables rules and iptables traffic.

use the command iptables -L -t nat to see the rules in the iptables NAT chains. 

[root@bkshost etc]# iptables -L -t nat
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination

Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination
MASQUERADE  all  --  192.168.1.0/24        anywhere

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

use the command iptables -L -t nat -v to see the rules in the iptables chains and the traffic for each rule. 

[root@bkshost etc]# iptables -L -t nat -v
Chain PREROUTING (policy ACCEPT 275K packets, 19M bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain POSTROUTING (policy ACCEPT 13846 packets, 1355K bytes)
 pkts bytes target     prot opt in     out     source               destination
99447 6447K MASQUERADE  all  --  *      eth1    172.22.0.0/24        0.0.0.0/0

Chain OUTPUT (policy ACCEPT 15307 packets, 1758K bytes)
 pkts bytes target     prot opt in     out     source               destination

WEB server on the inside network

If you have a WEB server on the inside network for example 192.168.1.30, which should be accessible from the outside. (eth1)

Example network
iptables -A PREROUTING -t nat -i eth1 -p tcp --dport 80 -j DNAT --to 192.168.1.30:80
iptables -A PREROUTING -t nat -i eth1 -p tcp --dport 443 -j DNAT --to 192.168.1.30:443

IP destination nat Forwarding with source nat

iptables -t nat -A PREROUTING -i eth1 -d 83.90.239.187  -j DNAT --to 192.168.139.103
iptables -t nat -A POSTROUTING -o eth0 -d 192.168.139.103 -j SNAT --to-source 172.16.4.17
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