Linux port forwarding is simple to do with iptables which may probably already being used as the firewall or part of the setting up a Linux gateway. In the Linux kernel, port forwarding is achieved by packet filter rules in iptables.
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Port forwarding also called “port mapping” commonly refers to the network address translator gateway changing the destination address and/or port of the packet to reach a host within a masqueraded, typically private, network.
Port forwarding can be used to allow remote computers (e.g., public machines on the Internet) to connect to a specific computer within a private network such as local area network (LAN), sothat xternal hosts can communicate with services provided by hosts within a LAN. For example, running a public HTTP server (port 80) on a host within a private LAN, or permitting secure shell ssh (port 22) access to hosts within the private LAN from the Internet.
In Unix/Linux box where port numbers below 1024 can only be listened by software running as root, port forwarding is also used to redirect incoming traffic from a low numbered port to software listening on a higher port. This software can be running as a normal user, which avoids the security risk caused by running as the root user.
iptables in Linux
iptables is a generic table structure for the definition of rulesets for network filtering framework by netfilter in Linux kernel.
In Linux box, iptables is implemented in Linux kernel as some kernel modules. Each rule within an IP table consists of a number of classifiers (iptables matches) and one connected action (iptables target). Users can build very powerfull firewall based on iptables which handles packets based on the type of packet activity and enqueues the packet in one of its builtin ‘tables’.
There are three important tables: mangle, filter and nat. The mangle table is responsible for the alteration of service bits in the TCP header. The filter queue is responsible for packet filtering. The nat table performs Network Address Translation (NAT). Each tables may have some built-in chains in which firewall policy rules can be placed.
The filter table has three built-in chains:
- Forward chain: Filters packets destined for networks protected by the firewall.
- Input chain: Filters packets destined for the firewall.
- Output chain: Filters packets originating from the firewall.
The nat table has the following built-in chains:
- Pre-routing chain: NATs packets when the destination address of the packet needs to be changed.
- Post-routing chain: NATs packets when the source address of the packet needs to be changed.
- Output chain: NATs packets originating from the firewall.
Below is a brief view of how packets are processed by the chains:
PACKET IN | PREROUTING--[routing]-->--FORWARD-->--POSTROUTING-->--OUT - nat (dst) | - filter - nat (src) | | | | INPUT OUTPUT - filter - nat (dst) | - filter | | `----->-----[app]----->------'
Note: if the packet is from the firewall, it will not go through the PREROUTING chain.
We only look into the packets that requires port forwarding which is the topic of this post.
The packet entering the firewall is inspected by the rules in the nat table’s PREROUTING chain to see whether it requires destination modification (DNAT). The packet is then routed by Linux router after leaving the PREROUTING chain. The packet which is destined for a “protected” network is filtered by the rules in the FORWARD chain of the filter table. The it will go through the packet undergoes SNAT in the POSTROUTING chain before arriving at the “protected” network. When the destination server decides to reply, the packet undergoes the same sequence of steps.
Port forwarding using iptables
This section assumes you have already set up the the Linux host as the gateway and configured the POSTROUTING rules as shown in Setting Up Gateway Using iptables and route on Linux for SNAT.
A port-forwarded packet will pass the PREROUTING chain in nat table, FORWARD chain in filter table, POSTROUTING chain in nat table and other chains. We need to add rules to these chains.
Let’s use a senario to introduce how to configure iptables to do port forwarding. Suppose our gateway can connect to both the Internet (0.0.0.0/0) and the LAN (192.168.1.0/24). The gateway’s eth0 interface has a public IP 126.96.36.199 while the eth1 has a LAN IP 192.168.1.1. Now, suppose that we have set up a HTTP server on 192.168.1.2:8080 and we want to provides service to the Internet through the public IP. We need to configure iptables to forward packets coming to port 80 of 188.8.131.52 to 8080 of 192.168.1.2 in LAN.
Below is the network topology:
Internet---------[router/firewall]-------------LAN 0.0.0.0/0 184.108.40.206 192.168.1.1 192.168.1.0/24
Normally we deny all incoming connections to a gateway machine by default because opening up all services and ports could be a security risk. We will only open the ports for the services that we will use. In this example, we will open port 80 for HTTP service.
First make sure that the IP forwarding is enabled on Linux following the “Enable Linux IP forwarding” Section in Setting Up Gateway Using iptables and route on Linux.
This is the rules to forward connections on port 80 of the gateway to the internal machine:
# iptables -A PREROUTING -t nat -i eth0 -p tcp --dport 80 -j DNAT --to 192.168.1.2:8080 # iptables -A FORWARD -p tcp -d 192.168.1.2 --dport 8080 -j ACCEPT
These two rules are straight forward. The first one specifies that all incoming tcp connections to port 80 should be sent to port 8080 of the internal machine 192.168.1.2. This rule alone doesn’t complete the job as described above that we deny all incoming connections by default. Then we accept the incoming connection to port 80 from eth0 which connect to the Internet with the publich IP by the second rule. From the process path in the “iptables” part, the packet will also pass the FORWARD chains. We add the second rule in FORWARD chain to allow forwarding the packets to port 8080 of 192.168.1.2.
By now, we have set up the the iptables rules for forwarding the 80 port. For other service, the method is similiar with the HTTP service.
The conntrack entries
The “nf_conntrack_*” kernel modules enables iptables to examine the status of connections by caching the related information for these connections. A cat of /proc/net/nf_conntrack (in some old Linux kernels, the file is /proc/net/ip_conntrack) will give a list of all the current entries in the conntrack database.
A conntrack entry looks like this:
ipv4 2 tcp 6 431581 ESTABLISHED src=220.127.116.11 dst=18.104.22.168 sport=53867 dport=80 packets=22 bytes=13861 src=192.168.1.2 dst=22.214.171.124 sport=8080 dport=53867 packets=14 bytes=3535 [ASSURED] mark=0 secmark=0 use=2
This entry contains all the information that the conntrack module maintains to know the state of a specific connection. We can find the version of ip protocal version and the decimal coding, the protocol and the normal decimal coding. After this, we get how long this conntrack entry should live. Next is the actual state that this entry is in at this present point of time. Then, we get the source IP address, destination IP address, source port and destination port. After that, we get the IPs and ports of both source and destination we expect of return packets.
In this entry we can find that the arriving connection is:
126.96.36.199:53867 --> 188.8.131.52:80
while the returning connection is:
192.168.1.2:8080 --> 184.108.40.206:53867
which reflects the port forwarding which we have set.