RETURN target
The RETURN target will cause the current packet to stop traveling through the chain where it hit the rule. If it is the subchain of another chain, the packet will continue to travel through the superior chains as if nothing had happened. If the chain is the main chain, for example the INPUT chain, the packet will have the default policy taken on it. The default policy is normally set to ACCEPT, DROP or similar.
For example, let's say a packet enters the INPUT chain and then hits a rule that it matches and that tells it to --jump EXAMPLE_CHAIN. The packet will then start traversing the EXAMPLE_CHAIN, and all of a sudden it matches a specific rule which has the --jump RETURN target set. It will then jump back to the INPUT chain. Another example would be if the packet hit a --jump RETURN rule in the INPUT chain. It would then be dropped to the default policy as previously described, and no more actions would be taken in this chain.
| Works under Linux kernel 2.3, 2.4, 2.5 and 2.6. |
SAME target
The SAME target works almost in the same fashion as the SNAT target, but it still differs. Basically, the SAME target will try to always use the same outgoing IP address for all connections initiated by a single host on your network. For example, say you have one /24 network (192.168.1.0) and 3 IP addresses (10.5.6.7-9). Now, if 192.168.1.20 went out through the .7 address the first time, the firewall will try to keep that machine always going out through that IP address.
Table 11-15. SAME target options
| Option | --to |
| Example | iptables -t mangle -A PREROUTING -s 192.168.1.0/24 -j SAME --to 10.5.6.7-10.5.6.9 |
| Explanation | As you can see, the --to argument takes 2 IP addresses bound together by a - sign. These IP addresses, and all in between, are the IP addresses that we NAT to using the SAME algorithm. |
| Option | --nodst |
| Example | iptables -t mangle -A PREROUTING -s 192.168.1.0/24 -j SAME --to 10.5.6.7-10.5.6.9 --nodst |
| Explanation | Under normal action, the SAME target is calculating the followup connections based on both destination and source IP addresses. Using the --nodst option, it uses only the source IP address to find out which outgoing IP the NAT function should use for the specific connection. Without this argument, it uses a combination of the destination and source IP address. |
| Works under Linux kernel 2.5 and 2.6. |
SECMARK target
The SECMARK target is used to set a security context mark on a single packet, as defined by SELinux and security systems. This is still somewhat in it's infancy in Linux, but should pick up more and more in the future. Since SELinux is out of the scope of this document, I suggest going to the Security-Enhanced Linux webpage for more information.
In brief, SELinux is a new and improved security system to add Mandatory Access Control (MAC) to Linux, implemented by NSA as a proof of concept. SELinux basically sets security attributes for different objects and then matches them into security contexts. The SECMARK target is used to set a security context on a packet which can then be used within the security subsystems to match on.
| The SECMARK target is only valid in the mangle table. |
Table 11-16. SECMARK target options
| Option | --selctx |
| Example | iptables -t mangle -A PREROUTING -p tcp --dport 80 -j SECMARK --selctx httpcontext |
| Explanation | The --selctx option is used to specify which security context to set on a packet. The context can then be used for matching inside the security systems of linux. |
SNAT target
The SNAT target is used to do Source Network Address Translation, which means that this target will rewrite the Source IP address in the IP header of the packet. This is what we want, for example, when several hosts have to share an Internet connection. We can then turn on ip forwarding in the kernel, and write an SNAT rule which will translate all packets going out from our local network to the source IP of our own Internet connection. Without doing this, the outside world would not know where to send reply packets, since our local networks mostly use the IANA specified IP addresses which are allocated for LAN networks. If we forwarded these packets as is, no one on the Internet would know that they were actually from us. The SNAT target does all the translation needed to do this kind of work, letting all packets leaving our LAN look as if they came from a single host, which would be our firewall.
The SNAT target is only valid within the nat table, within the POSTROUTING chain. This is in other words the only chain in which you may use SNAT. Only the first packet in a connection is mangled by SNAT, and after that all future packets using the same connection will also be SNATted. Furthermore, the initial rules in the POSTROUTING chain will be applied to all the packets in the same stream.
Table 11-17. SNAT target options
| Option | --to-source |
| Example | iptables -t nat -A POSTROUTING -p tcp -o eth0 -j SNAT --to-source 194.236.50.155-194.236.50.160:1024-32000 |
| Explanation | The --to-source option is used to specify which source the packet should use. This option, at its simplest, takes one IP address which we want to use for the source IP address in the IP header. If we want to balance between several IP addresses, we can use a range of IP addresses, separated by a hyphen. The --to--source IP numbers could then, for instance, be something like in the above example: 194.236.50.155-194.236.50.160. The source IP for each stream that we open would then be allocated randomly from these, and a single stream would always use the same IP address for all packets within that stream. We can also specify a range of ports to be used by SNAT. All the source ports would then be confined to the ports specified. The port bit of the rule would then look like in the example above, :1024-32000. This is only valid if -p tcp or -p udp was specified somewhere in the match of the rule in question. iptables will always try to avoid making any port alterations if possible, but if two hosts try to use the same ports, iptables will map one of them to another port. If no port range is specified, then if they're needed, all source ports below 512 will be mapped to other ports below 512. Those between source ports 512 and 1023 will be mapped to ports below 1024. All other ports will be mapped to 1024 or above. As previously stated, iptables will always try to maintain the source ports used by the actual workstation making the connection. Note that this has nothing to do with destination ports, so if a client tries to make contact with an HTTP server outside the firewall, it will not be mapped to the FTP control port. |