Chapter 11. Iptables targets and jumps

ACCEPT target

This target needs no further options. As soon as the match specification for a packet has been fully satisfied, and we specify ACCEPT as the target, the rule is accepted and will not continue traversing the current chain or any other ones in the same table. Note however, that a packet that was accepted in one chain might still travel through chains within other tables, and could still be dropped there. There is nothing special about this target whatsoever, and it does not require, nor have the possibility of, adding options to the target. To use this target, we simply specify -j ACCEPT.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

CLASSIFY target

The CLASSIFY target can be used to classify packets in such a way that can be used by a couple of different qdiscs (Queue Disciplines). For example, atm, cbq, dsmark, pfifo_fast, htb and the prio qdiscs. For more information about qdiscs and traffic controlling, visit the Linux Advanced Routing and Traffic Control HOW-TO webpage.

The CLASSIFY target is only valid in the POSTROUTING chain of the mangle table.

Works under Linux kernel 2.5 and 2.6.

CLUSTERIP target

The CLUSTERIP target is used to create simple clusters of nodes answering to the same IP and MAC address in a round robin fashion. This is a simple form of clustering where you set up a Virtual IP (VIP) on all hosts participating in the cluster, and then use the CLUSTERIP on each host that is supposed to answer the requests. The CLUSTERIP match requires no special load balancing hardware or machines, it simply does its work on each host part of the cluster of machines. It is a very simple clustering solution and not suited for large and complex clusters, neither does it have built in heartbeat handling, but it should be easily implemented as a simple script.

All servers in the cluster uses a common Multicast MAC for a VIP, and then a special hash algorithm is used within the CLUSTERIP target to figure out who of the cluster participants should respond to each connection. A Multicast MAC is a MAC address starting with 01:00:5e as the first 24 bits. an example of a Multicast MAC would be 01:00:5e:00:00:20. The VIP can be any IP address, but must be the same on all hosts as well.

Remember that the CLUSTERIP might break protocols such as SSH et cetera. The connection will go through properly, but if you try the same time again to the same host, you might be connected to another machine in the cluster, with a different keyset, and hence your ssh client might refuse to connect or give you errors. For this reason, this will not work very well with some protocols, and it might be a good idea to add separate addresses that can be used for maintenance and administration. Another solution is to use the same SSH keys on all hosts participating in the cluster.

The cluster can be loadbalanced with three kinds of hashmodes. The first one is only source IP (sourceip), the second is source IP and source port (sourceip-sourceport) and the third one is source IP, source port and destination port (sourceip-sourceport-destport). The first one might be a good idea where you need to remember states between connections, for example a webserver with a shopping cart that keeps state between connections, this load-balancing might become a little bit uneven -- different machines might get a higher loads than others, et cetera -- since connections from the same source IP will go to the same server. The sourceip-sourceport hash might be a good idea where you want to get the load-balancing a little bit more even, and where state does not have to be kept between connections on each server. For example, a large informational webpage with perhaps a simple search engine might be a good idea here. The third and last hashmode, sourceip-sourceport-destport, might be a good idea where you have a host with several services running that does not require any state to be preserved between connections. This might for example be a simple ntp, dns and www server on the same host. Each connection to each new destination would hence be "renegotiated" -- actually no negotiation goes on, it is basically just a round robin system and each host receives one connection each.

Each CLUSTERIP cluster gets a separate file in the /proc/net/ipt_CLUSTERIP directory, based on the VIP of the cluster. If the VIP is 192.168.0.5 for example, you could cat /proc/net/ipt_CLUSTERIP/192.168.0.5 to see which nodes this machine is answering for. To make the machine answer for another machine, lets say node 2, add it using echo "+2" >> /proc/net/ipt_CLUSTERIP/192.168.0.5. To remove it, run echo "-2" >> /proc/net/ipt_CLUSTERIP/192.168.0.5.

This target is in violation of the RFC 1812 - Requirements for IP Version 4 Routers RFC, so be wary of any problems that may arise. Specifically, section 3.3.2 which specifies that a router must never trust another host or router that says that it is using a multicast mac.

Works under late Linux 2.6 kernels, marked experimental.

CONNMARK target

The CONNMARK target is used to set a mark on a whole connection, much the same way as the MARK target does. It can then be used together with the connmark match to match the connection in the future. For example, say we see a specific pattern in a header, and we don't want to mark just that packet, but the whole connection. The CONNMARK target is a perfect solution in that case.

The CONNMARK target is available in all chains and all tables, but remember that the nat table is only traversed by the first packet in a connection, so the CONNMARK target will have no effect if you try to use it for subsequent packets after the first one in here. It can take one of four different options as seen below.

Works under Linux kernel 2.6.

CONNSECMARK target

The CONNSECMARK target sets a SELinux security context mark to or from a packet mark. For further information on SELinux, read more at the Security-Enhanced Linux homepage. The target is only valid in the mangle table and is used together with the SECMARK target, where the SECMARK target is used to set the original mark, and then the CONNSECMARK is used to set the mark on the whole connection.

SELinux is beyond the scope of this document, but basically it is an addition of Mandatory Access Control to Linux. This is more finegrained than the original security systems of most Linux and Unix security controls. Each object can have security attributes, or security context, connected to it, and these attributes are then matched to eachother before allowing or denying a specific task to be performed. This target will allow a security context to be set on a connection.

DNAT target

The DNAT target is used to do Destination Network Address Translation, which means that it is used to rewrite the Destination IP address of a packet. If a packet is matched, and this is the target of the rule, the packet, and all subsequent packets in the same stream will be translated, and then routed on to the correct device, host or network. This target can be extremely useful, for example,when you have a host running your web server inside a LAN, but no real IP to give it that will work on the Internet. You could then tell the firewall to forward all packets going to its own HTTP port, on to the real web server within the LAN. We may also specify a whole range of destination IP addresses, and the DNAT mechanism will choose the destination IP address at random for each stream. Hence, we will be able to deal with a kind of load balancing by doing this.

Note that the DNAT target is only available within the PREROUTING and OUTPUT chains in the nat table, and any of the chains called upon from any of those listed chains. Note that chains containing DNAT targets may not be used from any other chains, such as the POSTROUTING chain.

Since DNAT requires quite a lot of work to work properly, I have decided to add a larger explanation on how to work with it. Let's take a brief example on how things would be done normally. We want to publish our website via our Internet connection. We only have one IP address, and the HTTP server is located on our internal network. Our firewall has the external IP address $INET_IP, and our HTTP server has the internal IP address $HTTP_IP and finally the firewall has the internal IP address $LAN_IP. The first thing to do is to add the following simple rule to the PREROUTING chain in the nat table:

iptables -t nat -A PREROUTING --dst $INET_IP -p tcp --dport 80 -j DNAT \
--to-destination $HTTP_IP
	

Now, all packets from the Internet going to port 80 on our firewall are redirected (or DNAT'ed) to our internal HTTP server. If you test this from the Internet, everything should work just perfect. So, what happens if you try connecting from a host on the same local network as the HTTP server? It will simply not work. This is a problem with routing really. We start out by dissecting what happens in a normal case. The external box has IP address $EXT_BOX, to maintain readability.

1. Packet leaves the connecting host going to $INET_IP and source $EXT_BOX.

2. Packet reaches the firewall.

3. Firewall DNAT's the packet and runs the packet through all different chains etcetera.

4. Packet leaves the firewall and travels to the $HTTP_IP.

5. Packet reaches the HTTP server, and the HTTP box replies back through the firewall, if that is the box that the routing database has entered as the gateway for $EXT_BOX. Normally, this would be the default gateway of the HTTP server.

6. Firewall Un-DNAT's the packet again, so the packet looks as if it was replied to from the firewall itself.

7. Reply packet travels as usual back to the client $EXT_BOX.

Now, we will consider what happens if the packet was instead generated by a client on the same network as the HTTP server itself. The client has the IP address $LAN_BOX, while the rest of the machines maintain the same settings.

1. Packet leaves $LAN_BOX to $INET_IP.

2. The packet reaches the firewall.

3. The packet gets DNAT'ed, and all other required actions are taken, however, the packet is not SNAT'ed, so the same source IP address is used on the packet.

4. The packet leaves the firewall and reaches the HTTP server.

5. The HTTP server tries to respond to the packet, and sees in the routing databases that the packet came from a local box on the same network, and hence tries to send the packet directly to the original source IP address (which now becomes the destination IP address).

6. The packet reaches the client, and the client gets confused since the return packet does not come from the host that it sent the original request to. Hence, the client drops the reply packet, and waits for the "real" reply.

The simple solution to this problem is to SNAT all packets entering the firewall and leaving for a host or IP that we know we do DNAT to. For example, consider the above rule. We SNAT the packets entering our firewall that are destined for $HTTP_IP port 80 so that they look as if they came from $LAN_IP. This will force the HTTP server to send the packets back to our firewall, which Un-DNAT's the packets and sends them on to the client. The rule would look something like this:

iptables -t nat -A POSTROUTING -p tcp --dst $HTTP_IP --dport 80 -j SNAT \
--to-source $LAN_IP
	

Remember that the POSTROUTING chain is processed last of the chains, and hence the packet will already be DNAT'ed once it reaches that specific chain. This is the reason that we match the packets based on the internal address.

This last rule will seriously harm your logging, so it is really advisable not to use this method, but the whole example is still a valid one. What will happen is this, packet comes from the Internet, gets SNAT'ed and DNAT'ed, and finally hits the HTTP server (for example). The HTTP server now only sees the request as if it was coming from the firewall, and hence logs all requests from the internet as if they came from the firewall. This can also have even more severe implications. Take an SMTP server on the LAN, that allows requests from the internal network, and you have your firewall set up to forward SMTP traffic to it. You have now effectively created an open relay SMTP server, with horrenduously bad logging! One solution to this problem is to simply make the SNAT rule even more specific in the match part, and to only work on packets that come in from our LAN interface. In other words, add a --src $LAN_IP_RANGE to the whole command as well. This will make the rule only work on streams that come in from the LAN, and hence will not affect the Source IP, so the logs will look correct, except for streams coming from our LAN. You will, in other words, be better off solving these problems by either setting up a separate DNS server for your LAN, or to actually set up a separate DMZ, the latter being preferred if you have the money.

You think this should be enough by now, and it really is, unless considering one final aspect to this whole scenario. What if the firewall itself tries to access the HTTP server, where will it go? As it looks now, it will unfortunately try to get to its own HTTP server, and not the server residing on $HTTP_IP. To get around this, we need to add a DNAT rule in the OUTPUT chain as well. Following the above example, this should look something like the following:

iptables -t nat -A OUTPUT --dst $INET_IP -p tcp --dport 80 -j DNAT \
--to-destination $HTTP_IP
	

Adding this final rule should get everything up and running. All separate networks that do not sit on the same net as the HTTP server will run smoothly, all hosts on the same network as the HTTP server will be able to connect and finally, the firewall will be able to do proper connections as well. Now everything works and no problems should arise.

Everyone should realize that these rules only affect how the packet is DNAT'ed and SNAT'ed properly. In addition to these rules, you may also need extra rules in the filter table (FORWARD chain) to allow the packets to traverse through those chains as well. Don't forget that all packets have already gone through the PREROUTING chain, and should hence have their destination addresses rewritten already by DNAT.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

DROP target

The DROP target does just what it says, it drops packets dead and will not carry out any further processing. A packet that matches a rule perfectly and is then Dropped will be blocked. Note that this action might in certain cases have an unwanted effect, since it could leave dead sockets around on either host. A better solution in cases where this is likely would be to use the REJECT target, especially when you want to block port scanners from getting too much information, such as on filtered ports and so on. Also note that if a packet has the DROP action taken on it in a subchain, the packet will not be processed in any of the main chains either in the present or in any other table. The packet is in other words totally dead. As we've seen previously, the target will not send any kind of information in either direction, nor to intermediaries such as routers.

>	Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

DSCP target

This is a target that changes the DSCP(Differentiated Services Field) marks inside a packet. The DSCP target is able to set any DSCP value inside a TCP packet, which is a way of telling routers the priority of the packet in question. For more information about DSCP, look at the RFC 2474 - Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers RFC document.

Basically, DSCP is a way of differentiating different services into separate categories, and based on this, give them different priority through the routers. This way, you can give interactive TCP sessions (such as telnet, SSH, POP3) a very high fast connection, that may not be very suitable for large bulk transfers. If on the other hand the connection is one of low importance (SMTP, or whatever you classify as low priority), you could send it over a large bulky network with worse latency than the other network, that is cheaper to utilize than the faster and lower latency connections.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

ECN target

This target can be great, used in the correct way. Simply put, the ECN target can be used to reset the ECN bits from the IPv4 header, or to put it correctly, reset them to 0 at least. Since ECN is a relatively new thing on the net, there are problems with it. For example, it uses 2 bits that are defined in the original RFC for the TCP protocol to be 0. Some routers and other internet appliances will not forward packets that have these bits set to 1. If you want to make use of at least parts of the ECN functionality from your hosts, you could for example reset the ECN bits to 0 for specific networks that you know you are having troubles reaching because of ECN.

Please do note that it isn't possible to turn ECN on in the middle of a stream. It isn't allowed according to the RFC's, and it isn't possible anyways. Both endpoints of the stream must negotiate ECN. If we turn it on, then one of the hosts is not aware of it, and can't respond properly to the ECN notifications.

Works under Linux kernel 2.5 and 2.6.

LOG target options

The LOG target is specially designed for logging detailed information about packets. These could, for example, be considered as illegal. Or, logging can be used purely for bug hunting and error finding. The LOG target will return specific information on packets, such as most of the IP headers and other information considered interesting. It does this via the kernel logging facility, normally syslogd. This information may then be read directly with dmesg, or from the syslogd logs, or with other programs or applications. This is an excellent target to use to debug your rule-sets, so that you can see what packets go where and what rules are applied on what packets. Note as well that it could be a really great idea to use the LOG target instead of the DROP target while you are testing a rule you are not 100% sure about on a production firewall, since a syntax error in the rule-sets could otherwise cause severe connectivity problems for your users. Also note that the ULOG target may be interesting if you are using really extensive logging, since the ULOG target has support for direct logging to MySQL databases and suchlike.

Note that if you get undesired logging direct to consoles, this is not an iptables or Netfilter problem, but rather a problem caused by your syslogd configuration - most probably /etc/syslog.conf. Read more in man syslog.conf for information about this kind of problem. You may also need to tweak your dmesg settings. dmesg is the command that changes which errors from the kernel that should be shown on the console. dmesg -n 1 should prevent all messages from showing up on the console, except panic messages. The dmesg message levels matches exactly the syslogd levels, and it only works on log messages from the kernel facility. For more information, see man dmesg.

The LOG target currently takes five options that could be of interest if you have specific information needs, or want to set different options to specific values. They are all listed below.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

MARK target

The MARK target is used to set Netfilter mark values that are associated with specific packets. This target is only valid in the mangle table, and will not work outside there. The MARK values may be used in conjunction with the advanced routing capabilities in Linux to send different packets through different routes and to tell them to use different queue disciplines (qdisc), etc. For more information on advanced routing, check out the Linux Advanced Routing and Traffic Control HOW-TO. Note that the mark value is not set within the actual packet, but is a value that is associated within the kernel with the packet. In other words, you can not set a MARK for a packet and then expect the MARK still to be there on another host. If this is what you want, you will be better off with the TOS target which will mangle the TOS value in the IP header.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

MASQUERADE target

The MASQUERADE target is used basically the same as the SNAT target, but it does not require any --to-source option. The reason for this is that the MASQUERADE target was made to work with, for example, dial-up connections, or DHCP connections, which gets dynamic IP addresses when connecting to the network in question. This means that you should only use the MASQUERADE target with dynamically assigned IP connections, which we don't know the actual address of at all times. If you have a static IP connection, you should instead use the SNAT target.

When you masquerade a connection, it means that we set the IP address used on a specific network interface instead of the --to-source option, and the IP address is automatically grabbed from the information about the specific interface. The MASQUERADE target also has the effect that connections are forgotten when an interface goes down, which is extremely good if we, for example, kill a specific interface. If we would have used the SNAT target, we may have been left with a lot of old connection tracking data, which would be lying around for days, swallowing up useful connection tracking memory. This is, in general, the correct behavior when dealing with dial-up lines that are probably assigned a different IP every time they are brought up. In case we are assigned a different IP, the connection is lost anyways, and it is more or less idiotic to keep the entry around.

It is still possible to use the MASQUERADE target instead of SNAT even though you do have a static IP, however, it is not favorable since it will add extra overhead, and there may be inconsistencies in the future which will thwart your existing scripts and render them "unusable".

Note that the MASQUERADE target is only valid within the POSTROUTING chain in the nat table, just as the SNAT target. The MASQUERADE target takes one option specified below, which is optional.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

MIRROR target

Be warned, the MIRROR is dangerous and was only developed as an example code of the new conntrack and NAT code. It can cause dangerous things to happen, and very serious DDoS/DoS will be possible if used improperly. Avoif using it at all costs! It was removed from 2.5 and 2.6 kernels due to it's bad security implications!

The MIRROR target is an experimental and demonstration target only, and you are warned against using it, since it may result in really bad loops hence, among other things, resulting in serious Denial of Service. The MIRROR target is used to invert the source and destination fields in the IP header, and then to retransmit the packet. This can cause some really funny effects, and I'll bet that, thanks to this target, not just one red faced cracker has cracked his own box by now. The effect of using this target is stark, to say the least. Let's say we set up a MIRROR target for port 80 at computer A. If host B were to come from yahoo.com, and try to access the HTTP server at host A, the MIRROR target would return the yahoo host's own web page (since this is where the request came from).

Note that the MIRROR target is only valid within the INPUT, FORWARD and PREROUTING chains, and any user-defined chains which are called from those chains. Also note that outgoing packets resulting from the MIRROR target are not seen by any of the normal chains in the filter, nat or mangle tables, which could give rise to loops and other problems. This could make the target the cause of unforeseen headaches. For example, a host might send a spoofed packet to another host that uses the MIRROR command with a TTL of 255, at the same time spoofing its own packet, so as to seem as if it comes from a third host that uses the MIRROR command. The packet will then bounce back and forth incessantly, for the number of hops there are to be completed. If there is only 1 hop, the packet will jump back and forth 240-255 times. Not bad for a cracker, in other words, to send 1500 bytes of data and eat up 380 kbyte of your connection. Note that this is a best case scenario for the cracker or script kiddie, whatever we want to call them.

Works under Linux kernel 2.3 and 2.4. It was removed from 2.5 and 2.6 kernels due to it's inherent insecurity. Do not use this target!

NETMAP target

NETMAP is a new implementation of the SNAT and DNAT targets where the host part of the IP address isn't changed. It provides a 1:1 NAT function for whole networks which isn't available in the standard SNAT and DNAT functions. For example, lets say we have a network containing 254 hosts using private IP addresses (a /24 network), and we just got a new /24 network of public IP's. Instead of walking around and changing the IP of each and every one of the hosts, we would be able to simply use the NETMAP target like -j NETMAP -to 10.5.6.0/24 and voila, all the hosts are seen as 10.5.6.x when they leave the firewall. For example, 192.168.0.26 would become 10.5.6.26.

Works under Linux kernel 2.5 and 2.6.

NFQUEUE target

The NFQUEUE target is used much the same way as the QUEUE target, and is basically an extension of it. The NFQUEUE target allows for sending packets for separate and specific queues. The queue is identified by a 16-bit id.

This target requires the nfnetlink_queue kernel support to run. For more information on what you can do with the NFQUEUE target, see the QUEUE target.

Works under Linux kernel 2.6.14 and later.

NOTRACK target

This target is used to turn off connection tracking for all packets matching this rule. The target has been discussed at some length in the Untracked connections and the raw table section of the The state machine chapter.

The target takes no options and is very easy to use. Match the packets you wish to not track, and then set the NOTRACK target on the rules matching the packets you don't wish to track.

The target is only valid inside the raw table.

Works under late Linux 2.6 kernels.

QUEUE target

The QUEUE target is used to queue packets to User-land programs and applications. It is used in conjunction with programs or utilities that are extraneous to iptables and may be used, for example, with network accounting, or for specific and advanced applications which proxy or filter packets. We will not discuss this target in depth, since the coding of such applications is out of the scope of this tutorial. First of all it would simply take too much time, and secondly such documentation does not have anything to do with the programming side of Netfilter and iptables. All of this should be fairly well covered in the Netfilter Hacking HOW-TO.

As of kernel 2.6.14 the behavior of netfilter has changed. A new system for talking to the QUEUE has been deviced, called the nfnetlink_queue. The QUEUE target is basically a pointer to the NFQUEUE 0 nowadays. For programming questions, still see the above link. This requires the nfnetlink_queue.ko module.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

REDIRECT target

The REDIRECT target is used to redirect packets and streams to the machine itself. This means that we could for example REDIRECT all packets destined for the HTTP ports to an HTTP proxy like squid, on our own host. Locally generated packets are mapped to the 127.0.0.1 address. In other words, this rewrites the destination address to our own host for packets that are forwarded, or something alike. The REDIRECT target is extremely good to use when we want, for example, transparent proxying, where the LAN hosts do not know about the proxy at all.

Note that the REDIRECT target is only valid within the PREROUTING and OUTPUT chains of the nat table. It is also valid within user-defined chains that are only called from those chains, and nowhere else. The REDIRECT target takes only one option, as described below.

>	Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

REJECT target

The REJECT target works basically the same as the DROP target, but it also sends back an error message to the host sending the packet that was blocked. The REJECT target is as of today only valid in the INPUT, FORWARD and OUTPUT chains or their sub chains. After all, these would be the only chains in which it would make any sense to put this target. Note that all chains that use the REJECT target may only be called by the INPUT, FORWARD, and OUTPUT chains, else they won't work. There is currently only one option which controls the nature of how this target works, though this may in turn take a huge set of variables. Most of them are fairly easy to understand, if you have a basic knowledge of TCP/IP.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

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.

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.

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.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

TCPMSS target

The TCPMSS target can be used to alter the MSS (Maximum Segment Size) value of TCP SYN packets that the firewall sees. The MSS value is used to control the maximum size of packets for specific connections. Under normal circumstances, this means the size of the MTU (Maximum Transfer Unit) value, minus 40 bytes. This is used to overcome some ISP's and servers that block ICMP fragmentation needed packets, which can result in really weird problems which can mainly be described such that everything works perfectly from your firewall/router, but your local hosts behind the firewall can't exchange large packets. This could mean such things as mail servers being able to send small mails, but not large ones, web browsers that connect but then hang with no data received, and ssh connecting properly, but scp hangs after the initial handshake. In other words, everything that uses any large packets will be unable to work.

The TCPMSS target is able to solve these problems, by changing the size of the packets going out through a connection. Please note that we only need to set the MSS on the SYN packet since the hosts take care of the MSS after that. The target takes two arguments.

Works under Linux kernel 2.5 and 2.6.

TOS target

The TOS target is used to set the Type of Service field within the IP header. The TOS field consists of 8 bits which are used to help in routing packets. This is one of the fields that can be used directly within iproute2 and its subsystem for routing policies. Worth noting, is that if you handle several separate firewalls and routers, this is the only way to propagate routing information within the actual packet between these routers and firewalls. As previously noted, the MARK target - which sets a MARK associated with a specific packet - is only available within the kernel, and can't be propagated with the packet. If you feel a need to propagate routing information for a specific packet or stream, you should therefore set the TOS field, which was developed for this.

There are currently a lot of routers on the Internet which do a pretty bad job at this, so as of now it may prove to be a bit useless to attempt TOS mangling before sending the packets on to the Internet. At best the routers will not pay any attention to the TOS field. At worst, they will look at the TOS field and do the wrong thing. However, as stated above, the TOS field can most definitely be put to good use if you have a large WAN or LAN with multiple routers. You then in fact have the possibility of giving packets different routes and preferences, based on their TOS value - even though this might be confined to your own network.

The TOS target is only capable of setting specific values, or named values on packets. These predefined TOS values can be found in the kernel include files, or more precisely, the Linux/ip.h file. The reasons are many, and you should actually never need to set any other values; however, there are ways around this limitation. To get around the limitation of only being able to set the named values on packets, you can use the FTOS patch available at the Paksecured Linux Kernel patches site maintained by Matthew G. Marsh. However, be cautious with this patch! You should not need to use any other than the default values, except in extreme cases.

Note that this target is only valid within the mangle table and can't be used outside it.

Also note that some old versions (1.2.2 or below) of iptables provided a broken implementation of this target which did not fix the packet checksum upon mangling, hence rendering the packets bad and in need of retransmission. That in turn would most probably lead to further mangling and the connection never working.

The TOS target only takes one option as described below.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

TTL target

The TTL target is used to modify the Time To Live field in the IP header. One useful application of this is to change all Time To Live values to the same value on all outgoing packets. One reason for doing this is if you have a bully ISP which don't allow you to have more than one machine connected to the same Internet connection, and who actively pursues this. Setting all TTL values to the same value, will effectively make it a little bit harder for them to notice that you are doing this. We may then reset the TTL value for all outgoing packets to a standardized value, such as 64 as specified in the Linux kernel.

For more information on how to set the default value used in Linux, read the ip-sysctl.txt, which you may find within the Other resources and links appendix.

The TTL target is only valid within the mangle table, and nowhere else. It takes 3 options as of writing this, all of them described below in the table.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

ULOG target

The ULOG target is used to provide user-space logging of matching packets. If a packet is matched and the ULOG target is set, the packet information is multicasted together with the whole packet through a netlink socket. One or more user-space processes may then subscribe to various multicast groups and receive the packet. This is in other words a more complete and more sophisticated logging facility that is only used by iptables and Netfilter so far, and it contains much better facilities for logging packets. This target enables us to log information to MySQL databases, and other databases, making it much simpler to search for specific packets, and to group log entries. You can find the ULOGD user-land applications at the ULOGD project page.

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

What's next?

This chapter has discussed in detail each and every target that is available in Linux. This list is still growing as people write more and more target extensions for iptables and netfilter, and it is already quite extensive as you have seen. The chapter has also discussed the different target options available for each target.

The next chapter will delve into debugging your firewall scripts and what techniques are available for doing this. It will both show you moderate debugging techniques such as using bash and echo, to some more advanced tools such as nmap and nessus.