Iptables Tutorial 1.2.2 - Oskar Andreasson

Passive FTP works the opposite way. The FTP client tells the server that it wants some specific data, upon which the server replies with an IP address to connect to and at what port. The client will, upon receipt of this data, connect to that specific port, from its own port 20(the FTP-data port), and get the data in question. If you have an FTP server behind your firewall, you will in other words require this module in addition to your standard iptables modules to let clients on the Internet connect to the FTP server properly. The same goes if you are extremely restrictive to your users, and only want to let them reach HTTP and FTP servers on the Internet and block all other ports. Consider the following image and its bearing on Passive FTP.

Some conntrack helpers are already available within the kernel itself. More specifically, the FTP and IRC protocols have conntrack helpers as of writing this. If you can not find the conntrack helpers that you need within the kernel itself, you should have a look at the patch-o-matic tree within user-land iptables. The patch-o-matic tree may contain more conntrack helpers, such as for the ntalk or H.323 protocols. If they are not available in the patch-o-matic tree, you have a number of options. Either you can look at the CVS source of iptables, if it has recently gone into that tree, or you can contact the Netfilter-devel mailing list and ask if it is available. If it is not, and there are no plans for adding it, you are left to your own devices and would most probably want to read the Rusty Russell's Unreliable Netfilter Hacking HOW-TO which is linked from the Other resources and links appendix.

Conntrack helpers may either be statically compiled into the kernel, or as modules. If they are compiled as modules, you can load them with the following command

modprobe ip_conntrack_ftp

modprobe ip_conntrack_irc

modprobe ip_conntrack_tftp

modprobe ip_conntrack_amanda

Do note that connection tracking has nothing to do with NAT, and hence you may require more modules if you are NAT'ing connections as well. For example, if you were to want to NAT and track FTP connections, you would need the NAT module as well. All NAT helpers starts with ip_nat_ and follow that naming convention; so for example the FTP NAT helper would be named ip_nat_ftp and the IRC module would be named ip_nat_irc. The conntrack helpers follow the same naming convention, and hence the IRC conntrack helper would be named ip_conntrack_irc, while the FTP conntrack helper would be named ip_conntrack_ftp.

What's next?

This chapter has discussed how the state machine in netfilter works and how it keeps state of different connections. The chapter has also discussed how it is represented toward you, the end user and what you can do to alter its behavior, as well as different protocols that are more complex to do connection tracking on, and how the different conntrack helpers come into the picture.

The next chapter will discuss how to save and restore rulesets using the iptables-save and iptables-restore programs distributed with the iptables applications. This has both pros and cons, and the chapter will discuss it in detail.

Chapter 8. Saving and restoring large rule-sets

The iptables package comes with two more tools that are very useful, specially if you are dealing with larger rule-sets. These two tools are called iptables-save and iptables-restore and are used to save and restore rule-sets to a specific file-format that looks quite a bit different from the standard shell code that you will see in the rest of this tutorial.

Tip iptables-restore can be used together with scripting languages. The big problem is that you will need to output the results into the stdin of iptables-restore. If you are creating a very big ruleset (several thousand rules) this might be a very good idea, since it will be much faster to insert all the new rules. For example, you would then run make_rules.sh | iptables-restore.

Speed considerations

One of the largest reasons for using the iptables-save and iptables-restore commands is that they will speed up the loading and saving of larger rule-sets considerably. The main problem with running a shell script that contains iptables rules is that each invocation of iptables within the script will first extract the whole rule-set from the Netfilter kernel space, and after this, it will insert or append rules, or do whatever change to the rule-set that is needed by this specific command. Finally, it will insert the new rule-set from its own memory into kernel space. Using a shell script, this is done for each and every rule that we want to insert, and for each time we do this, it takes more time to extract and insert the rule-set.

To solve this problem, there is the iptables-save and restore commands. The iptables-save command is used to save the rule-set into a specially formatted text-file, and the iptables-restore command is used to load this text-file into kernel again. The best parts of these commands is that they will load and save the rule-set in one single request. iptables-save will grab the whole rule-set from kernel and save it to a file in one single movement. iptables-restore will upload that specific rule-set to kernel in a single movement for each table. In other words, instead of dropping the rule-set out of kernel some 30,000 times, for really large rule-sets, and then upload it to kernel again that many times, we can now save the whole thing into a file in one movement and then upload the whole thing in as little as three movements depending on how many tables you use.

As you can understand, these tools are definitely something for you if you are working on a huge set of rules that needs to be inserted. However, they do have drawbacks that we will discuss more in the next section.

Drawbacks with restore

As you may have already wondered, can iptables-restore handle any kind of scripting? So far, no, it cannot and it will most probably never be able to. This is the main flaw in using iptables-restore since you will not be able to do a huge set of things with these files. For example, what if you have a connection that has a dynamically assigned IP address and you want to grab this dynamic IP every-time the computer boots up and then use that value within your scripts? With iptables-restore, this is more or less impossible.

One possibility to get around this is to make a small script which grabs the values you would like to use in the script, then sed the iptables-restore file for specific keywords and replace them with the values collected via the small script. At this point, you could save it to a temporary file, and then use iptables-restore to load the new values. This causes a lot of problems however, and you will be unable to use iptables-save properly since it would probably erase your manually added keywords in the restore script. It is, in other words, a clumsy solution.

A second possibility is to do as previously described. Make a script that outputs rules in iptables-restore format, and then feed them on standard input of iptables-restore. For very large rulesets this would be to be preferred over running iptables itself, since it has a bad habit of taking a lot of processing power on very large rulesets as previously described in this chapter.

Another solution is to load the iptables-restore scripts first, and then load a specific shell script that inserts more dynamic rules in their proper places. Of course, as you can understand, this is just as clumsy as the first solution. iptables-restore is simply not very well suited for configurations where IP addresses are dynamically assigned to your firewall or where you want different behaviors depending on configuration options and so on.

Another drawback with iptables-restore and iptables-save is that it is not fully functional as of writing this. The problem is simply that not a lot of people use it as of today and hence there are not a lot of people finding bugs, and in turn some matches and targets will simply be inserted badly, which may lead to some strange behaviors that you did not expect. Even though these problems exist, I would highly recommend using these tools which should work extremely well for most rule-sets as long as they do not contain some of the new targets or matches that it does not know how to handle properly.

iptables-save

The iptables-save command is, as we have already explained, a tool to save the current rule-set into a file that iptables-restore can use. This command is quite simple really, and takes only two arguments. Take a look at the following example to understand the syntax of the command.

iptables-save [-c] [-t table]

The -c argument tells iptables-save to keep the values specified in the byte and packet counters. This could for example be useful if we would like to reboot our main firewall, but not lose byte and packet counters which we may use for statistical purposes. Issuing a iptables-save command with the -c argument would then make it possible for us to reboot without breaking our statistical and accounting routines. The default value is, of course, to not keep the counters intact when issuing this command.

The -t argument tells the iptables-save command which tables to save. Without this argument the command will automatically save all tables available into the file. The following is an example on what output you can expect from the iptables-save command if you do not have any rule-set loaded.

# Generated by iptables-save v1.2.6a on Wed Apr 24 10:19:17 2002

*filter

:INPUT ACCEPT [404:19766]

:FORWARD ACCEPT [0:0]

:OUTPUT ACCEPT [530:43376]

COMMIT

# Completed on Wed Apr 24 10:19:17 2002

# Generated by iptables-save v1.2.6a on Wed Apr 24 10:19:17 2002

*mangle

:PREROUTING ACCEPT [451:22060]

:INPUT ACCEPT [451:22060]

:FORWARD ACCEPT [0:0]

:OUTPUT ACCEPT [594:47151]

:POSTROUTING ACCEPT [594:47151]

COMMIT

# Completed on Wed Apr 24 10:19:17 2002

# Generated by iptables-save v1.2.6a on Wed Apr 24 10:19:17 2002

*nat

:PREROUTING ACCEPT [0:0]

:POSTROUTING ACCEPT [3:450]

:OUTPUT ACCEPT [3:450]

COMMIT

# Completed on Wed Apr 24 10:19:17 2002

This contains a few comments starting with a # sign. Each table is marked like *<table-name>, for example *mangle. Then within each table we have the chain specifications and rules. A chain specification looks like :<chain-name> <chain-policy> [<packet-counter>:<byte-counter>]. The chain-name may be for example PREROUTING, the policy is described previously and can, for example, be ACCEPT. Finally the packet-counter and byte-counters are the same counters as in the output from iptables -L -v. Finally, each table declaration ends in a COMMIT keyword. The COMMIT keyword tells us that at this point we should commit all rules currently in the pipeline to kernel.

The above example is pretty basic, and hence I believe it is nothing more than proper to show a brief example which contains a very small Iptables-save ruleset. If we would run iptables-save on this, it would look something like this in the output:

# Generated by iptables-save v1.2.6a on Wed Apr 24 10:19:55 2002

*filter

:INPUT DROP [1:229]

:FORWARD DROP [0:0]

:OUTPUT DROP [0:0]

-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT

-A FORWARD -i eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT

-A FORWARD -i eth1 -m state --state NEW,RELATED,ESTABLISHED -j ACCEPT

-A OUTPUT -m state --state NEW,RELATED,ESTABLISHED -j ACCEPT

COMMIT

# Completed on Wed Apr 24 10:19:55 2002

# Generated by iptables-save v1.2.6a on Wed Apr 24 10:19:55 2002

*mangle

:PREROUTING ACCEPT [658:32445]

:INPUT ACCEPT [658:32445]

:FORWARD ACCEPT [0:0]

:OUTPUT ACCEPT [891:68234]

:POSTROUTING ACCEPT [891:68234]

COMMIT

# Completed on Wed Apr 24 10:19:55 2002

# Generated by iptables-save v1.2.6a on Wed Apr 24 10:19:55 2002

*nat

:PREROUTING ACCEPT [1:229]

:POSTROUTING ACCEPT [3:450]

:OUTPUT ACCEPT [3:450]

-A POSTROUTING -o eth0 -j SNAT --to-source 195.233.192.1

COMMIT

# Completed on Wed Apr 24 10:19:55 2002

As you can see, each command has now been prefixed with the byte and packet counters since we used the -c argument. Except for this, the command-line is quite intact from the script. The only problem now, is how to save the output to a file. Quite simple, and you should already know how to do this if you have used linux at all before. It is only a matter of piping the command output on to the file that you would like to save it as. This could look like the following:

iptables-save -c > /etc/iptables-save

The above command will in other words save the whole rule-set to a file called /etc/iptables-save with byte and packet counters still intact.

iptables-restore

The iptables-restore command is used to restore the iptables rule-set that was saved with the iptables-save command. It takes all the input from standard input and can't load from files as of writing this, unfortunately. This is the command syntax for iptables-restore:

iptables-restore [-c] [-n]

The -c argument restores the byte and packet counters and must be used if you want to restore counters that were previously saved with iptables-save. This argument may also be written in its long form --counters.

The -n argument tells iptables-restore to not overwrite the previously written rules in the table, or tables, that it is writing to. The default behavior of iptables-restore is to flush and destroy all previously inserted rules. The short -n argument may also be replaced with the longer format --noflush.

To load a rule-set with the iptables-restore command, we could do this in several ways, but we will mainly look at the simplest and most common way here.

cat /etc/iptables-save | iptables-restore -c

The following will also work:

iptables-restore -c < /etc/iptables-save

This would cat the rule-set located within the /etc/iptables-save file and then pipe it to iptables-restore which takes the rule-set on the standard input and then restores it, including byte and packet counters. It is that simple to begin with. This command could be varied until oblivion and we could show different piping possibilities, however, this is a bit out of the scope of this chapter, and hence we will skip that part and leave it as an exercise for the reader to experiment with.

The rule-set should now be loaded properly to kernel and everything should work. If not, you may possibly have run into a bug in these commands.

What's next?

This chapter has discussed the iptables-save and iptables-restore programs to some extent and how they can be used. Both applications are distributed with the iptables package, and can be used to quickly save large rulesets and then inserting them into the kernel again.