Книга: Iptables Tutorial 1.2.2

Installation on Red Hat 7.1

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Installation on Red Hat 7.1

Red Hat 7.1 comes preinstalled with a 2.4.x kernel that has Netfilter and iptables compiled in. It also contains all the basic user-land programs and configuration files that are needed to run it. However, the Red Hat people have disabled the whole thing by using the backward compatible ipchains module. Annoying to say the least, and a lot of people keep asking different mailing lists why iptables doesn't work. So, let's take a brief look at how to turn the ipchains module off and how to install iptables instead.

Note The default Red Hat 7.1 installation today comes with a hopelessly old version of the user-space applications, so you might want to compile a new version of the applications as well as install a new and custom compiled kernel before fully exploiting iptables.

First of all you will need to turn off the ipchains modules so it won't start in the future. To do this, you will need to change some filenames in the /etc/rc.d/ directory-structure. The following command should do it:

chkconfig --level 0123456 ipchains off

By doing this we move all the soft links that points to the /etc/rc.d/init.d/ipchains script to K92ipchains. The first letter which per default would be S, tells the initscripts to start the script. By changing this to K we tell it to Kill the service instead, or to not run it if it was not previously started. Now the service won't be started in the future.

However, to stop the service from actually running right now we need to run another command. This is the service command which can be used to work on currently running services. We would then issue the following command to stop the ipchains service:

service ipchains stop

Finally, to start the iptables service. First of all, we need to know which run-levels we want it to run in. Normally this would be in run-level 2, 3 and 5. These run-levels are used for the following things:

• 2. Multiuser without NFS or the same as 3 if there is no networking.

• 3. Full multiuser mode, i.e. the normal run-level to run in.

• 5. X11. This is used if you automatically boot into Xwindows.

To make iptables run in these run-levels we would do the following commands:

chkconfig --level 235 iptables on

The above commands would in other words make the iptables service run in run-level 2, 3 and 5. If you'd like the iptables service to run in some other run-level you would have to issue the same command in those. However, none of the other run-levels should be used, so you should not really need to activate it for those run-levels. Level 1 is for single user mode, i.e, when you need to fix a screwedup box. Level 4 should be unused, and level 6 is for shutting the computer down.

To activate the iptables service, we just run the following command:

service iptables start

There are no rules in the iptables script. To add rules to an Red Hat 7.1 box, there is two common ways. Firstly, you could edit the /etc/rc.d/init.d/iptables script. This would have the undesired effect of deleting all the rules if you updated the iptables package by RPM. The other way would be to load the rule-set and then save it with the iptables-save command and then have it loaded automatically by the rc.d scripts.

First we will describe the how to set up iptables by cutting and pasting to the iptables init.d script. To add rules that are to be run when the computer starts the service, you add them under the start) section, or in the start() function. Note, if you add the rules under the start) section don't forget to stop the start() function in the start) section from running. Also, don't forget to edit a the stop) section either which tells the script what to do when the computer is going down for example, or when we are entering a run-level that doesn't require iptables. Also, don't forget to check out the restart section and condrestart. Note that all this work will probably be trashed if you have, for example, Red Hat Network automatically update your packages. It may also be trashed by updating from the iptables RPM package.

The second way of doing the set up would require the following: First of all, make and write a rule-set in a shell script file, or directly with iptables, that will meet your requirements, and don't forget to experiment a bit. When you find a set up that works without problems, or as you can see without bugs, use the iptables-save command. You could either use it normally, i.e. iptables-save > /etc/sysconfig/iptables, which would save the rule-set to the file /etc/sysconfig/iptables. This file is automatically used by the iptables rc.d script to restore the rule-set in the future. The other way is to save the script by doing service iptables save, which would save the script automatically to /etc/sysconfig/iptables. The next time you reboot the computer, the iptables rc.d script will use the command iptables-restore to restore the rule-set from the save-file /etc/sysconfig/iptables. Do not intermix these two methods, since they may heavily damage each other and render your firewall configuration useless.

When all of these steps are finished, you can deinstall the currently installed ipchains and iptables packages. This because we don't want the system to mix up the new iptables user-land application with the old preinstalled iptables applications. This step is only necessary if you are going to install iptables from the source package. It's not unusual for the new and the old package to get mixed up, since the rpm based installation installs the package in non-standard places and won't get overwritten by the installation for the new iptables package. To carry out the deinstallation, do as follows:

rpm -e iptables

And why keep ipchains lying around if you won't be using it any more? Removing it is done the same way as with the old iptables binaries, etc:

rpm -e ipchains

After all this has been completed, you will have finished with the update of the iptables package from source, having followed the source installation instructions. None of the old binaries, libraries or include files etc should be lying around any more.

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