Linux Security HOWTO : Network Security : System services and tcp_wrappers
Previous: Packet Sniffers
Next: Verify Your DNS Information

8.2. System services and tcp_wrappers

Before you put your Linux system on ANY network the first thing to look at is what services you need to offer. Services that you do not need to offer should be disabled so that you have one less thing to worry about and attackers have one less place to look for a hole.

There are a number of ways to disable services under Linux. You can look at your /etc/inetd.conf file and see what services are being offered by your inetd. Disable any that you do not need by commenting them out (# at the beginning of the line), and then sending your inetd process a SIGHUP.

You can also remove (or comment out) services in your /etc/services file. This will mean that local clients will also be unable to find the service (i.e., if you remove ftp, and try and ftp to a remote site from that machine it will fail with an "unknown service" message). It's usually not worth the trouble to remove services from /etc/services, since it provides no additional security. If a local person wanted to use ftp even though you had commented it out, they would make their own client that used the common FTP port and would still work fine.

Some of the services you might want to leave enabled are:

If you know you are not going to use some particular package, you can also delete it entirely. rpm -e packagename under the Red Hat distribution will erase an entire package. Under debian dpkg --remove does the same thing.

Additionally, you really want to disable the rsh/rlogin/rcp utilities, including login (used by rlogin), shell (used by rcp), and exec (used by rsh) from being started in /etc/inetd.conf. These protocols are extremely insecure and have been the cause of exploits in the past.

You should check /etc/rc.d/rc[0-9].d (on Red Hat; /etc/rc[0-9].d on Debian), and see if any of the servers started in those directories are not needed. The files in those directories are actually symbolic links to files in the directory /etc/rc.d/init.d (on Red Hat; /etc/init.d on Debian). Renaming the files in the init.d directory disables all the symbolic links that point to that file. If you only wish to disable a service for a particular run level, rename the appropriate symbolic link by replacing the upper-case S with a lower-case s, like this:

       root#  cd /etc/rc6.d
       root#  mv S45dhcpd s45dhcpd

If you have BSD-style rc files, you will want to check /etc/rc* for programs you don't need.

Most Linux distributions ship with tcp_wrappers "wrapping" all your TCP services. A tcp_wrapper (tcpd) is invoked from inetd instead of the real server. tcpd then checks the host that is requesting the service, and either executes the real server, or denies access from that host. tcpd allows you to restrict access to your TCP services. You should make a /etc/hosts.allow and add in only those hosts that need to have access to your machine's services.

If you are a home dialup user, we suggest you deny ALL. tcpd also logs failed attempts to access services, so this can alert you if you are under attack. If you add new services, you should be sure to configure them to use tcp_wrappers if they are TCP-based. For example, a normal dial-up user can prevent outsiders from connecting to his machine, yet still have the ability to retrieve mail, and make network connections to the Internet. To do this, you might add the following to your /etc/hosts.allow:

ALL: 127.

And of course /etc/hosts.deny would contain:

ALL: ALL

which will prevent external connections to your machine, yet still allow you from the inside to connect to servers on the Internet.

Keep in mind that tcp_wrappers only protects services executed from inetd, and a select few others. There very well may be other services running on your machine. You can use netstat -ta to find a list of all the services your machine is offering.


Linux Security HOWTO : Network Security : System services and tcp_wrappers
Previous: Packet Sniffers
Next: Verify Your DNS Information