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8. Network Security

Network security is becoming more and more important as people spend more and more time connected. Compromising network security is often much easier than compromising physical or local security, and is much more common.

There are a number of good tools to assist with network security, and more and more of them are shipping with Linux distributions.

8.1 Packet Sniffers

One of the most common ways intruders gain access to more systems on your network is by employing a packet sniffer on a already compromised host. This "sniffer" just listens on the Ethernet port for things like passwd and login and su in the packet stream and then logs the traffic after that. This way, attackers gain passwords for systems they are not even attempting to break into. Clear-text passwords are very vulnerable to this attack.

Example: Host A has been compromised. Attacker installs a sniffer. Sniffer picks up admin logging into Host B from Host C. It gets the admin's personal password as they login to B. Then, the admin does a su to fix a problem. They now have the root password for Host B. Later the admin lets someone telnet from his account to Host Z on another site. Now the attacker has a password/login on Host Z.

In this day and age, the attacker doesn't even need to compromise a system to do this: they could also bring a laptop or pc into a building and tap into your net.

Using ssh or other encrypted password methods thwarts this attack. Things like APOP for POP accounts also prevents this attack. (Normal POP logins are very vulnerable to this, as is anything that sends clear-text passwords over the network.)

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:


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.

8.3 Verify Your DNS Information

Keeping up-to-date DNS information about all hosts on your network can help to increase security. If an unauthorized host becomes connected to your network, you can recognize it by its lack of a DNS entry. Many services can be configured to not accept connections from hosts that do not have valid DNS entries.

8.4 identd

identd is a small program that typically runs out of your inetd server. It keeps track of what user is running what TCP service, and then reports this to whoever requests it.

Many people misunderstand the usefulness of identd, and so disable it or block all off site requests for it. identd is not there to help out remote sites. There is no way of knowing if the data you get from the remote identd is correct or not. There is no authentication in identd requests.

Why would you want to run it then? Because it helps you out, and is another data-point in tracking. If your identd is un compromised, then you know it's telling remote sites the user-name or uid of people using TCP services. If the admin at a remote site comes back to you and tells you user so-and-so was trying to hack into their site, you can easily take action against that user. If you are not running identd, you will have to look at lots and lots of logs, figure out who was on at the time, and in general take a lot more time to track down the user.

The identd that ships with most distributions is more configurable than many people think. You can disable it for specific users (they can make a .noident file), you can log all identd requests (We recommend it), you can even have identd return a uid instead of a user name or even NO-USER.

8.5 SATAN, ISS, and Other Network Scanners

There are a number of different software packages out there that do port and service-based scanning of machines or networks. SATAN, ISS, SAINT, and Nessus are some of the more well-known ones. This software connects to the target machine (or all the target machines on a network) on all the ports they can, and try to determine what service is running there. Based on this information, you can tell if the machine is vulnerable to a specific exploit on that server.

SATAN (Security Administrator's Tool for Analyzing Networks) is a port scanner with a web interface. It can be configured to do light, medium, or strong checks on a machine or a network of machines. It's a good idea to get SATAN and scan your machine or network, and fix the problems it finds. Make sure you get the copy of SATAN from metalab or a reputable FTP or web site. There was a Trojan copy of SATAN that was distributed out on the net. Note that SATAN has not been updated in quite a while, and some of the other tools below might do a better job.

ISS (Internet Security Scanner) is another port-based scanner. It is faster than Satan, and thus might be better for large networks. However, SATAN tends to provide more information.

Abacus is a suite of tools to provide host-based security and intrusion detection. Look at it's home page on the web for more information.

SAINT is a updated version of SATAN. It is web-based and has many more up-to-date tests than SATAN. You can find out more about it at:

Nessus is a free security scanner. It has a GTK graphical interface for ease of use. It is also designed with a very nice plugin setup for new port-scanning tests. For more information, take a look at:

Detecting Port Scans

There are some tools designed to alert you to probes by SATAN and ISS and other scanning software. However, if you liberally use tcp_wrappers, and look over your log files regularly, you should be able to notice such probes. Even on the lowest setting, SATAN still leaves traces in the logs on a stock Red Hat system.

There are also "stealth" port scanners. A packet with the TCP ACK bit set (as is done with established connections) will likely get through a packet-filtering firewall. The returned RST packet from a port that _had no established session_ can be taken as proof of life on that port. I don't think TCP wrappers will detect this.

8.6 sendmail, qmail and MTA's

One of the most important services you can provide is a mail server. Unfortunately, it is also one of the most vulnerable to attack, simply due to the number of tasks it must perform and the privileges it typically needs.

If you are using sendmail it is very important to keep up on current versions. sendmail has a long long history of security exploits. Always make sure you are running the most recent version from

Keep in mind that sendmail does not have to be running in order for you to send mail. If you are a home user, you can disable sendmail entirely, and simply use your mail client to send mail. You might also choose to remove the "-bd" flag from the sendmail startup file, thereby disabling incoming requests for mail. In other words, you can execute sendmail from your startup script using the following instead:

                # /usr/lib/sendmail -q15m
This will cause sendmail to flush the mail queue every fifteen minutes for any messages that could not be successfully delivered on the first attempt.

Many administrators choose not to use sendmail, and instead choose one of the other mail transport agents. You might consider switching over to qmail. qmail was designed with security in mind from the ground up. It's fast, stable, and secure. Qmail can be found at

In direct competition to qmail is "postfix", written by Wietse Venema, the author of tcp_wrappers and other security tools. Formerly called vmailer, and sponsored by IBM, this is also a mail transport agent written from the ground up with security in mind. You can find more information about postfix at

8.7 Denial of Service Attacks

A "Denial of Service" (DoS) attack is one where the attacker tries to make some resource too busy to answer legitimate requests, or to deny legitimate users access to your machine.

Denial of service attacks have increased greatly in recent years. Some of the more popular and recent ones are listed below. Note that new ones show up all the time, so this is just a few examples. Read the Linux security lists and the bugtraq list and archives for more current information.

You can find code for most exploits, and a more in-depth description of how they work, at using their search engine.

8.8 NFS (Network File System) Security.

NFS is a very widely-used file sharing protocol. It allows servers running nfsd and mountd to "export" entire filesystems to other machines using NFS filesystem support built in to their kernels (or some other client support if they are not Linux machines). mountd keeps track of mounted filesystems in /etc/mtab, and can display them with showmount.

Many sites use NFS to serve home directories to users, so that no matter what machine in the cluster they login to, they will have all their home files.

There is some small amount of security allowed in exporting filesystems. You can make your nfsd map the remote root user (uid=0) to the nobody user, denying them total access to the files exported. However, since individual users have access to their own (or at least the same uid) files, the remote root user can login or su to their account and have total access to their files. This is only a small hindrance to an attacker that has access to mount your remote filesystems.

If you must use NFS, make sure you export to only those machines that you really need to. Never export your entire root directory; export only directories you need to export.

See the NFS HOWTO for more information on NFS, available at

8.9 NIS (Network Information Service) (formerly YP).

Network Information service (formerly YP) is a means of distributing information to a group of machines. The NIS master holds the information tables and converts them into NIS map files. These maps are then served over the network, allowing NIS client machines to get login, password, home directory and shell information (all the information in a standard /etc/passwd file). This allows users to change their password once and have it take effect on all the machines in the NIS domain.

NIS is not at all secure. It was never meant to be. It was meant to be handy and useful. Anyone that can guess the name of your NIS domain (anywhere on the net) can get a copy of your passwd file, and use "crack" and "John the Ripper" against your users' passwords. Also, it is possible to spoof NIS and do all sorts of nasty tricks. If you must use NIS, make sure you are aware of the dangers.

There is a much more secure replacement for NIS, called NIS+. Check out the NIS HOWTO for more information:

8.10 Firewalls

Firewalls are a means of controlling what information is allowed into and out of your local network. Typically the firewall host is connected to the Internet and your local LAN, and the only access from your LAN to the Internet is through the firewall. This way the firewall can control what passes back and forth from the Internet and your LAN.

There are a number of types of firewalls and methods of setting them up. Linux machines make pretty good firewalls. Firewall code can be built right into 2.0 and higher kernels. The user-space tools ipfwadm for 2.0 kernels and ipchains for 2.2 kernels, allows you to change, on the fly, the types of network traffic you allow. You can also log particular types of network traffic.

Firewalls are a very useful and important technique in securing your network. However, never think that because you have a firewall, you don't need to secure the machines behind it. This is a fatal mistake. Check out the very good Firewall-HOWTO at your latest metalab archive for more information on firewalls and Linux.

More information can also be found in the IP-Masquerade mini-howto:

More information on ipfwadm (the tool that lets you change settings on your firewall, can be found at it's home page:

If you have no experience with firewalls, and plan to set up one for more than just a simple security policy, the Firewalls book by O'Reilly and Associates or other online firewall document is mandatory reading. Check out for more information. The National Institute of Standards and Technology have put together an excellent document on firewalls. Although dated 1995, it is still quite good. You can find it at Also of interest:

8.11 IP Chains - Linux Kernel 2.2.x Firewalling

Linux IP Firewalling Chains is an update to the 2.0 Linux firewalling code for the 2.2 kernel. It has many more features than previous implementations, including:

If you are currently using ipfwadm on your 2.0 kernel, there are scripts available to convert the ipfwadm command format to the format ipchains uses.

Be sure to read the IP Chains HOWTO for further information. It is available at

8.12 VPNs - Virtual Private Networks

VPN's are a way to establish a "virtual" network on top of some already-existing network. This virtual network often is encrypted and passes traffic only to and from some known entities that have joined the network. VPNs are often used to connect someone working at home over the public Internet to an internal company network.

If you are running a Linux masquerading firewall and need to pass MS PPTP (Microsoft's VPN point-to-point product) packets, there is a Linux kernel patch out to do just that. See: ip-masq-vpn.

There are several Linux VPN solutions available:

See also the section on IPSEC for pointers and more information.

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