BIND Domain Name System DNS

an article added by: Philip A Clare at 04092007


In: Categories » Computers and technology » Linux » BIND Domain Name System DNS

BIND (Berkley Internet Name Domain) is the most used DNS server on the Internet. Nowadays, every Linux distribution has a BIND package for DNS services.
The problem with BIND and any DNS server is that in order to be able to translate names into IP addresses it has to communicate with a whole lot of other DNS servers, and so, filtering DNS packets is not possible. DNS services are vital for internet connection; so in order to disrupt services to victims, attackers have a great interest in bringing down DNS servers. Although BIND is well known for its security issues, there are many vulnerable BIND servers out there, and so you have to be really careful running BIND. A DNS server survey at http://mydns.bboy.net/survey shows the popularity of BIND, and that there are still quite a lot of vulnerable versions out there. Here is some of my advice on what would provide a more secure BIND:

  • Don't use the BIND package that comes with your distribution of Linux; download the latest from BIND website (http://www.isc.org).
  • Place BIND in a chroot jail. This is the best thing to do to protect against remotely exploitable vulnerabilities in BIND that allow attackers to get a shell on the server running BIND. If you don't chroot your version of BIND and such a vulnerability is discovered, your Linux server and all data on it may be compromised before you have the time to upgrade.
  • Always apply patches and upgrade BIND whenever a bug is discovered or a new version comes out.
  • Secure zone transfers between primary and secondary DNS servers using DNS Transaction Signatures (TSIG).
  • Disable recursion and glue fetching to defend against DNS cache poisoning.

Although BIND is more popular and easier to configure, consider using TinyDNS, as it has proven to be more secure over the years.

Apache Web Server

The most popular web server is Apache, found at http://www.apache.org, which had some security issues in the past, whether they were Apache bugs or add-on modules' vulnerabilities. Here is some of my advice on what would provide a more secure Apache server:

  • Patch your server and try to keep it as up to date as possible.
  • Remove all sample scripts of add-on modules (mod_php, mod_cgi, mod_perl, etc.).
  • If running PHP, CGI, and other script languages, consider using suEXEC, a wrapper program called by Apache to allow it to call scripts from a different user ID than the one it uses for Apache.
  • Don't allow uploads of any scripts into your web server by untrusted parties.
  • Read about all vulnerabilities of any open-source projects that you install, such as PHPBB forums, for example.
  • Don't run the web server as root. Create a user with minimal rights to run the web server.
  • Modify the response token for your web server. It's harder for an attacker to bring it down when he or she doesn't know what web server you are running.

Version Control Systems

Version control systems provide tools for software developers to concurrently work on the same set of files and manage different versions of source code.
In Linux systems, the most popular version control system is CVS (Concurrent Versions System), used by many open-source software projects that allow anonymous access to their CVS repositories via the pserver protocol that runs on TCP port 2401 by default. A CVS server with remote access has the following vulnerabilities:

  • A heap-based buffer overflow that can be triggered by specially crafted entry lines. Exploit code for CVS servers was published on security lists, and allows attackers to execute arbitrary code on the CVS server.
  • There are some vulnerabilities in the implementation of other commands and functions that may be exploited by an authenticated user to cause Denial of Service or execute arbitrary code on the CVS server. Some of these may be exploited by anonymous users.

To protect against these vulnerabilities, consider the following steps:

  • Update CVS to the latest stable release. CVS can be found at http://www.cvshome.org.
  • Run the CVS server in a chroot jail.
  • Configure CVS to use the SSH protocol instead of the pserver protocol (which sends the passwords in plaintext).
  • If you don't allow anonymous access to your CVS server, try filtering port 2401 to allow only trusted hosts to connect to it.
  • Host the CVS server for anonymous read-only access on a stand-alone system.
  • Run the published exploits against your CVS servers.

Another version control system that gained popularity on Linux is subversion. A subversion repository can be remotely accessed via the svn protocol. The svn server runs on the TCP port 3690 by default and contains the following vulnerabilities:

  • A heap-based buffer overflow that can be exploited by unauthenticated attackers to execute arbitrary code on the subversion server.
  • A stack-based buffer overflow that can be triggered by a specially crafted get-date-rev svn command. In this way too an unauthenticated attacker can execute arbitrary code. For this vulnerability, multiple exploits were published on security lists.

To protect your subversion server against those vulnerabilities, consider the following steps:

1. Update your subversion software to the latest stable version from http://subversion.tigris.org/.
2. Configure subversion to use webDAV instead of the svn protocol.
3. If you don't allow anonymous access to your subversion server, try filtering the TCP port 3690 to allow only trusted hosts.
4. Run the published exploits against your subversion server.
5. Host the subversion server for anonymous read-only access on a stand-alone system.

Mail Transport Agents (MTA)

Email is one of the most popular services on the Internet and for a company it is a vital service in almost every department. SMTP (Send Mail Transport Protocol) is one of the oldest protocols on the Internet and it is used by MTAs to send email from the sender to the recipients. SMTP listens on the TCP port 25 by default, and if it is used to receive email from any email address on the Internet, it must not be filtered.
The most popular MTA for Linux is Sendmail, which had a lot of security issues including buffer overruns that could be remotely exploited to compromise the MTA server. Popular alternatives to Sendmail are Postfix, Qmail, Exim, and Courier-MTA.
MTAs' most popular problems are the following:

  • Vulnerabilities such as buffer overruns, heap overflows, etc., which can be used by remote or local attackers to compromise the server running the MTA.
  • Missconfiguration of the MTA allowing everyone to use it for sending mail. This is called open relay. Missconfigured MTAs as open relays immediately fall in the hands of spammers, which may cause big damages to your company by having your email server in one of the many email servers blacklists, plus the fact that all the spam consumes your bandwidth. You can check your mail server to see if it is an open relay at http://www.abuse.net/relay.html,  which runs a set of tests to see if there's any way for a spammer to use your email server to send mail to other people.
  • User-account database disclosure vulnerabilities.

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