Securing Name Servers on UNIX

The Domain Name System (DNS) is essential to the functioning of the Internet. The DNS organizes the Internet into distributed hierarchical domains. This hierarchical domain structure provides ease of administration and scalability. It must be kept secure.

In July of 1996, Eugene Kashpureff was able to hijack the www.internic.net (Internic) web site to www.alternic.net (Alternic). As a result, visitors to the Internic net site were directed to the Alternic web site. This was done without authorization from Internic. In late 1997, Kashpureff was arrested in Canada. He pleaded guilty to computer fraud in March of 1998. This incident serves to demonstrate the importance of DNS and the impact that a security attack on the DNS could have on organizations that provide services on the Internet. In today's era of e-commerce and “webification” of everything, DNS security is imperative.

BIND (Berkeley Internet Name Domain) is an implementation of DNS. I will describe here the vulnerabilities discovered in BIND and measures you can take to protect against them. I will assume you are familiar with the workings of the Internet and the DNS architecture.

Two major BIND versions are available today: BIND version 4.9 and the BIND 8 series. Most new development of BIND continues on the 8 series. The latest BIND, version 8.2.1, was released on June 21, 1999, and is available from http://www.isc.org/. In the 4.9 BIND series, the latest version of BIND is 4.9.7.

BIND is usually available as part of most UNIX-based operating systems. However, vendors tend to be behind in adapting to the latest BIND version. You can determine the version of BIND provided by the vendor of your operating system by checking the system log files.

The Internet Software Consortium (ISC) sponsors the development of BIND. The latest version of BIND provides many new features and security enhancements. Chief among these are full support for negative caching, the ability to run multiple virtual DNS servers, bug fixes from previous versions and performance enhancements. Table 1 compares some of the primary differences between BIND 8 and BIND 4.9.7. The ISC states the following about the two different streams of BIND:

Table 1

The risk to a BIND server may arise from a need for a functionality that can leave the BIND server susceptible to attacks, mis-configuration of BIND and vulnerabilities in BIND. The following vulnerabilities/issues in BIND could be exploited.

This vulnerability exists in all versions of BIND prior to version 4.9.6 and version 8.1.1. It allowed an intruder to cause a victim name server to query a remote name server controlled by the intruder. The remote name server would return bogus information to the victim name server. The bogus information would be cached on the victim name server for a period specified by the TTL field of the record returned by the remote name server. Very simply, this attack allowed the intruder to point the victim name server's host name IP address mapping to an alternate IP address of the intruder's choice. Eugene Kaspureff used cache poisoning to divert the traffic from www.internic.net to www.alternic.net.

BIND versions prior to BIND 4.9.7 and BIND 8.1.2 are vulnerable to this. This vulnerability allowed an intruder to gain root-level access on the victim name server, or just cause the server to crash. Earlier versions of BIND allowed the inverse-query feature (see Glossary). Actually, according to the DNS specification, the inverse queries are optional. By default, the servers are not configured to respond to fake queries. However, BIND 8 can be configured to provide fake responses to inverse queries. It is those servers configured to respond to fake queries that are vulnerable. The inverse-query feature code is disabled (commented out in source code) in BIND versions 4.9.7 and later.

BIND version 4.9.7 and 8.1.2 perform better bounds checking than the previous versions. The previous BIND version could be exploited to access an invalid memory location causing the server to crash. A crash leaves the name server unable to answer queries, which is a denial of service.