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The TCP/IP Guide

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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)
           9  Host Configuration and TCP/IP Host Configuration Protocols (BOOTP and DHCP)
                9  TCP/IP Dynamic Host Configuration Protocol (DHCP)
                     9  DHCP Client/Server Implementation, Features and Issues

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DHCP and BOOTP Interoperability
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DHCP For IP Version 6 (DHCPv6)
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DHCP Security Issues

DHCP was designed in the early 1990s, when the number of organizations on the Internet was relatively small. Furthermore, it was based on BOOTP, which was created in the 1980s when the Internet as we know it today barely even existed. In those days, Internet security wasn't a big issue, because it was mostly a small group of research and educational organizations using TCP/IP on the Internet. As a result, DHCP, like many protocols of that era, doesn't do much to address security concerns.

Actually, this is a bit understated. Not only does DHCP run over IP and UDP, which are inherently insecure, the DHCP protocol itself has in fact no security provisions whatsoever. This is a fairly serious issue in modern networks, because of the sheer power of DHCP: the protocol deals with critical configuration information. There are two different classes of potential security problems related to DHCP:

  • Unauthorized DHCP Servers: If a malicious person plants a “rogue” DHCP server, it is possible that this device could respond to client requests and supply them with spurious configuration information. This could be used to make clients unusable on the network, or worse, set them up for further abuse later on. For example, a hacker could exploit a bogus DHCP server to direct a DHCP client to use a router under the hacker's control, rather than the one the client is supposed to use.

  • Unauthorized DHCP Clients: A client could be set up that masquerades as a legitimate DHCP client and thereby obtain configuration information intended for that client; this could then be used to compromise the network later on. Alternately, a “bad guy” could use software to generate lots of bogus DHCP client requests to use up all the IP addresses in a DHCP server's pool. More simply, this could be used by a thief to steal an IP address from an organization for his own use.
Adding Security to DHCP

These are obviously serious concerns. The normal recommended solutions to these risks generally involve providing security at lower layers. For example, one of the most important techniques for preventing unauthorized servers and clients is careful control over physical access to the network: layer one security. Security techniques implemented at layer two may also be of use, for example, in the case of wireless LANs. Since DHCP runs over UDP and IP, one could use IPSec at layer three to provide authentication.

DHCP Authentication

To try to address some of the more specific security concerns within DHCP itself, in June 2001 the IETF published RFC 3118, Authentication for DHCP Messages. This standard describes an enhancement that replaces the normal DHCP messages with authenticated ones. Clients and servers check the authentication information and reject messages that come from invalid sources. The technology involves the use of a new DHCP option type, the Authentication option, and operating changes to several of the leasing processes to use this option.

Unfortunately, 2001 was pretty late in the DHCP game, and there are millions of DHCP clients and servers around that don't support this new standard. Both client and server must be programmed to use authentication for this method to have value. A DHCP server that supports authentication could use it for clients that support the feature and skip it for those that do not. However, the fact that this option is not universal means that it is not widely deployed, and most networks must rely on more conventional security measures.


Previous Topic/Section
DHCP and BOOTP Interoperability
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DHCP For IP Version 6 (DHCPv6)
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Version 3.0 - Version Date: September 20, 2005

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