DHCP Definition

The DHCP (Dynamic Host Configuration Protocol) is an extension of the Bootstrap Protocol (BOOTP) developed in 1985 to connect devices. Such as terminals and workstations without a hard disk to a Boot server. From which they receive their operating system.

DHCP was developed as a solution for large networks and laptops and therefore complements BOOTP. Among other things, for its ability to automatically assign reusable network addresses. And also for the existence of additional configuration possibilities.

After some first definitions of the protocol in 1993 in RFC 1531 and 1541. Its definitive specification came in 1997 with RFC 2131. The IANA (Internet Assigned Numbers Authority) provides the protocol of UDP ports 67 and 68 (for IPv6, ports 546 and 547). Also, reserved for the Bootstrap protocol.

The address assignment with DHCP is based on a client-server model. The terminal that wants to connect requests the IP configuration from a DHCP server, which, in turn, uses a database containing the assignable network parameters.

This server, a component of any modern ADSL router, can assign the following parameters to the client with the help of its database information:

  • Unique IP address
  • Subnet mask
  • Standard gateway
  • DNS servers
  • Proxy configuration via WPAD (Web Proxy Auto-Discovery Protocol).

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Is DHCP safe?

The Dynamic Host Configuration Protocol has a weak point and is its ability to be easily manipulated. As the client makes a call at discretion to all DHCP servers that could respond to his request. It would be relatively easy for an attacker to enter the network and impersonate one of them if he had access to it.

  • This so-called DHCP server “Rogue” (corrupt) tries to get ahead with its response to the legitimate server. And if it succeeds, it sends manipulated or unusable parameters.
  • If you do not carry a gateway, assign a subnet to each client or respond to all requests with the same IP address. This attacker could initiate a denial of service or Denial of Service attack on the network.
  • More dramatic, but feasible, would be the attempt to sneak into a router using fake data on the gateway and the DNS, so that it would be in a position to copy or divert data traffic.
  • This man-in-the-middle attack does not have, like the first, the purpose of causing a network crash, but of appropriating sensitive information such as bank details, passwords or postal addresses.
  • Whatever the type of attack, its architects need to have direct access to the network to abuse the DHCP protocol. So, do not forget to implement the necessary security measures that allow you to enjoy the advantages of this communication protocol without fear of suffering consequences of such a threat.

For the head of a local network is absolutely essential for protection against internal and external attempts to attack. And also, constant monitoring of all network processes with tools like Nagios.

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