DHCP (dynamic host configuration protocol)

Defination

The Dynamic Host Configuration Protocol (DHCP) is a set of rules used by communications devices such as a computer, router or network adapter to allow the device to request and obtain an IP address from a server which has a list of addresses available for assignment.

DHCP is a protocol used by networked computers (clients) to obtain IP addresses and other parameters such as the default gateway, subnet mask, and IP addresses of DNS servers from a DHCP server. It The DHCP server ensures that all IP addresses are unique, e.g., no IP address is assigned to a second client while the first client's assignment is valid (its lease has not expired). Thus IP address pool management is done by the server and not by a human network administrator.

DHCP emerged as a standard protocol in October 1993. As of 2006, RFC 2131 provides the latest (dated March 1997) DHCP definition. DHCP functionally became a successor to the older BOOTP protocol, whose leases were given for infinite time and did not support options. Due to the backward-compatibility of DHCP, very few networks continue to use pure BOOTP.

Overview

The Dynamic Host Configuration Protocol (DHCP) automates the assignment of IP addresses, subnet masks, default gateway, and other IP parameters. The assignment occurs when the DHCP-configured machine boots up or regains connectivity to a network. The DHCP client sends out a query requesting a response from a DHCP server on the locally attached network. The query is typically initiated immediately after booting up and before the client initiates any IP based communication with other hosts. The DHCP server then replies to the client with its assigned IP address, subnet mask, DNS server and default gateway information.

The assignment of the IP address generally expires after a predetermined period of time, before which the DHCP client and server renegotiate a new IP address from the server's predefined pool of addresses. Typical intervals range from one hour to several months, and can, if desired, be set to infinite (never expire). The length of time the address is available to the device to which it was assigned is called a lease, and is determined by the server.

Configuring firewall rules to accommodate access from machines who receive their IP addresses via DHCP is therefore more difficult because the remote IP address will vary from time to time. Administrators must usually allow access to the entire remote DHCP subnet for a particular TCP/UDP port.

Most home routers and firewalls are configured in the factory to be DHCP servers for a home network. An alternative to a home router is to use a computer as a DHCP server. ISPs generally use DHCP to assign clients individual IP addresses.

DHCP is a broadcast-based protocol. As with other types of broadcast traffic, it does not cross a router unless specifically configured to do so. Users who desire this capability must configure their routers to pass DHCP traffic across UDP ports 67 and 68. Home users, however, will practically never need this functionality.

Extent of DHCP usage

Most cable internet providers use DHCP to allocate IP addresses.

In the UK many broad-band ISP networks use DHCP, but xDSL providers make extensive use of "infinite lease", which amounts to assigning semi-static IPs.

In addition, many routers and other gateway devices provide DHCP support for networks running many computers being assigned private IP addresses.

Office networks also use DHCP, in particular when workers make extensive use of laptops which link directly to the in-house network only occasionally .

Network routers and often multilayer switches employ a DHCP relay agent, which relays DHCP "Discover" broadcasts from a LAN which does not include a DHCP server to a network which does have one. These devices may sometimes be configured to append information about the port from which a DHCP request originates (also known as option 82). One example of such a relay agent is the UDP Helper Address command employed by Cisco routers.

Security

Since DHCP servers provide IP addresses and thus network connectivity to anyone who has physical network access, DHCP simplifies network intrusion. While seasoned attackers will have no trouble finding usable IP addresses and other settings manually, amateur intruders may be grateful for the service.

If DHCP is used on an unprotected wireless LAN, anyone within range has access to the network, including use of internet connectivity and potentially access to data not otherwise protected. On a wired LAN, an attacker will need a physical connection which is more difficult to establish unnoticed.

IP address allocation

Depending on implementation, the DHCP server has three methods of allocating IP-addresses:

• manual allocation, where the DHCP server performs the allocation based on a table with MAC address - IP address pairs manually filled by the server administrator. Only requesting clients with a MAC address listed in this table get the IP address according to the table.
• automatic allocation, where the DHCP server permanently assigns to a requesting client a free IP-address from a range given by the administrator.
• dynamic allocation, the only method which provides dynamic re-use of IP addresses. A network administrator assigns a range of IP addresses to DHCP, and each client computer on the LAN has its TCP/IP software configured to request an IP address from the DHCP server when that client computer's network interface card starts up. The request-and-grant process uses a lease concept with a controllable time period. This eases the network installation procedure on the client computer side considerably.

This decision remains transparent to clients.

Some DHCP server implementations can update the DNS name associated with the client hosts to reflect the new IP address.

DHCP and firewalls

Firewalls usually have to permit DHCP traffic explicitly. Specification of the DHCP client-server protocol describes several cases when packets must have the source address of 0x00000000 or the destination address of 0xffffffff. Anti-spoofing policy rules and tight inclusive firewalls often stop such packets. Multi-homed DHCP servers require special consideration and further complicate configuration.

To allow DHCP, network administrators need to allow several types of packets through the server-side firewall. All DHCP packets travel as UDP datagrams; all client-sent packets have source port 68 and destination port 67; all server-sent packets have source port 67 and destination port 68. For example, a server-side firewall should allow the following types of packets:

• Incoming packets from 0.0.0.0 or dhcp-pool to dhcp-ip
• Incoming packets from any address to 255.255.255.255
• Outgoing packets from dhcp-ip to dhcp-pool or 255.255.255.255

where dhcp-ip represents any address configured on a DHCP server host and dhcp-pool stands for the pool from which a DHCP server assigns addresses to clients.

Example in ipfw firewall

To give an idea of how a configuration would look in production, the following rules for a server-side ipfirewall to allow DHCP traffic through. Dhcpd operates on interface rl0 and assigns addresses from 192.168.0.0/24 :

pass udp from 0.0.0.0,192.168.0.0/24 68 to me 67 in recv rl0
pass udp from any 68 to 255.255.255.255 67 in recv rl0

Example in Cisco IOS Extended ACL

The following entries are valid on a Cisco 3560 switch with enabled DHCP service. The ACL is applied to a routed interface, 10.32.73.129, on input. The subnet is 10.32.73.128/26.

10 permit udp host 0.0.0.0 eq bootpc host 10.32.73.129 eq bootps
20 permit udp 10.32.73.128 0.0.0.63 eq bootpc host 10.32.73.129 eq bootps
30 permit udp any eq bootpc host 255.255.255.255 eq bootps

Technical details

Schema of a typical DHCP session

DHCP uses the same two IANA assigned ports as BOOTP: 67/udp for the server side, and 68/udp for the client side.

DHCP operations fall into four basic phases. These phases are IP lease request, IP lease offer, IP lease selection, and IP lease acknowledgement.

After the client obtained an IP address, the client may start an address resolution query to prevent IP conflicts caused by address poll overlapping of DHCP servers.

DHCP discovery

The client broadcasts on the local physical subnet to find available servers. Network administrators can configure a local router to forward DHCP packets to a DHCP server on a different subnet. This client-implementation creates a UDP packet with the broadcast destination of 255.255.255.255 or subnet broadcast address.

A client can also request its last-known IP address (in the example below, 192.168.1.100). If the client is still in a network where this IP is valid, the server might grant the request. Otherwise, it depends whether the server is set up as authoritative or not. An authoritative server will deny the request, making the client ask for a new IP immediately. A non-authoritative server simply ignores the request, leading to an implementation dependent time out for the client to give up on the request and ask for a new IP.

DHCP offers

When a DHCP server receives an IP lease request from a client, it extends an IP lease offer. This is done by reserving an IP address for the client and sending a DHCPOFFER message across the network to the client. This message contains the client's MAC address, followed by the IP address that the server is offering, the subnet mask, the lease duration, and the IP address of the DHCP server making the offer.

The server determines the configuration, based on the client's hardware address as specified in the CHADDR field. Here the server, 192.168.1.1, specifies the IP address in the YIADDR field.

DHCP requests

When the client PC receives an IP lease offer, it must tell all the other DHCP servers that it has accepted an offer. To do this, the client broadcasts a DHCPREQUEST message containing the IP address of the server that made the offer. When the other DHCP servers receive this message, they withdraw any offers that they might have made to the client. They then return the address that they had reserved for the client back to the pool of valid addresses that they can offer to another computer. Any number of DHCP servers can respond to an IP lease request, but the client can only accept one offer per network interface card.

DHCP acknowledgement

When the DHCP server receives the DHCPREQUEST message from the client, it initiates the final phase of the configuration process. This acknowledgement phase involves sending a DHCPACK packet to the client. This packet includes the lease duration and any other configuration information that the client might have requested. At this point, the TCP/IP configuration process is complete.

The server acknowledges the request and sends the acknowledgement to the client. The system as a whole expects the client to configure its network interface with the supplied options.

DHCP information

The client sends a request to the DHCP server: either to request more information than the server sent with the original DHCPACK; or to repeat data for a particular application - for example, browsers use DHCP Inform to obtain web proxy settings via WPAD. Such queries do not cause the DHCP server to refresh the IP expiry time in its database.

DHCP releasing

The client sends a request to the DHCP server to release the DHCP and the client unconfigures its IP address. As clients usually do not know when users may unplug them from the network, the protocol does not define the sending of DHCP Release as mandatory

Client configuration parameters

A DHCP server can provide optional configuration parameters to the client. RFC 2132 defines the available DHCP options, which are summarized here.

(From Wikipedia)