Dynamic Host Configuration Protocol
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Dynamic Host Configuration Protocol (DHCP) is a protocol used by networked devices (clients) to obtain various parameters necessary for the clients to operate in an Internet Protocol (IP) network. By using this protocol, system administration workload greatly decreases, and devices can be added to the network with minimal or no manual configurations.
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[edit] Applicability
Dynamic Host Control Protocol is a way to administer network parameter assignment at a single DHCP server, or a group of such servers arranged in a fault-tolerant manner. Accomplished with the Dynamic Host Configuration Protocol (DHCP). Even in a network which has a few machines, Dynamic Host Control Protocol is useful, because a machine can be added by the local network with little effort.
Even for servers whose addresses rarely change, DHCP is recommended for setting their addresses, so if the servers need to be readdressed (RFC2071), the changes need to be made in as few places as possible. For the devices, such as routers and firewalls, that should not use DHCP, it is wise to put TFTP or SSH servers on the same machine that runs DHCP, again to centralize administration.
It is useful for directly assigning addresses to servers and desktop machines, and, through a PPP proxy, for dialup and broadband on-demand hosts, as well as for residential NAT gateways and routers. DHCP is usually not appropriate for infrastructure such as non-edge routers and DNS servers.
[edit] History
DHCP emerged as a standard protocol in October 1993, succeeding the BOOTP protocol. The current DHCP definition can be found in RFC 2131, while a proposed standard for DHCP over IPv6 (DHCPv6) can be found in RFC 3315.
[edit] Basic Protocol Operation
The Dynamic Host Configuration Protocol (DHCP) automates the assignment of IP addresses, subnet masks, default gateway, and other IP parameters. [1]
When a DHCP-configured client (be it a computer or any other network aware device) connects to a network, its DHCP client sends a broadcast query requesting necessary information from a DHCP server. The DHCP server manages a pool of IP addresses and information about client configuration parameters such as the default gateway, the domain name, the DNS servers, other servers such as time servers, and so forth. Upon receipt of a valid request the server will assign the computer an IP address, a lease (the length of time for which the allocation is valid), and other TCP/IP configuration parameters, such as the subnet mask and the default gateway. The query is typically initiated immediately after booting and must be completed before the client can initiate IP-based communication with other hosts.
DHCP provides three modes for allocating IP addresses. The best-known mode is dynamic, in which the client is provided a "lease" on an IP address for a period of time. Depending on the stability of the network, this could range from hours (a wireless network at an airport) to months (for desktops in a wired lab). At any time before the lease expires, the DHCP client can request renewal of the lease on the current IP address. A properly-functioning client will use the renewal mechanism to maintain the same IP address throughout its connection to a single network, otherwise it may risk losing its lease while still connected, thus disrupting network connectivity while it renegotiates with the server for its original or a new IP address.
The two other modes for allocation of IP addresses are automatic (also known as DHCP Reservation), in which the address is permanently assigned to a client, and manual, in which the address is selected by the client (manually by the user or any other means) and the DHCP protocol messages are used to inform the server that the address has been allocated.
The automatic and manual methods are generally used when finer-grained control over IP address is required (typical of tight firewall setups), although typically a firewall will allow access to the range of IP addresses that can be dynamically allocated by the DHCP server.
[edit] Security
Due to its standardization before Internet security became an issue, the basic DHCP protocol does not include any security provisions, potentially exposing it to two types of attacks:[2]
- Unauthorized DHCP Servers: as you can not specify the DHCP you want, an unauthorized server can respond to client requests, sending the client network configuration values that are beneficial to a hijacker. As an example, a hacker can configure the DHCP server to configure clients to a DNS server which has been poisoned.
- Unauthorized DHCP Clients: By masquerading as a legitimate client an unauthorized client can gain access to network configuration and an IP on a network it should otherwise not be allowed to connect to. Also, by flooding the DHCP server with requests for IP addresses it is possible for an attacker to exhaust the pool of available IP addresses, disrupting normal network activity.
To combat these threats RFC 3118 ("Authentication for DHCP Messages") introduced authentication information into DHCP messages allowing clients and servers to reject information from invalid sources. Although support for this protocol is widespread a large number of clients and servers still do not fully support authentication, forcing servers to support clients that do not support this feature. As a result other security measures are usually implemented around the DHCP server (such as IPsec) to ensure that only authenticated clients and servers are granted access to the network.
Wherever possible, DHCP-assigned addresses should be dynamically linked to a secure DNS server, to allow troubleshooting by name rather than by a potentially unknown address. Effective DHCP-DNS linkage requires having a file of either MAC addresses, or local names that will be sent to DNS, that uniquely identifies physical hosts.IP addresses and other parameters such as the default gateway, subnet mask, and IP addresses of DNS servers from a DHCP server. 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 network administrator.
[edit] IP address allocation
Depending on implementation, the DHCP server has three methods of allocating IP-addresses:
- dynamic allocation: 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 during network initialisation. The request-and-grant process uses a lease concept with a controllable time period, allowing the DHCP server to reclaim (and then allocate) IP addresses that are not renewed (dynamic re-use of IP addresses).
- automatic allocation: The DHCP server permanently assigns a free IP address to a requesting client from the range defined by the administrator.
- manual allocation: The DHCP server allocates an IP address based on a table with MAC address - IP address pairs manually filled in by the server administrator. Only requesting clients with a MAC address listed in this table will be allocated an IP address.
[edit] 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
[edit] 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 pass udp from me 67 to 192.168.0.0/24,255.255.255.255 68 out xmit rl0
[edit] 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
[edit] Technical details
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 pool overlapping of DHCP servers.
[edit] DHCP discovery
The client broadcasts on the 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 address.
[edit] 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.
[edit] 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.
[edit] 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.
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[edit] 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.
[edit] 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 mandate the sending of DHCP Release.
[edit] Client configuration parameters
A DHCP server can provide optional configuration parameters to the client. RFC 2132 describes the available DHCP options defined by Internet Assigned Numbers Authority (IANA) - DHCP and BOOTP PARAMETERS.
[edit] Options
DHCP Option 60 is an option for specifying the Vendor Class Identifier.[3] Within the DHCP there is default route. Option60 is the Vendor ID. Based on this option, you can make special decisions on the CPE to treat the STB. The biggest benefit with using DHCP option 60 is that you do not need to define a port to be bridge or router. Bridging is based on the MAC address of the option 60 so a switch could be connected to the SE567 and have both PC's and STB's on a single interface.
Option 60 can be used by DHCP clients to identify the vendor and functionality of a DHCP client. The information is a variable length string of characters or octets which has a meaning specified by the vendor of the DHCP client. One method that a DHCP client can communicate that it is using a certain type of hardware or firmware, is to set a value in its DHCP requests called the Vendor Class Identifier (VCI) (Option 60). This method has a DHCP server differentiate between the two kinds of CMs and process the requests from the two types of modems appropriately. Some types of set-top boxes also set the VCI (Option 60) to inform the DHCP server about the hardware type and functionality of the device. The value that this option is set to gives the DHCP server a hint about any required extra information that this client needs in a DHCP response.
[edit] See also
- Bootstrap Protocol (BOOTP)
- DHCP snooping
- Peg DHCP RFC 2322
- Preboot Execution Environment (PXE)
- Reverse Address Resolution Protocol (RARP)
- Rogue DHCP
- udhcpc - light version for embedded systems.
- Zero Configuration Networking (Zeroconf)
- Web Proxy Autodiscovery Protocol (WPAD)
[edit] References
- ^ Lemon, Ted; Droms, Ralph (2003). The DHCP handbook. Indianapolis: SAMS. ISBN 0-672-32327-3.
- ^ The TCP/IP Guide - Security Issues
- ^ RFC 2132, Section 9.13
[edit] External links
- RFC 2131 - Dynamic Host Configuration Protocol
- RFC 2132 - DHCP Options and BOOTP Vendor Extensions
- DHCP RFC - Dynamic Host Configuration Protocol RFC's (IETF)
- DHCP Server Security - This article looks at the different types of threats faced by DHCP servers and counter-measures for mitigating these threats.
- RFC 4242 - Information Refresh Time Option for Dynamic Host Configuration Protocol for IPv6
- DHCP Sequence Diagram - This sequence diagram covers several scenarios of DHCP operation.
- RFC 3046, Recommended Operation for Switches Running Relay Agent and Option 82 describes how DHCP option 82 works
- RFC 3942 - Reclassifying Dynamic Host Configuration Protocol Version Four (DHCPv4) Options
- RFC 4361 - Node-specific Client Identifiers for Dynamic Host Configuration Protocol Version Four (DHCPv4)
- DHCP Protocol Messages - A good description of the individual DHCP protocol messages.
- ISC DHCP - Internet Services Consortium's open source DHCP implementation.
- DHCP Tutorial
- BusyBox uDHCP client and server for embedded systems.
