Dynamic Host Configuration Protocol
The Dynamic Host Configuration Protocol is a network management protocol used on Internet Protocol networks whereby a DHCP server dynamically assigns an IP address and other network configuration parameters to each device on a network so they can communicate with other IP networks. A DHCP server enables computers to request IP addresses and networking parameters automatically from the Internet service provider, reducing the need for a network administrator or a user to manually assign IP addresses to all network devices. In the absence of a DHCP server, a computer or other device on the network needs to be manually assigned an IP address, or to assign itself an APIPA address, the latter of which will not enable it to communicate outside its local subnet.
DHCP can be implemented on networks ranging in size from home networks to large campus networks and regional Internet service provider networks. A router or a residential gateway can be enabled to act as a DHCP server. Most residential network routers receive a globally unique IP address within the ISP network. Within a local network, a DHCP server assigns a local IP address to each device connected to the network.
History
In 1984, the Reverse Address Resolution Protocol, defined in RFC 903, was introduced to allow simple devices such as diskless workstations to dynamically obtain a suitable IP address. However, because it acted at the data link layer it made implementation difficult on many server platforms, and also required that a server be present on each individual network link. RARP was superseded by the Bootstrap Protocol defined in RFC 951 in September 1985. This introduced the concept of a relay agent, which allowed the forwarding of BOOTP packets across networks, allowing one central BOOTP server to serve hosts on many IP subnets.DHCP is based on BOOTP but can dynamically allocate IP addresses from a pool and reclaim them when they are no longer in use. It can also be used to deliver a wide range of extra configuration parameters to IP clients, including platform-specific parameters. DHCP was first defined in RFC 1531 in October 1993; but due to errors in the editorial process, it was almost immediately reissued as RFC 1541.
Four years later the DHCPINFORM message type and other small changes were added by RFC 2131; which remains the standard for IPv4 networks.
DHCPv6 was initially described by RFC 3315 in 2003, but this has been updated by many subsequent RFCs. RFC 3633 added a DHCPv6 mechanism for prefix delegation, and stateless address autoconfiguration was added by RFC 3736.
Overview
defines how devices communicate within and across local networks on the Internet. A DHCP server can manage IP settings for devices on its local network, e.g., by assigning IP addresses to those devices automatically and dynamically.DHCP operates based on the client–server model. When a computer or other device connects to a network, the DHCP client software sends a DHCP broadcast query requesting the necessary information. Any DHCP server on the network may service the request. The DHCP server manages a pool of IP addresses and information about client configuration parameters such as default gateway, domain name, the name servers, and time servers. On receiving a DHCP request, the DHCP server may respond with specific information for each client, as previously configured by an administrator, or with a specific address and any other information valid for the entire network and for the time period for which the allocation is valid. A DHCP client typically queries for this information immediately after booting, and periodically thereafter before the expiration of the information. When a DHCP client refreshes an assignment, it initially requests the same parameter values, but the DHCP server may assign a new address based on the assignment policies set by administrators.
On large networks that consist of multiple links, a single DHCP server may service the entire network when aided by DHCP relay agents located on the interconnecting routers. Such agents relay messages between DHCP clients and DHCP servers located on different subnets.
Depending on implementation, the DHCP server may have three methods of allocating IP addresses:
;Dynamic allocation: A network administrator reserves a range of IP addresses for DHCP, and each DHCP client on the LAN is configured to request an IP address from the DHCP server during network initialization. The request-and-grant process uses a lease concept with a controllable time period, allowing the DHCP server to reclaim and then reallocate IP addresses that are not renewed.
;Automatic allocation: The DHCP server permanently assigns an IP address to a requesting client from the range defined by the administrator. This is like dynamic allocation, but the DHCP server keeps a table of past IP address assignments, so that it can preferentially assign to a client the same IP address that the client previously had.
;Manual allocation: Also commonly called static allocation and reservations.The DHCP server issues a private IP address dependent upon each client's client id, based on a predefined mapping by the administrator. This feature is variously called static DHCP assignment by DD-WRT, fixed-address by the dhcpd documentation, address reservation by Netgear, DHCP reservation or static DHCP by Cisco and Linksys, and IP address reservation or MAC/IP address binding by various other router manufacturers. If no match for the client's client ID or MAC address is found, the server may or may not fall back to either Dynamic or Automatic allocation.
DHCP is used for Internet Protocol version 4 and IPv6. While both versions serve the same purpose, the details of the protocol for IPv4 and IPv6 differ sufficiently that they may be considered separate protocols. For the IPv6 operation, devices may alternatively use stateless address autoconfiguration. IPv6 hosts may also use link-local addressing to achieve operations restricted to the local network link.
Operation
The DHCP employs a connectionless service model, using the User Datagram Protocol. It is implemented with two UDP port numbers for its operations which are the same as for the bootstrap protocol. UDP port number 67 is the destination port of a server, and UDP port number 68 is used by the client.DHCP operations fall into four phases: server discovery, IP lease offer, IP lease request, and IP lease acknowledgement. These stages are often abbreviated as DORA for discovery, offer, request, and acknowledgement.
The DHCP operation begins with clients broadcasting a request. If the client and server are on different subnets, a [|DHCP Helper or DHCP Relay Agent] may be used. Clients requesting renewal of an existing lease may communicate directly via UDP unicast, since the client already has an established IP address at that point. Additionally, there is a BROADCAST flag the client can use to indicate in which way it can receive the DHCPOFFER: 0x8000 for broadcast, 0x0000 for unicast. Usually, the DHCPOFFER is sent through unicast. For those hosts which cannot accept unicast packets before IP addresses are configured, this flag can be used to work around this issue.
Discovery
The DHCP client broadcasts a DHCPDISCOVER message on the network subnet using the destination address 255.255.255.255 or the specific subnet broadcast address. A DHCP client may also request its last known IP address. If the client remains connected to the same network, the server may grant the request. Otherwise, it depends whether the server is set up as authoritative or not. An authoritative server denies the request, causing the client to issue a new request. A non-authoritative server simply ignores the request, leading to an implementation-dependent timeout for the client to expire the request and ask for a new IP address.For example, if HTYPE is set to 1, to specify that the medium used is Ethernet, HLEN is set to 6 because an Ethernet address is 6 octets long. The CHADDR is set to the MAC address used by the client. Some options are set as well.
Offer
When a DHCP server receives a DHCPDISCOVER message from a client, which is an IP address lease request, the DHCP server reserves an IP address for the client and makes a lease offer by sending a DHCPOFFER message to the client. This message contains the client's client id, 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 DHCP server may also take notice of the hardware-level MAC address in the underlying transport layer: according to current RFCs the transport layer MAC address may be used if no client ID is provided in the DHCP packet.The DHCP 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 client's IP address in the YIADDR field.
Request
In response to the DHCP offer, the client replies with a DHCPREQUEST message, broadcast to the server, requesting the offered address. A client can receive DHCP offers from multiple servers, but it will accept only one DHCP offer. Based on required server identification option in the request and broadcast messaging, servers are informed whose offer the client has accepted. When other DHCP servers receive this message, they withdraw any offers that they have made to the client and return the offered IP address to the pool of available addresses.Acknowledgement
When the DHCP server receives the DHCPREQUEST message from the client, the configuration process enters its final phase. The 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 IP configuration process is completed.The protocol expects the DHCP client to configure its network interface with the negotiated parameters.
After the client obtains an IP address, it should probe the newly received address to prevent address conflicts caused by overlapping address pools of DHCP servers.
Information
A DHCP client may request more information than the server sent with the original DHCPOFFER. The client may also request repeat data for a particular application. For example, browsers use DHCP Inform to obtain web proxy settings via Web Proxy Auto-Discovery Protocol|.Releasing
The client sends a request to the DHCP server to release the DHCP information and the client deactivates its IP address. As client devices usually do not know when users may unplug them from the network, the protocol does not mandate the sending of DHCP Release.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 - DHCP and BOOTP PARAMETERS.A DHCP client can select, manipulate and overwrite parameters provided by a DHCP server. In Unix-like systems this client-level refinement typically takes place according to the values in the configuration file /etc/dhclient.conf.
Options
Options are octet strings of varying length. The first octet is the option code, the second octet is the number of following octets and the remaining octets are code dependent.For example, the DHCP message-type option for an offer would appear as 0x35, 0x01, 0x02, where 0x35 is code 53 for "DHCP message type", 0x01 means one octet follows and 0x02 is the value of "offer".
Documented in RFC 2132
The following tables list the available DHCP options, as listed in RFC 2132 and IANA registry.| Code | Name | Length | Notes |
| 0 | Pad | 0 octets | Can be used to pad other options so that they are aligned to the word boundary; is not followed by length byte |
| 1 | Subnet mask | 4 octets | Must be sent before the router option if both are included |
| 2 | Time offset | 4 octets | |
| 3 | Router | Multiples of 4 octets | Available routers, should be listed in order of preference |
| 4 | Time server | Multiples of 4 octets | Available time servers to synchronise with, should be listed in order of preference |
| 5 | Name server | Multiples of 4 octets | Available IEN 116 name servers, should be listed in order of preference |
| 6 | Domain name server | Multiples of 4 octets | Available DNS servers, should be listed in order of preference |
| 7 | Log server | Multiples of 4 octets | Available log servers, should be listed in order of preference. |
| 8 | Cookie server | Multiples of 4 octets | Cookie in this case means "fortune cookie" or "quote of the day", a pithy or humorous anecdote often sent as part of a logon process on large computers; it has nothing to do with cookies sent by websites. |
| 9 | LPR Server | Multiples of 4 octets | |
| 10 | Impress server | Multiples of 4 octets | |
| 11 | Resource location server | Multiples of 4 octets | |
| 12 | Host name | Minimum of 1 octet | |
| 13 | Boot file size | 2 octets | Length of the boot image in 4KiB blocks |
| 14 | Merit dump file | Minimum of 1 octet | Path where crash dumps should be stored |
| 15 | Domain name | Minimum of 1 octet | |
| 16 | Swap server | 4 octets | |
| 17 | Root path | Minimum of 1 octet | |
| 18 | Extensions path | Minimum of 1 octet | |
| 255 | End | 0 octets | Used to mark the end of the vendor option field |
| Code | Name | Length | Notes |
| 19 | IP forwarding enable/disable | 1 octet | |
| 20 | Non-local source routing enable/disable | 1 octet | |
| 21 | Policy filter | Multiples of 8 octets | |
| 22 | Maximum datagram reassembly size | 2 octets | |
| 23 | Default IP time-to-live | 1 octet | |
| 24 | Path MTU aging timeout | 4 octets | |
| 25 | Path MTU plateau table | Multiples of 2 octets |
| Code | Name | Length | Notes |
| 26 | Interface MTU | 2 octets | |
| 27 | All subnets are local | 1 octet | |
| 28 | Broadcast address | 4 octets | |
| 29 | Perform mask discovery | 1 octet | |
| 30 | Mask supplier | 1 octet | |
| 31 | Perform router discovery | 1 octet | |
| 32 | Router solicitation address | 4 octets | |
| 33 | Static route | Multiples of 8 octets | A list of destination/router pairs |
| Code | Name | Length | Notes |
| 34 | Trailer encapsulation option | 1 octet | |
| 35 | ARP cache timeout | 4 octets | |
| 36 | Ethernet encapsulation | 1 octet |
| Code | Name | Length | Notes |
| 37 | TCP default TTL | 1 octet | |
| 38 | TCP keepalive interval | 4 octets | |
| 39 | TCP keepalive garbage | 1 octet |
| Code | Name | Length | Notes |
| 40 | Network information service domain | Minimum of 1 octet | |
| 41 | Network information servers | Multiples of 4 octets | |
| 42 | Network Time Protocol servers | Multiples of 4 octets | |
| 43 | Vendor-specific information | Minimum of 1 octets | |
| 44 | NetBIOS over TCP/IP name server | Multiples of 4 octets | |
| 45 | NetBIOS over TCP/IP datagram Distribution Server | Multiples of 4 octets | |
| 46 | NetBIOS over TCP/IP node type | 1 octet | |
| 47 | NetBIOS over TCP/IP scope | Minimum of 1 octet | |
| 48 | X Window System font server | Multiples of 4 octets | |
| 49 | X Window System display manager | Multiples of 4 octets | |
| 64 | Network Information Service+ domain | Minimum of 1 octet | |
| 65 | Network Information Service+ servers | Multiples of 4 octets | |
| 68 | Mobile IP home agent | Multiples of 4 octets | |
| 69 | Simple Mail Transfer Protocol server | Multiples of 4 octets | |
| 70 | Post Office Protocol server | Multiples of 4 octets | |
| 71 | Network News Transfer Protocol server | Multiples of 4 octets | |
| 72 | Default World Wide Web server | Multiples of 4 octets | |
| 73 | Default Finger protocol server | Multiples of 4 octets | |
| 74 | Default Internet Relay Chat server | Multiples of 4 octets | |
| 75 | StreetTalk server | Multiples of 4 octets | |
| 76 | StreetTalk Directory Assistance server | Multiples of 4 octets |
| Code | Name | Length | Notes |
| 50 | Requested IP address | 4 octets | |
| 51 | IP address lease time | 4 octets | |
| 52 | Option overload | 1 octet | |
| 53 | DHCP message type | 1 octet | |
| 54 | Server identifier | 4 octets | |
| 55 | Parameter request list | Minimum of 1 octet | |
| 56 | Message | Minimum of 1 octet | |
| 57 | Maximum DHCP message size | 2 octets | |
| 58 | Renewal time value | 4 octets | |
| 59 | Rebinding time value | 4 octets | |
| 60 | Vendor class identifier | Minimum of 1 octet | |
| 61 | Client-identifier | Minimum of 2 octets | |
| 66 | TFTP server name | Minimum of 1 octet | |
| 67 | Bootfile name | Minimum of 1 octet |
| Code | Name | Length | Notes |
| 1 | DHCPDISCOVER | 1 octet | |
| 2 | DHCPOFFER | 1 octet | |
| 3 | DHCPREQUEST | 1 octet | |
| 4 | DHCPDECLINE | 1 octet | |
| 5 | DHCPACK | 1 octet | |
| 6 | DHCPNAK | 1 octet | |
| 7 | DHCPRELEASE | 1 octet | |
| 8 | DHCPINFORM | 1 octet |
Client vendor identification
An option exists 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 utilize to communicate to the server 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 .This method allows a DHCP server to differentiate between the two kinds of client machines and process the requests from the two types of modems appropriately. Some types of set-top boxes also set the VCI to inform the DHCP server about the hardware type and functionality of the device. The value this option is set to gives the DHCP server a hint about any required extra information that this client needs in a DHCP response.
Documented elsewhere
| Code | Name | Length | RFC |
| 82 | [|Relay agent information] | Minimum of 2 octets | RFC 3046 |
| 85 | Novell Directory Service servers | Minimum of 4 octets, multiple of 4 octets | RFC 2241 |
| 86 | NDS tree name | Variable | RFC 2241 |
| 87 | NDS context | Variable | RFC 2241 |
| 100 | Time zone, POSIX style | Variable | RFC 4833 |
| 101 | Time zone, tz database style | Variable | RFC 4833 |
| 119 | Domain search | Variable | RFC 3397 |
| 121 | Classless static route | Variable | RFC 3442 |
Relay agent information sub-options
The relay agent information option specifies container for attaching sub-options to DHCP requests transmitted between a DHCP relay and a DHCP server.| Code | Name | Length | RFC |
| 4 | Data-Over-Cable Service Interface Specifications device class | 4 octets | RFC 3256 |
Relaying
In small networks, where only one IP subnet is being managed, DHCP clients communicate directly with DHCP servers. However, DHCP servers can also provide IP addresses for multiple subnets. In this case, a DHCP client that has not yet acquired an IP address cannot communicate directly with the DHCP server using IP routing, because it does not have a routable IP address, does not know the link layer address of a router and does not know the IP address of the DHCP server.In order to allow DHCP clients on subnets not directly served by DHCP servers to communicate with DHCP servers, DHCP relay agents can be installed on these subnets. The DHCP client broadcasts on the local link; the relay agent receives the broadcast and transmits it to one or more DHCP servers using unicast. The relay agent stores its own IP address in field GIADDR field of the DHCP packet. The DHCP server uses the GIADDR-value to determine the subnet on which the relay agent received the broadcast, and allocates an IP address on that subnet. When the DHCP server replies to the client, it sends the reply to the GIADDR-address, again using unicast. The relay agent then retransmits the response on the local network.
In this situation, the communication between the relay agent and the DHCP server typically uses both a source and destination UDP port of 67.
Reliability
The DHCP ensures reliability in several ways: periodic renewal, rebinding, and failover. DHCP clients are allocated leases that last for some period of time. Clients begin to attempt to renew their leases once half the lease interval has expired. They do this by sending a unicast DHCPREQUEST message to the DHCP server that granted the original lease. If that server is down or unreachable, it will fail to respond to the DHCPREQUEST. However, in that case the client repeats the DHCPREQUEST from time to time, so if the DHCP server comes back up or becomes reachable again, the DHCP client will succeed in contacting it and renew the lease.If the DHCP server is unreachable for an extended period of time, the DHCP client will attempt to rebind, by broadcasting its DHCPREQUEST rather than unicasting it. Because it is broadcast, the DHCPREQUEST message will reach all available DHCP servers. If some other DHCP server is able to renew the lease, it will do so at this time.
In order for rebinding to work, when the client successfully contacts a backup DHCP server, that server must have accurate information about the client's binding. Maintaining accurate binding information between two servers is a complicated problem; if both servers are able to update the same lease database, there must be a mechanism to avoid conflicts between updates on the independent servers. A proposal for implementing fault-tolerant DHCP servers was submitted to the Internet Engineering Task Force, but never formalized.
If rebinding fails, the lease will eventually expire. When the lease expires, the client must stop using the IP address granted to it in its lease. At that time it will restart the DHCP process from the beginning by broadcasting a
DHCPDISCOVER message. Since its lease has expired, it will accept any IP address offered to it. Once it has a new IP address it will once again be able to use the network. However, since its IP address has changed, any ongoing connections will be broken.Modern application
As of 2018, DHCP remains widely-used to automatically assign IP addresses. Newer iterations for assigning IP addresses include DHCPv6 and SLAAC.Security
The base DHCP does not include any mechanism for authentication. Because of this, it is vulnerable to a variety of attacks. These attacks fall into three main categories:- Unauthorized DHCP servers providing false information to clients.
- Unauthorized clients gaining access to resources.
- Resource exhaustion attacks from malicious DHCP clients.
Because the DHCP server has no secure mechanism for authenticating the client, clients can gain unauthorized access to IP addresses by presenting credentials, such as client identifiers, that belong to other DHCP clients. This also allows DHCP clients to exhaust the DHCP server's store of IP addresses—by presenting new credentials each time it asks for an address, the client can consume all the available IP addresses on a particular network link, preventing other DHCP clients from getting service.
DHCP does provide some mechanisms for mitigating these problems. The [|Relay Agent Information Option] protocol extension allows network operators to attach tags to DHCP messages as these messages arrive on the network operator's trusted network. This tag is then used as an authorization token to control the client's access to network resources. Because the client has no access to the network upstream of the relay agent, the lack of authentication does not prevent the DHCP server operator from relying on the authorization token.
Another extension, Authentication for DHCP Messages, provides a mechanism for authenticating DHCP messages. As of 2002,
there were numerous security vulnerabilities identified against the security measures proposed by RFC 3118. This fact, combined with the introduction of 802.1x, slowed the deployment and take-rate of authenticated DHCP, and it has never been widely deployed.A 2010 book notes that:
here have been very few implementations of DHCP Authentication. The challenges of key management and processing delays due to hash computation have been deemed too heavy a price to pay for the perceived benefits.
Architectural proposals from 2008 involve authenticating DHCP requests using 802.1x or PANA. An IETF proposal was made for including EAP in DHCP itself, the so-called EAPoDHCP; this does not appear to have progressed beyond IETF draft level, the last of which dates to 2010.
IETF standards documents
- RFC 2131, Dynamic Host Configuration Protocol
- RFC 2132, DHCP Options and BOOTP Vendor Extensions
- RFC 3046, DHCP Relay Agent Information Option
- RFC 3397, Dynamic Host Configuration Protocol Domain Search Option
- RFC 3942, Reclassifying Dynamic Host Configuration Protocol Version Four Options
- RFC 4242, Information Refresh Time Option for Dynamic Host Configuration Protocol for IPv6
- RFC 4361, Node-specific Client Identifiers for Dynamic Host Configuration Protocol Version Four
- RFC 4436, Detecting Network Attachment in IPv4
- RFC 3442, Classless Static Route Option for Dynamic Host Configuration Protocol version 4