View source for Dynamic Host Configuration Protocol

{{short description|Principal protocol used to assign IPv4 addresses on an IPv4 network}}
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{{Internet protocol suite}}
The '''Dynamic Host Configuration Protocol''' ('''DHCP''') is a [[network protocol|network management protocol]] used on [[Internet Protocol]] (IP) networks for automatically assigning [[IP address]]es and other communication parameters to devices connected to the network using a [[client–server]] architecture.<ref name="TechTarget">{{Cite web |last=Gillis |first=Alexander S. |title=What is DHCP (Dynamic Host Configuration Protocol)? |url=https://searchnetworking.techtarget.com/definition/DHCP|access-date=19 February 2021 |work=TechTarget: SearchNetworking}}</ref>

The technology eliminates the need for individually configuring network devices manually, and consists of two network components, a centrally installed network DHCP [[Server (computing)|server]] and client instances of the [[protocol stack]] on each computer or device. When connected to the network, and periodically thereafter, a client [[Request–response|requests]] a set of parameters from the server using DHCP.

DHCP can be implemented on networks ranging in size from [[residential network]]s to large [[campus network]]s and regional ISP networks.<ref>{{cite book |last1=Peterson |first1=Larry L. |last2=Davie |first2=Bruce S. |date=2011 |url=https://books.google.com/books?id=BvaFreun1W8C&pg=PA372 |title=Computer Networks: A Systems Approach |publisher=Elsevier |isbn=978-0-12-385060-7 |edition=5th |access-date=March 21, 2019}}</ref> Many [[Router (computing)|routers]] and [[residential gateway]]s have DHCP server capability. Most residential network routers receive a [[Universally unique identifier|unique]] IP address within the ISP network. Within a local network, a DHCP server assigns a local IP address to each device.

DHCP services exist for networks running [[Internet Protocol version 4]] (IPv4), as well as version 6 ([[IPv6]]). The IPv6 version of the DHCP protocol is commonly called [[DHCPv6]].

==History==
The [[Reverse Address Resolution Protocol]] (RARP) was defined in 1984 for the configuration of simple devices, such as [[diskless workstation]]s, with a suitable IP address.{{Ref RFC|903}} Acting in the [[data link layer]], it made implementation difficult on many server platforms. It required that a server be present on each individual network link. RARP was superseded by the [[Bootstrap Protocol]] (BOOTP) defined in September 1985.{{Ref RFC|951}} This introduced the concept of a relay agent, which allowed the forwarding of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 was first defined in October 1993.{{Ref RFC|1531}}{{Ref RFC|1541}} It 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.<ref name="Network+ certification">Network+ Certification 2006 Published By Microsoft Press.</ref>

Four years later, the DHCPINFORM message type (used for [[Web Proxy Autodiscovery Protocol|WPAD]]) and other small changes were added. This definition, from 1997,{{Ref RFC|2131}} remains the core of the standard for IPv4 networks.

[[DHCPv6]] was initially defined in 2003.{{Ref RFC|3315}}  After updates by many subsequent RFCs, its definition was replaced in 2018,{{Ref RFC|8415}} where [[prefix delegation]] and [[stateless address autoconfiguration]] were now merged.

=={{Anchor|LEASE}}Overview==
[[Internet Protocol]] (IP) 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.<ref>{{Cite web |title=DHCP - Dynamic Host Configuration Protocol |url=https://routeripnet.com/dhcp/ |website=}}</ref>

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 [[Broadcasting (networking)|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 server]]s, and [[time server]]s. 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 (''lease'') is valid. A DHCP client typically queries 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 (computing)|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 a range defined by an 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: This method is also variously called ''static DHCP allocation'', ''fixed address allocation'', ''reservation'', and ''MAC/IP address binding''. An administrator maps a unique identifier (a ''client id'' or [[MAC address]]) for each client to an IP address, which is offered to the requesting client. DHCP servers may be configured to fall back to other methods if this fails.

DHCP services are used for [[Internet Protocol version 4]] (IPv4) and [[IPv6]]. The details of the protocol for IPv4 and IPv6 differ sufficiently that they may be considered separate protocols.<ref>{{cite book |title = The DHCP Handbook |year = 2003 |isbn = 978-0-672-32327-0 |first1= Ralph |last1=Droms |first2= Ted |last2=Lemon |publisher = [[SAMS Publishing]] |page = 436 }}</ref> For the IPv6 operation, devices may alternatively use [[IPv6 stateless address autoconfiguration|stateless address autoconfiguration]]. IPv6 hosts may also use [[link-local addressing]] to achieve operations restricted to the local network link.

==Operation==
[[File:DHCP session.svg|thumb|right|upright=1.2|An illustration of a typical non-renewing DHCP session; each message may be either a broadcast or a [[unicast]], depending on the DHCP client capabilities.{{Ref RFC|2131}}]]

The DHCP employs a [[connectionless]] service model, using the [[User Datagram Protocol]] (UDP). It is implemented with two UDP port numbers for its operations which are the same as for the bootstrap protocol ([[BOOTP]]). The server listens on UDP port number 67, and the client listens on UDP port number 68.

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 in different [[Broadcast domain|Broadcast Domains]], a [[#Relaying|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 (1 bit in 2 byte flags field, where all other bits are reserved and so are set to 0) the client can use to indicate in which way (broadcast or unicast) it can receive the DHCPOFFER: 0x8000 for broadcast, 0x0000 for unicast.{{Ref RFC|2131}} 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 [[Broadcasting (networking)|broadcasts]] a DHCPDISCOVER message on the network subnet using the destination address {{IPaddr|255.255.255.255}} (limited broadcast) or the specific subnet broadcast address (directed broadcast). A DHCP client may also request an IP address in the DHCPDISCOVER, which the server may take into account when selecting an address to offer.

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 (MAC address) is 6 octets long. The CHADDR is set to the MAC address used by the client. Some options are set as well.
{{APHD|start|title=Example Ethernet frame with a DHCPDISCOVER message}}
{{APHD|0|bits1=6|bits2=6|bits3=2|bits4=18|border4=bottom|field1={{MACaddr|00:05:3c:04:8d:59}}|hint1=Source MAC|field2={{MACaddr|ff:ff:ff:ff:ff:ff}}|hint2=Destination MAC (broadcast DHCPDISCOVERs)|field3={{Mono|0x0800}}|hint3=EtherType (indicating IPv4)|field4=&nbsp;|background4=mistyrose}}
{{APHD|4|bits1=0|border1=top|background1=mistyrose|field1=IPv4 packet, containing a UDP PDU with DHCP payload...}}
{{APHD|end}}
{{APHD|start|header=yew|title=IP Header}}
{{APHD|0|bits1=64|field1=IP header start}}
{{APHD|8|bits1=8|bits2=16|bits3=8|field1=TTL|field2=Protocol|value2={{Mono|17}} UDP|field3=Header Checksum}}
{{APHD|end}}
{{APHD|start|header=no|title=UDP Header}}
{{APHD|12|bits1=32|field1=Source Address|value1={{IPaddr|0.0.0.0}}}}
{{APHD|16|bits1=32|field1=Destination Address|value11={{IPaddr|255.255.255.255}}}}
{{APHD|20|bits1=16|bits2=16|field1=Source Port|value1=68|field2=Destination Port|value2=67}}
{{APHD|24|bits1=16|bits2=16|field1=Length|field2=Checksum}}
{{APHD|end}}
{{APHD|start|header=no|title=DHCP Payload: DHCPDISCOVER}}
{{APHD|28|bits1=8|bits2=8|bits3=8|bits4=8|field1=OP|value1={{Mono|0x01}}|hint1=BOOTREQUEST|field2=HTYPE|value2={{Mono|0x01}}|hint2=Ethernet|field3=HLEN|value3={{Mono|0x06}}|hint3=MAC addresses are 6 octets|field4=HOPS|value4={{Mono|0x00}}}}
{{APHD|32|bits1=32|field1=XID|value1={{Mono|0x3903F326}}}}
{{APHD|36|bits1=16|bits2=16|field1=SECS|value1={{Mono|0x0000}}|field2=FLAGS|value2={{Mono|0x0000}}}}
{{APHD|40|bits1=32|field1=CIADDR|value1=Client IP address: {{Mono|0x00000000}}}}
{{APHD|44|bits1=32|field1=YIADDR (Your IP address: {{Mono|0x00000000}}}}
{{APHD|48|bits1=32|field1=SIADDR|value1=Server IP address: {{Mono|0x00000000}}}}
{{APHD|52|bits1=32|field1=GIADDR|value1=Gateway IP address: {{Mono|0x00000000}}|hint1=A zero GIADDR means client and DHCP server are on the same subnet.}}
{{APHD|56|bits1=128|field1=CHADDR|value1=Client Hardware address: {{Mono|0x00053C04<br>0x8D590000<br>0x00000000<br>0x00000000}}}}
{{APHD|72|bits1=1536|field1=192 octets of 0s, or overflow space for additional options; BOOTP legacy.}}
{{APHD|264|bits1=32|field1=[[Magic cookie|Magic Cookie]]|value1={{Mono|0x63825363}}}}
{{APHD|end}}
{{APHD|start|header=no|title=DHCP Options (in [[Type-length-value|TLV]] format)}}
{{APHD|292|bits1=24|background1=linen|field1=First option: {{Mono|0x350101}}: Option 53 (DHCP Message Type) 1 octet (containing DHCPDISCOVER)|bits2=8|background2=linen|border2=right|field2=Second option:}}
{{APHD|324|bits1=32|background1=linen|border1=left|field1={{Mono|0x3204c0a80164}}: Option 50 (Request IP address) 4 octets (containing {{IPaddr|192.168.1.100}})}}
{{APHD|356|bits1=32|background1=linen|field1=Third option: {{Mono|0x370401030f06}}: Option: 55 (Parameter Request List) 4 octets|hint1=0x01: Request Subnet Mask; 0x03: Router; 0x0f: Domain Name|border1=right}}
{{APHD|388|bits1=8|background1=linen|border1=left|field1=PRL cont...|hint1=0x06: Domain Name Server|bits2=1|field2=<small>{{Mono|ff}}</small>|hint2=0xff Option end mark}}
{{APHD|end}}

===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 may contain the client's ''Client ID'' (Option 61, containing a unique value, traditionally a MAC address), 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 (as specified in the CHADDR field). This field must be used to identify the client, if no Client ID is provided in the DHCP packet.{{Ref RFC|2131|rsection=4.2}}

The DHCP server determines the configuration based on the client's hardware address as specified in the CHADDR (client hardware address) field. In the following example the server ({{IPaddr|192.168.1.1}}) specifies the client's IP address in the YIADDR (your IP address) field.
{{APHD|start|title=Example Ethernet frame with a DHCPOFFER message}}
{{APHD|0|bits1=6|bits2=6|bits3=2|bits4=18|border4=bottom|field1={{MACaddr|b4:0c:25:e3:7d:62}}|hint1=Source MAC (DHCP server)|field2={{MACaddr|00:05:3c:04:8d:59}}|hint2=Destination MAC (unicast to the client)|field3={{Mono|0x8000}}|hint3=EtherType (indicating IPv4)|field4=&nbsp;|background4=mistyrose}}
{{APHD|4|bits1=0|border1=top|background1=mistyrose|field1=IPv4 packet, containing a UDP PDU with DHCP payload...}}
{{APHD|end}}
{{APHD|start|header=yew|title=IP Header}}
{{APHD|0|bits1=64|field1=IP header start}}
{{APHD|8|bits1=8|bits2=16|bits3=8|field1=TTL|field2=Protocol ({{Mono|17}} UDP)|field3=Header Checksum}}
{{APHD|end}}
{{APHD|start|header=no|title=UDP Header}}
{{APHD|12|bits1=32|field1=Source Address ({{IPaddr|192.168.1.1}})}}
{{APHD|16|bits1=32|field1=Destination Address ({{IPaddr|192.168.1.100}})}}
{{APHD|20|bits1=16|bits2=16|field1=Source Port (67)|field2=Destination Port (68)}}
{{APHD|24|bits1=16|bits2=16|field1=Length|field2=Checksum}}
{{APHD|end}}
{{APHD|start|header=no|title=DHCP Payload: DHCPOFFER}}
{{APHD|28|bits1=8|bits2=8|bits3=8|bits4=8|field1=OP ({{Mono|0x02}})|hint1=BOOTREPLY|field2=HTYPE ({{Mono|0x01}})|field3=HLEN ({{Mono|0x06}})|field4=HOPS ({{Mono|0x00}})}}
{{APHD|32|bits1=32|field1=XID ({{Mono|0x3903F326}})}}
{{APHD|36|bits1=16|bits2=16|field1=SECS ({{Mono|0x0000}})|field2=FLAGS ({{Mono|0x0000}})}}
{{APHD|40|bits1=32|field1=CIADDR (Client IP address: {{Mono|0x00000000}})}}
{{APHD|44|bits1=32|field1=YIADDR  (Your IP address: {{Mono|0xC0A80164}} or {{IPaddr|192.168.1.100}})}}
{{APHD|48|bits1=32|field1=SIADDR (Server IP address: {{Mono|0xC0A80101}} or {{IPaddr|192.168.1.1}})}}
{{APHD|52|bits1=32|field1=GIADDR (Gateway IP address: {{Mono|0x00000000}})}}
{{APHD|56|bits1=128|field1=CHADDR (Client Hardware address: {{Mono|0x00053C04<br>0x8D590000<br>0x00000000<br>0x00000000}})}}
{{APHD|72|bits1=1536|field1=192 octets of 0s, or overflow space for additional options; BOOTP legacy.}}
{{APHD|264|bits1=32|field1=[[Magic cookie|Magic Cookie]] ({{Mono|0x63825363}})}}
{{APHD|end}}
{{APHD|start|header=no|title=DHCP Options (in [[Type-length-value|TLV]] format)}}
{{APHD|292|bits1=24|background1=linen|field1=First option: {{Mono|0x350102}}: Option 53 (DHCP Message Type) 1 octet (containing DHCPOFFER)|bits2=8|background2=linen|border2=right|field2=Second option:}}
{{APHD|324|bits1=32|background1=linen|border1=left|field1={{Mono|0x0104ffffff00}}: Option 1 (Subnet mask) 4 octets (containing {{IPaddr|255.255.255.0}})}}
{{APHD|356|bits1=32|background1=linen|field1=Third option: {{Mono|0x0304c0A80101}}: Option: 3 (Router) 4 octets (containing {{IPaddr|192.168.1.1}})|border1=right}}
{{APHD|388|bits1=8|bits2=24|background1=linen|border1=left|field1=Router cont...||background2=linen|border2=right|field2=Fourth option: {{Mono|0x330400015080}}: Option 51 (Address time) 4 octets (a 86400 second lease time)}}
{{APHD|420|bits1=16|background1=linen|border1=left|field1=Address time cont...|bits2=16|border2=bottom|field2=Fifth option:|background2=linen}}
{{APHD|452|bits1=96|background1=linen|border1=top-bottom|field1={{Mono|0x060c09070a0f09070a1009070a13}}:{{Break}}Option 6 (Domain Server) 14 octets (containing {{IPaddr|9.7.10.15}},{{IPaddr|9.7.10.16}},{{IPaddr|9.7.10.18}})}}
{{APHD|482|bits1=12|background1=linen|field1=&nbsp;|border1=top|field2=<small>{{Mono|ff}}</small>|hint2=0xff Option end mark}}
{{APHD|end}}

===Request===
In response to the DHCP offer, the client replies with a DHCPREQUEST message, broadcast to the server,{{Efn|name="optional-unicasts"|As an optional client behavior, some broadcasts, such as those carrying DHCP discovery and request messages, may be replaced with unicasts in case the DHCP client already knows the DHCP server's IP address.{{Ref RFC|2131}}{{rp|section=4.4.4}}}} requesting the offered address. A client can receive DHCP offers from multiple servers, but it will accept only one DHCP offer.

The client must send the ''server identification'' option in the DHCPREQUEST message, indicating the server whose offer the client has selected.{{Ref RFC|2131}}{{rp|Section 3.1, Item 3}} When other DHCP servers receive this message, they withdraw any offers that they have made to the client and return their offered IP address to the pool of available addresses.
{{APHD|start|title=Example Ethernet frame with a DHCPREQUEST message}}
{{APHD|0|bits1=6|bits2=6|bits3=2|bits4=18|border4=bottom|field1={{MACaddr|00:05:3c:04:8d:59}}|hint1=Source MAC|field2={{MACaddr|ff:ff:ff:ff:ff:ff}}|hint2=Destination MAC (broadcast DHCPREQUESTs)|field3={{Mono|0x8000}}|hint3=EtherType (indicating IPv4)|field4=&nbsp;|background4=mistyrose}}
{{APHD|4|bits1=0|border1=top|background1=mistyrose|field1=IPv4 packet, containing a UDP PDU with DHCP payload...}}
{{APHD|end}}
{{APHD|start|header=yew|title=IP Header}}
{{APHD|0|bits1=64|field1=IP header start}}
{{APHD|8|bits1=8|bits2=16|bits3=8|field1=TTL|field2=Protocol ({{Mono|17}} UDP)|field3=Header Checksum}}
{{APHD|end}}
{{APHD|start|header=no|title=UDP Header}}
{{APHD|12|bits1=32|field1=Source Address ({{IPaddr|0.0.0.0}})}}
{{APHD|16|bits1=32|field1=Destination Address ({{IPaddr|255.255.255.255}})}}
{{APHD|20|bits1=16|bits2=16|field1=Source Port (68)|field2=Destination Port (67)}}
{{APHD|24|bits1=16|bits2=16|field1=Length|field2=Checksum}}
{{APHD|end}}
{{APHD|start|header=no|title=DHCP Payload: DHCPREQUEST}}
{{APHD|28|bits1=8|bits2=8|bits3=8|bits4=8|field1=OP ({{Mono|0x01}})|hint1=BOOTREQUEST|field2=HTYPE ({{Mono|0x01}})|field3=HLEN ({{Mono|0x06}})|field4=HOPS ({{Mono|0x00}})}}
{{APHD|32|bits1=32|field1=XID ({{Mono|0x3903F326}})}}
{{APHD|36|bits1=16|bits2=16|field1=SECS ({{Mono|0x0000}})|field2=FLAGS ({{Mono|0x0000}})}}
{{APHD|40|bits1=32|field1=CIADDR (Client IP address: {{Mono|0x00000000}})}}
{{APHD|44|bits1=32|field1=YIADDR (Your IP address: {{Mono|0x00000000}})}}
{{APHD|48|bits1=32|field1=SIADDR (Server IP address: {{Mono|0xc0a80101}} or {{IPaddr|192.168.1.1}})}}
{{APHD|52|bits1=32|field1=GIADDR (Gateway IP address: {{Mono|0x00000000}})}}
{{APHD|56|bits1=128|field1=CHADDR (Client Hardware address: {{Mono|0x00053C04<br>0x8D590000<br>0x00000000<br>0x00000000}})}}
{{APHD|72|bits1=1536|field1=192 octets of 0s, or overflow space for additional options; BOOTP legacy.}}
{{APHD|264|bits1=32|field1=[[Magic cookie|Magic Cookie]] ({{Mono|0x63825363}})}}
{{APHD|end}}
{{APHD|start|header=no|title=DHCP Options (in [[Type-length-value|TLV]] format)}}
{{APHD|292|bits1=24|background1=linen|field1=First option: {{Mono|0x350103}}: Option 53 (DHCP Message Type) 1 octet (containing DHCPREQUEST)|bits2=8|background2=linen|border2=right|field2=Second option:}}
{{APHD|324|bits1=32|background1=linen|border1=left|field1={{Mono|0x3204c0a80164}}: Option 50 (Request IP address) 4 octets (containing {{IPaddr|192.168.1.100}})|hint1=Must match YIADDR from the DHCPOFFER packet.}}
{{APHD|356|bits1=32|background1=linen|field1=Third option: {{Mono|0x3604c0a801601}}: Option: 54 (DHCP Server) 4 octets  (containing {{IPaddr|192.168.1.1}})|hint1=Server Identification: tell which offer you accept.|border1=right}}
{{APHD|388|bits1=8|background1=linen|border1=left|field1=DHCP Server cont...|bits2=1|field2=<small>{{Mono|ff}}</small>|hint2=0xff Option end mark}}
{{APHD|end}}

===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.

{{APHD|start|title=Example Ethernet frame with a DHCPACK message}}
{{APHD|0|bits1=6|bits2=6|bits3=2|bits4=18|border4=bottom|field1={{MACaddr|b4:0c:25:e3:7d:62}}|hint1=Source MAC (DHCP server)|field2={{MACaddr|00:05:3c:04:8d:59}}|hint2=Destination MAC (unicast to the client)|field3={{Mono|0x8000}}|hint3=EtherType (indicating IPv4)|field4=&nbsp;|background4=mistyrose}}
{{APHD|4|bits1=0|border1=top|background1=mistyrose|field1=IPv4 packet, containing a UDP PDU with DHCP payload...}}
{{APHD|end}}
{{APHD|start|header=yew|title=IP Header}}
{{APHD|0|bits1=64|field1=IP header start}}
{{APHD|8|bits1=8|bits2=16|bits3=8|field1=TTL|field2=Protocol ({{Mono|17}} UDP)|field3=Header Checksum}}
{{APHD|end}}
{{APHD|start|header=no|title=UDP Header}}
{{APHD|12|bits1=32|field1=Source Address ({{IPaddr|192.168.1.1}})}}
{{APHD|16|bits1=32|field1=Destination Address ({{IPaddr|192.168.1.100}})}}
{{APHD|20|bits1=16|bits2=16|field1=Source Port (67)|field2=Destination Port (68)}}
{{APHD|24|bits1=16|bits2=16|field1=Length|field2=Checksum}}
{{APHD|end}}
{{APHD|start|header=no|title=DHCP Payload: DHCPACK}}
{{APHD|28|bits1=8|bits2=8|bits3=8|bits4=8|field1=OP ({{Mono|0x02}})|hint1=BOOTREPLY|field2=HTYPE ({{Mono|0x01}})|field3=HLEN ({{Mono|0x06}})|field4=HOPS ({{Mono|0x00}})}}
{{APHD|32|bits1=32|field1=XID ({{Mono|0x3903F326}})}}
{{APHD|36|bits1=16|bits2=16|field1=SECS ({{Mono|0x0000}})|field2=FLAGS ({{Mono|0x0000}})}}
{{APHD|40|bits1=32|field1=CIADDR (Client IP address: {{Mono|0x00000000}})}}
{{APHD|44|bits1=32|field1=YIADDR  (Your IP address: {{Mono|0xC0A80164}} or {{IPaddr|192.168.1.100}})}}
{{APHD|48|bits1=32|field1=SIADDR (Server IP address: {{Mono|0xC0A80101}} or {{IPaddr|192.168.1.1}})}}
{{APHD|52|bits1=32|field1=GIADDR (Gateway IP address: {{Mono|0x00000000}})}}
{{APHD|56|bits1=128|field1=CHADDR (Client Hardware address: {{Mono|0x00053C04<br>0x8D590000<br>0x00000000<br>0x00000000}})}}
{{APHD|72|bits1=1536|field1=192 octets of 0s, or overflow space for additional options; BOOTP legacy.}}
{{APHD|264|bits1=32|field1=[[Magic cookie|Magic Cookie]] ({{Mono|0x63825363}})}}
{{APHD|end}}
{{APHD|start|header=no|title=DHCP Options (in [[Type-length-value|TLV]] format)}}
{{APHD|292|bits1=24|background1=linen|field1=First option: {{Mono|0x350105}}: Option 53 (DHCP Message Type) 1 octet (containing DHCPACK)|bits2=8|background2=linen|border2=right|field2=Second option:}}
{{APHD|324|bits1=32|background1=linen|border1=left|field1={{Mono|0x0104ffffff00}}: Option 1 (Subnet mask) 4 octets (containing {{IPaddr|255.255.255.0}})}}
{{APHD|356|bits1=32|background1=linen|field1=Third option: {{Mono|0x0304c0A80101}}: Option: 3 (Router) 4 octets (containing {{IPaddr|192.168.1.1}})|border1=right}}
{{APHD|388|bits1=8|bits2=24|background1=linen|border1=left|field1=Router cont...||background2=linen|border2=right|field2=Fourth option: {{Mono|0x330400015080}}: Option 51 (Address time) 4 octets (a 86400 second lease time)}}
{{APHD|420|bits1=16|background1=linen|border1=left|field1=Address time cont...|bits2=16|border2=bottom|field2=Fifth option:|background2=linen}}
{{APHD|452|bits1=96|background1=linen|border1=top-bottom|field1={{Mono|0x060c09070a0f09070a1009070a13}}:{{Break}}Option 6 (Domain Server) 14 octets (containing {{IPaddr|9.7.10.15}},{{IPaddr|9.7.10.16}},{{IPaddr|9.7.10.18}})}}
{{APHD|482|bits1=12|background1=linen|field1=&nbsp;|border1=top|field2=<small>{{Mono|ff}}</small>|hint2=0xff Option end mark}}
{{APHD|end}}

===Selecting and configuring IP addresses===
When the server is reusing an IP address from its pool, it may first check (using [[Ping (networking utility)|ping]]) to see if it is not taken already.{{Ref RFC|2131}}{{rp|sec. 2.2}} This may happen if a host is configured manually with an IP address that lies within the DHCP scope.

Before claiming an IP address, the client should probe the newly received address (e.g. with [[Address Resolution Protocol|ARP]]), in order to find if there is another host present in the network with the proposed IP address.{{Ref RFC|2131}}{{rp|sec. 2.2}} If there is no reply, this address does not conflict with that of another host, so it is free to be used. If this probe finds another computer using that address, the client should broadcast a DHCPDECLINE to the DHCP server(s).

===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|WPAD]].

===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]] (IANA) -  DHCP and BOOTP PARAMETERS.<ref name=":0">{{cite web|url=https://www.iana.org/assignments/bootp-dhcp-parameters/bootp-dhcp-parameters.xhtml|title=Dynamic Host Configuration Protocol (DHCP) and Bootstrap Protocol (BOOTP) Parameters|publisher=iana.org|access-date=2018-10-16}}</ref>

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. This is called [[Type–length–value]] encoding. 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".

The following tables list the available DHCP options.{{Ref RFC|2132}}<ref name=":0" />

{| class="wikitable sortable"
|+ RFC 1497 (BOOTP Vendor Information Extensions) vendor extensions<ref name=rfc2132/>{{rp|Section 3}}
|-
! Code !! Name !! Length !! Notes
|-
| 0 || Pad || 0 [[Octet (computing)|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 || Client's subnet mask as per [https://datatracker.ietf.org/doc/html/rfc950 RFC 950]. If both the subnet mask and the router option (option 3) are included, the subnet mask option must be first.
|-
| 2 || Time offset || 4 octets ||Offset of the client's subnet in seconds from Coordinated Universal Time (UTC).  The offset is expressed as a two's complement 32-bit integer.  A positive offset indicates a location east of the zero meridian and a negative offset indicates a location west of the zero meridian.
|-
| 3 || Router || Multiples of 4 octets || Available routers, should be listed in order of preference
|-
| 4 || Time server || Multiples of 4 octets || Available [[Time Protocol]] 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 [[HTTP cookie|cookies sent by websites]].
|-
| 9 || LPR Server || Multiples of 4 octets ||A list of [[Line Printer Daemon protocol]] servers available to the client, should be listed in order of preference
|-
| 10 || Impress server || Multiples of 4 octets ||A list of Imagen Impress servers available to the client, should be listed in order of preference
|-
| 11 || Resource location server || Multiples of 4 octets ||A list of [[Resource Location Protocol]] servers available to the client, should be listed in order of preference
|-
| 12 || Host name || Minimum of 1 octet ||Name of the client. The name may be qualified with the local domain name.
|-
| 13 || Boot file size || 2 octets || Length of the boot image in 512B blocks
|-
| 14 || [[Merit Network|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
|}

{| class="wikitable sortable"
|+  IP layer parameters per host<ref name=rfc2132/>{{rp|Section 4}}
|-
! 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 ||
|}

{| class="wikitable sortable"
|+ IP Layer Parameters per Interface<ref name=rfc2132/>{{rp|Section 5}}
|-
! 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
|}

{| class="wikitable sortable"
|+  Link layer parameters per interface<ref name=rfc2132/>{{rp|Section 6}}
|-
! Code !! Name !! Length !! Notes
|-
| 34 || Trailer encapsulation option || 1 octet ||
|-
| 35 || ARP cache timeout || 4 octets ||
|-
| 36 || Ethernet encapsulation || 1 octet ||
|}

{| class="wikitable sortable"
|+  TCP parameters<ref name=rfc2132/>{{rp|Section 7}}
|-
! Code !! Name !! Length !! Notes
|-
| 37 || TCP default TTL || 1 octet ||
|-
| 38 || TCP keepalive interval || 4 octets ||
|-
| 39 || TCP keepalive garbage || 1 octet ||
|}

{| class="wikitable sortable"
|+  Application and service parameters<ref name=rfc2132/>{{rp|Section 8}}
|-
! 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]] (NTP) 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]] (SMTP) server || Multiples of 4 octets ||
|-
| 70 || [[Post Office Protocol]] (POP3) server || Multiples of 4 octets ||
|-
| 71 || [[Network News Transfer Protocol]] (NNTP) server || Multiples of 4 octets ||
|-
| 72 || Default [[World Wide Web]] (WWW) server || Multiples of 4 octets ||
|-
| 73 || Default [[Finger protocol]] server || Multiples of 4 octets ||
|-
| 74 || Default [[Internet Relay Chat]] (IRC) server || Multiples of 4 octets ||
|-
| 75 || [[StreetTalk]] server || Multiples of 4 octets ||
|-
| 76 || StreetTalk Directory Assistance (STDA) server || Multiples of 4 octets ||
|}

{| class="wikitable sortable"
|+  DHCP extensions<ref name=rfc2132/>{{rp|Section 9}}
|-
! 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 (T1) time value || 4 octets ||
|-
| 59 || Rebinding (T2) 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 ||
|}

===DHCP message types===

This table lists the DHCP message types, documented in RFC 2132, RFC 3203,<ref name="rfc3203">{{cite IETF
| title = DHCP reconfigure extension
| rfc = 3203
| last1 =T'joens
| first1 = Yves
| last2 = De Schrijver
| first2 = Peter
| date = December 2001
| publisher = [[IETF]]
| access-date = November 13, 2020
}}</ref> RFC 4388,<ref name="rfc4388">{{cite IETF
| title = Dynamic Host Configuration Protocol (DHCP) Leasequery
| rfc = 4388
| last1 = Woundy
| first1 = Rich
| last2 = Kinnear
| first2 = Kim
| date = February 2006
| publisher = [[IETF]]
| access-date = November 13, 2020
}}</ref> RFC 6926<ref name="rfc6926">{{cite IETF
| title = DHCPv4 Bulk Leasequery
| rfc = 6926
| last1 = Kinnear
| first1 = Kim
| last2 = Stapp
| first2 = Mark
| last3 = Rao
| first3 = D.T.V Ramakrishna
| last4 = Joshi
| first4 = Bharat
| last5 = Russell
| first5 = Neil
| last6 = Kurapati
| first6 = Pavan
| last7 = Volz
| first7 = Bernie
| date = April 2013
| publisher = [[IETF]]
| access-date = November 13, 2020
}}</ref> and RFC 7724.<ref name="rfc7724">{{cite IETF
| title = Active DHCPv4 Lease Query
| rfc = 7724
| last1 = Kinnear
| first1 = Kim
| last2 = Stapp
| first2 = Mark
| last3 = Volz
| first3 = Bernie
| last4 = Russell
| first4 = Neil
| date = December 2015
| publisher = [[IETF]]
| access-date = November 13, 2020
}}</ref>  These codes are the value in the DHCP extension 53, shown in the
table above.

{| class="wikitable sortable"
|+  DHCP message types
|-
! Code !! Name !! Length !! RFC
|-
| 1 || DHCPDISCOVER || 1 octet || rfc2132<ref name=rfc2132/>{{rp|Section 9.6}}
|-
| 2 || DHCPOFFER || 1 octet || rfc2132<ref name=rfc2132/>{{rp|Section 9.6}}
|-
| 3 || DHCPREQUEST || 1 octet || rfc2132<ref name=rfc2132/>{{rp|Section 9.6}}
|-
| 4 || DHCPDECLINE || 1 octet || rfc2132<ref name=rfc2132/>{{rp|Section 9.6}}
|-
| 5 || DHCPACK || 1 octet || rfc2132<ref name=rfc2132/>{{rp|Section 9.6}}
|-
| 6 || DHCPNAK || 1 octet || rfc2132<ref name=rfc2132/>{{rp|Section 9.6}}
|-
| 7 || DHCPRELEASE || 1 octet || rfc2132<ref name=rfc2132/>{{rp|Section 9.6}}
|-
| 8 || DHCPINFORM || 1 octet || rfc2132<ref name=rfc2132/>{{rp|Section 9.6}}
|-
| 9 || DHCPFORCERENEW ||  1 octet || rfc3203<ref name=rfc3203/>{{rp|Section 4}}
|-
| 10 || DHCPLEASEQUERY ||  1 octet || rfc4388<ref name=rfc4388/>{{rp|Section 6.1}}
|-
| 11 || DHCPLEASEUNASSIGNED ||  1 octet || rfc4388<ref name=rfc4388/>{{rp|Section 6.1}}
|-
| 12 || DHCPLEASEUNKNOWN ||  1 octet || rfc4388<ref name=rfc4388/>{{rp|Section 6.1}}
|-
| 13 || DHCPLEASEACTIVE ||  1 octet || rfc4388<ref name=rfc4388/>{{rp|Section 6.1}}
|-
| 14 || DHCPBULKLEASEQUERY ||  1 octet || rfc6926<ref name=rfc6926/>{{rp|Section 6.2.1}}
|-
| 15 || DHCPLEASEQUERYDONE ||  1 octet || rfc6926<ref name=rfc6926/>{{rp|Section 6.2.1}}
|-
| 16 || DHCPACTIVELEASEQUERY ||  1 octet || rfc7724<ref name=rfc7724/>{{rp|Section 5.2.1}}
|-
| 17 || DHCPLEASEQUERYSTATUS ||  1 octet || rfc7724<ref name=rfc7724/>{{rp|Section 5.2.1}}
|-
| 18 || DHCPTLS ||  1 octet || rfc7724<ref name=rfc7724/>{{rp|Section 5.2.1}}
|}

====Client vendor identification====
An option exists to identify the vendor and functionality of a DHCP client. The information is a [[Variable-length code|variable-length string]] of characters or octets which has a meaning specified by the vendor of the DHCP client. One method by which a DHCP client can 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 (VCI) (Option 60).

The value to which this option is set gives the DHCP server a hint about any required extra information that this client needs in a DHCP response. Some types of [[set-top boxes]] set the VCI to inform the DHCP server about the hardware type and functionality of the device. An [[Aruba Networks|Aruba]] campus [[wireless access point]], for example, supplies value 'ArubaAP' as option 60 in its DHCPDISCOVER message.<ref name='option60">{{cite web|title=Aruba DHCP Option 60|date=7 October 2020 |url=https://the-ethernets.com/2020/10/aruba-dhcp-option-60/}}</ref> The DHCP server can then augment its DHCPOFFER with an IP address of an Aruba [[wireless controller]] in option 43, so the access point knows where to register itself.

Setting a VCI by the client allows a DHCP server to differentiate between client machines and process the requests from them appropriately.

===Other extensions===
{| class="wikitable sortable"
|+ Documented DHCP options
|-
! Code !! Name !! Length !! RFC
|-
| 77 || User Class || Minimum of 2 octets || RFC 3004<ref name="ietf_spec_dhcp77">{{cite journal|last1=Stump|first1=G.|last2=Droms|first2=R.|last3=Gu|first3=Y.|last4=Vyaghrapuri|first4=R.|last5=Demirtjis|first5=A.|last6=Beser|first6=B.|last7=Privat|first7=J.|title=The User Class Option for DHCP|url=https://tools.ietf.org/html/rfc3004|website=IETF Documents|publisher=[[IETF]]|access-date=2 April 2024|doi=10.17487/RFC3004|date=November 2000|doi-access=}}</ref>
|-
| 82 || [[#Relay agent information sub-options|Relay agent information]] || Minimum of 2 octets || RFC 3046<ref name="ietf_spec_dhcp82">{{cite journal|last1=Patrick|first1=Michael|title=DHCP Relay Agent Information Option|url=https://tools.ietf.org/html/rfc3046|website=IETF Documents|publisher=[[IETF]]|access-date=22 July 2017|doi=10.17487/RFC3046|date=January 2001|doi-access=}}</ref>
|-
| 85 || [[Novell Directory Service]] (NDS) servers || Minimum of 4 octets, multiple of 4 octets || RFC 2241<ref name="ietf_spec_dhcp85-86-87">{{cite journal|last1=Provan|first1=Don|title=RFC 2241 – DHCP Options for Novell Directory Services|url=https://tools.ietf.org/html/rfc2241|website=IETF Documents|publisher=[[IETF]]|access-date=23 July 2017|doi=10.17487/RFC3256|date=November 1997|doi-access=|url-access=subscription}}</ref>{{rp|Section 2}}
|-
| 86 || NDS tree name || Variable || RFC 2241<ref name="ietf_spec_dhcp85-86-87"/>{{rp|Section 3}}
|-
| 87 || NDS context || Variable || RFC 2241<ref name="ietf_spec_dhcp85-86-87"/>{{rp|Section 4}}
|-
| 100 || [[Time zone]], POSIX style || Variable || RFC 4833<ref name="ietf_spec_dhcp100-101">{{cite journal|last1=Lear|first1=E.|last2=Eggert|first2=P.|title=Timezone Options for DHCP|url=https://tools.ietf.org/html/rfc4833|website=IETF Documents|publisher=[[IETF]]|date=April 2007|doi=10.17487/RFC4833 |access-date=28 June 2018}}</ref>
|-
| 101 || [[Time zone]], [[tz database]] style || Variable || RFC 4833<ref name="ietf_spec_dhcp100-101"/>
|-
| 114 || DHCP Captive-Portal || Variable || RFC 8910<ref>{{cite journal |last1=Kumari |first1=Warren |title=RFC 8910 - Captive-Portal Identification in DHCP and Router Advertisements (RAs) |url=https://tools.ietf.org/html/rfc8910 |website=ietf.org |date=September 2020 |publisher=IETF |access-date=25 March 2021}}</ref>
|-
| 119 || [[Search domain|Domain search]] || Variable || RFC 3397<ref name="ietf_spec_dhcp119">{{cite journal|doi=10.17487/RFC3397|last1=Bernard|first1=Aboba|last2=Stuart|first2=Cheshire|title=RFC 3397 – Dynamic Host Configuration Protocol (DHCP) Domain Search Option|url=https://tools.ietf.org/html/rfc3397|website=IETF Documents|publisher=[[IETF]]|date=November 2002|access-date=22 July 2017|doi-access=}}</ref>
|-
| 121 || Classless static route || Variable || RFC 3442<ref name="ietf_spec_dhcp121">{{cite ietf |url=https://tools.ietf.org/html/rfc3442 |title=The Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) |version=v. 4 |date=December 2002 |doi=10.17487/RFC3442 |rfc=3442 |last1=Lemon |first1=T. |last2=Cheshire |first2=S. |last3=Volz |first3=B. |doi-access=free}}</ref>
|-
| 209 || Configuration File || Variable || RFC 5071<ref name="rfc5071">{{cite journal |last1=Hankins |first1=David |title=RFC 5071 - Dynamic Host Configuration Protocol Options Used by PXELINUX |url=https://tools.ietf.org/html/rfc5071 |website=ietf.org |date=December 2007 |publisher=IETF |doi=10.17487/RFC5071 |access-date=25 March 2021}}</ref>
|-
| 210 || Path Prefix || Variable || RFC 5071<ref name="rfc5071"/>
|-
| 211 || Reboot Time || Variable || RFC 5071<ref name="rfc5071"/>
|}

====Relay agent information sub-options====
The relay agent information option (option 82) specifies container for attaching sub-options to DHCP requests transmitted between a DHCP relay and a DHCP server.<ref name="ietf_spec_dhcp82"/>

{| class="wikitable sortable"
|+ Relay agent sub-options
|-
! Code !! Name !! Length !! RFC
|-
| 1 || Agent Circuit ID || Minimum of 1 octet || RFC 3046<ref name="ietf_spec_dhcp82"/>
|-
| 2 || Agent Remote ID || Minimum of 1 octet || RFC 3046<ref name="ietf_spec_dhcp82"/>
|-
| 4 || Data-Over-Cable Service Interface Specifications (DOCSIS) device class || 4 octets || RFC 3256<ref name="ietf_spec_riasub4">{{cite journal|last1=Doug|first1=Jones|last2=Rich|first2=Woundy|title=RFC 3256 – The DOCSIS (Data-Over-Cable Service Interface Specifications) Device Class DHCP (Dynamic Host Configuration Protocol) Relay Agent Information Sub-option|url=https://tools.ietf.org/html/rfc3256|website=IETF Documents|publisher=[[IETF]]|access-date=23 July 2017|doi=10.17487/RFC3256|date=April 2002|doi-access=|url-access=subscription}}</ref>
|}

==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 a DHCP server not on the same subnet, as the client's broadcast can only be received on its own subnet.

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. A DHCP relay agent runs on a network device, capable of [[routing]] between the client's subnet and the subnet of the DHCP server. 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 IP addresses of the DHCP servers are manually configured in the relay agent.
The relay agent stores its own IP address, from the interface on which it has received the client's broadcast, in the ''GIADDR'' field of the DHCP packet.
The DHCP server uses the GIADDR-value to determine the subnet, and subsequently the corresponding address pool, from which to allocate an IP address.
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, using unicast (in most cases) to the newly reserved IP address, in an [[Ethernet frame]] directed to the client's MAC address.
The client should accept the packet as its own, even when that IP address is not yet set on the interface.{{Ref RFC|2131|rp=25}}
Directly after processing the packet, the client sets the IP address on its interface and is ready for regular IP communication, directly thereafter.

If the client's implementation of the IP stack does not accept unicast packets when it has no IP address yet, the client may set the ''broadcast'' bit in the FLAGS field when sending a DHCPDISCOVER packet.
The relay agent will use the {{IPaddr|255.255.255.255}} broadcast IP address (and the clients MAC address) to inform the client of the server's DHCPOFFER.

The communication between the relay agent and the DHCP server typically uses both a source and destination UDP port of 67.

==Client states==
[[File:Dhcp-client-state-diagram.svg|thumb|A simplified DHCP client state-transition diagram based on figure 5 of RFC 2131]]

A DHCP client can receive these messages from a server:{{Ref RFC|2131|rsection=4.4}}
* DHCPOFFER
* DHCPACK
* DHCPNAK

The client moves through DHCP states depending on how the server responds to the messages that the client sends.

==Reliability==
The DHCP ensures reliability in several ways: periodic renewal, rebinding,{{Ref RFC|2131|rsection=4.4.5}} 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.{{Ref RFC|2131|rsection=4.4.5 Paragraph 3}}   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,{{Ref RFC|2131|rsection=4.4.5 Paragraph 8}}{{Efn|The RFC calls for the client to wait one half of the remaining time until T2 before it retransmits the ''DHCPREQUEST'' packet}} 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,{{Ref RFC|2131|rsection=4.4.5 Paragraph 5}} the DHCP client will attempt to rebind, by broadcasting its ''DHCPREQUEST'' rather than unicasting it. Because it is [[Broadcasting (networking)|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.<ref>{{cite IETF
| title = DHCP Failover Protocol
| draft = draft-ietf-dhc-failover-12
| last1 = Droms | first1 = Ralph
| last2 = Kinnear | first2 = Kim
| last3 = Stapp | first3 = Mark
| last4 = Volz | first4 = Bernie
| last5 = Gonczi | first5 = Steve
| last6 = Rabil | first6 = Greg
| last7 = Dooley | first7 = Michael
| last8 = Kapur | first8 = Arun
| date = March 2003
| publisher = [[IETF]]
| access-date = May 9, 2010
}}</ref>{{Efn|The proposal provided a mechanism whereby two servers could remain loosely in sync with each other in such a way that even in the event of a total failure of one server, the other server could recover the lease database and continue operating. Due to the length and complexity of the specification, it was never published as a standard; however, the techniques described in the proposal are in wide use, with open-source and several commercial implementations.}}

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.{{Ref RFC|2131|rsection=4.4.5 Paragraph 9}}   At that time it will restart the DHCP process from the beginning by broadcasting a <code>DHCPDISCOVER</code> message. Since its lease has expired, it will accept any IP address offered to it. Once it has a new IP address (presumably from a different DHCP server) it will once again be able to use the network. However, since its IP address has changed, any ongoing connections will be broken.

==IPv6 networks==
The basic methodology of DHCP was developed for networks based on [[Internet Protocol version 4]] (IPv4). Since the development and deployment of [[IPv6]] networks, DHCP has also been used for assigning parameters in such networks, despite the inherent features of IPv6 for [[stateless address autoconfiguration]]. The IPv6 version of the protocol is designated as [[DHCPv6]].<ref name=":1">{{Cite web|url=https://www.networkworld.com/article/3297800/why-dhcps-days-might-be-numbered.html|title=Why DHCP's days might be numbered|last=Weinberg|first=Neal|date=2018-08-14|website=Network World|language=en|access-date=2019-08-07}}</ref>

==Security==
{{see also|DHCP snooping}}
The base DHCP does not include any mechanism for authentication.{{Ref RFC|3046|rsection=7}}
Because of this, it is vulnerable to a variety of attacks. These attacks fall into three main categories:{{Ref RFC|2131}}{{rp|sec. 7}}

* Unauthorized DHCP servers providing false information to clients.
* Unauthorized clients gaining access to resources.
* Resource exhaustion attacks from malicious DHCP clients.

Because the client has no way to validate the identity of a DHCP server, unauthorized DHCP servers (commonly called "[[rogue DHCP]]") can be operated on networks, providing incorrect information to DHCP clients.<ref name="Stapko2011"/>  This can serve either as a denial-of-service attack, preventing the client from gaining access to network connectivity,<ref name="Rountree2013">{{cite book |first= Derrick |last=Rountree |title = Windows 2012 Server Network Security: Securing Your Windows Network Systems and Infrastructure |url = https://books.google.com/books?id=NFzou_d4MGUC&pg=SA2-PA13 |year = 2013 |publisher = Newnes |isbn = 978-1-59749-965-1 |page = 22 }}</ref> or as a [[man-in-the-middle attack]].<ref name="Rooney2011">{{cite book |first= Timothy |last=Rooney |title = Introduction to IP Address Management |url = https://books.google.com/books?id=QgRDxkuI1MkC&pg=PA180 |year = 2010 |publisher = John Wiley & Sons |isbn = 978-1-118-07380-3 |page = 180 }}</ref>  Because the DHCP server provides the DHCP client with server IP addresses, such as the IP address of one or more DNS servers,{{Ref RFC|2131}}{{rp|sec. 7}} an attacker can convince a DHCP client to do its DNS lookups through its own DNS server, and can therefore provide its own answers to DNS queries from the client.<ref name="DNSRedirect">{{cite web |url = http://www.securelist.com/en/blog/208188095/TDSS_loader_now_got_legs |title = TDSS loader now got "legs" |first= Sergey |last=Golovanov (Kaspersky Labs) |date = June 2011 | archive-url=https://web.archive.org/web/20210125194521/https://securelist.com/tdss-loader-now-got-legs/30844/| archive-date=25 January 2021}}</ref> This in turn allows the attacker to redirect network traffic through itself, allowing it to eavesdrop on connections between the client and network servers it contacts, or to simply replace those network servers with its own.<ref name="DNSRedirect" />

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.<ref name="Stapko2011"/> 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.<ref name="Stapko2011">{{cite book |first= Timothy |last=Stapko |title = Practical Embedded Security: Building Secure Resource-Constrained Systems |url = https://books.google.com/books?id=Mly55VntuYMC&pg=PA39 |year = 2011 |publisher = Newnes |isbn = 978-0-08-055131-9 |page = 39 }}</ref>

DHCP does provide some mechanisms for mitigating these problems. The [[#Relay agent information sub-options|Relay Agent Information Option]] protocol extension{{Ref RFC|3046}} (usually referred to in the industry by its actual number as ''Option 82''<ref name="HensCaballero2008">{{cite book |first1 = Francisco J. |last1=Hens |first2 = José M. |last2=Caballero |title = Triple Play: Building the converged network for IP, VoIP and IPTV |url = https://books.google.com/books?id=aS1ZngveBIkC&pg=PA239 |year = 2008 |publisher = John Wiley & Sons |isbn = 978-0-470-75439-9 |page = 239 }}</ref><ref name="Ramirez2008">{{cite book |first= David H. |last=Ramirez |title = IPTV Security: Protecting High-Value Digital Contents |url = https://books.google.com/books?id=70tr_hSDULwC&pg=PA55 |year = 2008 |publisher = John Wiley & Sons |isbn = 978-0-470-72719-5 |page = 55 }}</ref>) 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.{{Ref RFC|3046}}{{rp|sec. 7}}

Another extension, Authentication for DHCP Messages{{Ref RFC|3118}} (RFC 3118), provides a mechanism for authenticating DHCP messages. As of 2002, this extension had not seen widespread adoption because of the problems of managing keys for large numbers of DHCP clients.<ref>{{cite web |url = http://www.ietf.org/mail-archive/web/dhcwg/current/msg00876.html |title = Implementation of RFC 3118 |first= Ted |last=Lemon |date = April 2002 }}</ref> A 2007 book about DSL technologies remarked that:<blockquote>[T]here 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.<ref name="GoldenDedieu2007">{{cite book |first1 = Philip |last1=Golden |first2 = Hervé |last2=Dedieu |first3 = Krista S. |last3=Jacobsen |title = Implementation and Applications of DSL Technology |url = https://books.google.com/books?id=Jjkd74jY47oC&pg=PA484 |year = 2007 |publisher = Taylor & Francis |isbn = 978-1-4200-1307-8 |page = 484 }}</ref></blockquote> A 2010 book notes that:<blockquote>[T]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.<ref name="Rooney2011b">{{cite book |first= Timothy |last=Rooney |title = Introduction to IP Address Management |url = https://books.google.com/books?id=QgRDxkuI1MkC&pg=PA181 |year = 2010 |publisher = John Wiley & Sons |isbn = 978-1-118-07380-3 |pages = 181–182 }}</ref></blockquote>

Architectural proposals from 2008 involve authenticating DHCP requests using [[802.1x]] or [[Protocol for Carrying Authentication for Network Access|PANA]] (both of which transport [[Extensible Authentication Protocol|EAP]]).<ref name="Copeland2008">{{cite book |first= Rebecca |last=Copeland |title = Converging NGN Wireline and Mobile 3G Networks with IMS |url = https://books.google.com/books?id=ruWv8RGkBGgC&pg=PA142 |year = 2008 |publisher = Taylor & Francis |isbn = 978-1-4200-1378-8 |pages = 142–143 }}</ref> An IETF proposal was made for including EAP in DHCP itself, the so-called <abbr>EAPoDHCP</abbr>;<ref name="PrasadMihovska2009">{{cite book |first1 = Ramjee |last1=Prasad |first2 = Albena |last2=Mihovska |title = New Horizons in Mobile and Wireless Communications: Networks, services, and applications |url = https://books.google.com/books?id=w9bEwBwd33MC&pg=PA339 |year = 2009 |publisher = Artech House |isbn = 978-1-60783-970-5 |page = 339 |volume = 2 }}</ref> this does not appear to have progressed beyond IETF draft level, the last of which dates to 2010.<ref>{{cite web |url=http://tools.ietf.org/search/draft-pruss-dhcp-auth-dsl-07 |title=Draft-pruss-DHCP-auth-DSL-07 - EAP Authentication Extensions for the Dynamic Host Configuration Protocol for Broadband |access-date=2013-12-12 |archive-url=https://web.archive.org/web/20150403091552/http://tools.ietf.org/search/draft-pruss-dhcp-auth-dsl-07 |archive-date=2015-04-03 }}</ref>

==IETF standards documents==
* {{IETF RFC|2131|link=no}}, Dynamic Host Configuration Protocol
* {{IETF RFC|2132|link=no}}, DHCP Options and BOOTP Vendor Extensions
* {{IETF RFC|3046|link=no}}, DHCP Relay Agent Information Option
* {{IETF RFC|3397|link=no}}, Dynamic Host Configuration Protocol (DHCP) Domain Search Option
* {{IETF RFC|3942|link=no}}, Reclassifying Dynamic Host Configuration Protocol Version Four (DHCPv4) Options
* {{IETF RFC|4242|link=no}}, Information Refresh Time Option for Dynamic Host Configuration Protocol for IPv6
* {{IETF RFC|4361|link=no}}, Node-specific Client Identifiers for Dynamic Host Configuration Protocol Version Four (DHCPv4)
* {{IETF RFC|4436|link=no}}, Detecting Network Attachment in IPv4 (DNAv4)
* {{IETF RFC|3442|link=no}}, Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) version 4
* {{IETF RFC|3203|link=no}}, DHCP reconfigure extension
* {{IETF RFC|4388|link=no}}, Dynamic Host Configuration Protocol (DHCP) Leasequery
* {{IETF RFC|6926|link=no}}, DHCPv4 Bulk Leasequery
* {{IETF RFC|7724|link=no}}, Active DHCPv4 Lease Query

==See also==
{{cols}}
* [[Boot Service Discovery Protocol]] (BSDP){{snd}} a DHCP extension used by Apple's [[NetBoot]]
* [[Comparison of DHCP server software]]
* {{Ref RFC|2322|ref=no}}
* [[Preboot Execution Environment]] (PXE)
* [[Reverse Address Resolution Protocol]] (RARP)
* [[Rogue DHCP]]
* [[UDP Helper Address]]{{snd}} a tool for routing DHCP requests across subnet boundaries
* [[Zeroconf]]{{snd}} Zero Configuration Networking
* [[Kea (software)|Kea]] – an open-source DHCP server developed by the [[Internet Systems Consortium]]
{{colend}}

==Notes==
{{Notelist}}

==References==
{{Reflist|30em}}

== External links ==

* {{commons category-inline|Dynamic Host Configuration Protocol (DHCP)}}

{{Authority control}}

[[Category:Application layer protocols]]
[[Category:Internet Standards]]
[[Category:Network service]]