Dynamic Host Configuration Protocol
| Internet protocol suite |
|---|
| Application layer |
| Transport layer |
| Internet layer |
| Link layer |
DHCP is a set of rules used by communications devices such as a computer, router or network adapter to allow the device to request and obtain an IP address from a server which has a list of addresses available for assignment.
DHCP is a protocol used by networked computers (clients) to obtain IP addresses and other parameters such as the default gateway, subnet mask, and IP addresses of DNS servers from a DHCP server. It facilitates access to a network because these settings would otherwise have to be made manually for the client to participate in the network.
The DHCP server ensures that all IP addresses are unique, e.g., no IP address is assigned to a second client while the first client's assignment is valid (its lease has not expired). Thus IP address pool management is done by the server and not by a human network administrator.
DHCP emerged as a standard protocol in October 1993. As of 2006, RFC 2131 provides the latest ([dated March 1997]) DHCP definition. DHCP functionally became a successor to the older BOOTP protocol, whose leases were given for infinite time and did not support options. Due to the backward-compatibility of DHCP, very few networks continue to use pure BOOTP.
The latest non-standard of the protocol, describing DHCPv6 (DHCP in an IPv6 environment), appeared in July 2003 as RFC 3315.
Overview
The Dynamic Host Configuration Protocol (DHCP) automates the assignment of IP addresses, subnet masks, default gateway, and other IP parameters. The assignment occurs when the DHCP-configured machine boots up or regains connectivity to a network. The DHCP client sends out a query requesting a response from a DHCP server on the locally attached network. The query is typically initiated immediately after booting up and before the client initiates any IP based communication with other hosts. The DHCP server then replies to the client with its assigned IP address, subnet mask, DNS server and default gateway information.
The assignment of the IP address generally expires after a predetermined period of time, before which the DHCP client and server renegotiate a new IP address from the server's predefined pool of addresses. Typical intervals range from one hour to several months, and can, if desired, be set to infinite (never expire). The length of time the address is available to the device it was assigned to is called a lease, and is determined by the server.
Configuring firewall rules to accommodate access from machines who receive their IP addresses via DHCP is therefore more difficult because the remote IP address will vary from time to time. Administrators must usually allow access to the entire remote DHCP subnet for a particular TCP/UDP port.
Most home routers and firewalls are configured in the factory to be DHCP servers for a home network. An alternative to a home router is to use a computer as a DHCP server. ISPs generally use DHCP to assign clients individual IP addresses.
DHCP is a broadcast-based protocol. As with other types of broadcast traffic, it does not cross a router unless specifically configured to do so. Users who desire this capability must configure their routers to pass DHCP traffic across UDP ports 67 and 68. Home users, however, will practically never need this functionality.
Extent of DHCP usage
Most cable internet providers use DHCP to allocate IP addresses.
In the UK many broad-band ISP networks use DHCP, but xDSL providers make extensive use of "infinite lease", which amounts to assigning semi-static IPs.
In addition, many routers and other gateway devices provide DHCP support for networks running many computers being assigned private IP addresses.
Office networks also use DHCP, in particular when workers make extensive use of laptops which link directly to the in-house network only occasionally .
Network routers and often multilayer switches employ a DHCP relay agent, which relays DHCP "Discover" broadcasts from a LAN which does not include a DHCP server to a network which does have one. These devices may be sometimes configured to append information about port from which DHCP request originates (also known as option 82). One example of such a relay agent is the UDP Helper Address command employed by Cisco routers.
Security
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DHCP and firewalls
Firewalls usually have to permit DHCP traffic explicitly. Specification of the DHCP client-server protocol describes several cases when packets must have the source address of 0x00000000 or the destination address of 0xffffffff. Anti-spoofing policy rules and tight inclusive firewalls often stop such packets. Multi-homed DHCP servers require special consideration and further complicate configuration.
To allow DHCP, network administrators need to allow several types of packets through the server-side firewall. All DHCP packets travel as UDP datagrams; all client-sent packets have source port 68 and destination port 67; all server-sent packets have source port 67 and destination port 68. For example, a server-side firewall should allow the following types of packets:
- Incoming packets from 0.0.0.0 or dhcp-pool to dhcp-ip
- Incoming packets from any address to 255.255.255.255
- Outgoing packets from dhcp-ip to dhcp-pool or 255.255.255.255
where dhcp-ip represents any address configured on a DHCP server host and dhcp-pool stands for the pool from which a DHCP server assigns addresses to clients
Example in ipfw firewall
To give an idea of how a configuration would look in production, the following rules for a server-side ipfirewall to allow DHCP traffic through. Dhcpd operates on interface rl0 and assigns addresses from 192.168.0.0/24 :
pass udp from 0.0.0.0,192.168.0.0/24 68 to me 67 in recv rl0 pass udp from any 68 to 255.255.255.255 67 in recv rl0 pass udp from me 67 to 192.168.0.0/24,255.255.255.255 68 out xmit rl0
Example in Cisco IOS Extended ACL
The following entries are valid on a Cisco 3560 switch with enabled DHCP service. The ACL is applied to a routed interface, 10.32.73.129, on input. The subnet is 10.32.73.128/26.
10 permit udp host 0.0.0.0 eq bootpc host 10.32.73.129 eq bootps 20 permit udp 10.32.73.128 0.0.0.63 eq bootpc host 10.32.73.129 eq bootps 30 permit udp any eq bootpc host 255.255.255.255 eq bootps
Technical details
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.
DHCP discovery
The client broadcasts on the local physical subnet to find available servers. Network administrators can configure a local router to forward DHCP packets to a DHCP server on a different subnet. This client-implementation creates a UDP packet with the broadcast destination of 255.255.255.255 or subnet broadcast address.
A client can also request its last-known IP address (in the example below, 192.168.1.100). If the client is still in a network where this IP is valid, the server might grant the request. Otherwise, it depends whether the server is set up as authoritative or not. An authoritative server will deny the request, making the client ask for a new IP immediately. A non-authoritative server simply ignores the request, leading to an implementation dependent time out for the client to give up on the request and ask for a new IP.
DHCP offers
When a DHCP server receives an IP lease request from a client, it extends an IP lease offer. This is done by reserving an IP address for the client and sending a DHCPOFFER message across the network to the client. This message contains the client's MAC address, followed by the IP address that the server is offering, the subnet mask, the lease duration, and the IP address of the DHCP server making the offer.
The server determines the configuration, based on the client's hardware address as specified in the CHADDR field. Here the server, 192.168.1.1, specifies the IP address in the YIADDR field.
DHCP requests
The client selects a configuration out of the DHCP "Offer" packets it has received and broadcasts it on the local subnet. Again, this client requests the 192.168.1.100 address that the server specified. In case the client has received multiple offers it specifies the server from which it has accepted the offer.
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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DHCP selection
When the client PC receives an IP lease offer, it must tell all the other DHCP servers that it has accepted an offer. To do this, the client broadcasts a DHCPREQUEST message containing the IP address of the server that made the offer. When the other DHCP servers receive this message, they withdraw any offers that they might have made to the client. They then return the address that they had reserved for the client back to the pool of valid addresses that they can offer to another computer. Any number of DHCP servers can respond to an IP lease request, but the client can only accept one offer per network interface card.
DHCP information
The client sends a request to the DHCP server: either to request more information than the server sent with the original DHCPACK; or to repeat data for a particular application - for example, browsers use DHCP Inform to obtain web proxy settings via WPAD. Such queries do not cause the DHCP server to refresh the IP expiry time in its database.
DHCP releasing
The client sends a request to the DHCP server to release the DHCP and the client unconfigures its IP address. As clients usually do not know when users may unplug them from the network, the protocol does not define the sending of DHCP Release as mandatory.
Client configuration parameters
A DHCP server can provide optional configuration parameters to the client. RFC 2132 defines the available DHCP options, which are summarized here.
RFC 1497 Vendor Extensions:
Data
Tag Name Length Meaning
--- ---- ------ -------
0 Pad Option 0 None
255 End Option 0 None
1 Subnet Mask 4 Subnet Mask Value
2 Time Offset 4 Time Offset in
Seconds from UTC
3 Router N×4 Router addresses
4 Time Server N×4 Timeserver addresses
5 Name Server N×4 IEN-116 Server addresses
6 Domain Server N×4 DNS Server addresses
7 Log Server N×4 Logging Server addresses
8 Quotes Server N×4 Quotes Server addresses
9 LPR Server N×4 Printer Server addresses
10 Impress Server N×4 Impress Server addresses
11 RLP Server N×4 N RLP Server addresses
12 Hostname N Hostname string
13 Boot File Size 2 Size of boot file in 512-octet
blocks
14 Merit Dump File N Client to dump and name
the file to dump it to
15 Domain Name N The DNS domain name of the
client
16 Swap Server 4 Swap Server address
17 Root Path N Path name for root disk
18 Extensions File N Path name for more BOOTP info
IP Layer Parameters per Host:
19 Forward On/Off 1 Enable/Disable IP Forwarding 20 SrcRte On/Off 1 Enable/Disable Non-Local Source Routing 21 Policy Filter N×8 Non-Local Source Routing Policy Filters 22 Max DG Assembly 2 Max Datagram Reassembly Size 23 Default IP TTL 1 Default IP Time to Live 24 MTU Timeout 4 Path MTU Aging Timeout 25 MTU Plateau N×2 Path MTU Plateau Table
IP Layer Parameters per Interface:
26 MTU Interface 2 Interface MTU Size 27 MTU Subnet 1 All Subnets are Local 28 Broadcast Address 4 Broadcast Address 29 Mask Discovery 1 Perform Mask Discovery 30 Mask Supplier 1 Provide Mask to Others 31 Router Discovery 1 Perform Router Discovery 32 Router Request 4 Router Solicitation Address 33 Static Route N×8 Static Routing Table
Link Layer Parameters per Interface:
34 Trailers 1 Trailer Encapsulation 35 ARP Timeout 4 ARP Cache Timeout 36 Ethernet 1 Ethernet Encapsulation
TCP Parameters:
37 Default TCP TTL 1 Default TCP Time to Live 38 Keepalive Time 4 TCP Keepalive Interval 39 Keepalive Data 1 TCP Keepalive Garbage
Application and Service Parameters:
40 NIS Domain N NIS Domain Name 41 NIS Servers N×4 NIS Server Addresses 42 NTP Servers N×4 NTP Server Addresses 43 Vendor Specific N Vendor Specific Information 44 NETBIOS Name Srv N×4 NETBIOS Name Servers 45 NETBIOS Dist Srv N×4 NETBIOS Datagram Distribution 46 NETBIOS Node Type 1 NETBIOS Node Type 47 NETBIOS Scope N NETBIOS Scope 48 X Window Font N×4 X Window Font Server 49 X Window Manager N×4 X Window Display Manager 64 NIS-Domain-Name N NIS+ v3 Client Domain Name 65 NIS-Server-Addr N×4 NIS+ v3 Server Addresses 68 Home-Agent-Addrs N×4 Mobile IP Home Agent Addresses 69 SMTP-Server N×4 Simple Mail Server Addresses 70 POP3-Server N×4 Post Office Server Addresses 71 NNTP-Server N×4 Network News Server Addresses 72 WWW-Server N×4 WWW Server Addresses 73 Finger-Server N×4 Finger Server Addresses 74 IRC-Server N×4 Chat Server Addresses 75 StreetTalk-Server N×4 StreetTalk Server Addresses 76 STDA-Server N×4 ST Directory Assist. Addresses
DHCP Extensions:
50 Address Request 4 Requested IP Address 51 Address Time 4 IP Address Lease Time 52 Option Overload 1 Overload "sname" or "file" 53 DHCP Msg Type 1 DHCP Message Type 54 DHCP Server Id 4 DHCP Server Identification 55 Parameter List N Parameter Request List 56 DHCP Message N DHCP Error Message 57 DHCP Max Msg Size 2 DHCP Maximum Message Size 58 Renewal Time 4 DHCP Renewal (T1) Time 59 Rebinding Time 4 DHCP Rebinding (T2) Time 60 Class Id N Vendor Class Identifier 61 Client Id N Client Identifier 66 Server-Name N TFTP Server Name 67 Bootfile-Name N Boot File Name
Newer extensions:
62 Netware/IP Domain N Netware/IP Domain Name
63 Netware/IP Option N Netware/IP sub Options
77 User-Class N User Class Information
78 Directory Agent N directory agent information
79 Service Scope N service location agent scope
80 Rapid Commit 0 Rapid Commit
81 Client FQDN N Fully Qualified Domain Name
82 Relay Agent Information N Relay Agent Information, RFC 3046
83 iSNS N Internet Storage Name Service
84 REMOVED/Unassigned
85 NDS Servers N Novell Directory Services
86 NDS Tree Name N Novell Directory Services
87 NDS Context N Novell Directory Services
88 BCMCS Controller Domain Name list
89 BCMCS Controller IPv4 address option
90 Authentication N Authentication
91-92 REMOVED/Unassigned
93 Client System N Client System Architecture
94 Client NDI N Client Network Device Interface
95 LDAP N Lightweight Directory Access Protocol
96 REMOVED/Unassigned
97 UUID/GUID N UUID/GUID-based Client Identifier
98 User-Auth N Open Group's User Authentication
99-111 REMOVED/Unassigned
112 Netinfo Address N NetInfo Parent Server Address
113 Netinfo Tag N NetInfo Parent Server Tag
114 URL N URL
115 REMOVED/Unassigned
116 Auto-Config N DHCP Auto-Configuration
117 Name Service Search N Name Service Search
118 Subnet Selection Option 4 Subnet Selection Option
119 Domain Search N DNS domain search list
120 SIP Servers DHCP Option N SIP Servers DHCP Option
121 Classless Static Route N Classless Static Route Option
Option
122 CCC N CableLabs Client Configuration
123 GeoConf Option 16 GeoConf Option
124 V-I Vendor Class Vendor-Identifying Vendor Class
125 V-I Vendor-Specific Vendor-Identifying Vendor-Specific
Information Information
126-127 Removed/Unassigned
128 PXE - undefined (vendor specific) (Tentatively Assigned - 23 June 2005)
128 Etherboot signature. 6 bytes: E4:45:74:68:00:00
128 DOCSIS "full security" server IP address
128 TFTP Server IP address (for IP Phone software load)
129 PXE - undefined (vendor specific) (Tentatively Assigned - 23 June 2005)
129 Kernel options. Variable length string
129 Call Server IP address
130 PXE - undefined (vendor specific) (Tentatively Assigned - 23 June 2005)
130 Ethernet interface. Variable length string.
130 Discrimination string (to identify vendor)
131 PXE - undefined (vendor specific) (Tentatively Assigned - 23 June 2005)
131 Remote statistics server IP address
132 PXE - undefined (vendor specific) (Tentatively Assigned - 23 June 2005)
132 802.1P VLAN ID
133 PXE - undefined (vendor specific) (Tentatively Assigned - 23 June 2005)
133 802.1Q L2 Priority
134 PXE - undefined (vendor specific) (Tentatively Assigned - 23 June 2005)
134 Diffserv Code Point
135 PXE - undefined (vendor specific) (Tentatively Assigned - 23 June 2005)
135 HTTP Proxy for phone-specific applications
136-149 Unassigned
150 TFTP server address (Tentatively Assigned - 23 June 2005)
150 Etherboot
150 GRUB configuration path name
151-174 Unassigned
175 Etherboot (Tentatively Assigned - 23 June 2005)
176 IP Telephone (Tentatively Assigned - 23 June 2005)
177 Etherboot (Tentatively Assigned - 23 June 2005)
177 PacketCable and CableHome (replaced by 122)
178-207 Unassigned
208 pxelinux.magic (string) = F1:00:74:7E (241.0.116.126) (Tentatively
Assigned - 23 June 2005)
209 pxelinux.configfile (text) (Tentatively Assigned - 23 June 2005)
210 pxelinux.pathprefix (text) (Tentatively Assigned - 23 June 2005)
211 pxelinux.reboottime (unsigned integer 32 bits) (Tentatively Assigned
- 23 June 2005)
212-219 Unassigned
220 Subnet Allocation Option (Tentatively Assigned - 23 June 2005)
221 Virtual Subnet Selection Option (Tentatively Assigned - 23 June 2005)
222-223 Unassigned
224-254 Private Use
See also
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 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.