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In the context of Computer Networking , Dynamic Host Configuration Protocol ('''DHCP''', Currently implemented as DHCPv6) is a Client-server Networking Protocol . A DHCP Server provides configuration parameters specific to the DHCP client host requesting, generally, information required by the client host to participate on an IP network. DHCP also provides a mechanism for allocation of IP Address es to client hosts. DHCP emerged as a Standard Protocol in October 1993. RFC 2131 provides the latest ( March 1997 ) DHCP definition. DHCP functionally became a successor to the older BOOTP protocol. Due to the backward-compatibility of DHCP, very few networks continue to use pure BOOTP. The Latest standard of the protocol, describing DHCPv6 (DHCP in a IPv6 environment), appeared in July 2003 as RFC 3315. IP ADDRESS ALLOCATION Depending on implementation, the DHCP server has three methods of allocating IP-addresses:
This decision remains transparent to clients. Some DHCP server implementations can update the DNS name associated with the client hosts to reflect the new IP address. They make use of the DNS update protocol established with RFC 2136.__FORCETOC__ CLIENT CONFIGURATION PARAMETERS A DHCP server can provide optional configurations to the client. RFC 2132 defines DHCP options. List of configurable options and their corresponding numbers: ''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 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 REMOVED/Unassigned 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 Unassigned 100 REMOVED/Unassigned 101 REMOVED/Unassigned 102-107 REMOVED/Unassigned 108 REMOVED/Unassigned 109 Unassigned 110 REMOVED/Unassigned 111 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 Removed/Unassigned 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 IMPLEMENTATIONS Microsoft introduced DHCP on their NT Server with Windows NT version 3.5 in late 1994. (Despite its billing as "a new feature from Microsoft", DHCP did not originate from Microsoft.) The Internet Software Consortium published DHCP software distributions for Unix variants with version 1.0.0 of the ISC DHCP Server released on December 6 1997 and a more RFC-compliant version 2.0 on June 22 1999 . Potential users can download this software from http://www.isc.org/sw/dhcp/ WIDE Project released their DHCP implementation in 1995 and a more stable version (1.4.0) in August 1997. One can freely download this from ftp://sh.wide.ad.jp/WIDE/free-ware/dhcp/ KAME Project released a DHCPv6 implementation, Now separately maintained. It uses the new home page http://wide-dhcpv6.sourceforge.net/ Novell has included a DHCP server in their NetWare operating system since version 5, released in 1998 . It integrates with Novell's directory service - Novell EDirectory . Weird Solutions has produced a variety of multi-platform DHCP implementations since 1997, targeted at both the Internet Service Provider and consumer markets. Other major implementations include:
Cisco Systems offers DHCP servers in routers and switches with their IOS software. Moreover, they offer Cisco Network Registrar (CNR) - a highly scalable and flexible DNS, DHCP and TFTP server. EXTENT OF DHCP USAGE Most Cable Internet providers in the United States of America use DHCP to allocate IP addresses. DSL providers in the US rarely use DHCP, preferring PPPoE instead. In the U.K. 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 Router s and other gateway-devices provide DHCP support for Networks of up to 255 computers, for assigning 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 Router s often 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. PROTOCOL ANATOMY DHCP uses the same two , and 68/udp for the Client Side . DHCP Discover 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 and also requests its last-known IP address (in the example below, 192.168.1.100) although the server may ignore this optional parameter.
DHCP Inform 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 Release 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. 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:
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 Ipfw firewall 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 SEE ALSO EXTERNAL LINKS
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