1 DHCP : DHCP : Dynamic Host Configuration Protocol :, Original by: Debie Beemsigne, Dominic Joy, Max Pereira, Ravidev Mohan, EL604, Fall 2001; Modified by Prof. M. Veeraraghavan  What is DHCP, and when & where is it used?  Components of DHCP  BOOTP and DHCP relation  DHCP message format  DHCP procedures  allocating new address  lease renewal  State machine  Questions & answers re. DHCP  Benefits of DHCP

2 What is DHCP ?  Dynamic Host Configuration Protocol  Used for dynamic allocation of IP addresses  used for hosts that run only client applications  Allows for host-specific configuration parameters to be delivered from a DHCP server to a host  DHCP can also be used to convey permanent IP address assignments to hosts  Server interfaces need permanent addresses because clients need to be able to reach them  Also, router interfaces should have permanent addresses for stability of routing data

3 Where is DHCP used?  Since class B and class C address spaces have been exhausted, service providers and enterprises use dynamically allocated IP addresses  e.g., a cable modem service provider who has many customers  since not all customers are simultaneously on the Internet, a client host dynamically obtains an address for a short period of time and releases it for use by some other client  e.g., used on the Poly campus on our wireless network, where many students, faculty and staff members use their wireless cards for access to the Poly campus network

4 Relevance of DHCP to wireless and mobile networking  If an end host only runs the “client” ends of applications  e.g. a web browser, but not a web server  e.g. Outlook to download messages delivered to a PC user’s incoming mail server, but not the mail server itself  e.g. Windows PCs have ftp clients but not ftp servers  you ftp into utopia, but do you typically ftp into your PC?  Then, the end host can simply connect to the network at any “point of attachment,” obtain a network address and start receiving information

5 Why is a dynamically obtained address sufficient for such hosts?  Because, such end hosts only make “outgoing calls”  When such an end host initiates a call (TCP connection), the called server receives an IP packet from the calling host with the host’s new temporary address and hence can, in turn, send the caller back requested data  These hosts do not receive “incoming calls,” i.e., no one calls them  Therefore, such end hosts do not need a permanent address that potentially “callees” will need to know  Question: can we categorize a cellular telephone as such an “end point”

6 Answer to question in previous slide  No, because a cell phone can be called  Therefore callees (calling parties) will need to know the network address of the cellular phone

7 Is DHCP needed only for wireless users or also wired?  DHCP can be used whether link to endpoint is “wired” or “wireless”  Even with an Ethernet NIC, a host can use DHCP to dynamically obtain an IP address  e.g. cable modem user; Ethernet from host to cable modem; cable from modem to Internet

8 DHCP for mobile users?  DHCP is not designed to handle mobility by itself  location management problem is to enable the delivery of calls to mobiles; but this means the callee needs the permanent address of the called mobile; so location management problem does not arise for DHCP hosts  handoff management requires a rerouting of packets when end host is in a connection; “costly” to update far end of connection with a new temporary address if DHCP is used to change addresses when a user moves

9 Components  DHCP client: a host using DHCP to obtain an IP address and other configuration information  DHCP server: a host that returns IP addresses and other configuration information  BOOTP relay agents: host or router that passes DHCP messages between DHCP clients and DHCP servers

10 DHCP uses BOOTP  Based on BOOTP  Uses BOOTP format for messages  Uses BOOTP relay agents to avoid having one DHCP server per network segment  BOOTP used to bootstrap hosts  First operation, get Ethernet and IP address for itself and the address of a server and bootfile name (diskless client)  Second operation, uses TFTP to download bootfile  BOOTP uses UDP  67 for the server port  68 for the client port

11 Differences between BOOTP and DHCP  Two differences  DHCP specifies a “lease time” for IP address usage allowing for allocation of the same address to another client upon lease expiry  DHCP can acquire all IP-related configuration information, more than in BOOTP  BOOTP clients can talk to DHCP servers (backward compatibility) (backward compatibility)  DHCP is an extension of BOOTP  Message format similar

12 Format of a DHCP message op (1)htype (1)hlen (1)hops(1) xid (4) secs (2)flags (2) ciaddr (4) yiaddr (4) siaddr (4) giaddr (4) chaddr (6) sname (64) file (128) options ( variable)

13 Format of a DHCP message cont.  Op - message op code / message type  1 = BootRequest,2 = BootReply  htype - hardware address type  hlen - hardware address length (i.e. ‘6’ for 10mbps Ethernet)  hops - client sets to 0, optionally used by relay agents when booting via a relay agent  xid - transaction ID, a random number chosen by the client, used by the client and server to associate messages and responses between a client and a server  secs – filled in by the client, seconds elapsed since client began address acquisition or renewal process  flags – “broadcast flag” used if client cannot accept unicast IP packets before IP layer is configured (“chicken-and-egg” problem)  destination IP address in IP header is a broadcast IP address instead of yiaddress and destination MAC address is all ones; then set this flag to “broadcast” mode. Server receiving this flag in Bootrequest will know to send the Bootreply in broadcast mode BACK

14 Format of a DHCP message cont.  ciaddr – client IP address; only filled in if client is in BOUND, RENEW or REBINDING state and can respond to ‘ARP’ requests  yiaddr – ‘your’ (client) IP address (set in DHCPOFFER)  siaddr – IP address of next server to use in bootstrap; returned in DHCPOFFER, DHCPACK by server  giaddr – relay agent IP address, used in booting via a relay agent  chaddr – client hardware addresses  sname – optional server host name, null terminated string  file – Boot file name, null terminated string; “generic” name or null in DHCPDISCOVER, fully qualified directory-path name in DHCPOFFER  options – optional parameters field (see RFC 1533) BACK

15 DHCP options  DHCP Message Type (must in all messages)  Requested IP Address  Server identifier  IP Address Lease Time  Renewal Time Value (T1)  Rebinding Time Value (T2)  Client identifier  DNS (Domain Name Server) option  specifies IP addresses of DNS servers available to client  Router option  specifies IP addresses of routers on the client’s subnet  Max Datagram Reassembly  Subnet Mask  Client Hostname  For all other options refer to RFC 2132

16 Types of DHCP messages  DHCPDISCOVER  DHCPOFFER  DHCPREQUEST  DHCPACK  DHCPNAK  DHCPDECLINE  DHCPRELEASE  DHCPINFORM

17 How does DHCP work?  When a client needs to start up TCP/IP operations, it broadcasts a request for address information. The DHCP server receives the request, assigns a new address for a specific time period (called a lease period) and sends it to the client together with the other required configuration information. This information is acknowledged by the client, and used to set up its configuration. The DHCP server will not reallocate the address during the lease period and will attempt to return the same address every time the client requests an address. The client may extend its lease with subsequent requests, and may send a message to the server before the lease expires telling it that it no longer needs the address so it can be released and assigned to another client on the network.

18 DHCP procedures  Obtaining a new address  Reusing a previously allocated address

19 Allocating new address ClientServer (selected)Server (not selected) Collects replies DHCP REQ. DHCPACK Initialization Complete Graceful Shutdown DHCPRELEASE Discard lease DHCPOFFER DHCP DISC. Client selects configuration; in DCHP REQ it accepts one server’s offer (server identifier option) and implicitly reject rest Also broadcast Sent on Ethernet broadcast address

20 How the server selects ‘ a new address  The client's current address as recorded in the client's current binding, ELSE  The client's previous address as recorded in the client's (now expired or released) binding, if that address is in the server's pool of available addresses and not already allocated, ELSE  The address requested in the 'Requested IP Address' option, if that address is valid and not already allocated, ELSE  A new address allocated from the server's pool of available addresses; the address is selected based on the subnet from which the message was received (if 'giaddr' is 0) or on the address of the relay agent that forwarded the message ('giaddr' is not 0).

21 Obtaining an IP address  DHCPDISCOVER is broadcast because client does not know IP address of DHCP server; BOOTP relay agents may relay it to other DHCP servers  One or more DHCP servers respond with DHCPOFFER, which carry yiaddr (Your IP address, i.e., client address)  recommended check: SHOULD instead of MUST  server uses ICMP echo request (ping) to check if address is free  ping needed because often a user will leave without graceful release of address

22 Obtaining an IP address contd.  If the client receives no DHCP offer before it times out, it retransmits DHCPDISCOVER  Client may wait for multiple replies and then choose one offer. It broadcasts DHCPREQUEST with ‘server identifier’ option included identifying the server whose offer it has accepted and ‘requested IP address’ option  Client SHOULD probe address with an ARP; if client detects that the address is already in use, it issues DHCPDECLINE

23 Obtaining an IP address contd.  Servers other than the one selected in the DHCPREQUEST will release their offered addresses, while the selected server will note the binding  If selected server cannot meet the needs of the DHCPREQ. it sends a DHCPNAK  If the client does not receive a DHCPACK or DHCPNAK before timeout it resends DHCPREQ  Retransmission attempts use exponential backoff times

24 Reuse of address Client Server DHCP REQ. Locates Config. DHCPACK Initialization Complete (Subsequent DHCP Packets Ignored) Servers with knowledge of the client's configuration parameters respond with a DHCPACK message to the client.

25 Reuse of IP address  DHCP REQUEST - client message to servers  requesting offered parameters from one server and implicitly declining offers from all others  confirming correctness of previously allocated address after, e.g., system reboot,  extending the lease on a particular network address.

26 Reuse of IP address cont.  DHCPACK - Server to client with configuration parameters, including committed network address.

27 State machine INIT/ REBOOT INIT REBOOTING SELECTING REQUESTINGREBINDING BOUNDRENEWING Send DHCPREQUEST DHCPNAK Restart Send DHCPDISCOVER DHCPNAK, Lease expired/Halt Network DHCPNAK/Halt Network T2 expires/ Broadcast DHCPREQUEST DHCPOFFER/ Send DHCPREQUEST DHCPOFFER/ Discard DHCPACK/ Record lease, set timersT1, T2 DHCPACK/Record lease set timersT1,T2 T1 expires/ Send DHCPREQUEST To leasing server (unicast) DHCPOFFER, DHCPACK DHCPNAK/Discard DHCPACK/ Record lease, set Timers T1,T2 DHCPNAK/ Discard DHCPACK + not accept Send DHCPDECLINE DHCPACK/Record lease, set timers init with known network addressInit with acquiring new network address T1 is earlier than T2, which should expire before lease expires

28 Timers  T1 is the time at which the client enters the RENEWING state and attempts to contact the server that originally issued the client's network address.  T2 is the time at which the client enters the REBINDING state and attempts to contact any server.  T1 MUST be earlier than T2  T2 MUST be earlier than the time at which the client's lease will expire.

29 Use of giaddr and bootp relays  By using BOOTP relay agents, we avoid having to place a DHCP server on every subnet  giaddr (gateway address) is poorly named  it exists to facilitate relaying of BOOTP messages and is not a general or gateway IP router address  giaddr should be 0 in DHCPDiscover (says BOOTPRequest in RFC 1542)  BOOTP relay agent should not be confused with an IP router: former terminates BOOTP messages, processes them and generates new ones; it does not simply forward packets  if giaddr is 0 when a BOOTP relay agent receives a message, it sets it to the IP address of the interface on which it was received in the new message it relays to the next agent  if giaddr is non-zero, then it MUST not be modified  allows the DHCP server to determine subnet of client  RFC 1542 explains this procedure well

30 Questions on DHCP  What happens if there is no DHCP server on a network and an IP host connects to it with the “Obtain IP address automatically” option selected?  Who makes and sells DHCP servers? Are these standalone entities or are they add-ons to IP routers?  Use a sniffer, capture DHCP messages and explain procedure and parameters.

31 What happens if there is no DHCP server on a network and an IP host connects to it with the “Obtain IP address automatically” option selected?

32 What happens if there is no DHCP server on a network and an IP host connects to it with the “Obtain IP address automatically” option selected? In static addressing, the following fields: Gateway, DNS Configuration and IP Address would have to be manually set for a host to have connectivity into the network. If there is no DHCP server, and no BOOTP relay agent, then no IP address will be assigned and hence host cannot communicate; In this case “Static Addressing” needs to be used

33 Who makes and sells DHCP servers? Are these standalone entities or are they add-ons to IP routers?  Commercial DHCP servers  Microsoft: DHCP server included in Windows NT Server 3.51, 4.0, Windows 2000  SunSoft: Solstice SolarNet PC-Admin 1.5 includes a DHCP/BOOTP server  Cisco IOS DHCP Server (IOS is used in routers)

34 Use sniffer and obtain a trace of a DHCP session Current IP configuration (before release)

35 IP configuration after release

36 Tcpdump output for a renew  17:26: :b0:c2:f3:15:80 0:50:4:fd:10: : > : xid:0x596f374e Y: S: G: [|bootp]  d4 b ff11 247a 80ee 7001  80ee c0 3bb  596f 374e ee 7090  80ee 1d19 80ee fd   0000  17:26: :50:4:fd:10:85 ff:ff:ff:ff:ff:ff : > : xid:0x596f374e [|bootp]  f5a  ffff ffff a8ac  596f 374e  fd   0000  17:26: :b0:c2:f3:15:80 0:50:4:fd:10: : > : xid:0x596f374e Y: G: [|bootp]  d9 b ff ee 7001  80ee c5 cb6b  596f 374e ee 7090  ee fd   0000  17:26: :50:4:fd:10:85 ff:ff:ff:ff:ff:ff : > : xid:0x596f374e [|bootp]  c f4a  ffff ffff ddee  596f 374e  fd   0000 Identify parameters and explain values

37 IP configuration after the renew

38 Benefits of using DHCP for dynamic address allocation  Address reuse  A service provider/enterprise with 1000 addresses can support a much larger number of subscribers/hosts with time multiplexing of addresses (one host uses an address, and releases it when done; another host is allocated the same address)  Removes error-prone manual host configuration process to set IP addresses, DNS server addresses, gateways, etc.  Configuration information can be administered from a single point.  Major network resource changes (e.g. a router changing an interface address) requires an update of only the DHCP server, rather than every system.  Caveat: cannot allocate temporary addresses to servers/routers

39 References   RFC 2131: DHCP   RFC 2132: DHCP options   RFC 951: Bootstrap protocol (BOOTP)   RFC 1542: BOOTP relay agents   Steven M. Glass, “Use of DHCP in Mobile IP,” 2000, dhcpinmip/index.htm   Charles Perkins, “Mobile IP”, Addison Wesley, 1998