CMPT 471 Networking II

1. CMPT 471Networking II DHCP

2. Bootstrapping Autoconfiguration • Bootstrapping is a process by which a client (diskless or with limited storage) can obtain information ranging from IP addresses to bootloading files (from a server or servers) • Bootstrapping uses this information to load OS into volatile storage and customize the environment for a particular host (configure the host) • At one time configuration of host specific information like addresses, was done manually as each host was added to a network or network information was updated. • Autoconfiguration allows us to automate this procedure, or isolate the need for updates to one or more configuration servers. Automation of these procedures is now also important for supporting mobility of hosts.

3. Protocols: 1 • Protocols used for bootstrapping include RARP and BOOTP (BOOTstrap Protocol). • RARP, Reverse Address Resolution Protocol, (RFC 903) is limited, supplying only IP address of the booting host. This is adequate only on very simple networks • RARP was superceded by BOOTP which could supply more information(RFC 951, 1542, 2132), but was appropriate only when • workstations rarely move from network to network • Each workstation had its own fixed IP address

4. Protocols: 2 • Networks today do not conform to either of these constraints. • Workstations are commonly moved from network to network • Workstations will want an IP address from the network they are attached to, not a static IP address • To extend the functionality of BOOTP, DHCP (Dynamic Host Configuration Protocol) was developed • DHCP is fully backward compatible with BOOTP, providing necessary extensions to that protocol.

5. DHCP configuration • Hosts of a particular hardware type will have a common operating system/environment. They will also need host specific configuration information • In addition to the IP address BOOTP and DHCP servers can supply other configuration information including netmask (prefix length), gateway address, desired IP address, DNS server address, bootstrap file name and other information (some vendor specific) • BOOTP and DHCP messages are encapsulated in UDP packets which are in turn encapsulated in IP packets

6. Encapsulation BOOTP or DHCP BOOTP or DHCP message BOOTP or DHCP message UDP header IP header UDP header BOOTP or DHCP message UDP header BOOTP or DHCP message Ethernet header IP header

7. Uses • BOOTP / DHCP can be used to boot a host with very little non volatile (permanent) memory. • A minimal version of the OS can be stored in the permanent memory and used at boot to request a full boot image and configuration • BOOTP / DHCP can be used on a host which requires only configuration information • Host had complete OS and all common required software elements. • Host needs host specific configuration information

8. Sending the DHCP request • DHCP messages are sent in IP packets through port 67 • DHCP is being used to determine the IP address of the booting host • Therefore, DCHP requests must be broadcast to the local network • The DHCP server will receive the request if it is on the local network • The DHCP server need not be on the local network segment. DHCP relay agents (routers or multihomed hosts) can forward DHCP requests and replies to other network segments

9. Sending the DHCP reply • The DHCP server knows the IP address of the booting host, so it can address a unicast IP packet to that host • The booting host does not know its own IP address and IP specifies (RFC 1122) that a host should drop packets not destined for one of its own IP addresses. • This seems to imply that the reply must also be broadcast. However, RFC 1122 does not specifically state what should happen if the host does not know its own IP address • The later BOOTP specification (RFC 1542) does specify that IP packets should not be ignored if the host does not yet know its own IP address • BOOTP and DHCP replies are usually unicast, but can be broadcast. They are sent through port 68

10. But how does IP layer get a packet not sent to its IP address? • The host sending the DHCP reply will • know the IP address of the host that sent the DHCP request • assign the IP address for this host (so now it knows both the IP and Ethernet addresses of the requesting host) • In most cases insert the IP/ethernet address information into the ARP cache of the machine constructing the DHCP reply • Be able to unicast a reply to the requesting host

11. But how does IP layer get a packet not sent to its IP address? • When the DHCP reply is received • The Ethernet address the reply is delivered to is the host making the DHCP request • The Ethernet packet has its header removed and the remaining data is passed to the IP layer • The IP destination address in the DHCP request is the IP of the requesting machine but the requesting machine does not know this yet.

12. Two step boot process • The DHCP server maintains a static database of network information that can be sent to the booting host in the DHCP reply • The DHCP reply provides information necessary to obtain a boot image, not the boot image itself. • DHCP allows this information to be used to access and download a boot image. • Boot image may reside on the DHCP server or on another host • Allows configuration of multiple groups of machines with similar hardware or software needs (ie different kernel)

13. DHCP message structure Comer 2000: fig 23.5

14. DHCP message structure: 1 • OP field is 1 for BOOTP request, 2 for BOOTP reply • HTYPE (hardware type as for ARP), HLEN (in octets), and Client address specify the booting hosts hardware address • Hops is set to 0 by the booting host and incremented by the DHCP relay agent before forwarding to an adjacent network • Transaction id is used to match requests and replies • Secs is the # of seconds since the first DHCP request • Only one flag is used. It determines if the reply is to be broadcast or unicast (default is unicast) • Client IP address is either 0.0.0.0 or an IP address being requested by the booting client

15. DHCP message structure: 2 • Your ip address is the IP address supplied by the server (may not be the requested address). Must be included since it cannot be recovered if the reply is broadcast • Server IP address is used to indicate where the boot image can be obtained • Router IP address is used by a relay agent. Booting host sets to 0.0.0.0, first relay agent inserts its own IP address before forwarding to adjacent network. DHCP server returns reply to first relay agent which is responsible for unicast or broadcast on the local network segment.

16. BOOTP message structure: 3 • Server Name is used if a response is requested from a specific BOOTP server • Boot file name, booting host specifies type of boot file, server sends address of image • Vendor Specific Information Field: Includes options. This is where netmask, server addresses (DNS, NIS,…), and actual vendor specific data is inserted. Maximum length of field is 64 bytes. If more than 64 bytes of options are needed the BOOTP server can specify a file containing the option and vendor specific information. Each option in the field or the file is given as a tag, a length, and the data

17. Structure of options: samples Option code Option data Option length (octets) Lease time 51 Message type 4 53 1 Option data 1 4 Subnet mask 4 6 DNS server IP address requested IP address 4 50 1st and 2nd router IP address 3 8 255

18. Message type • The message type is specified as an option. • The option has option code 53, length 1, and data • DHCPOFFER 2 • DHCPPACK 5 • DHCPREQUEST 3 • DHCPDISCOVER 1 • DHCPDECLINE 4 • DHCPNAK 6 • DHCPRELEASE 7 • DHCPINFORM 8 • The IP type field indicates DHCP, not the type of DHCP message

19. Dynamic Configuration • BOOTP is based upon a static database configured by the network administrator • To make the system work in networks where hosts are added or removed frequently or in large networks you really want a dynamic, automatically updated system. In such a system the administrator no longer needs to manually update the system each time a change occurs. • DHCP is the extension to BOOTP that allows dynamic configuration.

20. Lifetime of Dynamic Entries • Three timers are used within DHCP • Lease time: the length of time for which the assigned IP address will remain valid (seconds) • Renewal time: length of time before the client should request an extension of the lease time from its chosen DHCP server (seconds). Default is 50% of Lease time. • Rebinding time: length of time before the client should request an extension of lease from all accessible DHCP servers (seconds). Default is 87.5% of Lease time

21. DHCP: use of renewal timer • If client’s renewal time expires • Client requests extension from it’s DHCP server (Request unicast to DHCP server) • if no reply is received request is transmitted with exponential backoff (4, 8, 16, …, 64 seconds) until a response is received or a set number of retransmission occur • If lease extension is granted update lease time, renewal time and rebinding time to reflect extension • If lease is not extended terminate lease (send DHCPRELEASE message

22. Use of rebinding timer • If client’s rebinding timer expires • Client broadcasts an extension request to all DHCP servers • if no reply is received request is transmitted with exponential backoff (4, 8, 16, … 64 seconds) until a response is received or until a set number of retransmission occurs • If lease extension is granted update lease time, renewal time, rebinding time and identity of selected DHCP server to reflect extension granted by a different server • If lease is not extended terminate lease (send DHCPRELEASE message)

23. DHCP state machine INIT REBOOT DHCPREQUEST DHCPNAK Comer 200 0: fig 23.4

24. Initial configuration: Summary • Client begins in INITIALIZE, if it boots without a valid IP address (no address, expired address or invalid address)

25. Initial configuration: Summary • Four messages are exchanged. • A DHPDISCOVER message is broadcast by the host • Each server/relay client finds an IP address for the host and sends it to the host in a DHCPOFFER message • The host chooses a server from received offer messages (usually takes the first) then broadcasts a DHCPREQUEST • When the server receives the DHCPREQUEST message it confirms the IP address assignment by sending a DHCPACK back to the host.

26. Initial configuration: 1 • A DHPDISCOVER message is broadcast so it reaches all DHCP servers or relay clients on the attached network • Ethernet: destination address all 1’s, source address is ethernet address of host making DHCP request • IP: source address 0.0.0.0, destination address 255.255.255.255 • UDP : source port 68, destination port 67 • Client address and your address 0.0.0.0, client hardware address is the Ethernet address of the host making the DHCP request • Options: needs Message type and parameter request list

27. Initial configuration: 2 • Each server/relay client finds an IP address for the host and sends it to the host in a DHCPOFFER message • Ethernet: destination address is the client Ethernet address, source address is Ethernet address of replying DHCP server • IP: source address is the address of the replying server, destination address is the client address (or broadcast) • UDP : source port 68, destination port 67 • Client address 0.0.0.0 and your address is set, client hardware address as for DHCPDISCOVER packet • Options: needs Message type, lease time, renewal time, server IP address, rebinding time, any subnet mask

28. Initial configuration: 3 • The client chooses a server from received offer messages (usually takes the first) then broadcasts a DHCPREQUEST including the server address and suggested least time and rebinding time • The chosen server chooses an IP address, places the address and configuration information into its database and replies with a DHCPACK containing the information • It is recommended that the server probe the IP address it is assigning to the client • Other servers know that they have not been chosen and release the IP address they offered for later use. • When the client receives the DHCPACK it is recommended that it should use an ARP request to confirm its assigned address is not in use.

29. Initial configuration: 4 client DHCP DISCOVER DHCP REQUEST DHCP PACK DHCP OFFER Selected router client DHCP DISCOVER DHCP REQUEST Offered Address returned to pool DHCP OFFER unselected router

30. INIT REBOOT DHCPREQUEST DHCPNAK Initial configuration: transitions Comer 2000: fig 23.4

31. DHCP message structure Comer 2000: fig 23.5

32. Rebooting before lease expires • When a client is rebooted or turned on and it has recorded a previous lease • It checks to see if the has expired • If the lease has expired it goes to INIT state and begins negotiating a new IP address • If the lease has not expired it goes into INIT-REBOOT state and attempts to confirm the lease is still valid • The client broadcasts a DHCPREQUEST, including the IP address to be confirmed as a requested IP address in the options • When the server receives the request it checks that the address is available and is on the correct network then sends a DHCPACK if it is. • When the client receives the DHCPACK it is recommended that it should use an ARP request to confirm its assigned address is not in use.

33. INIT REBOOT DHCPREQUEST DHCPNAK Reboot before lease expires: transitions Comer 2000: fig 23.4

34. DHCP message structure Comer 2000: fig 23.5

35. Moving to a new network • When a client is rebooted or turned on and it has recorded a previous lease • It checks to see if the has expired • If the lease has not expired it goes into INIT-REBOOT state and attempts to confirm the lease is still valid • The client broadcasts a DHCPREQUEST, including the IP address to be confirmed as a requested IP address in the options • When the server receives the request it checks if the address is on the correct network since the host has moved it is not • The server sends a DCHPNAK to the client • The client moves into the INIT state and start a new initialization

36. Moving to a new network: transitions INIT REBOOT DHCPREQUEST DHCPNAK Comer 2000: fig 23.4

37. Extending a lease: 1 • When the client negotiates the lease a renewal time and a rebinding time can be set (default is ½ lease time and 87.5% of lease time) • At the renewal time is reached the client sends a DHCPREQUEST message to the specified server requesting that the lease be extended • If the server agrees it sends a DHCPACK back to the client to extend the lease • If the server disagrees it sends a DHCPNAK and the client must disconnect from the network terminating the lease

38. Extending a lease: 2 • If the server does not reply to the DHCPREQUEST sent to renew the lease • the client will retransmit (unicast) with exponential backoff (after 4, 8, 16, … seconds). • Eventually the client will enter rebind state • then DHCPREQUESTS are broadcast to all servers. • If another server can extend the lease that server will respond an renew the lease and move the client back to the bound state • If no server responds and the lease expires the client returns to the initialize state

39. INIT REBOOT DHCPREQUEST DHCPNAK Extending a lease: transitions Comer 2000: fig 23.4

40. INIT REBOOT DHCPREQUEST DHCPNAK Releasing a lease: transitions Comer 2000: fig 23.4

41. DHCP software • A commonly used DHCP package used on linux networks is the ISC (Internet software consortium) DHCP package • To help understand how this package is used read the linux documentation for • The DHCP relay agent, dhcrelay (posted) • The DHCP client, dhclient • The DHCP server, dhcpd