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CMPT 471Networking II

DHCP

© Janice Regan, 2006-2013

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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.

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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RARP was superceded by BOOTP which could supply more information (RFC 951, 1542, 2132), but was appropriate only when workstations rarely move from network to network Each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

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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.

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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

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Encapsulation BOOTP or DHCP

BOOTP or DHCP message

BOOTP or DHCP message

BOOTP or DHCP messageUDP header

UDP headerIP header

BOOTP or DHCP messageUDP headerIP headerEthernet header

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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

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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

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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

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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

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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.

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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)

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DHCP message structure

Comer 2000: fig 23.5

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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

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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.

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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

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51

Structure of options: samples

Message type

Option code Option length (octets) Option data

53 1 Option data

1 4 Subnet mask

6 4 DNS server IP address

50 4 requested IP address

255

3 8 1st and 2nd router IP address

4 Lease time

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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

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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.

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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

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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

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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)

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DHCP state machine

Comer 2000: fig 23.4

INIT REBOOT

DHCPREQUEST

DHCPNAK

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Initial configuration: Summary Client begins in INITIALIZE, if it boots without a valid IP

address (no address, expired address or invalid address)

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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.

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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

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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

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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.

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Initial configuration: 4client

DHCP DISCOVER

DHCP OFFER

DHCP DISCOVER

DHCP REQUEST

DHCP OFFERDHCP REQUEST

DHCP PACK

Offered Address returned to pool

client

Selected router

unselected router

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Initial configuration: transitions

Comer 2000: fig 23.4

INIT REBOO

T

DHCPREQUEST DHCPNAK

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DHCP message structure

Comer 2000: fig 23.5

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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.

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Reboot before lease expires: transitions

Comer 2000: fig 23.4

INIT REBOOT

DHCPREQUEST DHCPNAK

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DHCP message structure

Comer 2000: fig 23.5

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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

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Moving to a new network: transitions

Comer 2000: fig 23.4

INIT REBOO

T

DHCPREQUESTDHCPNAK

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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

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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

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Extending a lease: transitions

Comer 2000: fig 23.4

INIT REBOO

T

DHCPREQUESTDHCPNAK

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Releasing a lease: transitions

Comer 2000: fig 23.4

INIT REBOO

T

DHCPREQUESTDHCPNAK

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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