System Configuration: DHCP and Autoconfiguration Chapter 6

DHCP: Dynamic Host Configuration Protocol Goal: allow host to dynamically obtain its IP address from network server when it joins network can renew its lease on address in use allows reuse of addresses (only hold address while connected/“on”) support for mobile users who want to join network (more shortly) DHCP overview: host broadcasts “DHCP discover” msg [optional] DHCP server responds with “DHCP offer” msg [optional] host requests IP address: “DHCP request” msg DHCP server sends address: “DHCP ack” msg

DHCP client-server scenario / / / DHCP server arriving DHCP client needs address in this network

DHCP server: arriving client DHCP discover src : , 68 dest.: ,67 yiaddr: transaction ID: 654 DHCP offer src: , 67 dest: , 68 yiaddrr: transaction ID: 654 lifetime: 3600 secs DHCP request src: , 68 dest:: , 67 yiaddrr: transaction ID: 655 lifetime: 3600 secs DHCP ACK src: , 67 dest: , 68 yiaddrr: transaction ID: 655 lifetime: 3600 secs DHCP client-server scenario

DHCP: more than IP addresses DHCP can return more than just allocated IP address on subnet:  address of first-hop router for client  name and IP address of DNS sever  network mask (indicating network versus host portion of address)

 connecting laptop needs its IP address, addr of first-hop router, addr of DNS server: use DHCP router with DHCP server built into router  DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in Ethernet  Ethernet frame broadcast (dest: FFFFFFFFFFFF ) on LAN, received at router running DHCP server  Ethernet demuxed to IP demuxed, UDP demuxed to DHCP DHCP UDP IP Eth Phy DHCP UDP IP Eth Phy DHCP DHCP: example

DHCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server  encapsulation of DHCP server, frame forwarded to client, demuxing up to DHCP at client DHCP: example router with DHCP server built into router DHCP UDP IP Eth Phy DHCP UDP IP Eth Phy DHCP  client now knows its IP address, name and IP address of DSN server, IP address of its first-hop router

Highlights Address Pools Leases Lease duration: minutes or days Client’s request requested lease duration, a copy of the address it is already using or last used, and other parameters Server’s response Based on client’s information (including the requesting MAC address) in addition to other exogenous information (e.g., the time of day, the interface on which the request was received)

DHCP Key difference from old BOOTP Leases Address Allocation Automatic Dynamic Manual

DHCP/BOOTP Message Format

Some options Common options include Pad (0) Subnet Mask (1) Router Address (3) Domain Name Server (6) Domain Name (15) Requested IP Address (50) Address Lease Time (51) DHCP Message Type (53) Server Identifier (54) Parameter Request List (55) DHCP Error Message (56) Lease Renewal Time (58) Lease Rebinding Time (59) Client Identifier (61) Domain Search List (119) End (255).

DHCP Message Types DHCPDISCOVER (1) DHCPOFFER (2) DHCPREQUEST (3) DHCPDECLINE (4) DHCPACK (5) DHCPNAK (6) DHCPRELEASE (7) DHCPINFORM (8) DHCPFORCERENEW (9) DHCPLEASEQUERY (10) DHCPLEASEUNASSIGNED (11) DHCPLEASEUNKNOWN (12) DHCPLEASEACTIVE (13)

Protocol Operation The IPv6 header is of fixed size (40 bytes) and contains 128-bit source and destination addresses. The Next Header field is used to indicate the presence and types of additional extension headers that follow the IPv6 header, forming a daisy chain of headers that may include special extensions or processing directives. Application data follows the header chain, usually immediately following a transport-layer header.

DHCP Message Types T: Lease Time T1: Renewal Time: start renewal with same server after this time expires Typically T/2 T2: Rebinding Time: renewal must be done with same server by T2. If not, then rebinding of the address is needed which means doing it from scratch while indicating that the client already has an IP address (in the client addr) Typically 7T/8

Lease Timers: An Example (tcpipguide.com)

DHCP client state machine

IPv6 Address Lifecycle Tentative: can be used only for IPv6 Neighbor Discovery DAD: Duplicate Address Detection is performed in this state Optimistic: It is like Tentative, but an address is allocated for some limited use Deprecated: can be used for existing connections, but not for new connections DAD: Duplicate Address Detection

Duplicate Address Detection (DAD) send one or more ICMPv6 Neighbor Solicitation messages If a Neighbor Advertisement message is received in response, DAD has failed, and the address being checked is abandoned

DHCPv6

Location Information GEOCONF option(“Geographical configuration”) Latitude, longitude, altitude GEOCONF_CIVIC option Country, city, district etc. More complex as the categories vary by country and characters in different languages have complex representations Why are these options needed?

Mobility and Handoff Information DHCP options to convey information about cellular configuration and handovers between different networks Types of such options Information Services available networks Command Services functions for controlling link parameters Event Services notification of link status changes

DHCP Snooping Implemented in some switches which limits DHCP traffic to those addresses that are in a list Two problems can be potentially addressed Rogue DHCP server is limited in the damage it can do Allocation limited to a set of MAC addresses

Stateless Address Autoconfiguration (SLAAC) Consider a network that has a single link and suppose there are no DHCP servers Single-Link or Link Local Addresses Automatic Private IP Addressing Generate a random address ( through using the 16-bit subnet mask ) Check for conflicts using ACD (Address Conflict Detection)