Lecture 13: IP autoconfiguration

Transcription

1 DD2393/EP2120 p Lecture 13: IP autoconfiguration Autoconfiguration in IP: stateful and stateless autoconfiguration Olof Hagsand KTH CSC 1

2 Literature Forouzan Chapter 18 All sections. BOOTP is in itself not relevant but necessary for historical reasons in order to understand DHCP The chapter only handles stateful autoconfiguration Forouzan Chapter 26.4 and 28.4 Stateless autoconfiguration in IPv6 2

3 Autoconfiguration requirements Suppose you connect a computer with no configured state to a network. What does it take to communicate? To a directly connected node? To a node on the Internet? For computers connected to a local link: IP address and subnet mask (IP address of a name server)* For Internet access a computer also needs: IP address of a router (default gateway) IP address of a name server* Additional info in both cases: Default TTL, Time servers, Printers Other services *: not strictly necessary 3

4 Different autoconf variants 1) Stateful autoconfiguration A central server keeps track of the address of every individual host DHCP This is the most widely used method today 2) Stateless autoconfiguration (SLAAC) Automatic address assignment by the hosts themselves Server only supplies non-host specific information Use of link-local addresses Mainly IPv6 (but IPv4 has it now too) 3) Zeroconf No servers hosts assign and require all information automatically Not widely deployed 4

5 Stateful autoconfiguration Historical development: RevARP --> BOOTP --> DHCP, Stateful DHCPv6 5

6 BOOTP Reverse ARP (RARP) translates MAC IP addr But only contains an IP address Requires fixed hw addresses BOOTP (RFC 951) is a lot more powerful than RARP Client/Server Protocol Specify a boot image for disk-less clients BOOTP sends requests/replies over UDP Easy to write a user space server Client does not need a full TCP/IP stack to run BOOTP But BOOTP is not dynamic uses a static binding between MAC and IP addresses 6

7 DHCP Dynamic Host Configuration Protocol DHCP provides dynamic configuration Client can get a temporary address, and move from network to network DHCP is backward compatible with BOOTP BOOTP client can request a static configuration from a DHCP server Allows reuse of address, which avoids having to tie up addresses for systems which are not currently connected, or have been removed Extended with many other services: name and time servers, etc. Stateful DHCPv6 is the corresponding protocol in IPv6 There is a also a stateless DHCPv6 in IPv6 with no unique address assignment 7

8 DHCP simplified operation Client 68 src: dst: Server Address: C UDP 68 UDP Request UDP src: S dst: or C Reply 67 UDP IP header IP datagram UDP datagram UDP header DHCP request/reply 20 bytes 8 bytes 300 bytes 8

9 DHCP simplified operation 1. DHCP server uses UDP port 67 and waits for client 2. Client sends initial requests (discover) encapsulated in UDP using: dst IP address dst port no 67 src IP address src port no Server replies (DHCPOFFER) with either broadcast or unicast message using UDP destination port 68 The unicast IP address of the client can be used in server s reply message if client s IP address has been manually added in the ARP table Note that the server will know the client s MAC address Only place in IP where client has fixed port! 9

10 Relay agent Client A relay agent (proxy) is used so a DHCP server can serve several subnets A relay agent is a router that sends local requests to a remote server, and relays replies back to the subnet. The relay agent 'tunnels' the orignal request to the server. Relay agent Broadcast UDP Request UDP Server 68 UDP Reply 67 UDP 10

11 Options and message types Lots of semantics is implemented in the DHCP options field or Vendor-specific field Uses TLV : Type-Length-Value Subnetmask Address of routers, nameservers, timeservers, hostname, etc. Message type examples DHCPDISCOVER DHCPOFFER DHCPREQUEST DHCPACK DHCPNACK DHCPRELEASE type length value 11

12 Example of a simple server config file shared network LOCAL NET { Default lease time 5000; Max lease time 10000; option domain name "exp.se"; option domain name servers ; option domain name servers ; subnet netmask { option routers ; option subnet mask ; option broadcast address ; range ; host armitage { fixed address ; hardware ethernet 00:61:e0:42:a5:bc; } host molly { fixed address ; hardware ethernet 00:a3:e0:42:6b:ff; } } } 12

13 Initialization First discover a server, then request address lease Discover is sent on broadcast with as source Offer includes an IP address Request/Ack commits the use of the ip address Client Server DISCOVER OFFER REQUEST ACK 13

14 Expiration After 50% of lease, client needs to renew If server NACKs, client needs to start over (shown below) If server does not respond, client continues until lease expires Client Server REQUEST REQUEST NACK RELEASE 14

15 Client state machine Boot S: DHCPNACK C: DHCPDISCOVER Initializing Selecting Client C Server C: DHCPREQUEST S: DHCPOFFER Requesting S S:DHCPACK C: DHCPRELEASE 50% Expired? C: DHCPREQUEST Bound Expired? or S: DHCPNACK Renewing S: DHCPACK Rebinding 87.5% Expired? C: DHCPREQUEST 15

16 Stateless autoconfiguration Stateless autoconf - SLAAC Nodes can start communicating directly Router has only non-host specific data Does not keep track of individual host addresses Mainly for IPv6 1. Create a link-local address 2. Get a global prefix from a router 3. Create a global address Client Router Global destination 16

17 1. How to create LLAs FE80/10 is used for IPv6 link-local addresses /16 is allocated for IPv4 Alt 1: A host allocates an LLA by mac-derived address (see below) Alt 2: A host allocates an LLA by a random function A host probes this address by sending an ICMPv6 neighbor solicitation with the new address as target and ::/128 as source. A reply (neighbor advertisement) means a conflicting address (try a new) 48 bit MAC address 64 bits FE80:: FFFE 17

18 2. Get a global prefix from the router Router configured with global prefixes that hosts can create addresses from Router sends ICMPv6 router advertisments (RA) on the sub-network periodically Hosts may send ICMPv6 router solicitations to trigger router RAs 18

19 3. Create a global address Use the global prefix obtained by RA to create a global address Either mac-derived or random function This is a global routable address If you have a routable address, you should always use that instead of a LLA Also, link-local addresses should never be configured in DNS except if you use mdns - where you can use local names 48 bit MAC address 64 bits Global prefix FFFE 19

20 RA versus DHCP Using router advertisments (RA) for stateless autoconfiguration in IPv6 has its limits and is controversial RA only provides address, subnet and next-hop. But a host (almost) always needs name servers, for example. So there is a need for DHCP in most networks anyway So why would you need RA at all if you have to set up a DHCP server? Good question. Many therefore question the use of RA However, RA is much more lightweight than DHCP And you can use stateless DHCP together with RA. Client Router Server 20

21 Zeroconf Allowing two hosts to communicate without infra-structure Typically for ad-hoc meetings on a wireless network Communicate with new people in public Use a service (eg printer) you pass by This has worked well in proprietary network technologies Apple's Appletalk, Microsoft NETBIOS Novell IPX UPnP (Universal plug and play) But IP has always been bad at this What do you need to make it work? 21

22 IP Zeroconf requirements 1.Allocate addresses without a DHCP server IPv4 Link-Local Addressing 2.Translate between names and IP addresses without a DNS server Multicast DNS (mdns) introduces a new top_level-domain: local. 3.Find services, like printers, without a directory server DNS Service Discovery (SRV posts) 4.Allocate IP Multicast addresses without a server 22

23 Exercise: DHCP Autoconfiguration Assume two hosts A and B, an IP sub-network N (a WaveLAN), a router R, and a DNS server D. A, R and D are connected to N, while B is accessible on the Internet via R. Assume further that A is just being connected to N it has no configured IPv4 address on that interface. Describe what is necessary in terms of all protocol messages for A to send a UDP packet to B given only its name? Your answer should include DHCP, DNS, ARP, and IP. This is mainly an exercise in understanding all actions necessary to send an IP message starting from scratch. 23

24 Exercise: Multi-homing in IPv6 Consider a host A that wishes to start a communication with another remote host B. A is on a multi-homed site with two border routers peering with two different ISPs. Your site has two different IPv6 address blocks from each of the ISP, and thus two different prefixes. Host A can therefore use two different addresses A1 and A2. Assume that the same situation is true for B: it has addresses B1 and B2. Discuss the following questions: How does A obtain the two source addresses A1 and A2? How does A obtain the destination addresses (B1 and B2)? How does A decide which source address to use (A1 or A2)? How does A decide which destination address to use (B1 or B2)? When B receives a packet from A, which source/destination addresses does it use in a reply? Suppose A and B have started to communicate using a pair of addresses. Can the address be changed when the session is up? 24