Ethernet Addresses & Resolution

Transcription

1 Ethernet Addresses & Resolution A data link such as Ethernet or a token ring has its own addressing scheme When an Ethernet frame is sent from one host to another, it is the 48-bit Ethernet address that determines the destination The first 28-bits are the organization that made the Ethernet card, the second 28-bits are randomly assigned by the manufacturer The device driver software never looks at the destination IP address in the IP datagram 9/17/2002 ICSS420 - ARP - RARP 1

2 ARP Address resolution provides a mapping between two different forms of addresses 32-bit IP addresses and whatever the data link uses ARP (address resolution protocol) is a protocol used to do address resolution in the TCP/IP protocol suite (RFC826) ARP provides a dynamic mapping from an IP address to the corresponding hardware address 9/17/2002 ICSS420 - ARP - RARP 2

3 Basic Idea ARP is required on multi-access channels and relies on the ability to broadcast The protocol is simple: broadcast a packet containing the IP address of the destination machine the machine with that address, or possibly a server, sends a reply containing the hardware address upon receipt the hardware address is used to send the original packet 9/17/2002 ICSS420 - ARP - RARP 3

4 ARP Cache Essential to the efficient operation of ARP is the maintenance of a cache on each host The cache maintains the recent IP to physical address mappings Each entry is aged (usually the lifetime is 20 minutes) forcing periodic updates of the cache ARP replies are often broadcast so that all hosts can update their caches 9/17/2002 ICSS420 - ARP - RARP 4

5 arp Commnd The arp(8) command on a Unix system can be used to see the contents of the ARP cache kiev> arp -a Net to Media Table Device IP Address Mask Flags Phys Addr hme0 redshirt :60:08:8b:e9:aa hme0 tiger :00:20:85:f6:8d hme0 thunderbolt :00:20:9a:af:60 hme0 starfury :00:20:9a:af:79 hme0 cs3-router :10:11:09:f0:28 hme0 epsilon :00:20:86:71:c0 hme0 mordor :00:20:96:01:ad hme0 itlabman :00:c0:65:d7:b9 hme0 joanne :05:02:59:51:52 hme0 laurie :05:02:79:c4:20 hme0 kiev SP 08:00:20:9e:f2:99 9/17/2002 ICSS420 - ARP - RARP 5

6 ARP Packet Format Hardware Type Protocol Type Hardware Size Protocol Size Operation Sender s Hardware Address (for Ethernet 6 bytes) Sender s Protocol Address (for IP 4 bytes) Target Hardware Address Target Protocol Address Destination IP Address 9/17/2002 ICSS420 - ARP - RARP 6

7 Proxy ARP Proxy ARP lets a router answer ARP requests on one of its networks for a host on another of its networks This fools the sender of the ARP request into thinking that the router is the destination The router is acting as a proxy agent for the destination, relaying packets to it from other hosts 9/17/2002 ICSS420 - ARP - RARP 7

8 Proxy ARP Proxy ARP is also known as promiscuous ARP or the ARP hack The names come from the other use of proxy ARP: to hide two physical networks from each other, with a router between the two This has been used to separate hosts running two different versions of TCP/IP 9/17/2002 ICSS420 - ARP - RARP 8

9 Gratuitous ARP Gratuitous ARP occurs when a host sends an ARP request looking for its own IP address This can happen at bootstrap time Gratuitous ARP provides two features it lets a host determine if another host is already configured with the same IP address if the host sending the gratuitous ARP has just changed its hardware address, the packet causes other hosts on the net to update their ARP cache entries 9/17/2002 ICSS420 - ARP - RARP 9

10 Issues Many people ARP to be a dangerous protocol a bogus host can issue a gratuitous ARP and change cache entries a bogus host can send replies giving its own hardware address (instead of the target) Broadcasting can be expensive excessive use of bandwidth CPU costs 9/17/2002 ICSS420 - ARP - RARP 10

11 Reverse Address Resolution Protocol When a system boots, it typically gets its IP address from a file How does a system, without a disk, get its IP address? Since each system has a unique hardware address, that hardware address can be used to lookup the corresponding IP address RARP (RFC903) does exactly that 9/17/2002 ICSS420 - ARP - RARP 11

12 RARP Packet Format The format is exactly the same as ARP except some of the numbers change The RARP request is broadcast and the reply is sent to the requester Unlike ARP, designated RARP server(s) that handles RARP requests 9/17/2002 ICSS420 - ARP - RARP 12