CSCE 515: Computer Network Programming Chin-Tser Huang University of South Carolina

2/1/20052 Need for Address Resolution Need to map from an IP address to a hardware address Frames exchanged at hardware level must be addressed to correct interface Mapping between IP address and hardware address may change Need to map from a hardware address to an IP address For systems without a disk (diskless workstations or X terminals)

2/1/20053 ARP and RARP 32-bit Internet address 48-bit Ethernet address ARPRARP Provide dynamic mapping between IP address and hardware address

2/1/20054 ARP Request For computer i to get hardware address of computer j, i broadcasts a rqst message with IP address of j to the subnetwork Internet i j r default router switch rqst(ipa.j)

2/1/20055 ARP Reply If j sees a rqst message from i with its IP address, j sends a rply message with its IP address and hardware address to i Internet i j r default router switch rply(ipa.j,hda.j)

2/1/20056 ARP and RARP Packet Format Ethernet destination addr Ethernet source addr frame type hard type prot type op sender Ethernet addr sender IP addr target Ethernet addr target IP addr Ethernet header28-byte ARP request/reply hard size prot size

2/1/20057 ARP Cache Maintain recent mappings from Internet addresses to hardware addresses Make ARP operations efficient Timeout is 20 minutes for completed entry and 3 minutes for incomplete entry Timeout for an entry restarted every time it is referenced

2/1/20058 Proxy ARP A router answers ARP request on one of its networks for a host on another of its networks Sender of ARP request will regard the router as destination host and send IP datagram to the router Router acting as proxy agent will forward IP datagram to destination

2/1/20059 Proxy ARP Example slipbsdisunsvr4 netb gemini modem Ethernet subnet Ethernet subnet SLIP ARP request for ARP reply Router configured to act as proxy ARP agent for sun

2/1/ Gratuitous ARP A host sends an ARP request looking for its own IP address Usually done at bootstrap time Two features provided by gratuitous ARP Let a host determine if another host is already configured with same IP address Let other hosts update their cache entries

2/1/ arp Command Available at /usr/sbin -a option to display all entries in ARP cache -n option to print IP addresses instead of host names -d option to delete an entry from ARP cache (superuser only) -s option to add a permanent entry to ARP cache (superuser only) Keyword temp Keyword pub

2/1/ Ethereal A free network protocol analyzer for Unix and Windows Examine data from a live network or from a capture file on disk Provide a rich display filter language and the ability to view the reconstructed stream of a TCP session

2/1/ Ethereal at Unix Lab To sniff packets at Unix lab with Ethereal, use the following four machines at SWGN 1D39 regulus.cse.sc.edu pollux.cse.sc.edu nunki.cse.sc.edu menkar.cse.sc.edu

2/1/ Next Class IP Read TI Ch. 3 Homework 1 will be passed out