ARP Address Resolution Protocol

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ARP (Address Resolution Protocol)
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Code Introduction
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ARP Structure
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ARP Functions

• arpwhohas
• arprequest
• arpintr
• in_arpinput
• arptimer

• arptfree
• arpresolve
• arplookup
• arp_rtrequest

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arpwhohas Function

• The arpwhohas function is normally called by
  arpresolve to broadcast an ARP request.

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arprequest Function

• The arprequest function is called by arpwhohas to
  broadcast an ARP request.
• Allocate and initialize mbuf

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arprequest Function

• Initialize pointers
• Fill in Ethernet header
• Set Ethernet destination address to Ethernet
  Broadcast Address
• Ethernet type is ETHERTYPE_ARP
• Fill in ARP fields
• Fill in sa_family and call interface output
  function

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arpintr Function

• Process the Frame
• Hardware type specifies Ethernet address
• The size of frame gt the size of arphdr 2 HW
  address 2 protocol address
• Call in_arpinput
• ETHERTYPE_IP
• ETHERTYPE_IPTRAILERS

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in_arpinput Function

• Its called by arpintr to process each received
  ARP request or ARP reply
• Validate senders hardware address
• Check senders IP address
• Search routing table for match with senders IP
  address

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• Update existing entry or fill in new entry
• Check if senders hardware address changed
• Record senders hardware address
• Update newly resolved ARP entry
• If this host is the target
• Check if this host is a proxy server for target
• Form proxy reply
• Complete construction of ARP reply packet
• Fill in sockaddr with Ethernet header

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arptimer Function

• Called every 5 min.
• Set next timeout
• Check all ARP entries

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arptfree Function

• Invalidate entries in use
• the link-layer address length is set to 0
• the la_asked counter is reset to 0
• the RTF_REJECT flag is cleared
• Delete unreferenced entries

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arpresolve Function

• Obtain the Ethernet address for an IP address
• If destination Ethernet address is known
• allow ether_output to queue the IP datagram on
  the interfaces output queue
• If doesnt know the Ethernet address
• Held the datagram
• Send ARP request
• Avoid ARP flooding

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• Handle broadcast and multicast destinations
• Check ARP entry for validity
• the expiration time is 0 or the expiration time
  is greater than current time
• the family of the socket address struct pointed
  to by rt_gateway is AF_LINK
• the link-level address is nonzero
• Hold only most recent IP datagram

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• Send ARP request but avoid ARP flooding
• Never send s more than 1 ARP request in any
  second to a destination.
• If a reply is not received after 5 ARP request,
  the RTF_REJECT flag in the routing table is set
  and the expiration timer is set for 20 sec.
• After 20 sec. pause in ARP request, arpresolve
  will send ARP request to the destination again.

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arplookup Function

• It calls the routing function rtalloc1 to look up
  an ARP entry in the Internet routing table.
• If arplookup succeeds, a pointer is return to the
  corresponding llinfo_arp structure
• Otherwise a null pointer is return
• A new entry should be created