Network Protocols Address Resolution Protocol ARP TDC375 Autumn 2010 11 John Kristoff DePaul University 1 ARP overview Primarily used by IP to find a layer 2 address Problem statement What L2 daddr do you use If L3 daddr is on your subnet L2 daddr L3 host Else L2 daddr is the router Use ARP on the local net to find the L2 daddr TDC375 Autumn 2010 11 John Kristoff DePaul University 2 Typical ARP process Step 2 Receiver Step 1 Sender Put in own L2 L3 saddrs Is that my L3 daddr Fill in known L3 daddr Fill in missing fields Send to L2 broadcast daddr Reply directly to sender TDC375 Autumn 2010 11 John Kristoff DePaul University 3 ARP frame format diagram courtesy of http www netbook cs purdue edu TDC375 Autumn 2010 11 John Kristoff DePaul University 4 Variations of ARP Inverse ARP get a L2 daddr when L3 is known Reverse ARP IP address auto configuration DHCP ARP Used to validate a DHCP lease Gratutious ARP update others of your mapping UnARP notify others to flush your mapping Proxy ARP router that ARPs for non local host TDC375 Autumn 2010 11 John Kristoff DePaul University 5 Some ARP security thoughts Hosts and routers build maintain ARP table cache This might be a good thing to monitor few do Learn ARP mappings we didn t initiate Responders usually cache sender s mapping Hosts seeing the broadcast and having the sender s mapping cached usually refresh Lack of security means MiTM attacks possible LAN switches with port security TDC375 Autumn 2010 11 John Kristoff DePaul University 6
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