CSCI6268L19 - Foundations of Network and Foundations of...

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Unformatted text preview: Foundations of Network and Foundations of Network and Computer Security Computer Security J J ohn Black Lecture #19 Oct 14 th 2009 CSCI 6268/TLEN 5550, Fall 2009 Sending a UDP packet • Assume IPv4 – Get IP address via DNS • Domain Name Service • Distributed database mapping textual names to IP addresses • Insecure – DNS spoofing – More on this later – Ok, so we have an IP address – And we presumably have a port # Pack it Up! Message UDP Header Src IP, Dest IP, Len, Chksm, TTL Src Port, Dest Port, Len, Chksm Eth Header IP Header Src addr, Dest addr, Chksm Ethernet addresses are called “MAC addresses” Ethernet checksum is actually appended to end of packet Ethernet MTU is 1500 bytes Routing on a Network • Usually done via OSPF or LSP for LANs – Open Shortest Path First, Link-State Protocol – These protocols assume “modest sized” networks – A routing protocol decides how to forward packets based on routing tables • BGP is used on backbone – Border Gateway Protocol – Routes using incomplete information Local Routing Table • Our local routing table (on host of user1) is not going to have a route to IP of user2 – Routing table will therefore send our packet to the gateway – Gateway is the machine/router on the “edge” of the network responsible for processing all incoming/outgoing traffic from/to the LAN • NAT boxing, firewalling, and other stuff is usually done here as well Getting to the Gateway • How to we route to the IP address of the gateway on our local Ethernet? – ARP (Address Resolution Protocol) • Translates IP addresses into MAC addresses • Caches old lookups, so we probably already have the MAC address of the gateway • If not, we send an ARP Request to the LAN, including the IP address whose MAC we seek • Owner (ie, the gateway) sends ARP Reply with his MAC address and we cache it – Usually, all other machines who hear the ARP Reply cache it as well – Leads to attacks… more later Sending to the Gateway...
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This note was uploaded on 03/11/2010 for the course CS 6268 taught by Professor Black during the Spring '09 term at University of Colombo.

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CSCI6268L19 - Foundations of Network and Foundations of...

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