TCP IP Illustrated

TCP IP Illustrated

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Unformatted text preview: pt key (which terminates tcpdump) and we won't show the number of packets received and dropped. (Dropped packets are those that arrived faster than tcpdump could keep up with. Since the examples in the text were often run on an otherwise idle network, this is always 0.) sun % tcpdump -e tcpdump: listening on le0 09:11:22.642008 0:0:c0:6f:2d:40 ff:ff:f f:ff:ff:ff arp 60: arp who-has svr4 tell bsdi 09:11:22.644182 0;0:c0:c2:9b:26 0:0:c0:6f:2d:40 arp 60: arp reply svr4 is-at 0:0:c0:c2:9b:26 09:11:22.644839 0:0:c0:6f:2d:40 0:0:c0:c2:9b:26 ip 60: bsdi.1030 > svr4.discard: S 596459521:596459521(0) win 4096 <mss 1024> [tos 0x10] 09:11:22.649842 0:0:c0:c2:9b:26 0:0:c0:6f:2d:40 ip 60: svr4.discard > bsdi.1030: S 3562228225:3562228225(0) ack 596459522 win 4096 <mss 1024> 09:11:22.651623 0:0:c0:6f:2d:40 0:0:c0:c2:9b:26 ip 60: bsdi.1030 > svr4.discard: . ack 1 win 4096 [tos 0x10] 4 other packets that we don't show type our interrupt key to terminate ^? 9 packets received by filter 0 packets dropped by kernel Figure A.3 tcpdump output for Figure 4.4. A.5 Security Considerations It should be obvious that tapping into a network's traffic lets you see many things you shouldn't see. For example, the passwords typed by users of applications such as Telnet and FTP are transmitted across the network exactly as the user enters them. (This is called the cleartext representation of the password, in comparison to the encrypted representation. It is the encrypted representation that is stored in the Unix password file, normally /etc/passwd or /etc/shadow.) Nevertheless, there are many times when a network administrator needs to use a tool such as tcpdump to diagnose network problems. Our use of tcpdump is as a learning tool, to see what really gets transmitted across the file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/append_a.htm (6 of 7) [12/09/2001 14.47.59] Appendix A: The tcpdump Program network. Access to tcpdump, and similar vendor-supplied utilities, depends on the system. Under SunOS, for example, acc...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.

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