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Network Debugging

Network Debugging. Organizational Communications and Technologies Prithvi Rao H. John Heinz III School of Public Policy and Management Carnegie Mellon University. Objectives. Present some examples of debugging tools. Readings.

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Network Debugging

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  1. Network Debugging Organizational Communications and Technologies Prithvi Rao H. John Heinz III School of Public Policy and Management Carnegie Mellon University

  2. Objectives • Present some examples of debugging tools

  3. Readings Suggested reading: Class notes Practical Internetworking with TCP/IP and UNIX (not required)

  4. Tools for Debugging • Most tools are available for Unix based systems (most servers are likely to be UNIX) • Some tools are freely available (traceroute and dig) • Some proprietary tools exist (etherfind)

  5. Ping • Simple yet valuable network debugging tool • Sends an ICMP echo request message to remote host • Remote host sends back an ICMP echo and reply message • Sending an echo is called “pinging” • Good baseline test of connectivity • Successful ping implies that IP packets can be exchanged • Network routing is also tested with pingExample % ping unix5.andrew.cmu.edu unix5.andrew.cmu.edu is alive

  6. Ping • Variant of first form of pingExample% ping -s akasha.tic.com 5 no response from akasha.tic.com64 bytes from akasha.tic.com (192.12.23.130) icmp_seq = 0, time = 6 ms 64 bytes from akasha.tic.com (192.12.23.130) icmp_seq = 1, time = 5 ms64 bytes from akasha.tic.com (192.12.23.130) icmp_seq =2, time = 5 ms In this case command times out after 5 seconds

  7. Ping • Echo request and echo reply are part of ICMP • Ping does not rely on application server running on remote host • Relies on network interface card to be configured properly • Unix host in single-user-mode will respond to ping commands

  8. Remote Script • Script that permits running of arbitrary command on remote machine • Uses the remote shell (rsh) protocolExample % remote uptime –h unix5.andrew.cmu.edu Host: unix5.andrew.cmu.edu 8:30 am up 5 days <time> <#users> <load average>

  9. Netstat • Netstat is jack-of-all-trades network tool • Can display connections, interfaces, routing tables and traffic statistics • Active connection display • Statistics display • Interfaces • Routing

  10. Routing: netstat -r • Displays kernel routing tableExample% netstat –r Destination Gateway Flags Refcnt Use Interface 127.0.0.1 127.0.01 UH 12 244870 le0 default 192.12.23.132 UG 0 51 le0 192.12.23.128 192.12.23.129 U 16 8248341 le0

  11. Routing: netstat -r • Gateway is the IP address of the next hop to which to send address • Flags is the status of each route • Refcnt is the current number of active TCP connections • Use is the total number of IP packets sent using route • Interface is the logical name of the local interface

  12. Routing: netstat -i • Can display status of all interfaces Example% netstat –i –n Name MTU Net/Dest Address Ipkts Ierrs Opkts Oerrs Collle0 1500 192.12.23.128192.12.23.1298141411 0 7902647 0 61

  13. Routing: netstat -i • Name is logical name of network • MTU is size in bytes of MTU interface • Net/Dest IP address of network to which interface is connected or that of end-point of link • Address is local IP address of interface • Ipkts is count of datalink frames received on link since last bootstrap • Ierrs is number of datalink frames received with errors and dropped by interface • Opkts is datalink frames sent on interface since last boot • Oerrs count of frames not sent due to output errors • Coll is count of collisions detected by this interface

  14. Routing: netstat -f • Permits looking at TCP and UDP packets onlyExample% netstat –f inetActive internet connectionsProto Rec-Q Req-Q Local Address Foreign Address (state)tcp 0 4096 kiwilabs.com kia.smtp ESTABLISHED

  15. Routing: netstat -f • Proto is the protocol (TCP or UDP) • Recv-Q number of bytes in socket input queue • Send-Q number of bytes in socket output queue • Local address set of period separated names • Foreign address is remote socket address given in format of local address • (state) is current state of TCP connection. Field is always empty for UDP connections

  16. Traceroute • Traces route an IP packet takes to destination host • Takes single hostname argument and lists all intermediate router • Sends three UDP messages encapsulated in an IP packet and records the round-trip time in milliseconds for each message sent to intermediate router • Lost message or router that does not respond is denoted with a “*”Example% traceroute unix5.andrew.cmu.edu

  17. ARP: Address Resolution Protocol • Arp command permits the examining and modifying of local ARP cacheExample% arp -a

  18. Etherfind • Specific to Sun for tracing Ethernet frames • Performs functions of a network analyzer • Output can be piped to other unix tools (grep, awk, sed) • Can display selective frames • Mainly useful for TCP/IIPExample% etherfind –i le0 –v –t greater 0

  19. nslookup • Simple tool for querying DNS servers • Without arguments user is prompted for queries Example% nslookup <unix5> > unix5> ls kiwilabs.com> set type = pttr> set type = any

  20. Summary • Presented examples of network debugging tools • Discussed the use of these tools for various purposes

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