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

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)

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 ping Example % ping unix5.andrew.cmu.edu unix5.andrew.cmu.edu is alive

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

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

Remote Script Script that permits running of arbitrary command on remote machine Uses the remote shell (rsh) protocol Example % remote uptime –h unix5.andrew.cmu.edu Host: unix5.andrew.cmu.edu 8:30 am up 5 days

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

Routing: netstat -r Displays kernel routing table Example % netstat –r Destination Gateway Flags Refcnt Use Interface UH le0 default UG 0 51 le U le0

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

Routing: netstat -i Can display status of all interfaces Example % netstat –i –n Name MTU Net/Dest Address Ipkts Ierrs Opkts Oerrs Coll le

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

Routing: netstat -f Permits looking at TCP and UDP packets only Example % netstat –f inet Active internet connections Proto Rec-Q Req-Q Local Address Foreign Address (state) tcp kiwilabs.com kia.smtp ESTABLISHED

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

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

ARP: Address Resolution Protocol Arp command permits the examining and modifying of local ARP cache Example % arp -a

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/IIP Example % etherfind –i le0 –v –t greater 0

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

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