Network Debugging
ifconfig
ifconfig is the primary command to use for debugging
network interface problems, especially ifconfig -a. If
necessary, ifconfig -a statements can be inserted into
rc scripts to track interface condition during the boot process.
The first thing to check is that all values from ifconfig -a
are as expected (FLAGS=UP and RUNNING, MTU=1500 for ethernet,
INET=IP address, NETMASK correct (255.255.252.0 for Princeton),
BROADCAST correct, ETHER=ethernet address).
netstat
netstat provides useful information regarding traffic flow.
In particular, netstat -i lists statistics for each interface,
netstat -s provides a full listing of several counters,
and netstat -rs provides routing table statistics.
netstat -k provides a useful summary of several network-related
statistics, but this option is officially unsupported and may be removed
in a future release.
Here are some of the issues that can be revealed with netstat:
netstat -i: (Collis+Ierrs+Oerrs)/(Ipkts+Opkts)
> 2%: This may indicate a network hardware issue.netstat -i: (Collis/Opkts) > 10%: The interface
is overloaded. Traffic will need to be reduced or redistributed to
other interfaces or servers.netstat -i: (Ierrs/Ipkts) > 25%: Packets are
probably being dropped by the host, indicating an overloaded network
(and/or server). Retransmissions can be dropped by reducing the
rsize and wsize mount parameters to 2048
on the clients. Note that this is a temporary workaround, since this
has the net effect of reducing maximum NFS throughput on the segment.netstat -s: If significant numbers of packets arrive
with bad headers, bad data length or bad checksums, check the network
hardware.netstat -i: If there are more than 120 collisions/second,
the network is overloaded. See the suggestions above.netstat -i: If the sum of input and output packets is
higher than about 600 for a 10Mbs interface or 6000 for a 100Mbs interface,
the network segment is too busy. See the suggestions above.
snoop
snoop provides a snapshot of network traffic. This utility
gives a definitive answer to the question whether packets are arriving
at their destination.
ping and traceroute
ping -sRv (or traceroute, if available) can provide useful routing
information that may pinpoint the source of network congestion.
