AIX 5L Problem Determination - IBM Redbooks

address of the named server and try to ping it. If you can, then it is reachable. If
not, try different name resolution ordering.
Note: When you can ping the name server, it does not mean that the named
daemon is active on this system.
By default, resolver routines attempt to resolve names using BIND and DNS. If
the /etc/resolv.conf file does not exist, or if BIND or DNS could not find the name,
NIS is queried (if it is running). NIS is authoritative over the local /etc/hosts, so
the search will end here if it is running. If NIS is not running, then the local
/etc/hosts file is searched. If none of these services could find the name, then the
resolver routines return with HOST_NOT_FOUND. If all of the services are
unavailable, then the resolver routines return with SERVICE_UNAVAILABLE.
If you want to change the name resolution ordering so that NIS takes precedence
over the BIND and DNS, your /etc/netsvc.conf file should look like the following
example:
# cat /etc/netsvc.conf
hosts = nis,bind
You can override this setting by using the NSORDER environment variable:
# export NSORDER=local,bind
In this situation the /etc/hosts file will be examined for name resolution first.
8.3.1 The tcpdump and iptrace commands
You may need to see the real data crossing the wire to solve a problem. There
are two commands that let you see every incoming and outgoing packet from
your interface: tcpdump and iptrace.
The tcpdump command prints out the headers of packets captured on a specified
network interface. The following example shows a telnet session between hosts
9.3.240.59 and 9.3.240.58:
# tcpdump -i tr0 -n -I -t dst host 9.3.240.58
9.3.240.59.44183 > 9.3.240.58.23: S 1589597023:1589597023(0) win 16384 [tos 0x10]
9.3.240.58.23 > 9.3.240.59.44183: S 1272672076:1272672076(0) ack 1589597024 win
15972
9.3.240.59.44183 > 9.3.240.58.23: . ack 1 win 15972 [tos 0x10]
9.3.240.59.44183 > 9.3.240.58.23: . ack 1 win 15972 [tos 0x10]
9.3.240.59.44183 > 9.3.240.58.23: P 1:16(15) ack 1 win 15972 [tos 0x10]
9.3.240.59.44183 > 9.3.240.58.23: P 1:16(15) ack 1 win 15972 [tos 0x10]
9.3.240.59.44183 > 9.3.240.58.23: . ack 6 win 15972 [tos 0x10]
186 IBM ^ Certification Study Guide - AIX 5L Problem Determination Tools and Techniques