System information

Troubleshooting TCP/IP
Book Title
7-104
Possible Problem Solution
Default gateway is not
specified or is misconfigured
on local or remote host
If hosts are not running routed, a default gateway should be configured.
Step 1 Determine whether the local and remote hosts have a default gateway
specification. Use the following UNIX command:
unix-host% netstat -rn
Check the output of this command for a default gateway specification.
Step 2 If the default gateway specification is incorrect, or if it is not present at
all, you can change or add a default gateway using the following UNIX
command at the local host:
unix-host% route add default address 1
where address is the IP address of the default gateway (the router local to
the host). The value 1 indicates that the specified gateway is one hop
away.
You might need to reboot the host for this change to take effect.
Step 3 It is recommended that you specify a default gateway as part of the boot
process. Specify the IP address of the gateway in the /etc/defaultrouter
UNIX host file. This filename might be different on your UNIX system.
If you are working with a PC or a Macintosh, consult the corresponding
documentation to determine how to set the default gateway.
Misconfigured or
missing routed default
routes
Step 1 If the host is running routed, use the netstat -rn UNIX command to view
the host’s routing table. The entry with Destination “default” denotes the
default route.
Step 2 The default route entry should point to the router that has the route to the
remote host. If there is no default route entry, use the route UNIX
command to manually configure the default gateway:
unix-host% route add default address 1
DNS
1
host table is incomplete If the DNS receives a lookup request for a host name that is not in its cache, it
cannot reply to the request, and the client cannot establish a connection.
Step 1 At the UNIX prompt, enter the following command:
unix-host% host address
where address is the IP address of a server, router, or other network node.
Step 2 If the result of this host command is Host not found, but you can open
the connection using the host’s IP address rather than its name, try
connecting to other hosts using their names. If connections to other hosts
can be opened using their names, then the host table might be incomplete.
Add hostname-to-address mappings to the DNS cache for every host on
the network.
Step 3 If you cannot open any connections using host names, the DNS might not
be up and running. For troubleshooting information, see the following
problem, “DNS is not up and running.”
DNS is not up and running If issuing the host command at the UNIX prompt returns a Host not found
message, but you are able to open a connection using the host’s IP address, the
DNS might not be up and running.
Consult the DNS software documentation or your system administrator for
information on configuring and enabling
the DNS.