UNIX for Dummies Questions & Answers

Hi,

not very clear what are you really looking for, but maybe this can help:
If you're trying to look for what network services the server is acting as a client (that is, applications running on the server that are acting as clients with regard to some network services):
1. netstat: look at external ports/services the server has in state ESTABLISHED or CLOSE_WAIT; run this command if you have a login account on the server. You could find services provided by the server itself (i.e. applications from the monitored server connecting to TCP/UDP sockets on the server itself).
2. a network traffic sniffing tool (i.e. tcpdump, wireshark, etc.) may also do the job, provided they are run for a sufficiently long time (many applications may have permanet traffic with external services while others may just connect once in a while). You don't have to run tcpdump or wireshark on the machine itself, as long as you have a machine in the same subnet as the server you are monitoring with a network interface capable of 'promiscuous mode'.

When you asked for nmap usage, remember that nmap is generally used to discover what network services are listening on the target server. Unless you run nmap on the target server itself, the results returned by nmap may be less comprehensive than the results returned by running netstat (this time, you would look to results with TCP/UDP sockets in state LISTENING), because a firewall may filter some sockets.

see ya
fra

For example, the web server depends on DNS. But I'm not sure if the web server would be a DNS client?

But how do you identify services a server is a client for? Is it if the port is not a common port number?

Basically, I need to find out about server relationships- how to identify what services a server is dependent on. Like how to use CLI for example to find out that email depends on DNS.

Thanks, I appreciate any help!