How Do I find the Port Number For an Internet Service?

[keiusui]

Using a Perl script, how would I find out what port number sendmail uses?

2006-02-22 Retitled by g0n, as per Monastery guidelines
Original title: 'port number'

[jbrugger]

monarch is right.
On your own system you'd could cat /etc/services as root (i'd never do that in a script), but if you've got a proper configured linux like system, certain ports are reserved for proper use, indeed smtp (mail) connects to 25, pop connects to 110 and secure mail connects to 465.
There are many many more as you probably know. But why do you want to know to how to find out what port number sendmail uses in Perl?

[monarch]

print `nmap -sS -v -v $hostname` is one way for detecting the port number on a remote system.

For detecting what is going on in your own system you might want to try print `netstat -ap` and look for what ports sendmail has bound to.

Assuming you really want to port scan yourself you could try Net::Telnet and try connecting to every port you can and see if you get the introductory SMTP message. Or implement it yourself using IO::Socket::INET.

Personally I would just try port 25 as a first guess, however.