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[root@Innova ~]# nmap -vv localhost
Starting Nmap 4.53 ( http://insecure.org ) at 2009-10-02 08:04 IST
Initiating SYN Stealth Scan at 08:04
Scanning localhost.localdomain (127.0.0.1) [1714 ports]
Discovered open port 80/tcp on 127.0.0.1
Discovered open port 443/tcp on 127.0.0.1
Discovered open port 22/tcp on 127.0.0.1
Discovered open port 8080/tcp on 127.0.0.1
Discovered open port 139/tcp on 127.0.0.1
Discovered open port 111/tcp on 127.0.0.1
Discovered open port 2049/tcp on 127.0.0.1
Discovered open port 445/tcp on 127.0.0.1
Discovered open port 977/tcp on 127.0.0.1
Discovered open port 8009/tcp on 127.0.0.1
Discovered open port 631/tcp on 127.0.0.1
Discovered open port 3306/tcp on 127.0.0.1
Completed SYN Stealth Scan at 08:04, 0.08s elapsed (1714 total ports)
Host localhost.localdomain (127.0.0.1) appears to be up ... good.
Interesting ports on localhost.localdomain (127.0.0.1):
Not shown: 1702 closed ports
PORT STATE SERVICE
22/tcp open ssh
80/tcp open http
111/tcp open rpcbind
139/tcp open netbios-ssn
443/tcp open https
445/tcp open microsoft-ds
631/tcp open ipp
977/tcp open unknown
2049/tcp open nfs
3306/tcp open mysql
8009/tcp open ajp13
8080/tcp open http-proxy
Read data files from: /usr/share/nmap
Nmap done: 1 IP address (1 host up) scanned in 0.157 seconds
Raw packets sent: 1714 (75.416KB) | Rcvd: 3440 (144.504KB)
I want to configure sendmail and so do I need to open port 25.
Any idea how can I open that port?
An open port means there is an application currently listening on the port. The fact that you don't see it opened means no application is currently using that port (which is fine since you _intend_ to use it).
Basically, just starting sendmail will actually 'open' the port.
Note though that you may need to configure your router/firewall or such in order for it to forward traffic on the given port (though this is highly improbable on port numbers under 1024).
Hope this helps, don't hesitate if you have further questions.
That's what I meant by a sane setup; by default all ports are closed unless explicitly set open. See my link.
BTW, default (again security issue) for sendmail's cfg is to only listen on 127.0.0.1 ie localhost.
You have to fixup the cfg file and restart sendmail.
Modify /etc/mail/sendmail.mc to listen on all interfaces
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