NETSTAT

If netstat is not installed, use apt-get or yum to install net-tools.

[root@server1 ~]# yum install net-tools

The netstat -l option can be used to only display connections with a status of LISTEN. A port that is listening is considered to be open, and vice versa. In this example, the machine is listening for IPv4 connections on port 53 (DNS), 631 (IPP), 123 (NTP).

[root@server1 ~]# netstat -l
Active Internet connections (w/o servers)
Proto  Recv-Q  Send-Q  Local Address    Foreign Address   State
tcp    0       0       localhost:53     0.0.0.0:*         LISTEN
tcp    0       0       localhost:631    0.0.0.0:*         LISTEN
tcp    0       0       ::1:631          [::]:*            LISTEN
udp    0       0       0.0.0.0:ntp      0.0.0.0:*         LISTEN

The netstat -p (process ID, name) option adds a column that displays the PID and process name.

[root@server1 ~]# netstat -lp
Active Internet connections (w/o servers)
Proto  Recv-Q  Send-Q  Local Address    Foreign Address   State    PID/Program Name
tcp    0       0       localhost:53     0.0.0.0:*         LISTEN   11938/dnsmasq
tcp    0       0       localhost:631    0.0.0.0:*         LISTEN   10294/cupsd
tcp    0       0       ::1:631          [::]:*            LISTEN   10294/cupsd

NMAP

Nmap can be used to display the open ports. In this example, ports 25 (SMTP) and 80 (HTTP) are open. -sS performs a TCP SYN Scan

[root@server1 ~]# nmap -sS localhost
Nmap scan report for localhost (127.0.0.1)
Host is up (0.00026s latency).
Not shown: 991 closed ports
PORT STATE SERVICE
25/tcp open smtp
80/tcp open http

To detect the operating system, add the -O option.

[root@server1 ~]# nmap -sT -O localhost