Linux scan for open ports

A nice command to scan open ports for a host/ip is:

nmap -sS -O 127.0.0.1

You will get an answer like this:

PORT     STATE    SERVICE
22/tcp   open     ssh
25/tcp   open     smtp
80/tcp   open     http
110/tcp  open     pop3
111/tcp  open     rpcbind
119/tcp  open     nntp
135/tcp  filtered msrpc
137/tcp  filtered netbios-ns
138/tcp  filtered netbios-dgm
139/tcp  filtered netbios-ssn
445/tcp  filtered microsoft-ds
1720/tcp filtered H.323/Q.931
3306/tcp open     mysql
8080/tcp open     http-proxy
8081/tcp open     blackice-icecap
8443/tcp open     https-alt
Device type: general purpose

If you need to see the open ports from inside (your server) use netstat:

netstat -a

You will get something like:

Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address           Foreign Address         State
tcp        0      0 *:cvspserver            *:*                     LISTEN
tcp        0      0 *:mysql                 *:*                     LISTEN
tcp        0      0 *:netbios-ssn           *:*                     LISTEN
tcp        0      0 *:sunrpc                *:*                     LISTEN
tcp        0      0 *:http-alt              *:*                     LISTEN
tcp        0      0 *:microsoft-ds          *:*                     LISTEN
tcp        0      0 *:www-http              *:*                     LISTEN
tcp        0      0 *:ssh                   *:*                     LISTEN
tcp        0      0 *:https                 *:*                     LISTEN
udp        0      0 *:netbios-ns            *:*
udp        0      0 *:netbios-dgm           *:*
udp        0      0 *:tftp                  *:*
.......

Active UNIX domain sockets (servers and established)
Proto RefCnt Flags       Type       State         I-Node Path
unix  2      [ ACC ]     STREAM     LISTENING     5170   @/tmp/hald-local/dbus-B                                                             cG6NtYfGm
unix  2      [ ACC ]     STREAM     LISTENING     7389   /var/lib/mysql/mysql.so                                                             ck
unix  2      [ ]         DGRAM                    8071089 /var/lib/dhcp/dev/log
unix  2      [ ]         DGRAM                    8071091 /var/lib/named/dev/log
unix  2      [ ]         DGRAM                    2177   @/org/kernel/udev/udevd
unix  2      [ ]         DGRAM                    5171   @/org/freedesktop/hal/u                                                             dev_event
............