nmap can be used with following command:

# nmap -sS -O 127.0.0.1

The answer will be something like:

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2010-07-08 11:31 CEST
Interesting ports on localhost.localdomain (127.0.0.1):
Not shown: 1663 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
25/tcp   open  smtp
80/tcp   open  http
110/tcp  open  pop3
111/tcp  open  rpcbind
143/tcp  open  imap
993/tcp  open  imaps
995/tcp  open  pop3s
3306/tcp open  mysql
8009/tcp open  ajp13
8080/tcp open  http-proxy
8443/tcp open  https-alt
No exact OS matches for host (If you know what OS is running on it, see http://www.insecure.org/cgi-bin/nmap-submit.cgi).
......
 
Uptime 15.472 days (since Wed Jun 23 00:12:46 2010)
 
Nmap finished: 1 IP address (1 host up) scanned in 9.558 seconds

and nmap can be used to scan remote hosts (keep in mind that many admins will notice this as hack attempt). Simple replace 127.0.0.1 with host IP address.

Another way to check open ports is via netstat. Netstat can show hidden ports and programs associated with ports. Simple execute next command as root:

# netstat -nap

The output list can be very long (depending from your network activity).

Note: Use this tips carefully. I’m not responsible for any potential damage…

Install tftp-server with

# yum install tftp-server xinetd

Now you will need to set up some things before you can start and use tftp server

# nano /etc/xinetd.d/tftp

service tftp
{
        socket_type             = dgram
        protocol                = udp
        wait                    = yes
        user                    = root
        server                  = /usr/sbin/in.tftpd
        server_args             = -s /var/lib/tftpboot
        disable                 = no
        per_source              = 11
        cps                     = 100 2
        flags                   = IPv4
}

To enable tftp server, set “disable” to no. Also, check server_args. In this case, all files will be/must be in /var/lib/tftpboot. If you have any problems with permissions, try to chmod 777 /var/lib/tftpboot and fix perms later.

Restart xinetd to apply settings you just made

# service xinetd restart

Setting “disable” to yes will disable tftp server and for security reasons, you should disable tftp whenever you don’t need it.

TFTP has been implemented on top of the User Datagram Protocol (UDP) using port number 69. TFTP is designed to be small and easy to implement, therefore, lacks most of the features of a regular FTP. TFTP only reads and writes files (or mail) from/to a remote server. It cannot list directories, and currently has no provisions for user authentication.

In TFTP, any transfer begins with a request to read or write a file, which also serves to request a connection. If the server grants the request, the connection is opened and the file is sent in fixed length blocks of 512 bytes. Each data packet contains one block of data, and must be acknowledged by an acknowledgment packet before the next packet can be sent. A data packet of less than 512 bytes signals termination of a transfer. If a packet gets lost in the network, the intended recipient will timeout and may retransmit his last packet (which may be data or an acknowledgment), thus causing the sender of the lost packet to retransmit that lost packet. The sender has to keep just one packet on hand for retransmission, since the lock step acknowledgment guarantees that all older packets have been received. Notice that both machines involved in a transfer are considered senders and receivers. One sends data and receives acknowledgments, the other sends acknowledgments and receives data.