Study Notes

IP protocol group contains the following four items: IP(Internet protocol),TCP,UDP and ICMP.I’ll explain these words here.

IP: abbreviation of Internet Protocol,defined in RFC791,is a protocol without connection which means each data packet is delivered to the network and routed to the target host.IP protocol could not guaranteed all the packets would be sent to the target and even in the original order.

TCP: abbreviation of Transmisson Control Protocol,defined in RFC793.TCP enable two hosts to establish a connection and exchange streams of data.It can guatanee both the integrity and the sequence of packets.IP uses ip address to route the packets to the target,but Ports are uses by TCP to route the packets to the identified processes of the target operating systems.

UDP: abbreviation of User Datagram Protocol,defined in RFC 768,which is a protocol based on no connection as TCP.As same as IP,UDP could not guarantee the integrity and sequence of packets,however,UDP uses ports to route the packets to the identified processes running in target host.

ICMP: abbreviation of Internet Control Message Protocol,defined in RFC 792.ICMP is used to help other three protocols to do those what they could not resolve.for example,we could use ICMP to identify whether the specified system is running or tested by pinging.

Issue the following telnet command to connect any running service of target
system:

# telnet <host> <portnumber> 

Last afternoon,it is my first time to connect remote host with telnet command.when I was still happy,I found no commands of Linux could be executed in the console.however,some classic dos commands like dir or cd could be used to show the directory or return the up level. I tried many Linux commands,but all failed.It confused my a lot because I could not find the way to operate the remote machine.
Finally,I had to ask the custome for the reason.He said what I logged by telnet early is a firewall device.He also tell me to connect to the really host by telnet after logging in the firewall system.I did what he said,and logged in the remote target system by telnet successfully,of course all Linux commands work well.

I found the speed was slower than operation in localhost.and when I used VI to edit /etc/fstab,I could not insert or delete the letters.Another engineer siting by me tried this again,but he even could not save his working on the /etc/fstab.It’s strange.

X-windows system
X-windows is the GUI environment of UNIX,and its port number starts from 6000 to 6063.”xhost” tools is used to protect the security of X-window system.
server access control program for X:

# xhost + system-name
# xhost +
# whatis xhost

Show who is logged on and what they are doing:

# whatis w

TCP/IP interface configuration and router utility:

# whatis ip