Traceroute is a program that shows you the route taken by packets through a network. It traces the route of packets from source to destination. It is commonly used when your network doesn’t work as well and you want to examine where can be the problem. Traceroute sends a UDP packet to the destination taking advantage of ICMP’s messages. ICMP has two types of messages: error-reporting messages and query messages. Query messages are generally used to diagnose network problems (the ping tool uses ICMP’s query messages). The error-reporting messages as the name suggest report errors if any in the IP packet; it uses Destination unreachable and Time exceeded errors message. It works by theses steps
Traceroute creates a UDP packet from the source to destination with a TTL(Time-to-live) = 1
The UDP packet reaches the first router where the router decrements the value of TTL by 1, thus making our UDP packet’s TTL = 0 and hence the packet gets dropped.
Noticing that the packet got dropped, it sends an ICMP message (Time exceeded) back to the source.
Traceroute makes a note of the router’s address and the time taken for the round-trip.
It sends two more packets in the same way to get an average value of the round-trip time. Usually, the first round-trip takes longer than the other two due to the delay in ARP finding the physical address, the address stays in the ARP cache during the second and the third time and hence the process speeds up.
The steps that have occurred up til now, occur again and again until the destination has been reached. The only change that happens is that the TTL is incremented by 1 when the UDP packet is to be sent to next router/host.
Once the destination is reached, Time exceeded ICMP message is NOT sent back this time because the destination has already been reached.
But, the UDP packet used by Traceroute specifies the destination port number to be one that is not usually used for UDP. Hence, when the destination computer verifies the headers of the UDP packet, the packet gets dropped due to the improper port being used and an ICMP message (this time – Destination Unreachable) is sent back to the source.
When Traceroute encounters this message, it understands that the destination has been reached. Even the destination is reached 3 times to get the average of the round-trip time.
A stateless Linux server is a centralized server in which no state exists on the single workstations. There may be scenarios when a state of a partilcuar system is meaningful (A snap shot is taken then) and the user wants all the other machines to be in that state. This is where the stateless Linux server comes into picture.
Features
It stores the prototypes of every machine.
It stores snapshots taken for those systems.
It stores home directories for those system.
Uses LDAP containing information of all systems to assist in finding out which snapshot (of state) should be running on which system.
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Internal commands are stored in the; same level as the operating system while external commands are stored on the hard disk among the other utility programs.
List The Three Main Parts Of An Operating System Command
The three main parts are the command, options and arguments.
What Is The Difference Between An Argument And An Option (or Switch)?
An argument is what the command should act on: it could be a filename, directory or name. An option is specified when you want to request additional information over and above the basic information each command supplies.
What Is The Purpose Of Online Help?
Online help provides information on each operating system command, the syntax, the options, the arguments with descriptive information.
Name Two Forms Of Security.
Two forms of security are Passwords and File Security with permissions specified.
What Command Do You Type To Find Help About The Command Who?
$ man who
What Is The Difference Between Home Directory And Working Directory?
Home directory is the directory you begin at when you log into the system. Working directory can be anywhere on the system and it is where you are currently working.
Which Directory Is Closer To The Top Of The File System Tree, Parent Directory Or Current Directory?
The parent directory is above the current directory, so it is closer to the root or top of the file system.
What Are Two Subtle Differences In Using The More And The Pg Commands?
With the more command you display another screenful by pressing the spacebar, with pg you press the return key.
The more command returns you automatically to the UNIX shell when completed, while pg waits until you press return.
When Is It Better To Use The More Command Rather Than Cat Command?
It is sometimes better to use the more command when you are viewing a file that will display over one screen.
How Many Vi Editor Modes Do You Know?
Three modes -
Command mode: letters or sequence of letters interactively command vi.
Insert mode: Text is inserted.
Command line mode: enter this mode by typing ":" and entry command line at the foot of the screen.
How Can You Terminate Vi Session?
Use command: ZZ that is save changes and quit.
Use command line: ":wq" that is write changes and quit.
Use command line: ":q!" to ignore changes and quit.