127.0.0.1 is used for 'localhost'

/ Kari Hurtta

hurtta@Hurtta06k:~$ head -1 /etc/hosts
127.0.0.1 localhost
hurtta@Hurtta06k:~$

On Aug 1, 3:34*am, aark...@gmail.com wrote:
> The following is the definition of loop back *address ,i saw in the
> book tcp/ip guide
>
> special range of addresses ,127.0.0.0 to 127.255.255.255 , is set
> aside for loop back functionality.IP datagrams sent by a host *to a
> 127.x.x.x loop back address are not passed down to the data link layer
> for transmission,instead they loopback
> to the source device at the IP level. In essence this short circuits
> the normal protocol stack; data is sent by a device's layer 3 IP
> implementation and immediately received by it.


There are a number of TCP/IP stacks where the loopback driver is, in
fact, a driver at the same level as a normal Ethernet card device
driver. It just doesn't talk to much real hardware. Whether that
approach is used, or the packet flow is actually short circuited that
at the IP layer is entirely implementation dependent.


> This loopback range is used for testing the TCP/IP protocol
> implementation on a host. since the lower layers are short
> circuited ,sending to a loopback address allows you to isolate and
> test the higher layers ,IP and above without interference from lower
> layers. 127.0.0.1 is the address most commonly used for testing
> purposes
>
> now my question is as follows
>
> 1)This loopback range is used for testing the TCP/IP protocol
> implementation on a host. since the lower layers are short
> circuited ,sending to a loopback address allows you to isolate and
> test the higher layers
>
> can any *one give example(s) for this or a situation where this is
> used ?


The loopback address is often used when a service resides on the same
machine as a client. As far as usage goes, that's far more common
than for testing the TCP/IP stack.

On 2008-08-01 04:34:06 -0400, aarklon@gmail.com said:

> can any one give example(s) for this or a situation where this is
> used ?

Usually, more than one loopback address can be defined on most OS's
that have a TCP/IP stack - small, embedded operating systems may not be
able to do this, however.

A previous poster mentioned services on a host being attached to the
loopback interface - very common and efficient use of resources on the
host, especially programmatically - write once, and a client / server
application can even run on a single box. A lot of the common X-Windows
software suites make extensive use of this (KDE, Gnome) in popular
Linux distributions.

Another use for loopbacks that's common is security. SSH tunneling,
where the SSH secure-shell software and server is used to encrypt
otherwise unencrypted data, often makes use of loopback interfaces. SSH
attaches a listener process to a loopback address and port the user
specifies, also connecting to the remote SSH server, which will forward
traffic (if the configuration permits) to the desired "real"
destination. The "real" destination can also be an interface on the
remote server, providing an encrypted connection for any service. I use
this to "wrap" my POP3 e-mail and some other services directly to the
hosts the services reside on.

Directly addressing your post - loopbacks can be really useful for
simulating a client-server connection between two pieces of an
application, and taking a packet capture to debug issues - the traffic
stays on your host, where you can enjoy complete control over what the
application does and get packet dumps anytime you want, etc.

/dmfh

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