Loop back address - TCP-IP

This is a discussion on Loop back address - TCP-IP ; Hi all, 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 ...

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Thread: Loop back address

  1. Loop back address

    Hi all,

    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.

    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 ?

  2. Re: Loop back address

    127.0.0.1 is used for 'localhost'

    / Kari Hurtta

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

  3. Re: Loop back address

    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.

  4. Re: Loop back address

    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

    --
    _ __ _
    __| |_ __ / _| |_ 01100100 01101101
    / _` | ' \| _| ' \ 01100110 01101000
    \__,_|_|_|_|_| |_||_| dmfh(-2)dmfh.cx


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