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loopback address RRS feed

  • Question

  • Hi,

    I wanted to know what is the use of the Loop back address. What importance and when it is used and when it is helpful.

    C:\>netstat -ano|find /i "1620"
      TCP    0.0.0.0:1485           0.0.0.0:0              LISTENING       1620
      TCP    10.xx.xx.xx:1485     10.xxx.xx.xxx:50751    ESTABLISHED     1620
      TCP    10.xxx.xx.xx:1485     10.xxx.xx.xxx:61294    ESTABLISHED     1620
      TCP    127.0.0.1:63373        0.0.0.0:0              LISTENING       1620
      TCP    [::]:1485              [::]:0                 LISTENING       1620
      TCP    [::1]:63373            [::]:0                 LISTENING       1620

    FROM SQL Server ERRORLOG

    2012-08-13 19:50:41.97 spid7s      Server name is 'xxxxx\SQL2008'. This is an informational message only. No user action is required.
    2012-08-13 19:50:42.77 Server      The certificate [Cert Hash(sha1) "889C2785E3AFD5C0987E3B69436B6BF589572884"] was successfully loaded for encryption.
    2012-08-13 19:50:42.92 Server      Server is listening on [ 'any' <ipv6> 1485].
    2012-08-13 19:50:42.92 Server      Server is listening on [ 'any' <ipv4> 1485].
    2012-08-13 19:50:42.92 Server      Server local connection provider is ready to accept connection on [ \\.\pipe\SQLLocal\SQL2008 ].
    2012-08-13 19:50:42.92 Server      Server named pipe provider is ready to accept connection on [ \\.\pipe\MSSQL$SQL2008\sql\query ].
    2012-08-13 19:50:42.93 Server      Server is listening on [ ::1 <ipv6> 63373].
    2012-08-13 19:50:42.93 Server      Server is listening on [ 127.0.0.1 <ipv4> 63373].

    Thanks in Advance.

    Monday, August 13, 2012 2:27 PM

Answers

  • 2012-08-13 19:50:42.92 Server      Server is listening on [ 'any' <ipv6> 1485].
    2012-08-13 19:50:42.92 Server      Server is listening on [ 'any' <ipv4> 1485].
    2012-08-13 19:50:42.92 Server      Server local connection provider is ready to accept connection on [ \\.\pipe\SQLLocal\SQL2008 ].
    2012-08-13 19:50:42.92 Server      Server named pipe provider is ready to accept connection on [ \\.\pipe\MSSQL$SQL2008\sql\query ].
    2012-08-13 19:50:42.93 Server      Server is listening on [ ::1 <ipv6> 63373].
    2012-08-13 19:50:42.93 Server      Server is listening on [ 127.0.0.1 <ipv4> 63373].

    That's not the port for the "common" tcp/ip connections. Is there a following entry after this in the log?

        Dedicated admin connection support was established for listening locally on port 63373.

    Then this port is used by DAC = Dedicated Admin Connection; it's used for trouble shooting the SQL Server, if e.g. one process stress the SQL Server, so that normal connection do work/can't be connected.


    Olaf Helper
    * cogito ergo sum * errare humanum est * quote erat demonstrandum *
    Wenn ich denke, ist das ein Fehler und das beweise ich täglich
    Blog Xing


    • Edited by Olaf HelperMVP Tuesday, August 14, 2012 11:09 AM
    • Proposed as answer by amber zhang Wednesday, August 15, 2012 2:33 AM
    • Marked as answer by amber zhang Monday, August 20, 2012 2:01 AM
    Tuesday, August 14, 2012 11:08 AM

All replies

  • Hello,

    The loopback device = ip address 127.0.0.1 is used, when you want ot establish a local tcp/ip connection, because for this protocol you always need an ip address.


    Olaf Helper
    * cogito ergo sum * errare humanum est * quote erat demonstrandum *
    Wenn ich denke, ist das ein Fehler und das beweise ich täglich
    Blog Xing

    • Proposed as answer by Janos BerkeMVP Tuesday, August 14, 2012 7:34 AM
    Monday, August 13, 2012 2:33 PM
  • Loopback address is a special IP number (127.0.0.1) that is designated for the software loopback interface of a machine. The loopback interface has no hardware associated with it, and it is not physically connected to a network.

    The loopback interface allows IT professionals to test IP software without worrying about broken or corrupted drivers or hardware.


    Please click the Mark as Answer or Vote As Helpful if a post solves your problem or is helpful!

    Monday, August 13, 2012 7:36 PM
  • Thanks for the replies.

    @Olaf Helper - Is this mandatory? If i disable it, if there any impact on sql server. From sql server perpective, how it is utilized what is impact i dont have it? I can have shared memory if i want to have a local connection to sql server..

    Tuesday, August 14, 2012 4:37 AM
  • It is possible to config that SQL Server is listening on select network adapter, as the loopback device is; see Configure Network Protocols and Components. On the first tab you can set "Listen on all" to "False", then you can deactivate the adapter separately.

    But why, does it disturb anything/anyone if the SQL Server is listing on the loopback device?


    Olaf Helper
    * cogito ergo sum * errare humanum est * quote erat demonstrandum *
    Wenn ich denke, ist das ein Fehler und das beweise ich täglich
    Blog Xing

    Tuesday, August 14, 2012 6:36 AM
  • Basically, i have a client asking for this. He claims my sql listens at 1485 and it is using another port as well 63373. As per security, they wanted to restrict the port no to 1485.. So, i am unsure what is use of this loop back address and can we disable it and what kind of impact if loop bacck address for sql server.

    2012-08-13 19:50:42.92 Server      Server is listening on [ 'any' <ipv6> 1485].
    2012-08-13 19:50:42.92 Server      Server is listening on [ 'any' <ipv4> 1485].
    2012-08-13 19:50:42.92 Server      Server local connection provider is ready to accept connection on [ \\.\pipe\SQLLocal\SQL2008 ].
    2012-08-13 19:50:42.92 Server      Server named pipe provider is ready to accept connection on [ \\.\pipe\MSSQL$SQL2008\sql\query ].
    2012-08-13 19:50:42.93 Server      Server is listening on [ ::1 <ipv6> 63373].
    2012-08-13 19:50:42.93 Server      Server is listening on [ 127.0.0.1 <ipv4> 63373].

    Tuesday, August 14, 2012 10:48 AM
  • 2012-08-13 19:50:42.92 Server      Server is listening on [ 'any' <ipv6> 1485].
    2012-08-13 19:50:42.92 Server      Server is listening on [ 'any' <ipv4> 1485].
    2012-08-13 19:50:42.92 Server      Server local connection provider is ready to accept connection on [ \\.\pipe\SQLLocal\SQL2008 ].
    2012-08-13 19:50:42.92 Server      Server named pipe provider is ready to accept connection on [ \\.\pipe\MSSQL$SQL2008\sql\query ].
    2012-08-13 19:50:42.93 Server      Server is listening on [ ::1 <ipv6> 63373].
    2012-08-13 19:50:42.93 Server      Server is listening on [ 127.0.0.1 <ipv4> 63373].

    That's not the port for the "common" tcp/ip connections. Is there a following entry after this in the log?

        Dedicated admin connection support was established for listening locally on port 63373.

    Then this port is used by DAC = Dedicated Admin Connection; it's used for trouble shooting the SQL Server, if e.g. one process stress the SQL Server, so that normal connection do work/can't be connected.


    Olaf Helper
    * cogito ergo sum * errare humanum est * quote erat demonstrandum *
    Wenn ich denke, ist das ein Fehler und das beweise ich täglich
    Blog Xing


    • Edited by Olaf HelperMVP Tuesday, August 14, 2012 11:09 AM
    • Proposed as answer by amber zhang Wednesday, August 15, 2012 2:33 AM
    • Marked as answer by amber zhang Monday, August 20, 2012 2:01 AM
    Tuesday, August 14, 2012 11:08 AM