Antwort: Re: Antwort: Re: timeout by DNS? [Virus checked]

From: Vesna Vranesic (vesna.vranesic_at_RZB.AT)
Date: 11/23/04

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    Date:         Tue, 23 Nov 2004 14:10:19 +0100
To: aix-l@Princeton.EDU

    The algorithm says: try to reach the primary after RES_TIMEOUT for the
    first time and retry for RES_RETRY times in sum.
    The default value says concrete: if you can’t find the primary wait 5 sec
    and try again. After that, try another 3 times: wait
    another 10 sec, try again; wait 20 sec, try again; and finally, wait 40 sec
    and try for the 4th time to reach the primary. If
    the 4th try fails, look for the secondary.

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    Hi Holger,

    that is o.k. You change the values and it works (now 15 seconds). But what
    is the reason that the client don't go after 5 seconds over the second
    entrie in the /etc/resolv.conf.
    I think the client try to connect the first nameserver, after 5 seconds to
    the second nameserver. If the second nameserver is o.k. the nslookup comes
    back. After 5 seconds, not 15 seconds!

    I don't understand this!

    Best regards,
    Frank

                          Holger.VanKoll@SW
                          ISSCOM.COM An:
    aix-l@Princeton.EDU
                          Gesendet von: IBM Kopie:
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                          23.11.2004 13:49
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    for me it works, see below
    what values did you set those variables to?

    (0)root@sbe16098 173# time nslookup gd2y5d
    *** Can't find server name for address 93.47.226.200:No response from
    server
    Server: dnsc.swissptt.ch
    Address: 10.32.17.13

    Name: gd2y5d.swissptt.ch
    Address: 10.49.184.130

    real 1m15.02s
    user 0m0.00s
    sys 0m0.00s
    (0)root@sbe16098 174# export RES_RETRY=2
    (0)root@sbe16098 175# export RES_TIMEOUT=5
    (0)root@sbe16098 176# time nslookup gd2y5d
    *** Can't find server name for address 93.47.226.200:No response from
    server
    Server: dnsc.swissptt.ch
    Address: 10.32.17.13

    Name: gd2y5d.swissptt.ch
    Address: 10.49.184.130

    real 0m15.01s
    user 0m0.00s
    sys 0m0.00s
    (0)root@sbe16098 177# unset RES_RETRY RES_TIMEOUT
    (0)root@sbe16098 178# time nslookup gd2y5d
    *** Can't find server name for address 93.47.226.200:No response from
    server
    Server: dnsc.swissptt.ch
    Address: 10.32.17.13

    Name: gd2y5d.swissptt.ch
    Address: 10.49.184.130

    real 1m15.01s
    user 0m0.00s
    sys 0m0.00s

    -----Original Message-----
    From: IBM AIX Discussion List [mailto:aix-l@Princeton.EDU] On Behalf Of
    fmu@OERAG.DE
    Sent: Tuesday, November 23, 2004 1:35 PM
    To: aix-l@Princeton.EDU
    Subject: Antwort: Re: timeout by DNS? [Virus checked]

    Hi Vesna,

    thanks for your answer.
    Your explanation is good and something about this I found in the
    Internet.
    Which I do not understand, if the first entry is wrong or the first
    nameserver was crashed, the second entry should function after 5
    seconds.
    But why functioned that not and it does run all cycles through (75s).

    Best regards,
    Frank Mueller

                          Vesna Vranesic
                          <vesna.vranesic@R An:
    aix-l@Princeton.EDU
                          ZB.AT> Kopie:
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                          23.11.2004 13:17
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    The quotation I have found few years ago about this issue:

    "On AIX, a DNS query will wait a total of 75 seconds before

    failing over to the second nameserver entry in the resolv.conf.
    The 75 sec timeout period comes from two values RES_RETRY and
    RES_TIMEOUT. The way it works is the dns client will try to contact the
    dns server, then if it doesn't get an answer back it will wait 5 seconds
    and try again, then it will wait 10 secs and try again, 20 sec and try
    again, 40 secs and try again.
    .
    5 + 10 + 20 + 40 = 75 seconds
    If you want to change these on the dns client do the following:
    .
    # export RES_RETRY=value
    # export RES_TIMEOUT=value
    .
    The timeout is defined by two values :
          RES_TIMEOUT (default 5 sec)
          RES_RETRY (defautl 4 times)
          Each retry the initial timeout value is multipied by 2 .
          Example :
          5 + 10 + 20 + 40 = 75 <--- default timeout .
          The Cust can export other values to change this behaviour .
          Example :
          export RES_TIMEOUT=10
          export RES_RETRY=2
    .
          10 + 20 = 30 seconds timeout
    .
    In order for these to remain in effect after a reboot you need to vi

    /etc/environment and add these in the file."

    I am not sure if something has changed recently (read: for AIX5L).

    Ciao
    Vesna

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    | | hdkutz@HDKUTZ.DE |
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    | | 23.11.2004 12:48 |
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    On Tue, Nov 23, 2004 at 12:25:38PM +0100, Holger.VanKoll@SWISSCOM.COM
    wrote:
    > >Now I change the first entrie to a wrong IP-adress for the first
    > nameserver, the nslookup needs a long time to go to the second
    > nameserver.
    > then, dont do that ;-)
    >
    > >Now my question is, what is the parameter to tune the timeout for
    DNS?
    >
    > well, the timeouts for nslookup , host, dig are different - so it
    > looks like its up to the application nslookup has (afair) a
    -timeout=option
    >
    > I doubt there is a system-wide parameter that you can tune
    >
    > if dns-response time is critical, run a local caching-only nameserver
    Hmm. Why isn't "options rotate" like in Linux or Solaris implemented in
    AIX?
    Recently we had a problem with our Production Nameserver. All Solaris
    and Linux Boxes went fine, only AIX got into trouble resolving Names
    from the secondary Nameserver, waiting for timeout of the primary DNS.

    Cheers,
    ku 

    --
C-3PO:
        Sir, the possibility of successfully navigating
        an asteroid field is approximately 3,720 to 1!
Han Solo:
        Never tell me the odds!
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