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PM31474: SMTP SHOULD WRITE A MESSAGE TO THE CONSOLE WHEN IT DISCARDS A TRUNCATED DNS RESPONSE

A fix is available

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APAR status

  • Closed as program error.

Error description

  • SMTP has it's own resolver function and uses the UDP protocol to
    communicate with the DNS. If a DNS response has the truncate bit
    set (meaning important information is missing from the reply
    because the information would not fit in a UDP packet), SMTP
    discards the reply.  This can cause mail to not be delivered if
    SMTP is not able to resolve the hostname to an IP address.  The
    only way to tell this is happening is to turn on RESOLVER TRACE
    in SMTP and look for message
    EZA5526I 01/26/11 14:35:41# 1903 Truncation Bit set in
       Nameserver Response
    .
    SMTP should issue a message to the console when it discards
    the first truncated DNS reply.  This provides a way to know
    a problem may be going on that needs to be investigated.
    .
    KEYWORDS: SMTP SMTPD truncation truncated not sending mail
    delivering hung
    .
    VERIFICATION STEPS:
    F smtp_jobname,SMSG,QU will show mail is on the resolver queues:
    EZA5594I SMSG QUEUE Output
    ----- Mail Queues -----
    Spool Queue:         0
    Undeliverable Queue: 0
    --- Resolver Queues ---
    Process Queue:       0
    Send Queue:          0
    Wait Queue:          100
    Retry Queue:         50
    Completed Queue:     0
    Error Queue:         0
       *END OF OUTPUT*
    .
    Turn on resolver trace with:
     F smtp_jobname,SMSG,TRACE
    Resolver trace will show message
    EZA5526I
    .
    To turn off resolver trace, isssue:
    F smtp_jobname,SMSG,NOTRACE
    

Local fix

  • There are a couple of things that can be done:
    1) Change SMTP to use IPMAILERADDRESS x.x.x.x where x.x.x.x is
       the IP address of the mail server and code RESOLVERUSAGE NO.
    2) Change the DNS so that it doesn't return so much information
       so that it does not turn on the truncate bit in the replies
    

Problem summary

  • ****************************************************************
    * USERS AFFECTED: All users of the IBM Communications Server   *
    *                 for z/OS Version 1 Release(s) 11, and 12     *
    *                 IP: SMTP                                     *
    ****************************************************************
    * PROBLEM DESCRIPTION: SMTP does not write a message to the    *
    *                      system console when it discards a       *
    *                      truncated DNS response                  *
    ****************************************************************
    * RECOMMENDATION:                                              *
    ****************************************************************
    SMTP does not generate a console message when it discards a
    truncated DNS response. The customer must do a TSO command
    "SMSG <SMTP_STARTED_PROCEDURE> QUEUE" to see all the mail
    messages that are still on the resolver queues waiting for
    retry. They would then activate resolver traces by doing a
    TSO command "SMSG <SMTP_STARTED_PROCEDURE> TRACE". In reviewing
    the resolver traces the following message is found:
    EZA5526I 02/03/11 13:25:08# 1 Truncation Bit set in Nameserver
    Response
    +-------------------------------------------------------------+
    + Please check our Communications Server for OS/390 homepages +
    + for common networking tips and fixes.  The URL for these    +
    + homepages can be found in Informational APAR II11334.       +
    +-------------------------------------------------------------+
    

Problem conclusion

  • SMTP will generate a new message EZA5208E once when this
    condition is first detected. Customers can automate on this
    message to then activate resolver trace to determine which
    domain name server(s) have the problem and scope of the
    problem. For example, if all mail messages are being blocked
    or just a few due to an unusual mailbox name.
    
    ****************************************************************
    * FUNCTION AFFECTED: Communications Server TCP/IP (PM31474)    *
    *                     SMTP                                     *
    ****************************************************************
    * DESCRIPTION      : Documentation updates                     *
    ****************************************************************
    * TIMING           : None                                      *
    ****************************************************************
    Added new message EZA5208E and updated z/OS IP Diagnostic
    Guide.
    
    ----------------------------------------------------------------
    z/OS Communications Server IP Messages:
    Volume 1 (EZA)
    Document Number SC31-8783
    ----------------------------------------------------------------
    EZA5208E <mm/dd/yy> <hh:mm:ss> DNS response was truncated
    so a name cannot be resolved
    
    Explanation:
    SMTP has received a response to a domain name server (DNS)
    query. The response was truncated because it was too large to
    fit in a UDP packet. SMTP supports only UDP protocol for
    name resolution. Because the DNS response was incomplete,
    it was discarded. As a result, the mail messages remain
    on the resolver queues and will be retried. This message will
    be generated only the first time the situation is detected.
    
    <mm/dd/yy> is the date (month/day/year) when the DNS response
    was received.
    
    <hh:mm:ss> is the time (hours:minutes:seconds) when the DNS
    response was received.
    
    Examples:
    
    SMTP output file with resolver traces active:
    EZA5526I 02/03/11 13:25:08#   1 Truncation Bit set in Nameserver
    Response
    EZA5208E 02/03/11 13:25:08 DNS response was truncated so
    a name cannot be resolved
    
    System console:
    $HASP100 SMTP     ON STCINRDR
    IEF695I START SMTP     WITH JOBNAME SMTP     IS ASSIGNED TO USER
    IBMUSER, GROUP SYS1
    $HASP373 SMTP     STARTED
    IEF403I SMTP - STARTED - TIME=13.40.18
    EZY1876I SMTP STACK FUNCTIONS STARTED WITH PARAMETER SMTP,PARM=,
    ERRFILE(SYSERR),NOSPIE/.
    EZA5208E 02/03/11 13:40:20 DNS response was truncated so
    a name cannot be resolved
    
    System Action: SMTP continues.
    
    Operator Response: None.
    
    User Response: Not applicable.
    
    System Programmer Response:
    Contact the administrator for the name server and inform them
    of the problem. If the problem can be resolved at the DNS,
    SMTP will automatically recover because it will retry the
    DNS query. The retry time of the DNS query depends on your
    SMTP configuration setting, see the RESOLVERRETRYINT statement
    in the z/OS IP Configuration Reference.
    
    If you are using the IPMAILERNAME statement and the problem
    cannot be resolved at the DNS, you can switch to using the
    IPMAILERADDRESS statement until the DNS is modified. If you
    do switch to the address indicated by the IPMAILERADDRESS
    statement and you want all the mail sent to that address,
    then set the RESOLVERUSAGE statement to NO. See the SMTP
    configuration statements IPMAILERNAME and IPMAILERADDRESS
    in the z/OS IP Configuration Reference.
    
    You can determine if the problem is resolved by entering
    the TSO command SMSG <SMTP_STARTED_TASK_NAME> QUEUE. If the
    problem is resolved, the number of mail messages on the
    resolver queues will decrease.
    
    Problem Determination:
    If you are using multiple DNS servers, you might need
    to determine which server returned this response.
    You can dynamically activate resolver traces by entering
    the TSO command SMSG <SMTP_STARTED_TASK_NAME> TRACE. Review
    the resolver traces in the SMTP output file. Look for
    message EZA5526I, which contains the DNS query number, to
    determine which DNS is issuing this response and what
    name is being resolved.
    
    Source: z/OS Communications Server TCP/IP: SMTP
    
    Module: SMTPRES
    
    Routing code: 2,10
    
    Descriptor code: 4
    
    Automation: This message is a candidate for automation. You can
    use this message to alert you to a DNS server problem.
    
    ----------------------------------------------------------------
    z/OS Communications Server IP Diagnosis:
    Document Number GC31-8782
    ----------------------------------------------------------------
    Chapter: Diagnosing Simple Mail Transfer Protocol (SMTP)
    problem
    
    Under the following section add new first bullet:
    If the problem is that mail was not forwarded to a remote TCP/IP
    host:
    
    Has the host name been resolved to an IP address?
    1. Check the system console or the SMTP output log for an
    EZA5208E message. If this is present, then SMTP has received
    a domain name server (DNS) response with the truncation bit
    on in the packet. SMTP discards these packets because the
    DNS response is not complete. SMTP will continue to retry
    these DNS queries for a configured amount of time. But if
    the DNS response is the same the mail message will
    eventually become undeliverable. Use RESOLVER traces to
    determine the DNS sending the response and what is the
    unresolved name. Look for message EZA5526I in the resolver
    traces. Changes are needed to the DNS to resolve this problem.
    
    Note that the CSSMTP application handles the DNS truncation
    bit differently since it uses the system resolver whereas SMTP
    has its own resolution code. So migration to the new CSSMTP
    application would avoid this problem.
    
    2. Run RESOLVER trace to see if the host name is resolved
    correctly. The RESOLVER trace is explained in SMTP RESOLVER
    trace.
    
    * Cross Reference between External and Internal Names
    EZAAA00H (MSSMTP  )  EZABB01X (SMTP    )  EZABB025 (SMTPRES )
    EZAAA00H (MSSMTP  )  EZABB01X (SMTP    )  EZABB025 (SMTPRES )
    

Temporary fix

Comments

APAR Information

  • APAR number

    PM31474

  • Reported component name

    TCP/IP V3 MVS

  • Reported component ID

    5655HAL00

  • Reported release

    1B0

  • Status

    CLOSED PER

  • PE

    NoPE

  • HIPER

    NoHIPER

  • Special Attention

    NoSpecatt / Xsystem

  • Submitted date

    2011-01-28

  • Closed date

    2011-02-21

  • Last modified date

    2011-04-04

  • APAR is sysrouted FROM one or more of the following:

  • APAR is sysrouted TO one or more of the following:

    UK65090 UK65091

Modules/Macros

  • EZAAA00H EZABB01X EZABB025
    

Publications Referenced
GC31878211SC31878311GC31878210SC31878310 

Fix information

  • Fixed component name

    TCP/IP V3 MVS

  • Fixed component ID

    5655HAL00

Applicable component levels

  • R1B0 PSY UK65090

       UP11/03/22 P F103

  • R1C0 PSY UK65091

       UP11/03/22 P F103

Fix is available

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Document Information

Modified date:
04 April 2011