PH66092: MQ Z/OS: ABN= 0C4-00000004 LOC=CSQXBNTM.????????+00ED2 WITH THE PSW IN CSQXBNT1

APAR status

• Closed as program error.

Error description

• The CHIN log has errors: DUMP TITLE=CSQ1,ABN=
  0C4-00000004,C=MQ900.930.CHIN,M=CSQXDISP,L
  OC=CSQXBNTM.????????+00ED2 CSQX112E +CSQ1 CSQXDISP Dispatcher
  process error, TCB=008B41D8 reason=0C4000-00000004. In the dump,
  the PSW for the ABEND0C4 is in module CSQXBNT1 at offset x'8D0'.
  The problem occurs when MQ attempts to use a reverse DNS lookup
  to establish the hostname associated with the IP address of a
  new channel connection, and the DNS server is returning a large
  number of matching hostnames. In the reported case, DNS returned
  35 hostnames. MQ has logic to handle long lists of hostnames by
  limiting the number which get copied into the channel's storage.
  However, there are errors in the logic that manages these lists:
  1) If tracing is active, an attempt to record the oversized list
  is made using a trace call with the wrong number of parameters.
  This results in residual data being treated as a length for the
  data to be traced. If the residual data is a valid length then
  the wrong data will be traced but processing will continue. If
  the residual data represents a bad length (e.g. a very large
  value that causes unallocated storage to be accessed), then the
  ABEND0C4 can occur. 2) If tracing is inactive or the trace
  succeeds, then the code truncates the buffer size for storing
  the hostnames, but it goes on to try to copy all the hostnames
  into the buffer. This results in data over-running the end of
  the allocated buffer and potentially overlaying storage
  allocated for other purposes.
  

Local fix

• If possible, reduce the number of host names in DNS associated
  with a particular ip address. If that is not possible, the
  reverse lookup can be disabled with the MQ comomand: ALTER QMGR
  REVDNS(DISABLED) REVDNS(ENABLED) is required for matching
  against CHLAUTH rules that contain host names and to include the
  host name in error messages. If you use the hostname provided in
  channel connection error messages as an aid in problem
  diagnosis, use the DISABLED setting only as a temporary
  workaround.
  

Problem summary

• ****************************************************************
  * USERS AFFECTED: All users of IBM MQ for z/OS Version 9       *
  *                 Release 3 Modification 0 and Release 4       *
  *                 Modification 0.                              *
  ****************************************************************
  * PROBLEM DESCRIPTION: When a channel initiator attempts to    *
  *                      establish a channel connection, the job *
  *                      may abend with code 0C4-00000004. This  *
  *                      abend occurs in module CSQXBNTM during  *
  *                      reverse DNS lookup processing.          *
  ****************************************************************
  The failure occurs when the remote ip address of incoming
  channels resolved to a large number of hostname aliases. MQ
  processes these hostnames in two passes: first to calculate
  buffer size needed, then to allocate the buffer and copy the
  hostnames. Current limits to the entries is 32 or 2048 bytes.
  Errors in the logic start to appear once these limits are hit:
  * If trace is active, the incorrect parameter is passed into the
  trace routine causing an abend in the tracing code.
  * In the first pass, a check is present to prevent a buffer that
  is greater than the limit, however this logic is missing in the
  second pass and so all the aliases are processed and wrote
  beyond the allocated buffer, leading to an 0C4.
  * If channel events are enabled and a large number hostnames are
  present, the fixed buffer is not big enough to hold all
  hostnames leading to an FFST being issued.
  

Problem conclusion

• This APAR corrects the logic in hostname processing to prevent
  both tracing and buffer overrun errors. The fix ensures that
  trace calls use valid parameters and that hostname copying
  respects buffer size limits. The buffer for channel events now
  dynamically reallocates storage to support an increased number
  of hostname entries in cases where an FFST would have been
  issued.
  

Temporary fix

Comments

APAR Information

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

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

  UO07728 UO07729

Modules/Macros

• CMQXREVN CSQIRECP CSQMSTRT CSQXCRHN
  

Fix information

• Fixed component name

  IBM MQ Z/OS V9

• Fixed component ID

  5655MQ900

Applicable component levels