PQ95774: TCP LAYER SENDS PACKET WITH BOTH SYN AND RST FLAGS ON WHEN CONNECTION ESTABLISHMENT TIMEOUT OCCURS

Obtain the fix for this APAR.

APAR status

• Closed as unreproducible.

Error description

• FINREVERSAL of apar pq70634....See apar pq70634 for problem
  description details
  

Local fix

Problem summary

• ****************************************************************
  * USERS AFFECTED: All users of the IBM Communications Server   *
  *                 for z/OS Version 1 Release 4 & 5 IP          *
  ****************************************************************
  * PROBLEM DESCRIPTION: Outbound TCP packets have the SYN and   *
  *                      RST flags on in the same packet.        *
  ****************************************************************
  * RECOMMENDATION:                                              *
  ****************************************************************
  The TCP layer will determine a connection should be reset and
  builds an outbound packet with the RST flag set on.  The SYN and
  FIN flags from the previous outbound packet on the connection
  will remain unchanged.  This may result in SYN/RST or FIN/RST
  flag combination being on in the packet.  The flag combination
  is not a violation of the TCP protocol.
  There are several references on the Internet that indicate the
  flag combination may be an attempt at intrusion and should thus
  be discarded.
  +-------------------------------------------------------------+
  + 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

Temporary fix

Comments

• Routine TCSNDINT has been amended to explicitly turn off the
  SYN and FIN flags when setting RST.
  

APAR Information

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

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

  UQ95144 UQ95145 PK01910

Modules/Macros

• EZBTCFWR EZBTCSND TCSNDINT
  

Fix information

• Fixed component name

  TCP/IP V3 MVS

• Fixed component ID

  5655HAL00

Applicable component levels

• R140 PSY UQ95144

     UP04/12/16 P F412

• R150 PSY UQ95145

     UP04/12/16 P F412

Fix is available

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