OA61535: REMOTE HOST IDENTIFICATION HAS CHANGED may occur after enabling DSA support for host keys

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• z/OS OpenSSH clients may encounter the message:
  FOTS1317 WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED
  on z/OS OpenSSH 2.4 (OpenSSH 7.6p1) after enabling
  client support for deprecated DSA (ssh-dss) host keys.
  
  
  
  The DSA host key support for clients can be enabled by modifying
  the system-wide ssh_config file and including keyword:
  HostKeyAlgorithms=+ssh-dss
  
  or specifying this keyword in the per-user configuration file
  ($HOME/.ssh/config) or on the command line with invocation flag:
  -oHostKeyAlgorithms=+ssh-dss
  
  
  After enabling DSA host key support for the z/OS clients (ssh,
  scp, sftp) the FOTS1317 condition may arise if the remote
  server has additional non-DSA host keys available for use.
  
  
  
  ANALYSIS:
  The ssh-dss algorithm (DSA) was removed from the
  HostKeyAlgorithms list in OpenSSH 2.4 (openSSH 7.6).  Adding it
  back via the HostKeyAlgorithms keyword with "+ssh-dss" option
  may still fail as the DSA support is restored as the lowest
  priority option, and when the HostKeyAlgorithms keyword is
  specified, the client is required to use the higher priority
  keys if they are available.  Without the keyword, the priority
  is given to existing keys in the client's known_hosts (or system
  ssh_known_hosts) files.
  
  
  KNOWN IMPACT:
  The problem surfaces if DSA host key support is enabled with the
  HostKeyAlgorithms keyword, and the server offers non-DSA host
  keys and StrictHostKeyChecking is not set to "no".
  
  
  VERIFICATION STEPS:
  1.  Must be running z/OS OpenSSH 2.4 (OpenSSH 7.6p1).
  2.  Must have StrictHostKeyChecking set to a value other than
      "no".
  3.  Must enable the use of DSA host keys, and the remote server
      offers keys other than DSA which are not in the user's
      known_hosts file.
  

Local fix

• BYPASS/CIRCUMVENTION:
  If possible do one of the following:
  
  1. Specify HostKeyAlgorithms=ssh-dss
     This method will explicitly use only DSA host keys.
  
  
  2. Change the order of the host key algorithms in the client
     configurations.  This requires an explict and complete list
     of the desired algorithms in the HostKeyAlgorithms keyword.
     Refer to the OpenSSH User Guide for the default list.
  
  3. Use ssh-keyscan to add the higher priority host key into the
     user's known_host file.
  
  4. Remove the DSA host key for the remote server from the
     user's known_hosts file and connect interactively to the
     remote system to where the user will be prompted to
     automatically add the server's non-DSA key to the
     known_hosts file.
  
  
  
  
  
  KEYWORDs:
  5655M2301
  msgFOTS1317
  

Problem summary

• ****************************************************************
  * USERS AFFECTED: z/OS users of IBM z/OS V2R4 OpenSSH.         *
  ****************************************************************
  * PROBLEM DESCRIPTION: The existing host key in the            *
  *                      known_hosts file is not chosen, instead *
  *                      an attempt is made to use a             *
  *                      non-existent host key, which will fail. *
  ****************************************************************
  The order of the appended host key algorithms which is specified
  by the HostkeyAlgorithms=+ option will be at the end of
  the host key algorithms list without scanning the known_host
  file to reorder the host key algorithms list.
  

Problem conclusion

• When using HostkeyAlgorithms to merely append or remove
  algorithms from the default set (i.e. HostkeyAlgorithms=+/-...).
  Program retains the default behavior of preferring the
  algorithms that have existing keys in known_hosts.
  

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:

  UJ05922

Modules/Macros

• FOTSXSSH
  

Fix information

• Fixed component name

  OPENSSH FOR Z/O

• Fixed component ID

  5655M2301

Applicable component levels

• R240 PSY UJ05922

     UP21/07/21 P F107

Fix is available

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