Troubleshooting
Problem
If a network load-balancing utility, such as VIPA or F5, is in use; incoming z/OS OpenSSH client connections can fail. (Clients are utilities such as ssh, scp, and sftp)
Symptom
OpenSSH client connections fail.
Cause
When an IP load-balancer is in use, the destination host and client IP addresses might not be able of being reverse mapped.
Diagnosing The Problem
To verify, collect a server-side (sshd) trace of the incoming connection, which fails and determine whether the failure is related to reverse mapping DNS addresses, similar to:
debug3: Trying to reverse map address xxx.xxx.xxx.xxx. fatal: FOTS1450 Timeout before authentication for xxx.xxx.xxx.xxx
Resolving The Problem
Resolve this problem by updating the sshd_config (/etc/ssh/sshd_config) file and update the UseDNS option to "no":
UseDNS no
(You might need to uncomment or define it. The default is "no", prior to z/OS 2.4 the default was "yes".) Then, restart sshd to pick up the changed configuration.
This setting prevents sshd from trying to reverse map the IP address of the incoming connection.
Product Synonym
ssh; sshd; sftp; scp
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Document Information
Modified date:
03 February 2026
UID
ibm10732171