OpenSSH Troubleshooting
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Internal
Connection Takes a Long Time
In most cases this is caused by the server attempting to use DNS to get the IP name of the client. It is fixed by turning 'UseDNS' off. For more details, see:
Troubleshooting sshd Server
Execute the sshd Server in Foreground
One method to troubleshoot a sshd server in an attempt to figure out why a specific ssh connection does not work as intended, is to stop the current sshd server (stopping the server won't drop the existing connections) and start it in foreground:
systemctl stop sshd /usr/sbin/sshd -d [-D] [-d] [-d]
sshd running in foreground behaves differently than the same binary running in background - for example, a foreground server will allow root logging with a public key without password, while the background server won't, so this method has limited usefulness in some cases.