SSH Doctor

Attribution: Sourced from steipete/agent-scripts by Peter Steinberger.

Use when SSH connects then closes before auth, Remote Login seems advertised but unusable, or Mac SSH needs diagnosis.

Rules

• Do not print secrets, tokens, full env, or broad secret grep output.
• Validate locally first: loopback failure means server-side sshd/launchd/config; loopback success plus remote failure means network/firewall/filter/listen path.
• Report suspicious config lines before changing /etc/ssh/sshd_config.
• Prefer non-interactive SSH:

ssh -o RequestTTY=no -o RemoteCommand=none HOST 'hostname; id -un'

Baseline

hostname; id -un; sw_vers
ipconfig getifaddr en0
sudo systemsetup -getremotelogin
sudo systemsetup -setremotelogin on
sudo launchctl print system/com.openssh.sshd 2>&1 | head -80
sudo launchctl kickstart -k system/com.openssh.sshd
sudo lsof -nP -iTCP:22 -sTCP:LISTEN
nc -vz 127.0.0.1 22
ssh -4 -F /dev/null -o RequestTTY=no -o RemoteCommand=none USER@127.0.0.1 'hostname; id -un'

Config Audit

sudo sshd -T 2>&1 | egrep -i '^(allowusers|denyusers|allowgroups|denygroups|listenaddress|maxstartups|logingracetime|usepam|passwordauthentication|pubkeyauthentication|authenticationmethods)'
sudo egrep -n '^[[:space:]]*(AllowUsers|DenyUsers|AllowGroups|DenyGroups|Match|MaxStartups|LoginGraceTime|ListenAddress|AuthenticationMethods|UsePAM|PasswordAuthentication|PubkeyAuthentication)\b' /etc/ssh/sshd_config /etc/ssh/sshd_config.d/* 2>/dev/null || true

Suspicious:

• DenyUsers matching target user
• Restrictive AllowUsers / AllowGroups
• Match block accidentally applying
• Tiny MaxStartups or LoginGraceTime
• ListenAddress missing target interface

Logs

sudo log show --last 30m --predicate 'process == "sshd" OR process == "launchd"' --style compact | tail -160

Stale sshd-session Fix

Inspect first:

sudo launchctl print system/com.openssh.sshd 2>&1 | egrep 'active count|copy count|state =|last exit code|runs ='
ps -axo pid,ppid,uid,user,state,lstart,etime,comm,args | awk '/sshd-session:/ && !/awk/ {print}'

If stale sessions are blocking new SSH:

ps -axo pid=,args= | awk '/sshd-session: / && !/awk/ {print $1}' | xargs sudo kill -TERM
sleep 2
ps -axo pid=,args= | awk '/sshd-session: / && !/awk/ {print}'

Firewall (remote failure only)

Only after loopback works but remote fails:

sudo /usr/libexec/ApplicationFirewall/socketfilterfw --getglobalstate
sudo lsof -nP -iTCP:22 -sTCP:LISTEN

Closeout

Report:

• Root cause
• Exact commands changed
• Validation output, redacted as needed
• Whether remote should retry