Debug: SSH connection troubleshooting

SSH connection fails, hangs, or disconnects unexpectedly.

Systematic SSH debugging:

# 1. Verbose output (most useful)
ssh -vvv user@host 2>&1 | head -100
# Look for: where it gets stuck
# - DNS resolution
# - TCP connection
# - Key exchange
# - Authentication

# 2. Check basic connectivity
ping host                           # Is host reachable?
nc -zv host 22                      # Is SSH port open?
nmap -p 22 host                     # Port scan

# 3. Common issues:

# Permission denied (publickey)
ssh -i ~/.ssh/mykey user@host       # Specify key explicitly
ls -la ~/.ssh/                       # Check permissions
chmod 700 ~/.ssh
chmod 600 ~/.ssh/id_rsa
chmod 644 ~/.ssh/id_rsa.pub
chmod 644 ~/.ssh/authorized_keys     # On server

# Connection refused
# -> sshd not running on server
# -> Wrong port (try ssh -p 2222 user@host)
# -> Firewall blocking

# Connection timed out
# -> Host unreachable or firewall dropping packets
# -> Try: ssh -o ConnectTimeout=10 user@host

# Connection drops after idle
# Add to ~/.ssh/config:
# Host *
#   ServerAliveInterval 60
#   ServerAliveCountMax 3

# Too many authentication failures
# -> ssh-agent has too many keys loaded
ssh-add -l                           # List loaded keys
ssh-add -D                           # Remove all
ssh-add ~/.ssh/specific_key          # Add only needed key
# Or: ssh -o IdentitiesOnly=yes -i key user@host

# 4. Check server-side
sudo tail -f /var/log/auth.log       # Debian/Ubuntu
sudo journalctl -u sshd -f           # systemd
sudo sshd -T                         # Test config

SSH failures happen at multiple layers (DNS, TCP, crypto, auth). The -vvv flag reveals exactly which stage fails, making targeted debugging possible.

Debugging SSH connections to remote servers