SSH Essentials

Basic Connection

ssh user@hostname
ssh user@hostname -p 2222
ssh -i ~/.ssh/id_rsa user@hostname
ssh -v user@hostname        # verbose (debug)
ssh -J jumphost user@target # jump host / bastion

SSH Keys

# Generate key
ssh-keygen -t ed25519 -C "your@email.com"
ssh-keygen -t rsa -b 4096 -C "your@email.com"

# Copy key to server
ssh-copy-id user@hostname
ssh-copy-id -i ~/.ssh/id_rsa.pub user@hostname

# Check loaded keys
ssh-add -l

# Add key to agent
ssh-add ~/.ssh/id_rsa
ssh-add -K ~/.ssh/id_rsa   # macOS: add to keychain

Port Forwarding & Tunneling

# Local port forwarding (access remote service locally)
ssh -L 8080:localhost:80 user@remote
ssh -L 3306:db-server:3306 user@bastion    # MySQL through bastion

# Remote port forwarding (expose local service to remote)
ssh -R 8080:localhost:3000 user@remote

# Dynamic SOCKS proxy
ssh -D 1080 user@remote

# Persistent tunnel with autossh
autossh -M 0 -N -L 3306:localhost:3306 user@remote

File Transfers

# SCP
scp file.txt user@host:/remote/path/
scp user@host:/remote/file.txt /local/path/
scp -r /local/dir user@host:/remote/dir

# rsync (preferred for directories)
rsync -avz /local/dir/ user@host:/remote/dir/
rsync -avz --delete /local/dir/ user@host:/remote/dir/
rsync -avz -e "ssh -p 2222" /local/dir/ user@host:/remote/dir/

SSH Config (~/.ssh/config)

Host myserver
    HostName 192.168.1.100
    User deploy
    Port 22
    IdentityFile ~/.ssh/id_ed25519
    ServerAliveInterval 60
    ServerAliveCountMax 3

Host bastion
    HostName bastion.example.com
    User admin

Host prod-db
    HostName 10.0.0.5
    User deploy
    ProxyJump bastion

ssh myserver     # uses config above

Security Best Practices

# /etc/ssh/sshd_config best practices:
PermitRootLogin no
PasswordAuthentication no
PubkeyAuthentication yes
AllowUsers deploy
MaxAuthTries 3
LoginGraceTime 30
ClientAliveInterval 300
ClientAliveCountMax 2

# After changes:
sshd -t          # test config
systemctl restart sshd

Troubleshooting

ssh -vvv user@host          # max verbosity
journalctl -u sshd -f       # server logs
tail -f /var/log/auth.log   # auth attempts
ss -tlnp | grep :22         # check sshd listening

Tips

• Always use ed25519 keys (faster, more secure than RSA 2048)
• Never disable StrictHostKeyChecking in production
• Use SSH agent forwarding (-A) sparingly — security risk
• ProxyJump is the modern replacement for ProxyCommand
• ServerAliveInterval 60 prevents timeout on idle connections