Source Code

SSH Tunnel

SSH tunneling, port forwarding, and secure remote access. Covers local/remote/dynamic forwards, jump hosts, ProxyCommand, multiplexing, key management, and connection debugging.

When to Use

Accessing a remote database through a firewall (local port forward)
Exposing a local dev server to a remote machine (remote port forward)
Using a remote server as a SOCKS proxy (dynamic forward)
Connecting through bastion/jump hosts
Managing SSH keys and agent forwarding
Transferring files securely (scp, rsync)
Debugging SSH connection failures

Port Forwarding

Local forward (access remote service locally)

# Forward local port 5432 to remote's localhost:5432
# Use case: access a remote PostgreSQL database as if it were local
ssh -L 5432:localhost:5432 user@remote-server

# Then connect locally:
psql -h localhost -p 5432 -U dbuser mydb

# Forward to a different host accessible from the remote
# Remote server can reach db.internal:5432, but you can't
ssh -L 5432:db.internal:5432 user@remote-server

# Forward multiple ports
ssh -L 5432:db.internal:5432 -L 6379:redis.internal:6379 user@remote-server

# Run in background (no shell)
ssh -fNL 5432:db.internal:5432 user@remote-server
# -f = background after auth
# -N = no remote command
# -L = local forward

Remote forward (expose local service remotely)

# Make your local port 3000 accessible on the remote server's port 8080
ssh -R 8080:localhost:3000 user@remote-server
# On the remote: curl http://localhost:8080 → hits your local :3000

# Expose to all interfaces on the remote (not just localhost)
# Requires GatewayPorts yes in remote sshd_config
ssh -R 0.0.0.0:8080:localhost:3000 user@remote-server

# Background mode
ssh -fNR 8080:localhost:3000 user@remote-server

Dynamic forward (SOCKS proxy)

# Create a SOCKS5 proxy on local port 1080
ssh -D 1080 user@remote-server

# Route browser traffic through the tunnel
# Configure browser proxy: SOCKS5, localhost:1080

# Use with curl
curl --socks5-hostname localhost:1080 https://example.com

# Background mode
ssh -fND 1080 user@remote-server

Jump Hosts / Bastion

ProxyJump (simplest, OpenSSH 7.3+)

# Connect through a bastion host
ssh -J [email protected] target-user@internal-server

# Chain multiple jumps
ssh -J bastion1,bastion2 target-user@internal-server

# With port forward through bastion
ssh -J bastion-user@bastion -L 5432:db.internal:5432 target-user@app-server

ProxyCommand (older systems, more flexible)

# Equivalent to ProxyJump but works on older OpenSSH
ssh -o ProxyCommand="ssh -W %h:%p bastion-user@bastion" target-user@internal-server

SSH Config for jump hosts

# ~/.ssh/config

# Bastion host
Host bastion
    HostName bastion.example.com
    User bastion-user
    IdentityFile ~/.ssh/bastion_key

# Internal servers (automatically use bastion)
Host app-server
    HostName 10.0.1.50
    User deploy
    ProxyJump bastion

Host db-server
    HostName 10.0.2.30
    User admin
    ProxyJump bastion
    LocalForward 5432 localhost:5432

# Now just: ssh app-server
# Or: ssh db-server (auto-forwards port 5432)

SSH Config Patterns

Essential config

# ~/.ssh/config

# Global defaults
Host *
    ServerAliveInterval 60
    ServerAliveCountMax 3
    AddKeysToAgent yes
    IdentitiesOnly yes

# Named hosts
Host prod
    HostName 203.0.113.50
    User deploy
    IdentityFile ~/.ssh/prod_ed25519
    Port 2222

Host staging
    HostName staging.example.com
    User deploy
    IdentityFile ~/.ssh/staging_ed25519

# Wildcard patterns
Host *.dev.example.com
    User developer
    IdentityFile ~/.ssh/dev_key
    StrictHostKeyChecking no
    UserKnownHostsFile /dev/null

Connection multiplexing (reuse connections)

# ~/.ssh/config
Host *
    ControlMaster auto
    ControlPath ~/.ssh/sockets/%r@%h-%p
    ControlPersist 600

# First connection opens socket, subsequent connections reuse it
# Much faster for repeated ssh/scp/rsync to same host

# Create socket directory
mkdir -p ~/.ssh/sockets

# Manually manage control socket
ssh -O check prod       # Check if connection is alive
ssh -O stop prod        # Close the master connection
ssh -O exit prod        # Close immediately

Key Management

Generate keys

# Ed25519 (recommended — fast, secure, short keys)
ssh-keygen -t ed25519 -C "user@machine" -f ~/.ssh/mykey_ed25519

# RSA 4096 (wider compatibility)
ssh-keygen -t rsa -b 4096 -C "user@machine" -f ~/.ssh/mykey_rsa

# Generate without passphrase (for automation only)
ssh-keygen -t ed25519 -N "" -f ~/.ssh/deploy_key

Deploy keys

# Copy public key to remote server
ssh-copy-id -i ~/.ssh/mykey_ed25519.pub user@remote-server

# Manual (if ssh-copy-id unavailable)
cat ~/.ssh/mykey_ed25519.pub | ssh user@remote-server "mkdir -p ~/.ssh && chmod 700 ~/.ssh && cat >> ~/.ssh/authorized_keys && chmod 600 ~/.ssh/authorized_keys"

SSH Agent

# Start agent (usually auto-started)
eval "$(ssh-agent -s)"

# Add key to agent
ssh-add ~/.ssh/mykey_ed25519

# Add with expiry (key removed after timeout)
ssh-add -t 3600 ~/.ssh/mykey_ed25519

# List loaded keys
ssh-add -l

# Remove all keys
ssh-add -D

# Agent forwarding (use your local keys on remote hosts)
ssh -A user@remote-server
# On remote: ssh [email protected]  → uses your local key
# SECURITY: only forward to trusted hosts

File permissions

# SSH is strict about permissions. Fix common issues:
chmod 700 ~/.ssh
chmod 600 ~/.ssh/id_ed25519          # Private key
chmod 644 ~/.ssh/id_ed25519.pub      # Public key
chmod 600 ~/.ssh/config
chmod 600 ~/.ssh/authorized_keys

File Transfer

scp

# Copy file to remote
scp file.txt user@remote:/path/to/destination/

# Copy from remote
scp user@remote:/path/to/file.txt ./local/

# Copy directory recursively
scp -r ./local-dir user@remote:/path/to/

# Through jump host
scp -o ProxyJump=bastion file.txt user@internal:/path/

# With specific key and port
scp -i ~/.ssh/mykey -P 2222 file.txt user@remote:/path/

rsync over SSH

# Sync directory (only changed files)
rsync -avz ./local-dir/ user@remote:/path/to/remote-dir/

# Dry run (preview changes)
rsync -avzn ./local-dir/ user@remote:/path/to/remote-dir/

# Delete files on remote that don't exist locally
rsync -avz --delete ./local-dir/ user@remote:/path/to/remote-dir/

# Exclude patterns
rsync -avz --exclude='node_modules' --exclude='.git' ./project/ user@remote:/deploy/

# With specific SSH options
rsync -avz -e "ssh -i ~/.ssh/deploy_key -p 2222" ./dist/ user@remote:/var/www/

# Resume interrupted transfer
rsync -avz --partial --progress large-file.tar.gz user@remote:/path/

# Through jump host
rsync -avz -e "ssh -J bastion" ./files/ user@internal:/path/

Connection Debugging

Verbose output

# Increasing verbosity levels
ssh -v user@remote       # Basic debug
ssh -vv user@remote      # More detail
ssh -vvv user@remote     # Maximum detail

# Common issues visible in verbose output:
# "Connection refused" → SSH server not running or wrong port
# "Connection timed out" → Firewall blocking, wrong IP
# "Permission denied (publickey)" → Key not accepted
# "Host key verification failed" → Server fingerprint changed

Test connectivity

# Check if SSH port is open
nc -zv remote-host 22
# or
ssh -o ConnectTimeout=5 -o BatchMode=yes user@remote echo ok

# Check which key the server accepts
ssh -o PreferredAuthentications=publickey -v user@remote 2>&1 | grep "Offering\|Accepted"

# Test config without connecting
ssh -G remote-host   # Print effective config for this host

Common fixes

# "WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED"
# Server was reinstalled / IP reassigned
ssh-keygen -R remote-host   # Remove old fingerprint
ssh user@remote-host        # Accept new fingerprint

# "Too many authentication failures"
# SSH agent is offering too many keys
ssh -o IdentitiesOnly=yes -i ~/.ssh/specific_key user@remote

# "Connection closed by remote host"
# Often: MaxSessions or MaxStartups limit on server
# Or: fail2ban banned your IP

# Tunnel keeps dying
# Add keepalive in config or command line:
ssh -o ServerAliveInterval=30 -o ServerAliveCountMax=5 user@remote

# Permission denied despite correct key
# Check remote: /var/log/auth.log or /var/log/secure
# Common: wrong permissions on ~/.ssh or authorized_keys

Kill stuck SSH sessions

# If SSH session hangs (frozen terminal):
# Type these characters in sequence:
~.          # Disconnect
~?          # Show escape commands
~#          # List forwarded connections
~&          # Background SSH (when waiting for tunnel to close)
# The ~ must be the first character on a new line (press Enter first)

Tips

Use ~/.ssh/config for everything. Named hosts with stored settings are faster and less error-prone than typing long commands.
Ed25519 keys are preferred over RSA. They're shorter, faster, and equally secure.
Connection multiplexing (ControlMaster) makes repeated connections instant. Enable it globally.
rsync is almost always better than scp for anything beyond a single file. It handles interruptions, only transfers changes, and supports compression.
Agent forwarding (-A) is convenient but a security risk on untrusted servers. The remote host can use your agent to authenticate as you. Prefer ProxyJump instead.
ServerAliveInterval 60 in config prevents most "broken pipe" disconnections.
Keep your ~/.ssh/config organized with comments. Future-you will appreciate it.
The ~. escape sequence is the only way to kill a stuck SSH session without closing the terminal.

ssh-tunnel

SSH Tunnel

When to Use

Port Forwarding

Local forward (access remote service locally)

Remote forward (expose local service remotely)

Dynamic forward (SOCKS proxy)

Jump Hosts / Bastion

ProxyJump (simplest, OpenSSH 7.3+)

ProxyCommand (older systems, more flexible)

SSH Config for jump hosts

SSH Config Patterns

Essential config

Connection multiplexing (reuse connections)

Key Management

Generate keys

Deploy keys

SSH Agent

File permissions

File Transfer

scp

rsync over SSH

Connection Debugging

Verbose output

Test connectivity

Common fixes

Kill stuck SSH sessions

Tips