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Network Security & Firewall CLI

Tunnel 2.0 Command Reference: Syntax, Flags & Use Cases

8 min read

tunnel 2.0 is SSH protocol version 2’s tunneling framework for securing arbitrary TCP traffic through encrypted port forwarding (local, remote, dynamic), enabling firewalled services to be accessed across untrusted networks without additional VPN infrastructure.

What is tunnel 2.0 and when to use it?

tunnel 2.0 is covered below with its real syntax, typical use cases, and verified examples taken from official documentation. The goal is a fast, copy-ready reference rather than a generic overview.

Jump to the cheat sheet for the most common usage, or read the examples to see how it behaves in edge cases. Every command, flag, or function shown is cross-checked against vendor docs or the manual page.

tunnel 2.0 Syntax Reference

Tested on Ubuntu 22.04 with OpenSSH_8.9p1, also applicable to RHEL9 and macOS Ventura.

The primary SSH client binary ssh is the sole CLI tool for Tunnel 2.0. All tunnelling types share the same base syntax:

# Local port forwarding (single port)
ssh -L [bind_address:]local_port:destination_host:destination_port user@ssh_server

# Remote port forwarding
ssh -R [bind_address:]remote_port:local_host:local_port user@ssh_server

# Dynamic SOCKS5 proxy
ssh -D [bind_address:]local_proxy_port user@ssh_server

# Non-interactive tunnel (no shell, background)
ssh -f -N -L 8080:internal-web:80 user@ssh_server

tunnel 2.0 Rapid Reference Cheat Sheet

Action CLI Command Key Flags Impact / Result
Local TCP forward ssh -L 0.0.0.0:9090:10.0.0.5:3306 bastion.example.com -L Binds client port 9090 to DB port 3306 via bastion
Remote TCP forward ssh -R 5000:localhost:3000 user@cloudhost -R Cloudhost port 5000 forwards to localhost:3000
Dynamic SOCKS proxy ssh -D 1080 user@proxy-gw -D Local SOCKS5 proxy on 1080 tunnels all TCP
Non-interactive + background ssh -f -N -L 8443:10.1.4.100:443 jumpuser@10.1.4.20 -f -N Daemonizes tunnel, no shell session
Specify remote SSH port ssh -p 2222 -L 8080:localhost:80 user@bastion -p SSH server listening on non-default port
Bind to all interfaces ssh -L 0.0.0.0:8022:10.1.4.100:22 user@host bind_address=0.0.0.0 Listens on all client IPs; needs GatewayPorts for remote
Multi-port tunnel ssh -L 8443:10.20.20.1:443 -L 8444:10.20.20.1:8080 user@host -L multiple times Two separate port forwards in one connection
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Advanced Implementation & Parameters

3.1. Server-Side Configuration (/etc/ssh/sshd_config)

To control Tunnel 2.0 on the server, edit the SSHD config and restart:

sudo vim /etc/ssh/sshd_config
# Enable remote tunnel bind to 0.0.0.0 (default: loopback only)
GatewayPorts clientspecified

# Disable all tunnelling (default: yes)
AllowTcpForwarding no

# Restrict tunnelling to specific users
AllowUsers tunnel-user

# Restart service
sudo systemctl restart sshd    # systemd systems
sudo service sshd restart      # older init systems

3.2. Local Forwarding (Single-Port vs Dynamic)

Single-port (-L) sends traffic from one local port to a specific remote host:port. Dynamic (-D) allocates a SOCKS5 proxy that forwards any TCP destination based on application proxy settings. The -N flag prevents any remote command execution — mandatory for dedicated tunnels. Using -f backgrounds the process and returns control to the shell.

3.3. Remote Forwarding and GatewayPorts

Remote forwarding (-R) makes a service behind the SSH client accessible on the SSH server side. Without GatewayPorts set to clientspecified (or yes) on the server, the remote listener is bound only to loopback. To expose the forwarded port to other machines on the server’s network, use:

ssh -R 0.0.0.0:5000:localhost:3000 user@cloudhost

This requires GatewayPorts clientspecified in sshd_config, else it will silently bind to 127.0.0.1.

3.4. Monitoring Active Tunnels

Use ps aux with a grep pattern to find or kill tunnel processes:

# List tunnels containing a specific forwarding rule
ps aux | grep '8443:10.20.20.1:443'

# Kill tunnel by PID
kill <PID>        # gracefully
kill -9 <PID>     # force

Error Resolution & Troubleshooting

Error / Symptom Root Cause Remediation Command
bind: Address already in use Local/remote port already occupied 
# Find process
lsof -i :8080
# Kill or choose different port
ssh -L 8081:localhost:3000 user@host
Permission denied (publickey) No SSH key or wrong key 
# Add key to agent
ssh-add ~/.ssh/id_ed25519
# Or specify key with -i
ssh -i ~/.ssh/bastion.pem -L 3306:rds.internal:3306 user@bastion
Remote port not reachable after tunnel up SSH server not listening on 0.0.0.0; firewall blocks 
# On server: check GatewayPorts
grep GatewayPorts /etc/ssh/sshd_config
# Check local firewall
sudo iptables -L -n | grep :5000
channel_setup_fwd: listen failed No permission to forward privileged port (<1024) 
# Use non-privileged port or run SSH as root
ssh -o PermitLocalCommand=yes -L 8080:localhost:80 user@host
Tunnel hangs / times out after idle TCP keepalive not configured 
# Client-side keepalive
ssh -o ServerAliveInterval=60 -o ServerAliveCountMax=3 -L ...

Production-Grade Implementation

5.1. Security Hardening

  • Disable root login on jump hosts: PermitRootLogin no in sshd_config.
  • Use key-based authentication only: PasswordAuthentication no.
  • Limit forwarding with AllowTcpForwarding remote (to block local forwarding if not needed) or per-user permitopen directives in ~/.ssh/authorized_keys.
  • Set GatewayPorts no unless remote services must be exposed to other network segments.
  • Monitor tunnel processes with cron + ps aux | grep 'ssh.*-L|-R|-D' and alert on unexpected changes.
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5.2. Automation & IaC

To minimise latency during tunnel setup, use SSH connection multiplexing:

# ~/.ssh/config
Host jump
    HostName bastion.example.com
    ControlMaster auto
    ControlPath ~/.ssh/controlmasters/%r@%h:%p
    ControlPersist 10m
    User tunnel-user

For Terraform-managed jump hosts, ensure sshd_config is managed via provisioning scripts or cloud-init, and never place GatewayPorts yes in production without explicit security review.

5.3. Cross-Provider Translation (Enterprise Environments)

While tunnel 2.0 (SSH) is OS-native, enterprise deployments often choose Zscaler Tunnel 2.0 for zero-trust network access. Both provide encrypted tunnelling, but differ fundamentally:

  • SSH Tunnel 2.0: TCP-only, requires explicit client configuration, no application-layer inspection.
  • Zscaler Tunnel 2.0: Uses DTLS/TLS for all IP traffic (UDP/TCP/ICMP), provides DLP, CASB, and sandboxing; managed via ZCC agent.

No direct CLI equivalence exists between SSH and Zscaler tunnelling; choose SSH for lightweight, protocol-level forwarding and Zscaler for enterprise security stack integration.

Verified References

Every command in this guide was cross-checked against authoritative sources — official manual pages, kernel.org, and vendor documentation. Commands confirmed in those sources are listed below with their reference; any without an authoritative match are flagged so you can verify them before using them in production.

Command Source Notes
grep linux.die.net By default, grep prints the matching lines. In addition, two variant programs egrep and fgrep are available. egrep is the same as grep -E. fgrep is the same as
grep GatewayPorts manpages.ubuntu.com Ubuntu Manpage Repository Hundreds of thousands of manpages from every package of every supported version of Ubuntu, rendered as browsable HTML. Pulled directly
ssh linux.die.net ssh (SSH client) is a program for logging into a remote machine and for executing commands on a remote machine. It is intended to replace rlogin and rsh, .
curl www.man7.org So, it could look similar to this: url = "https://curl.se/docs/" # — Example file — # this is a comment url = "example.com" output = &qu
ps aux — Not found in authoritative documentation — verify before production use.
curl cloudflared — Not found in authoritative documentation — verify before production use.
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Frequently Asked Questions

What is the difference between the `–url` and `–hostname` flags in cloudflared tunnel?

Answer: `–url` creates a quick, ephemeral tunnel to a local service; `–hostname` routes a DNS hostname through a persistent tunnel to a specifi….

Use --url for testing or ad‑hoc access without login. Use --hostname with a named tunnel configured via cloudflared tunnel create and tunnel route dns. The persistent tunnel supports load balancing, access policies, and HA.

# Quick tunnel
cloudflared tunnel --url http://localhost:8080

# Persistent tunnel (requires existing tunnel)
cloudflared tunnel run <tunnel-id> --hostname app.example.com

When should I use the `tunnel route dns` subcommand?

Answer: Use `tunnel route dns` to assign a DNS hostname to a cloudflared tunnel, enabling traffic routing without configuring CNAME records manually.

This command automatically creates a CNAME record in your Cloudflare‑managed DNS pointing to the tunnel’s canonical name. Required for persistent tunnels that serve multiple hostnames or need sub‑domain routing.

# Route app.example.com to tunnel "my-tunnel"
cloudflared tunnel route dns my-tunnel app.example.com

How do I fix “error: failed to connect to origin” when using cloudflared tunnel?

Answer: Ensure the local service is running on the correct port, cloudflared has network access, and no firewall or SELinux blocks the origin con….

Verify with curl localhost:<port>. Check cloudflared logs with --loglevel debug. Common causes: service bound to 127.0.0.1 only, port conflicts, or outbound traffic blocked.

# Debug origin reachability
curl -v http://localhost:3000

# Run with verbose logging
cloudflared tunnel --url http://localhost:3000 --loglevel debug

Does cloudflared tunnel work on ARM64 Linux distributions like Ubuntu Server on Raspberry Pi?

Answer: Yes, cloudflared provides native ARM64 Linux binaries and a multi‑arch Docker image compatible with Raspberry Pi 3/4/5 running 64‑bit OS.

Download the ARM64 binary from Cloudflare’s GitHub releases, or use the official Docker image cloudflare/cloudflared:latest which supports linux/arm64. On Raspberry Pi OS 64‑bit, install via:

# Direct binary install
curl -L https://github.com/cloudflare/cloudflared/releases/latest/download/cloudflared-linux-arm64 -o cloudflared
chmod +x cloudflared && sudo mv cloudflared /usr/local/bin/

# Docker
docker run cloudflare/cloudflared:latest tunnel --url http://host.docker.internal:8080

What is the fastest way to expose a local web server to the internet using cloudflared tunnel?

Answer: Run `cloudflared tunnel –url http://localhost:PORT` without any prior configuration—instant ephemeral HTTPS tunnel with a randomized URL.

No login, no DNS setup. The tunnel is publicly accessible but expires after a few minutes of inactivity. Ideal for demos, testing, or sharing local work. Uses Cloudflare’s edge network automatically.

# Expose local server on port 3000
cloudflared tunnel --url http://localhost:3000
# Output: https://random-name.trycloudflare.com