The Proxy Trick That Gets Past IP-Based Firewalls

Understanding IP-Based Firewalls and Their Limitations

IP-based firewalls, those venerable sentinels of network security, allow or deny traffic based on the source or destination IP address. Yet, their stoicism is not without chinks. A firewall configured to only permit requests from whitelisted IP ranges can be circumvented if a request appears to originate from within that trusted enclave. This is the essence of the “proxy trick” — a method both elegant and subversive, reminiscent of Dumas’ masterful intrigues.

---

The Proxy Trick: Bypassing IP-Based Firewalls with Internal Relays

At its core, the proxy trick leverages an intermediary machine — typically within the firewall’s trusted range — to relay traffic. By carefully orchestrating requests through this proxy, an external actor may masquerade as an insider, slipping past the firewall’s rigid facade.

Types of Proxies Used

Proxy Type	Description	Typical Use Case	Complexity	Stealthiness
SSH Tunnel	Securely forwards local ports through SSH	Administrative access, testing	Low	High
HTTP CONNECT	Utilizes HTTP CONNECT method for tunneling arbitrary TCP traffic	Web and API access	Medium	Medium
SOCKS Proxy	Generic relay for TCP connections, often via SSH or dedicated proxy server	Web browsing, data exfiltration	Low	High
Reverse Proxy	Forwards traffic from external clients to internal services	Access to internal web servers	High	Medium

---

Step-by-Step: Implementing the Proxy Trick with SSH Tunneling

Assume you have credentials for a jump box within the firewall.

1. Establish an SSH Tunnel

bash
ssh -L 8080:internal.service.local:80 [email protected]

This command binds port 8080 on your local machine to port 80 of internal.service.local, relayed via jumpbox.corp.com.

1. Access the Internal Service

In your browser or with curl:

bash
curl http://localhost:8080/

The request appears to originate from jumpbox.corp.com, which the firewall trusts.

Resource: SSH Port Forwarding Tutorial

---

Crafting HTTP CONNECT Proxies with socat and ncat

If the firewall permits outbound HTTP/HTTPS, the HTTP CONNECT method is an exquisite tool. One can deploy a lightweight proxy on a trusted host:

1. Deploy a Proxy with Socat

On the internal host:

bash
socat TCP-LISTEN:3128,reuseaddr,fork TCP:target.internal:80

1. Configure Your Client

Point your browser or tool to use the internal host as an HTTP proxy (port 3128).

Reference: Socat Documentation

---

SOCKS Proxies: SSH’s Subtlest Feature

SSH’s dynamic forwarding can transmute your session into a SOCKS proxy:

ssh -D 1080 [email protected]

Configure your application to route traffic through localhost:1080. The traffic, after being encrypted and ferried through the jump box, emerges within the sanctum of the internal network.

Resource: OpenSSH SOCKS Proxy Guide

---

Reverse Proxies: Turning the Firewall Inside Out

For more persistent or complex needs, a reverse proxy such as ngrok or frp can be installed on a trusted internal host. The reverse proxy establishes an outbound tunnel to an external server, then relays inbound connections back to the internal network — thus, the firewall’s directionality is subverted.

Sample frp Configuration (on Internal Host)

[common]
server_addr = external.server.com
server_port = 7000

[web]
type = http
local_port = 80
custom_domains = myinternal.example.com

---

Comparison: Proxy Approaches for Firewall Evasion

Method	Setup Required on Internal Host	External Control	Encryption	Audit Evasion	Suitable For
SSH Tunnel	SSH server	Yes	Yes	Moderate	Occasional manual access
HTTP CONNECT	HTTP proxy (e.g., socat)	Yes	No	Low	Web/API traffic
SOCKS Proxy	SSH server	Yes	Yes	High	Web, CLI tools, automation
Reverse Proxy	Custom software (ngrok, frp)	Yes	Varies	Low	Persistent web access

---

Practical Countermeasures

• Network Segmentation: Restrict internal proxy hosts’ access wherever possible.
• Egress Filtering: Disable or limit outbound connections from internal hosts.
• Proxy Detection: Monitor for anomalous proxy processes (e.g., socat, ncat, SSH with -L/-D flags).
• Zero Trust Models: Move beyond IP-based trust; employ mutual TLS, authenticated reverse proxies, or per-user access controls.

---

Further Reading

• The Art of the Pivot: Post-Exploitation with Proxies
• SSH Tunneling Explained
• Reverse Proxy Patterns and Security
• FRP – Fast Reverse Proxy

---

Example: Full Exploitation Workflow

Suppose a web application is accessible only from an internal IP range. The following sequence illustrates a typical exploitation:

1. Compromise a host inside the firewall (jump box).
2. Establish an SSH SOCKS proxy:

bash
ssh -D 1080 [email protected]

1. Route browser or tool traffic through the proxy.
2. Access the internal web application via the local SOCKS proxy.
3. Optional: Deploy a reverse proxy (frp, ngrok) for persistent access.

---

Summary Table: Actionable Steps and Tools

Step	Tool/Command Example	Purpose
Local port forwarding (SSH)	ssh -L 8080:target:80 user@jumpbox	Access single internal service
Dynamic SOCKS proxy (SSH)	ssh -D 1080 user@jumpbox	Proxy arbitrary traffic
HTTP CONNECT proxy (socat/ncat)	socat TCP-LISTEN:3128,fork TCP:target:80	Web/API proxying
Reverse proxy (frp)	See frp config above	Persistent, external-to-internal access

---

In the realm of network security, the proxy trick is a testament to the ingenuity of those who, like the Count of Monte Cristo, seek passage where none is granted. The lesson for defenders: trust not the IP address, for it is but a mask that may be donned by friend or foe alike.