Invisible Proxying for Non-Proxy Aware Thick Clients

In this tutorial you will be guided through the process of configuring desktop applications to proxy traffic through Caido.

Thick Clients

“Thick clients” refer to desktop applications installed on your computer. These applications perform the majority of their processes client-side and occasionally communicate:

• With a database server (two-tier architecture).
• With an external backend server that communicates with a database server (three-tier architecture).

While web applications that run inside a browser can be configured to use a proxy via the connection settings or an extension like FoxyProxy, non-proxy aware thick clients ignore system proxy settings and do not have proxy setting options themselves.

So, in order to pass the HTTP traffic that these thick clients generate through Caido, you will need to set up “invisible proxying”.

Invisible Proxying

In invisible proxying, Caido acts as the destination server that the thick client application is expecting to communicate with directly.

NOTE

In this tutorial we will demonstrate setting up invisible proxying using a simple Node.js script that will act as a thick client communicating with http://www.example.com/. To follow along, ensure you have Node.js installed and create a file named thick-client.js with the following content:

const url = process.argv[2];

if (!url) {
  console.error("Usage: node fetch-test.js <url>");
  process.exit(1);
}

// Disable SSL verification (FOR TESTING ONLY).
process.env.NODE_TLS_REJECT_UNAUTHORIZED = '0';

fetch(url)
  .then((res) => res.text())
  .then((body) => {
    console.log("Response:");
    console.log(body);
  })
  .catch((err) => {
    console.error("Fetch error:", err);
  });

DNS Resolution

In order for Caido to capture the traffic, the domain name of a destination server needs to resolve to Caido’s listening address.

TIPS

• To discover the domain/domains the thick client is communicating with, use a network traffic inspection tool like Wireshark and filter traffic by the DNS protocol.

• To discover the IP address of a domain name, run the terminal command:

nslookup www.example.com

This can be done by adding 127.0.0.1 www.example.com as an entry to either:

• The C:\Windows\System32\drivers\etc\hosts file in Windows.
• The /etc/hosts file in Linux and macOS.

Port Binding / Forwarding

The thick client application will expect the destination server to be running on either port 80 (for HTTP) or 443 (for HTTPS). However, ports below 1024 are considered privileged ports which only bind to services running with root/administrative privileges.

DANGER

Running Caido with root/administrative privileges is NOT recommended. Doing so will create issues later on since any resource created by Caido will be owned by the root/administrator user. DO NOT DO THIS.

Instead, you must either:

• Preferred Method: Use port forwarding to redirect traffic intended for ports 80 and 443 to Caido's listening port.
• Use alternative port binding methods available to certain operating systems.

---

Windows

On Windows, you can use the netsh (Network Shell) command-line utility to setup port forwarding. Open Command Prompt as Administrator and run:

netsh interface portproxy add v4tov4 listenport=80 listenaddress=127.0.0.1 connectport=8080 connectaddress=127.0.0.1

netsh interface portproxy add v4tov4 listenport=443 listenaddress=127.0.0.1 connectport=8080 connectaddress=127.0.0.1

TIPS

View any active rules with:

netsh interface portproxy show all

Remove the rules with:

netsh interface portproxy delete v4tov4 listenport=80 listenaddress=127.0.0.1

netsh interface portproxy delete v4tov4 listenport=443 listenaddress=127.0.0.1

---

macOS

On macOS, you can use the pfctl (Packet Filter) command-line utility to setup port forwarding by writing a redirection rule in a pf.conf file. Open the /etc/pf.conf file and add:

rdr pass on lo0 inet proto tcp from any to any port 80 -> 127.0.0.1 port 8080
rdr pass on lo0 inet proto tcp from any to any port 443 -> 127.0.0.1 port 8080

Reload the rules with:

sudo pfctl -f pf.conf

Ensure Packet Filter is enabled with:

sudo pfctl -e

INFO

As Packet Filter hijacks port 8080, Caido's user interface will no longer load on that port. Currently, the only workaround to this is to use the CLI parameter --ui-listen 8081 to bind another port for the UI.

WARNING

Alternatively, to bind ports 80 and 443 on macOS without using root permissions or port fowarding, you can instead configure Caido to listen on all interfaces (0.0.0.0).

However, we do NOT recommend doing this in untrusted networks since this allows ANY computer on the same network as you to proxy through your computer.

---

Linux

On Linux, you can use the iptables command-line utility to setup port forwarding. Open a terminal and run:

sudo iptables -t nat -A OUTPUT -p tcp -d 127.0.0.1 --dport 80 -j REDIRECT --to-port 8080

sudo iptables -t nat -A OUTPUT -p tcp -d 127.0.0.1 --dport 443 -j REDIRECT --to-port 8080

TIPS

View any active rules with:

sudo iptables -t nat -L OUTPUT -n -v --line-numbers

Remove the rules with:

sudo iptables -t nat -F OUTPUT

Alternatively, on Linux, you can grant the Caido CLI permission to bind to ports 80 and 443 with:

sudo setcap 'cap_net_bind_service=+ep' ./path/to/caido-cli

Ensure you grant permission to the CLI, NOT the desktop application. Usually, the binary will be found under resources/bin/caido-cli in your installation directory.

---

Enable Invisible Proxying

To enable invisible proxying:

1. In the launch window, click on the button attached to an instance and select Edit.

2. Then, click on Advanced to expand the drop-down settings menu options and click on the Enable invisible proxying checkbox.

3. Click on the Save button to update and save the configuration.

DNS Rewrite

The target domain will now resolve to Caido. However, Caido will also resolve the domain to itself, since DNS queries will check the hosts file before being sent to a resolver. In order for Caido to pass the request along to the actual destination server, you must create a DNS Rewrite rule to preserve the original IP address of the target domain.

To create a rule:

1. Click on the account button in the top-right corner of the Caido user-interface, select Settings, and open the Network tab.

2. Scroll down and click on the + Add Rewrite button.

3. Click on the Use static IP checkbox and type in the IP address in the Redirect to static IP input field.

4. Add www.example.com to the Included Hosts list and click on the + Create button to save the rule.

TIP

Glob syntax (*) is supported to account for varying subdomains and top-level domains/extended top-level domains.

Testing

To test the configuration, navigate to the directory in which the thick-client.js file is saved to and enter:

node thick-client.js http://www.example.com/

And:

node thick-client.js https://www.example.com/

Each time the script is executed, a new request will be proxied through Caido.

TIP

You may need to flush the DNS cache.

ipconfig /flushdns