Hey guys! Today, we're diving deep into the world of OSQuid, focusing on how to set it up for transparently proxying SCHTTP (Secure Hypertext Transfer Protocol) traffic on specific ports. This is super useful when you want to intercept and analyze secure web traffic without explicitly configuring clients to use a proxy. Let's break it down step by step, ensuring you understand the ins and outs of this powerful setup.

Understanding OSQuid and SCHTTP

Before we jump into the configuration, let's get our terms straight. OSQuid is a high-performance, open-source proxy server that supports various protocols, including HTTP, HTTPS, and, of course, SCHTTP. It's widely used for caching, filtering, and monitoring web traffic. SCHTTP, on the other hand, is simply HTTPS—HTTP over a secure, encrypted connection. When we talk about transparently proxying SCHTTP, we mean that the clients (like web browsers) don't even realize their traffic is being intercepted and processed by the proxy server. This is crucial for environments where you can't or don't want to modify client configurations, such as in corporate networks or when analyzing malware traffic.

The beauty of OSQuid lies in its flexibility and extensibility. It allows you to define rules and policies for handling different types of traffic, making it an invaluable tool for network administrators and security professionals. By transparently proxying SCHTTP, you gain the ability to inspect encrypted traffic, enforce security policies, and even perform real-time content filtering. However, it's super important to handle encrypted traffic responsibly and ethically. Always ensure you have the necessary permissions and comply with privacy regulations when intercepting and analyzing secure communications.

Setting up OSQuid for transparent SCHTTP proxying involves configuring the server to listen on specific ports and intercept traffic destined for those ports. This requires careful planning and configuration to avoid disrupting legitimate traffic and ensuring that the proxy server can handle the load. We'll walk through the necessary steps, including configuring the firewall, setting up OSQuid, and verifying that everything is working correctly. By the end of this guide, you'll have a solid understanding of how to transparently proxy SCHTTP traffic using OSQuid.

Prerequisites

Before we get started, make sure you have the following:

• A Linux server: This guide assumes you're using a Linux-based server (like Ubuntu or CentOS). You'll need root access to install and configure OSQuid.
• OSQuid installed: If you haven't already, you'll need to install OSQuid. You can usually do this via your distribution's package manager (e.g., apt-get install squid or yum install squid).
• Basic networking knowledge: Understanding of TCP/IP networking, ports, and firewall rules is essential.
• Root privileges: You'll need root or sudo access to modify system configurations.

Step-by-Step Configuration

1. Install and Configure OSQuid

First, let’s ensure OSQuid is installed. Open your terminal and run:

sudo apt-get update
sudo apt-get install squid

Or, if you're on a CentOS/RHEL system:

sudo yum update
sudo yum install squid

Once installed, the main configuration file is typically located at /etc/squid/squid.conf. Always back up the original configuration file before making any changes. This way, if something goes wrong, you can easily revert to the original state. Here's how to back it up:

sudo cp /etc/squid/squid.conf /etc/squid/squid.conf.backup

2. Configure OSQuid for Transparent Proxying

Now, let’s modify the squid.conf file. Open it with your favorite text editor (e.g., nano or vim):

sudo nano /etc/squid/squid.conf

Add or modify the following lines to enable transparent proxying. First, you need to define the HTTP port that OSQuid will listen on. A common choice is port 3128:

http_port 3128 transparent

Next, configure the access control lists (ACLs) to allow connections. Make sure these lines are present and adjusted to your network. The following ACLs allow access from your local network. Replace 192.168.1.0/24 with your actual network address if it's different:

acl localnet src 192.168.1.0/24
acl SSL_ports port 443
acl Safe_ports port 80          # http
acl Safe_ports port 21          # ftp
acl Safe_ports port 443         # https
acl Safe_ports port 70          # gopher
acl Safe_ports port 210         # wais
acl Safe_ports port 1025-65535  # unregistered ports
acl Safe_ports port 280         # http-mgmt
acl Safe_ports port 488         # gss-http
acl Safe_ports port 591         # filemaker
acl Safe_ports port 777         # multiling http
acl CONNECT method CONNECT

http_access deny !Safe_ports
http_access deny CONNECT !SSL_ports
http_access allow localnet
http_access allow localhost
http_access deny all

These ACL rules ensure that only safe ports are accessed and that the proxy allows connections from your local network. Adjust the localnet ACL to match your internal network's IP range. This is crucial for preventing unauthorized access to your proxy server.

3. Configure Firewall Rules

To transparently proxy traffic, you need to redirect incoming traffic on ports 80 (HTTP) and 443 (HTTPS) to OSQuid's port (3128). This is typically done using iptables. Here are the commands to achieve this:

sudo iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j REDIRECT --to-port 3128
sudo iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 443 -j REDIRECT --to-port 3128

Replace eth0 with your network interface. These commands redirect incoming HTTP and HTTPS traffic to OSQuid's port. The -t nat option specifies that we're working with the NAT table, and -A PREROUTING adds a rule to the PREROUTING chain, which is processed before routing decisions are made. The -i eth0 option specifies the input interface, and -p tcp specifies the TCP protocol. The --dport option specifies the destination port, and -j REDIRECT specifies the target action, which is to redirect the traffic to the specified port.

To make these rules persistent across reboots, you might need to save them depending on your Linux distribution. For example, on Debian-based systems, you can use:

sudo apt-get install iptables-persistent
sudo netfilter-persistent save

On CentOS/RHEL, you can use:

sudo iptables-save > /etc/sysconfig/iptables

4. Configure SSL Bumping (Optional but Recommended)

To inspect HTTPS traffic, you'll need to configure SSL bumping (also known as SSL interception). This involves generating a certificate authority (CA) and configuring OSQuid to use it. Be extremely cautious when implementing SSL bumping, as it can raise privacy and security concerns. Ensure you have the necessary legal and ethical permissions before proceeding.

First, generate a CA using openssl:

openssl req -new -newkey rsa:2048 -nodes -x509 -days 3650 -keyout myCA.pem -out myCA.pem

Follow the prompts to create your CA certificate. This certificate will be used to sign the certificates of the websites you're intercepting. Next, configure OSQuid to use this CA. Add the following lines to your squid.conf:

https_port 3129 intercept ssl-bump generate-host-certificates=on dynamic_cert_mem_cache_size=4MB cert=/etc/squid/myCA.pem key=/etc/squid/myCA.pem
ssl_bump peek step1 all
ssl_bump bump step2 all
ssl_bump splice all

These lines configure OSQuid to intercept HTTPS traffic, generate certificates on the fly, and use your CA to sign them. The ssl_bump peek step1 all and ssl_bump bump step2 all lines enable SSL bumping for all traffic. The ssl_bump splice all line specifies that traffic should be spliced (i.e., not intercepted) if SSL bumping fails.

Finally, you need to install the CA certificate on the client machines that will be accessing the proxied traffic. This is necessary so that the clients trust the certificates generated by OSQuid. The exact procedure for installing the certificate varies depending on the operating system and browser. For example, on Windows, you can import the certificate into the Trusted Root Certification Authorities store. On macOS, you can add the certificate to the Keychain Access utility.

5. Restart OSQuid

After making these changes, restart OSQuid to apply the new configuration:

sudo systemctl restart squid

Or, if you're using an older system:

sudo service squid restart

6. Verify the Configuration

To verify that transparent proxying is working, you can use curl or a web browser on a client machine within your network. For example, use curl with the -v option to see the traffic details:

curl -v https://www.example.com

If OSQuid is correctly intercepting the traffic, you should see details in the curl output indicating that the connection is being proxied. Alternatively, you can check the OSQuid access logs, typically located at /var/log/squid/access.log, to see if traffic is being logged.

Troubleshooting

• OSQuid not starting: Check the configuration file for syntax errors. Use squid -k parse to test the configuration.
• Traffic not being redirected: Ensure that the iptables rules are correctly configured and that the network interface is correct.
• SSL errors: Make sure the CA certificate is correctly installed on the client machines.
• Access denied errors: Review your ACL rules in squid.conf to ensure that traffic is being allowed.

Conclusion

Transparently proxying SCHTTP traffic with OSQuid can provide valuable insights into network traffic and enhance security. Remember to handle SSL bumping responsibly and ethically. By following these steps, you can set up OSQuid to transparently proxy SCHTTP traffic on specific ports, giving you greater control and visibility over your network. Good luck, and happy proxying!