I’ve recently set up Squid as a transparent proxy for a security project. What should have been relatively straightforward had me browsing through tutorials from over 10 years ago that don’t quite work any more. After comparing this prehistoric knowledge with some supposedly up-to-date documentation, I managed to understand enough about Squid to get a minimal transparent proxy configuration on a modern version of Linux hosted in the cloud.

With the hopes of saving someone some time that may be embarking on a similar journey, I thought I’d write this post. We discuss HTTP transparent proxying at the start, but provide resources for allowing support for HTTPS.

The following instructions have been tested on Ubuntu 18.04 deployed in Azure, and Ubuntu 20.04 on DigitalOcean.

Installing Squid

This part is straightforward, so just follow the normal install procedure for your operating system/package manager. I used Ubuntu, so installing Squid was as easy as sudo apt install squid.

Before we continue, it’s worth checking if Squid is able to run at this point (which may not be the case if something is using Squid’s default port, for example). It should be running after installation, which you can check with systemctl status squid. If squid is not running, try to fix anything at this point.

Configuring Squid

Configuration file

Now the most important part - the configuration. The config is stored under /etc/squid/squid.conf, but before we make any changes I like to make a copy of the original:

sudo cp /etc/squid/squid.conf /etc/squid/squid.conf.orig

Next, edit the configuration file with your favourite text editor:

sudo vim /etc/squid/squid.conf

And enter this minimal configuration:

http_access allow all

http_port 3128 intercept

The http_access parameter should ideally be narrowed down as described in the Squid documentation, but to eliminate potential errors we will permit anything to access the proxy.

The http_port states which port Squid will listen at, for which we keep the default 3128. We will redirect traffic to this port using iptables soon. intercept is needed to make Squid act as a transparent proxy.

Nothing else is necessary for a working configuration as of the time of writing this post, unlike what some other tutorials may lead you to believe. Note that in its current state, there will be a warning printed in the Squid logs whenever it’s started, stemming from the fact that a non-transparent port is not open. If you’d like to silence that, you can have Squid listen at a vacant port by adding e.g. http_port 3129 to the configuration.

Finally, we can restart Squid:

sudo systemctl restart squid

This should be it for the Squid configuration! Make sure to check if it’s working, as described earlier in the post. If it’s not, good places to start are the journalctl entries for squid, and the access and log files by default located at /var/log/squid/access.log and /var/log/squid/cache.log, respectively.

Enabling IP forwarding

Since we’re configuring a transparent proxy, we need to configure IP forwarding on the system:

sudo sysctl net.ipv4.ip_forward=1

To make this configuration persistent, modify /etc/sysctl.conf and uncomment the line:

#net.ipv4.ip_forward=1

iptables Rules

To get the kernel to forward packets received at port 80 to Squid, we need the following iptables rule:

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

Make sure to modify the above rule, or add additional ones if you have other interfaces apart from eth0 that you’d like to forward traffic from. Those can be found using e.g. ip link show or ifconfig. This rule makes it so that only external traffic will be send to Squid, and all traffic originating at the machine will reach its destination and not cause a cycle.

If at any point you make a mistake with your configuration, you can flush all existing iptables NAT rules:

sudo iptables -t nat -F

Or list any existing rules using:

sudo iptables -t nat -L

Closing Notes - HTTPS Support, Gateway Setup, Spoofed Requests

You should now have a minimal Squid transparent proxy running. Make sure to configure the machine as the default gateway for whichever machines you’d like to transparently proxy data for.

To enable transparent proxying of HTTPS traffic, I recommend suntong’s guide.

Note that Squid is unable to resolve the original destination of packets that have had their destination IP spoofed (source). To resolve those properly, I’ve had luck using Privoxy in intercepting mode as I describe in this post.

Thanks for reading, and I hope that this post helped anyone struggling with Squid!