transparently proxy squid in a docker container

[Justin Michael Schwartzbeck <justinmschw@xxxxxxxxx>]

Hi all,

For some years I have used squid 3.5 with SSL bump and transparent proxy locally on my laptop. I have been using the following in my squid.conf:


ssl_bump server-first all
http_port 3128
http_port 3129 intercept
http_port 3130 ssl-bump intercept generate-host-certificates=on dynamic_cert_mem_cache_size=4MB cert=/etc/squid/ssl/bluestar.crt key=/etc/squid/ssl/bluestar.pem

So if I want to manually set the proxy on the client side, I use port 3128, but by default all http/https traffic is redirected to port 3129 and 3130, respectively. Here are my iptables rules:


iptables -t nat -A OUTPUT -p tcp -m tcp --dport 80 -m owner --uid-owner root -j RETURN
iptables -t nat -A OUTPUT -p tcp -m tcp --dport 80 -m owner --uid-owner dockeruser -j RETURN
iptables -t nat -A OUTPUT -p tcp -m tcp --dport 80 -j REDIRECT --to-ports 3129
iptables -t nat -A OUTPUT -p tcp -m tcp --dport 443 -m owner --uid-owner root -j RETURN
iptables -t nat -A OUTPUT -p tcp -m tcp --dport 443 -m owner --uid-owner dockeruser -j RETURN
iptables -t nat -A OUTPUT -p tcp -m tcp --dport 443 -j REDIRECT --to-ports 3130

dockeruser is the user that starts the docker container, and proxy is the actual squid user. I didn't know which one I needed a rule for, so I just chose both.

As I said before, this worked great when I was running squid 3.5 on bare metal. Now I am running squid 4 in a docker container. I am seeing the following error many times in the squid logs when I try to use the transparent proxy:

2021/02/24 01:45:17| WARNING: Forwarding loop detected for:

GET /success.txt HTTP/1.1

User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Firefox/78.0

Accept: */*

Accept-Language: en-US,en;q=0.5

Accept-Encoding: identity,gzip,deflate

Pragma: no-cache

Via: 1.1 19deb96addda (squid/4.11)

X-Forwarded-For: 172.18.0.1

Cache-Control: no-cache

Host: detectportal.firefox.com

And from firefox I see this:

WARNING: Forwarding loop detected for

SSL_ERROR_RX_RECORD_TOO_LONG

I feel like I am very close, but I'm not sure what I am missing. Does someone else know of a better way to do this? I had assumed that since I publish the ports, I should be able to redirect to them the same way I would if squid were running locally.

I would appreciate any help in figuring this out.

Thanks,

-Justin

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