Requests looping in transparent proxy mode

[fazio91]

Problem Description

When attempting to set up a mitmproxy server as a Transparent Proxy following the official guide from https://docs.mitmproxy.org/stable/howto-transparent/, the requests reaching the proxy server keep looping indefinitely instead of being processed correctly.

Steps to reproduce the behavior:

1. Launch an Amazon EC2 instance (Amazon Linux 2023).
2. Download and extract the mitmproxy binary from https://mitmproxy.org/.
   yum install -y iptables
wget https://downloads.mitmproxy.org/11.0.1/mitmproxy-11.0.1-linux-x86_64.tar.gz
tar -zxvf mitmproxy-11.0.1-linux-x86_64.tar.gz
3. Enable IP forwarding and adjust system settings:
   sysctl -w net.ipv4.ip_forward=1
sysctl -w net.ipv6.conf.all.forwarding=1
sysctl -w net.ipv4.conf.all.send_redirects=0
4. Set up iptables rules to redirect traffic:
   iptables -t nat -A PREROUTING -i enX0 -p tcp --dport 80 -j REDIRECT --to-port 8080
iptables -t nat -A PREROUTING -i enX0 -p tcp --dport 443 -j REDIRECT --to-port 8080
ip6tables -t nat -A PREROUTING -i enX0 -p tcp --dport 80 -j REDIRECT --to-port 8080
ip6tables -t nat -A PREROUTING -i enX0 -p tcp --dport 443 -j REDIRECT --to-port 8080
5. Start mitmproxy in transparent mode:
   ./mitmproxy --mode transparent --showhost --set block_global=false
6. From another host, send a request:
   curl -x http://my-aws-server:8080 http://my-destination.ext

I also tried the steps outlined in section 5 of the guide (starting mitmproxy as another user, adjusting firewall rules and verifying packet ownership), but the issue persists. The requests always loop.

System Information

Mitmproxy: 11.0.1 binary
Python: 3.12.7
OpenSSL: OpenSSL 3.3.2 3 Sep 2024
Platform: Linux-6.1.115-126.197.amzn2023.x86_64-x86_64-with-glibc2.34