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Transparent Socket Forwarding with SPR and MITMProxy

· 2 min read

Update​

This post has become a guide which is being kept up to date, check it out!

Overview​

In this post we'll show how PLUS members can add a mitmproxy plugin to their SPR setup, and then use the Programmable Firewall (PFW) plugin to redirect traffic through mitmproxy with DNAT forwarding.

We do not need to configure our clients with proxy settings to point to mitmproxy, or rewrite DNS responses, since we are using the PFW feature to do the redirection.

This plugin is available on github.

Prepare the plugin​

from the SPR directory, typically /home/spr/super

cd plugins
git clone https://github.com/spr-networks/spr-mitmproxy
echo [\"plugins/spr-mitmproxy/docker-compose.yml\"] > ../configs/base/custom_compose_paths.json
cd spr-mitmproxy
docker compose build

Configure SPR​

  1. Navigate to the SPR UI. Add mitmproxy under the Plugins page
  • be sure its been added to configs/base/custom_compose_paths.json as above
  • Enable it by toggling the slider
  1. Add mitmweb0 to the custom interface rules. You can verify your container's network address in the Container tab -> Under Firewall-> Custom Interface Access Add a new rule, make sure mitmproxy has wan at least to access the internet.

  1. Create a forwarding rule to the container web interface :8081. Pick an arbitrary IP in the subnet -- although not the same one as the container as that confuses dnat.

  2. Create a site forward rule with PFW for traffic to intercept

Using mitmproxy​

Then make a curl request from any of the LAN devices, and it should populate on the mitmweb host. This was the :8081 host that was earlier defined

Leveraging Transparent Sockets​

Behind the scenes, mitmproxy is using transparent sockets with DNAT. Inside the container network, we establish dnat rules to mitmproxy from incoming ports 80, 443.

#!/bin/bash

nft -f - << EOF
table inet nat {
        chain prerouting {
                type nat hook prerouting priority filter; policy accept;
                tcp dport { 80, 443 } dnat ip to 127.0.0.1:9999
        }
}
EOF

mitmweb -p 9999 -m transparent --web-host 0.0.0.0

We'd love to hear from you​

We're always thrilled to get feedback on plugins people would like to see, and we're excited to hear about what people will be able to do with mitmproxy running alongside SPR. Drop a line at outreach[at]supernetworks.org or join us on discord