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Golang TProxy provides an easy to use wrapper for the Linux Transparent Proxy functionality.

Transparent Proxy (TProxy for short) provides the ability to transparently proxy traffic through a userland program without the need for conntrack overhead caused by using NAT to force the traffic into the proxy.

Another feature of TProxy is the ability to connect to remote hosts using the same client information as the original client making the connection. For example, if the connection 10.0.0.1:50073 -> 8.8.8.8:80 was intercepted, the service could make a connection to 8.8.8.8:80 pretending to come from 10.0.0.1:50073.

The linux kernel and IPTables handle diverting the packets back into the proxy for those remote connections by matching incoming packets to any locally bound sockets with the same details.

This is done in three steps. (Please note, this is from my understanding of how it works, which may be wrong in some places, so please correct me if I have described something wrong)

Step 1 - Binding a listener socket with the IP_TRANSPARENT socket option

Preparing a socket to receive connections with TProxy is really no different than what is normally done when setting up a socket to listen for connections. The only difference in the process is before the socket is bound, the IP_TRANSPARENT socket option.

syscall.SetsockoptInt(fileDescriptor, syscall.SOL_IP, syscall.IP_TRANSPARENT, 1)

Step 2 - Setting the IP_TRANSPARENT socket option on outbound connections

Same goes for making connections to a remote host pretending to be the client, the IP_TRANSPARENT socket option is set and the Linux kernel will allow the bind so along as a connection was intercepted with those details being used for the bind

Step 3 - Adding IPTables and routing rules to redirect traffic in both directions

Finally IPTables and routing rules need to be setup to tell Linux to redirect the desired traffic to the proxy application.

First make a new chain in the mangle table called DIVERT and add a rule to direct any TCP traffic with a matching local socket to the DIVERT chain

iptables -t mangle -N DIVERT
iptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT

Then in the DIVERT chain add rules to add routing mark of 1 to packets in the DIVERT chain and accept the packets

iptables -t mangle -A DIVERT -j MARK --set-mark 1
iptables -t mangle -A DIVERT -j ACCEPT

And add routing rules to direct traffic with mark 1 to the local loopback device so the Linux kernal can pipe the traffic into the existing socket.

ip rule add fwmark 1 lookup 100
ip route add local 0.0.0.0/0 dev lo table 100

Finally add a IPTables rule to catch new traffic on any desired port and send it to the TProxy server

iptables -t mangle -A PREROUTING -p tcp --dport 80 -j TPROXY --tproxy-mark 0x1/0x1 --on-port 8080

To test this out and see it work, try running the example in example/tproxy_example.go on a virtual machine and route some traffic through it.

Contributing

To contribute to this project, please follow this guide:

  1. Create an issue detailing your planned contribution
  2. Fork this repository and implement your contribution
  3. Create a pull request linking back to the issue
  4. Await approval and merging

TODOs

[x] Add support for proxying UDP connections