I'm trying to understand how TPROXY works in an effort to build a transparent proxy for Docker containers.
After lots of research I managed to create a network namespace, inject an veth interface into it and add TPROXY rules. The following script worked on a clean Ubuntu 18.04.3:
ip netns add ns0
ip link add br1 type bridge
ip link add veth0 type veth peer name veth1
ip link set veth0 master br1
ip link set veth1 netns ns0
ip addr add 192.168.3.1/24 dev br1
ip link set br1 up
ip link set veth0 up
ip netns exec ns0 ip addr add 192.168.3.2/24 dev veth1
ip netns exec ns0 ip link set veth1 up
ip netns exec ns0 ip route add default via 192.168.3.1
iptables -t mangle -A PREROUTING -i br1 -p tcp -j TPROXY --on-ip 127.0.0.1 --on-port 1234 --tproxy-mark 0x1/0x1
ip rule add fwmark 0x1 tab 30
ip route add local default dev lo tab 30
After that I launched a toy Python server from Cloudflare blog:
import socket
IP_TRANSPARENT = 19
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
s.setsockopt(socket.IPPROTO_IP, IP_TRANSPARENT, 1)
s.bind(('127.0.0.1', 1234))
s.listen(32)
print("[+] Bound to tcp://127.0.0.1:1234")
while True:
c, (r_ip, r_port) = s.accept()
l_ip, l_port = c.getsockname()
print("[ ] Connection from tcp://%s:%d to tcp://%s:%d" % (r_ip, r_port, l_ip, l_port))
c.send(b"hello world\n")
c.close()
And finally by running ip netns exec ns0 curl 1.2.4.8 I was able to observe a connection from 192.168.3.2 to 1.2.4.8 and receive the "hello world" message.
The problem is that it seems to have compatibility issues with Docker. All worked well in a clean environment, but once I start Docker things start to go wrong. It seems like the TPROXY rule was no longer working. Running ip netns exec ns0 curl 192.168.3.1 gave "Connection reset" and running ip netns exec ns0 curl 1.2.4.8 timed out (both should have produced the "hello world" message). I tried restoring all iptables rules, deleting ip routes and rules generated by Docker and shutting down Docker, but none worked even if I didn't configure any networks or containers.
What is happening behind the scenes and how can I get TPROXY working normally?