I have a TPROXY-supported proxy server running on my machine (listening
on 127.0.0.1:8080). I configured iptables to redirect traffic to this
proxy server using `TPROXY` target as below:
```sh
ip route add local default dev lo table 100
ip rule add fwmark 1 table 100
iptables -t mangle -N PROXY
# Ignore private IPs
iptables -t mangle -A PROXY -d 10.0.0.0/8 -j RETURN
iptables -t mangle -A PROXY -d 100.64.0.0/10 -j RETURN
iptables -t mangle -A PROXY -d 127.0.0.0/8 -j RETURN
iptables -t mangle -A PROXY -d 169.254.0.0/16 -j RETURN
iptables -t mangle -A PROXY -d 172.16.0.0/12 -j RETURN
iptables -t mangle -A PROXY -d 192.0.0.0/24 -j RETURN
iptables -t mangle -A PROXY -d 224.0.0.0/4 -j RETURN
iptables -t mangle -A PROXY -d 240.0.0.0/4 -j RETURN
iptables -t mangle -A PROXY -d 255.255.255.255/32 -j RETURN
iptables -t mangle -A PROXY -d 192.168.0.0/16 -j RETURN
iptables -t mangle -A PROXY -p tcp -j TPROXY --on-port 8080 --on-ip
127.0.0.1 --tproxy-mark 1
iptables -t mangle -A PROXY -p udp -j TPROXY --on-port 8080 --on-ip
127.0.0.1 --tproxy-mark 1
iptables -t mangle -A PREROUTING -j PROXY
iptables -t mangle -N PROXY_SELF
# Ignore private IPs
iptables -t mangle -A PROXY_SELF -d 10.0.0.0/8 -j RETURN
iptables -t mangle -A PROXY_SELF -d 100.64.0.0/10 -j RETURN
iptables -t mangle -A PROXY_SELF -d 127.0.0.0/8 -j RETURN
iptables -t mangle -A PROXY_SELF -d 169.254.0.0/16 -j RETURN
iptables -t mangle -A PROXY_SELF -d 172.16.0.0/12 -j RETURN
iptables -t mangle -A PROXY_SELF -d 192.0.0.0/24 -j RETURN
iptables -t mangle -A PROXY_SELF -d 224.0.0.0/4 -j RETURN
iptables -t mangle -A PROXY_SELF -d 240.0.0.0/4 -j RETURN
iptables -t mangle -A PROXY_SELF -d 255.255.255.255/32 -j RETURN
iptables -t mangle -A PROXY_SELF -d 192.168.0.0/16 -j RETURN
iptables -t mangle -A PROXY_SELF -m mark --mark 2 -j RETURN
iptables -t mangle -A PROXY_SELF -p tcp -j MARK --set-mark 1
iptables -t mangle -A PROXY_SELF -p udp -j MARK --set-mark 1
iptables -t mangle -A OUTPUT -j PROXY_SELF
```
It works as expected for locally-generated network. However, there's a
problem with Docker container networks. As I didn't change any of its
default network/iptables configuration, it uses internet-connected
network device (`eth0`) as gateway to pass outgoing networks from. When
I enable iptables tproxy to redirect traffic to the proxy server, all
traffic originated from my machine is properly passed through the proxy
server, but the traffic originated from inside Docker container does not
reach the proxy server, and fails to access the internet. How can I
resolve it?
I also used `LOG` iptables target for every chain in every table to see
how packets traverse through, and to debug (or guess) what the issue is.
From what I understood, for locally-generated packets as they pass
through `mangle` table `OUTPUT` chain, they are marked with fwmark 1,
then routed to `lo` device (instead of the default internet-connected
device `eth0`) because of the `ip route`, and `ip rule` commands, and
they will then received on `lo` device, which get redirected to the
proxy server by `tproxy` target defined in `mangle` table's `PREROUTING`
chain, and the rest of the proxying process. On the other hand, for
packets sent from Docker container, they are first received on `docker0`
(default Docker bridge network device), and once they are redirected by
`tproxy` target, the packet _hangs_ in `nat` table `INPUT` chain, which
AFAIK is just before they are handed over to the proxy server process.
As I don't see any log message from proxy server, I guess these packets
are being dropped (?) somehow. I tested the followings as well:
- Running the proxy server bounded to `0.0.0.0`
- Enabling `net.ipv4.ip_forward` kernel option
- Letting `docker0` packets to pass through the `mangle` table
`PREROUTING` chain first by just marking them with fwmark 1 (without
jumping to `tproxy`), and let `ip route` to re-route them back to `lo`
(similar to what happens for locally-generated packets), and then apply
tproxy redirection for the associated packet that is received later on
on the `lo` device. This works as expected for the _forwared_ packet,
but the corresponding packet that is received on `lo` device hangs right
after the `mangle` table's `PREROUTING` chain and does no proceed.
But with no results...
Any help is appreciated.
As I'm new to iptables advanced networking on Linux (started completely
since 3 days ago), please also let me know if there is any more
information that needs to be provided.
Cheers
