redsocks + iptables behavior [closed] - linux

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The following is the network topology of the test bed where I use redsocks and iptables to access youtube from LAN, provided that I have a ssh dynamic forwarding established between Linux and VPS:
|
outside | inside firewall
|
| LAN 192.168.1.0/24
| ----+--------------------+-----------
| |.2 | .3
+-----+ | +------+----+ +----+----+
| | | | | | |
Youtube <-->| VPS <============> Linux | | Windows |
| | | | | | |
+-----+ | +-----------+ +---------+
|
|
The Linux box and Windows box are on the same LAN inside the firewall, which blocks any access to say Youtube. The VPS is outside the firewall and it can access Youtube. From within the Linux box I can ssh -D to the VPS, making VPS acts as a SOCKS5 proxy.
To further make the SOCKS5 proxy "transparent" to the processes (e.g., browser) on the Linux box, I did the following:
run a SOCKS5 client software redsocks to talk with the SOCKS5 server, for handling the SOCKS5 protocols.
add some iptable rules, to direct youtube-bound traffic to redsocks process
This works fine for the Linux box itself:
echo 1 > /proc/sys/net/ipv4/ip_forward
/sbin/iptables -F -t nat
/sbin/iptables -t nat -X REDSOCKS
/sbin/iptables -t nat -N REDSOCKS
/sbin/iptables -t nat -A OUTPUT -d $YOUTUBE -j REDSOCKS
/sbin/iptables -t nat -A REDSOCKS -p TCP -d $YOUTUBE -j REDIRECT --to-ports $REDSOCKS_PORT
/sbin/iptables -t nat -A REDSOCKS -p UDP -d $YOUTUBE -j REDIRECT --to-ports $REDSOCKS_PORT
/sbin/iptables -t nat -A REDSOCKS -p ICMP -d $YOUTUBE -j REDIRECT --to-ports $REDSOCKS_PORT
/sbin/iptables -t nat -A REDSOCKS -j RETURN
Now I want let Windows box also to access Youtube "transparently", with some software configuration. My idea is as the following:
Change the routing table on Windows machine such that all Youtube-bound traffic will be routed to the Linux box (192.168.1.2)
Setup iptables "somehow" to do SNAT for the Youtube-bound traffic from Windows
After some tests, I encountered the following problem:
the REDIRECT (or DNAT) target only applicable for PREROUTING/OUTPUT hooks (e.g., in the iptable settings for redsocks, the REDSOCKS chain is hooked on OUTPUT chain)
the SNAT (or MASQUERADE) target only applicable for POSTROUTING hook
What I want to do for the Windows machine is firstly SNAT and then REDIRECT, but as SNAT is happening on POSTROUTING hook, it seems it's too later (in the packet processing flow) to have it to be processed by PREROUTING/OUTPUT hooks...
Is there a way out here?
PS 1. I found a working solution (below in the reply from myself), although I don't really understand why it worked.
PS 2. I started looking into source code of redsocks, one thing I found so far is that redsocks use getsockopt(,SO_ORIGINAL_DST,) to obtain the destination address before DNAT. What's not clear to me is the following processing, and src/dst ip address of the return packet from redsocks. Don't know if there is an introduction about redsocks implementation...

Answer 1

I found a solution which does not require SNAT:
let redsocks listen on 192.168.1.2 instead of 127.0.0.1, by changing /etc/redsocks.conf
change REDIRECT --to-ports 12345 target in all rules in REDSOCKS chain to DNAT --to-destination 192.168.1.2:12345 target
add a rule in PREROUTING chain to handle "youtube-bound traffic from windows" by REDSOCKS chain: /sbin/iptables -t nat -A PREROUTING -d $youtube -s $win10 -j REDSOCKS
add a route entry on Windows to route youtube-bound traffic to 192.168.1.2
With some more learning and tests, I think I know why the new config works, and why the previous config does not work.
The first is the behavior of REDIRECT:
for locally oriented traffic, the new destination ip address after REDIRECT is 127.0.0.1
for external traffic, the new destination ip address after REDIRECT is NOT 127.0.0.1, but 192.168.1.2
That's why the previous config does not work, since in previous config, redsocks listen on 127.0.0.1, but iptables REDIRECTs to 192.168.1.2.
The new config let redsocks listen on 192.168.1.2 and explicitly use DNAT to change destination of both locally generated and external traffic to 192.168.1.2, so it works.
I also tested another config, almost the same as the 2nd config, but letting redsocks listen on 127.0.0.1, and use DNAT explicitly set destination ip to 127.0.0.1. This way, externally generated traffic will be directly to 127.0.0.1, which are regarded as martian packets by kernel. I also need to set /proc/sys/net/ipv4/conf/eth0/route_localnet to 1, then it works for windows machine.
At this point, to me, it seems every piece of information fit together :)