Forwarding RDP via a Linux machine using iptables: Not working

Question

I have a Linux machine and a Windows machine behind a router that implements NAT (the diagram might be overkill, but was fun to make):

I am forwarding RDP port (3389) on the router to the Linux machine because I want to audit RDP connections. For the Linux machine to forward RDP traffic, I wrote these iptables rules:

iptables -t nat -A PREROUTING -p tcp --dport 3389 -j DNAT --to-destination win-box
iptables -A FORWARD -p tcp --dport 3389 -j ACCEPT

The port is listening on the Windows machine:

C:\Users\nimmy>netstat -a

Active Connections

  Proto  Local Address          Foreign Address        State
  (..snip..)
  TCP    0.0.0.0:3389           WIN-BOX:0         LISTENING
  (..snip..)

And the port is forwarding on the Linux machine:

# tcpdump port 3389
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
01:33:11.451663 IP shieldsup.grc.com.56387 > linux-box.myapt.lan.ms-wbt-server: Flags [S], seq 94663035, win 8192, options [mss 1460], length 0
01:33:11.451846 IP shieldsup.grc.com.56387 > win-box.myapt.lan.ms-wbt-server: Flags [S], seq 94663035, win 8192, options [mss 1460], length 0

However, I am not getting any successful RDP connections from the outside. The port is not even responding:

C:\Users\outside-nimmy>telnet example.com 3389
Connecting To example.com...Could not open connection to the host, on port 3389: Connect failed

Any ideas?

Update

Per @Zhiqiang Ma, I looked at nf_conntrack proc file during a connection attempt and this is what I see (192.168.3.1 = linux-box, 192.168.3.5 = win-box):

# cat /proc/net/nf_conntrack | grep 3389
ipv4     2 tcp      6 118 SYN_SENT src=4.79.142.206 dst=192.168.3.1 sport=43142 dport=3389 packets=6 bytes=264 [UNREPLIED] src=192.168.3.5 dst=4.79.142.206 sport=3389 dport=43142 packets=0 bytes=0 mark=0 secmark=0 zone=0 use=2

2nd update

Got tcpdump on the router and it seems that win-box is sending an RST packet:

21:20:24.767792 IP shieldsup.grc.com.45349 > linux-box.myapt.lan.3389: S 19088743:19088743(0) win 8192 <mss 1460>
21:20:24.768038 IP shieldsup.grc.com.45349 > win-box.myapt.lan.3389: S 19088743:19088743(0) win 8192 <mss 1460>
21:20:24.770674 IP win-box.myapt.lan.3389 > shieldsup.grc.com.45349: R 721745706:721745706(0) ack 755785049 win 0

Why would Windows be doing this?

Solution 1

Add port in iptables rules?:

iptables -t nat -A PREROUTING -p tcp --dport 3389 -j DNAT --to-destination win-box:3389
iptables -A FORWARD -p tcp --dport 3389 -j ACCEPT

I am not very sure it's the reason. But I usually do it in this way: http://www.systutorials.com/816/port-forwarding-using-iptables/

You can all try flush the tables first: iptables -t nat -F; iptables -F and then add these two rules in case other rules in your iptables block the connection.

You may also

cat /proc/net/nf_conntrack

and see the content there. Each forwarding connection has entries there.

Note: MASQUERADE is required as well if the outbound route from windows does not by default pass through the iptables box; see comments below ( you may need to unhide).

Solution 2

I saw you solved the issue with MASQUERADE. I didn't notice that last comment was hidden, so I had to solve the question for my own, thanks to the great Iptables Tutorial (look for it in Freshmeat). I did almost the same as you, but doing a SNAT instead of MASQUERADE, since the linux box has a static local IP. MASQUERADE would be more appropriate if the linux box had its IP given by DHCP, otherwise it's told to be a more processor consuming task.

I didn't need any FORWARD rule, although I had to

echo 1 | sudo tee /proc/sys/net/ipv4/ip_forward

Comments

[Arjan]

Hi there! 8 out of 10 posts refer to your own blog. Though you do quote some information too: is referring to that blog really needed?

[NukaRakuForgotEmail]

@Zhigiang Ma: I added an update.

[ericzma]

@Nimmy Lebby I think Router is the gateway of both linux-box and win-box. Am I right? Then do you mind make the linux-box the gateway of the win-box? That may solve the problem. The incoming packet goes in this way: Internet -> Router -> linux-box -> win-box. But the outgoing packet goes in different way: win-box -> Router -> Internet. linux-box is confused and mark the incoming packet as "[UNREPLIED]". I use port-forwarding on the gateway, which works quite well and I believe the two rules by now should work if linux-box is win-box's gateway.

[ericzma]

@Nimmy Lebby: If you just want audit the RDP connections on linux-box, I think you may log and drop the packet received by linux-box. And leave the Router to forward RDP connections to win-box.

[NukaRakuForgotEmail]

I enabled Masquerading /sbin/iptables -t nat -A POSTROUTING MASQUERADE and that did the trick. Thanks!

[Daniel Farrell]

Yeah, this seems like the correct answer to me. SNAT sould work well for connections that come from a known IP range, MASQUERADE worked well for my case ( the outbound proxy was not the default route back on the windows side)

[ASR]

you missed -j. The command is /sbin/iptables -t nat -A POSTROUTING -j MASQUERADE