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Getting iptables not to reply

+1 vote
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If you have an 'accept' rule for a service that is not currently running, is it possible to have iptables to simply not respond instead of reporting the port as 'closed'? During a port scan at grc.com, if the router doesn't reply the port will be reported as 'stealth'.

posted Sep 30, 2013 by Abhay Kulkarni

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Do you want to respond with ACK, FIN, or RST?
Err should've been FIN, SYN, or RST

1 Answer

+1 vote

So what happens when you hit a machine that has a port closed? Well: this happens (telnet 127.0.0.30 443):

tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on lo, link-type EN10MB (Ethernet), capture size 65535 bytes 20:42:07.706751 IP 127.0.0.1.34056 > 127.0.0.30.443: Flags [S], seq  2590507944, win 43690, options [mss 65495,sackOK,TS val 3717641 ecr  0,nop,wscale 7], length 0
20:42:07.706778 IP 127.0.0.30.443 > 127.0.0.1.34056: Flags [R.], seq 0, ack 2590507945, win 0, length 0

In response to my TCP SYN packet on loopback ip 127.0.0.30:443, which does not have a service running, you see the kernel respond with a TCP RST.

Abhay: You're talking about a router. Is the router being hit here on a closed port, or is some machine behind the router being hit on a closed port?

Only option I see, which really sucks and __I DO_NOT_RECOMMEND_THIS__ because it may also DROP legitimate RST packets when the service actually wants to send it:

iptables -I OUTPUT -p tcp -s 127.0.0.30 --sport 443 --tcp-flags RST RST -j DROP
answer Sep 30, 2013 by Ahmed Patel
Just FYI, the RST sending code is normal behaviour (RFC793):
Quote:
If the state is CLOSED (i.e., TCB does not exist) then all data in the incoming segment is discarded. An incoming segment containing a RST is discarded. An incoming segment not containing a RST causes a RST to be sent in response.
If your output policy is default drop (ie, allow only what you use - you find interresting stuff like the Pandora music service doing weird stuff which is cool anyway), you get that behavior. Since we're dealing with output, you can only allow users or a white list of apps. I'd log the hell out of stuff if you're going to do this either way.
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