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FYI regarding nmap-payloads, Snort evasion, etc.
From: Daniel Miller <bonsaiviking () gmail com>
Date: Fri, 20 Apr 2012 16:37:27 -0500


I ran across this while testing scan types against Snort IDS. Two of the payloads (xdmcp for 177/udp and Amanda for 10080/udp) trigger two default rules (sid:1867 and sid:634) when directed from external to internal addresses.

After some thought, I considered implementing an option to turn off payloads, listing it under IDS evasion methods. However, after digging in the code, I found out that using --data-length 0 would have the exact same effect (as far as I am aware).

A few more notes from my testing (which is far from complete):

* Nmap's ICMP echo request is detected specifically as Nmap because it contains no data. Setting --data-length 48 works to simulate ping from Linux.

* The --scan-delay option slows down EVERYTHING. ARP ping, dns resolution, version detection, you name it. (Haven't tested NSE scripts).

* To evade the low (default) sensitivity portscan detection built into Snort, --scan-delay 15.5s works pretty well. That's VERY SLOW! (and generic. There are different thresholds for different scan types.)

* Host detection can be done a little faster with -PY, since anything not TCP, UDP, or ICMP is lumped into a single tracker with higher detection thresholds (lower sensitivity). It's not as accurate as some other detection types, but (unlike -PO50 or others) it's less likely to be alerted on.

* There are detection rules for 161,T:705,U:162 that will trigger on any traffic at all. May want to avoid these ports.

Note that this testing was with default rules on Snort. There are no guarantees that I even set it up correctly, so YMMV. My basic takeaway was that if you really need to avoid IDS, plan on doing lots of separate scans. My current best effort involves 3 separate invocations of nmap just to do host discovery, then 1 for name resolution (on up hosts only), and 2 separate scans for ports.

Hope it was helpful!
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