Nmap Announce mailing list archives
Re: firewalk meets nmap - TTL
From: Fyodor <fyodor () insecure org>
Date: Sat, 4 Nov 2000 19:57:11 -0800 (PST)
On Thu, 2 Nov 2000, Lance Spitzner wrote:
firewalk meets nmap
Yeah, I enthusiastically agree that Nmap could benefit from some traceroute-like exploration via TTL manipulation. That is one of the reasons I added a '<traceroute type="udp" port="41702">' section to the example XML output I sent last August ( http://lists.insecure.org/nmap-hackers/2000/Jul-Sep/0039.html ). Now there are a couple obvious advantages to adding this traceroute functionality: 1) Traceroute output is deeply useful in mapping a network to determine its basic setup/architecture. Most people do traceroute anyway, and Nmap could do it faster and in a more flexible fashion that has a higher chance of getting through. 2) The XML output with traceroute would allow a smart frontend to draw a network graph representing the connections between networks as well as different icons based on the machine type (a la Cheops). Or even a Ballista-style Network fly-through :). As Lance mentioned, the more subtle advantage lies in the use of TTL to deduce filtering characteristics. I consider this particularly important in the case of UDP, where Nmap has trouble differentiating between ports that are not responding because they are open and those that are not responding because they are filtered. For example, lets scan some random @Home machine: amy~# nmap -O -sUS 24.176.215.14 Starting nmap V. 2.54BETA7 ( www.insecure.org/nmap/ ) Interesting ports on c206945-a.sfmissn1.sfba.home.com (24.176.215.14): (The 3096 ports scanned but not shown below are in state: closed) Port State Service 137/tcp filtered netbios-ns 137/udp open netbios-ns 138/tcp filtered netbios-dgm 138/udp open netbios-dgm 139/tcp filtered netbios-ssn 139/udp open netbios-ssn 520/udp open route 641/tcp open unknown 653/tcp open unknown 1080/tcp filtered socks TCP Sequence Prediction: Class=trivial time dependency Difficulty=7 (Trivial joke) Remote operating system guess: Windows NT4 / Win95 / Win98 As you can see, @Home is apparently filtering some of the TCP ports. But what about those 6 "open" UDP ports? Are they really open, or do they just appear open because @Home is silently dropping the packets? Things really hit the fan when ISP's drop 31337/udp . I get panicked mails every week saying "ACK!!! Nmap says I am infected with Back Orifice!". One solution would be to send application-specific probes to each non-responsive UDP port to elicit a response (eg Netbios queries to 139, SNMP queries to 161, etc). But coding this would be a huge effort, and it wouldn't work against services that require authentication info before they will respond. Another approach is to play TTL games. As Lance noted, some (but not all) firewalls may apply the filtering rules to drop packets before they decrement the TTL and apply the time exceeded test. This makes it easy to determine filtering rules, since time exceeded messages are only returned by the firewall for allowed ports. In a similar vein, medium and large sized networks may apply the filters several hops before the final destination machine. So a traceroute to UDP/137 may stop several hops before a traceroute to UDP/45000 . This likely means that 137 is filtered. This technique does not require the filtering order in the approach above, but it does require that the filter allows outbound TTL exceeded messages. So the change I was considering adding to Nmap would take the @Home results given above and: 1) Send traceroute probes to the "open" UDP ports ( 137,138,139,520,641,653) to determine the highest TTL that results in a time exceeded response. 2) Do the same to a known-closed UDP port (for example, port udp/1 in the above is "closed" even though it isn't shown because it is one of the 3096 Nmap omitted from output). 3) In the cases where the MAX TTL from a #1 port is less than that from step #2, that port can be marked filtered. Otherwise, we have to leave it as "open" (even though it is still possibly filtered). So now, lets try this against the machine above to see if we can determine whether 137/udp is open or filtered: First we do a traceroute against 137/udp: amy# hping -I eth0:0 -t 1 -2 -V -p 137 --traceroute 24.176.215.14 using eth0:0, addr: 208.184.74.98, MTU: 1500 HPING 24.176.215.14 (eth0:0 24.176.215.14): udp mode set, 28 headers + 0 data bytes 1->TTL 0 during transit from 208.184.74.97 (opium.lnxnet.net) 2->TTL 0 during transit from 216.200.177.1 (sdsl-216-200-177-1.dsl.sjc.megapath.net) 3->TTL 0 during transit from 209.249.140.3 (main4-209-249-140.sjc.above.net) 4->TTL 0 during transit from 208.184.102.193 (core5-main4-oc3-2.sjc.above.net) 5->TTL 0 during transit from 208.184.102.206 (core3-core5-oc48.sjc2.above.net) 6->TTL 0 during transit from 208.185.175.134 (athome-above.sjc2.above.net) 7->TTL 0 during transit from 24.7.75.178 (bb3-pos3-1.rdc1.sfba.home.net) 8->TTL 0 during transit from 10.0.254.243 (unknown host name) 9->TTL 0 during transit from 10.84.17.10 (unknown host name) 10->TTL 0 during transit from 24.9.239.185 (ubr1.sfmissn1.sfba.home.net) So we saw up to 10 hops to port 137. Note the 10.* reserved internal addresses. Way to go @Home! :). Now lets try against a known-closed port (I'll use 1): amy~# hping -I eth0:0 -t 1 -2 -V -p 1 --traceroute 24.176.215.14 using eth0:0, addr: 208.184.74.98, MTU: 1500 HPING 24.176.215.14 (eth0:0 24.176.215.14): udp mode set, 28 headers + 0 data bytes 1->TTL 0 during transit from 208.184.74.97 (opium.lnxnet.net) 2->TTL 0 during transit from 216.200.177.1 (sdsl-216-200-177-1.dsl.sjc.megapath.net) 3->TTL 0 during transit from 209.249.140.3 (main4-209-249-140.sjc.above.net) 4->TTL 0 during transit from 208.184.102.193 (core5-main4-oc3-2.sjc.above.net) 5->TTL 0 during transit from 208.184.102.206 (core3-core5-oc48.sjc2.above.net) 6->TTL 0 during transit from 208.185.175.134 (athome-above.sjc2.above.net) 7->TTL 0 during transit from 24.7.75.178 (bb3-pos3-1.rdc1.sfba.home.net) 8->TTL 0 during transit from 10.0.254.243 (unknown host name) 9->TTL 0 during transit from 10.84.17.10 (unknown host name) 10->TTL 0 during transit from 24.9.239.185 (ubr1.sfmissn1.sfba.home.net) ICMP Port Unreachable from 24.176.215.14 (c206945-a.sfmissn1.sfba.home.com) So here again we have a maximum of 10 hops. D'oh! This does not give us enough evidence to conclude that filtering is in place (even though I am 99% sure it is). For networks "vulnerable" to this type of filtering detection, that second traceroute would show more hops than the first one before the port unreachables started. But even though they aren't 100% effective (as demonstrated above), the TTL tricks are probably worth adding. I may get to them at some point. But the best way to lobby for their inclusion is to send me a patch :). By the way, Nmap currently uses a random (per execution) TTL values that various between 37 and 64 or so. Cheers, Fyodor -------------------------------------------------- For help using this (nmap-hackers) mailing list, send a blank email to nmap-hackers-help () insecure org . List run by ezmlm-idx (www.ezmlm.org).
Current thread:
- firewalk meets nmap - TTL Lance Spitzner (Nov 03)
- RE: firewalk meets nmap - TTL Ofir Arkin (Nov 03)
- RE: firewalk meets nmap - TTL (tested) Lance Spitzner (Nov 04)
- RE: firewalk meets nmap - TTL (tested) Ofir Arkin (Nov 04)
- RE: firewalk meets nmap - TTL (tested) Lance Spitzner (Nov 04)
- Re: firewalk meets nmap - TTL Fyodor (Nov 05)
- Re: firewalk meets nmap - TTL Mikael Olsson (Nov 08)
- RE: firewalk meets nmap - TTL Ofir Arkin (Nov 03)