Penetration Testing mailing list archives
an anternative to port-knoking using the OpenBSD pf only
From: poplix <poplix () papuasia org>
Date: Mon, 23 Jan 2006 22:44:52 +0100
Hi there, I wish to propose an alternative to port knoking that uses the native OpenBSD's pf code only. The idea is to use the pf's passive os fingerprinter to authenticate initial SYN packets. With a tool (or kernel patch) able to rewrite packets header is possibleto use a specific sequence of header fields as a key to validate packets.
Rewriting tcp/ip headers can be useful to change the non standard tcp/ipstack behavior in order to spoof your os to programs that perform analisys
of the headers values to identify oss. This well known method topassively fingerprint oss is based on the analisys of those tcp/ip headers fields that are vendor specific such as tcp options, window size, ttl, ecc.
There are several programs out there that implements this tecnique: p0f, ettercap, OpenBSD pf, ecc but deciveing this kind of programs canresult in waste of time. In fact it's well known that packets header can be
easily forged or manipulated so os fingerprinting (ospf) is used for statistics, performance improvements or similar only.But this is not the point: is possible to use osfp spoof in conjunction with
OBSD pf as an alternative to port knoking. Port knoking is useful to increase stealthness of the internet servers, but it requires additional software to run on those systems increasing security risks. Using ospf spoof is possible to have stealthness using the native OBSD kernel code only. As mentioned above OpenBSD pf has a builtin passive os fingerprinter that can be used in pf rules, an example can be: pass in proto tcp from any os "Windows 2000" This rule allows packets from any Windows 2000 machine. With this feature is possible to block all incoming packets except those that come from a specific operating system.The idea is to use os fingerprints as a key. An user can invent a specific
sequence of header values that will identify his fake os, add it to fingerprints database and use it in the firewall. The result is an OBSD machine that is totally stealth to port scans but the owner can log into it using his specific set of header values. I've tested this tecnique with a tool I'm writing that is capable of rewriting outgoing (and incoming) packets, but maybe fragroute with some modifications can do the same. The Test: I've a linux box capable of setting arbitrary header values (usingtripp) so I choosed a set of tcp/ip headers fields that are not listed
in osfp database and then I added this sequence naming it "poplix". I added the invented os fingerprint to pf os databese by inserting this line to /etc/pf.os (my os fingerprints database) 8192:128:0:60:M3884,N,W0,N,N,T:poplix:::poplix fi Now I can configure pf with the following ruleset: block in on rl0 all pass in from any os poplix keep state This new os fingerprint entry will identify my packets and allow them to pass through firewall. Linux client runs tripp with the following rule: OUT (tcp.flags='syn') set( tcp.win='8192',ip.ttl='128',ip.flags.df='0', tcp.options='mss=3884+noop+wss=0+noop+noop+ts=TS' );This rule tells tripp to rewrite all outgoing tcp packets with the SYN
flag set only allowing my first handshake packet to match "poplix" fingerprint. Result: root@sd:# ssh papuasia.org -l poplix ssh: connect to host papuasia.org port 22: Connection timed out root@sd:# root@sd:~# tripp -o eth0 -f rules -g 10.0.0.138 -S & rewriter for outgoing traffic started root@sd:~# ssh papuasia.org -l poplix poplix () papuasia org's password: ********** poplix () papuasia org:~$ Note that is also possible to kill tripp after the connection is estabilished (in this case on password request). Notes on tripp: Please note that tripp is in a very embrional state: ip options are not processed, not all tcp options are supported, most icmp types are dropped (echo request end echo reply only pass through tripp), tripp works on ethernet only and it has been tested on linux only. For more info visit http://tripp.dynalias.org I hope that someone can find it useful, ciao poplix ------------------------------------------------------------------------------Audit your website security with Acunetix Web Vulnerability Scanner: Hackers are concentrating their efforts on attacking applications on your website. Up to 75% of cyber attacks are launched on shopping carts, forms, login pages, dynamic content etc. Firewalls, SSL and locked-down servers are futile against web application hacking. Check your website for vulnerabilities to SQL injection, Cross site scripting and other web attacks before hackers do! Download Trial at:
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Current thread:
- an anternative to port-knoking using the OpenBSD pf only poplix (Jan 23)
- Re: an anternative to port-knoking using the OpenBSD pf only Joachim Schipper (Jan 24)
- <Possible follow-ups>
- RE: an anternative to port-knoking using the OpenBSD pf only Shenk, Jerry A (Jan 28)