[Snort-users] Alerting on internal TCP connection attempts to non-existent services or hosts .

Stark, Vernon L. Vernon.Stark at ...383...
Tue Dec 3 16:59:21 EST 2013

We have good luck with rules that look for the SYN packet used to begin a TCP session.  So, you might try the following:

alert tcp any any -> any 3389 (sid:100000; msg:"RDP Detected"; flags:S; )

-----Original Message-----
From: James Lay [mailto:jlay at ...13475...] 
Sent: Tuesday, December 03, 2013 4:51 PM
To: snort-users at lists.sourceforge.net
Subject: Re: [Snort-users] Alerting on internal TCP connection attempts to non-existent services or hosts .

On 2013-12-03 06:58, Jonathan Heard wrote:
> Alerting on internal TCP connection attempts to non-existent services 
> or hosts
> Hi All,
>  I'm trying to configure snort in a closed network (i.e. no internet) 
> and I really want to be able to receive alerts if snort ever sees 
> particular types of connection on the wire, regardless of whether it 
> actually reaches a host. e.g. If someone so much as tries to establish 
> a telnet or ftp connection from an internal host to any ip address, I 
> want to know about it.
>  Snort is running in passive mode and is receiving all traffic for 
> analysis via an ERSPAN session (i.e. snort is decoding almost 100% GRE 
> inbound). It's version " GRE (Build 132)" and I compiled it 
> myself from source using mainly the default config options.
>  At present a rule such as:
>  alert tcp any any -> any 3389 (sid:100000; msg:"RDP Detected";)
>  ...only fires when I establish a successful TCP connection between 
> two hosts on the Monitored VLAN - This includes just using 'telnet 
> <ip_address> 3389'. However if I use the IP address of either a 
> non-existent server in the subnet, or a server which is not listening 
> on port 3389 then snort doesn't log any alerts for this rule :-(
>  If I run 'snort -v' I can see the captured TCP packet leaving the 
> host which initiated the connection, so I know snort is seeing it - 
> But I cannot find a way to make it react.
>  The packet is represented by snort -v as follows (with some info
> redacted):
>  <Date/Time> <SRC_IP>:58978 -> <DEST_IP>:3389  TCP TTL: 128 TOS:0x0 
> ID:XXXXX IpLen: 20 DgmLen:52 DF
>  ******S* Seq: 0xXXXXXXXX Ack: 0x0 Win: 0x2000 TcpLen: 32  TCP Options 
> (6) => MSS: 1260 NOP WS: 8 NOP NOP SackOK
>  I'm using the snort.conf which comes with the snort free subscription 
> ruleset - I've tried stripping it back to a very basic config with 
> most of the preprocessors and stock rules disabled but the behaviour 
> remains the same.
>  Is it possible to achieve this and if so how please?
>  Many Thanks in advance
>  Jonathan

Wonder if something like the below would work:

alert tcp any 3389 -> any any (msg:"RDP RST Packet Detected"; flow:stateless; flags:RA+; sid:100000;)

This should alert on the reset packet sent from the machine that doesn't have 3389 open on it.


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