Snort mailing list archives
Re: False positives with UDP Portscan PROTO255
From: Jeff Kell <jeff-kell () utc edu>
Date: Sat, 05 Mar 2005 18:47:56 -0500
Mike Lieberman wrote:
I have doubts about some of the messages I am getting from Snort (using rules for 2.3). For instance the following portscan message is from ns1.sprintlink.net to ns1.netwright.net. We see DNS server to DNS Server traffic labeled as port scans. In the case below, unless Sprint’s primary name server ( as well as many others from [have]) has been compromised, these ‘portscans’ would actually have to be something related to BIND.
Any significant number of DNS queries within a short time (depending on your portscan settings) will do this because the traffic is connectionless. Although you and I know these are query/response, the generic portscan preprocessor doesn't.
If you consider a number of queries to a given host (which can precisely be the case if you have a 'cacheing-only' forwarding server on one side) you have sequentially increasing source [ephemeral] ports querying the host on udp/53. The replies look like a fixed source port (udp/53) going back to those sequentially increasing ephemeral ports on the same host.
And that is the generic "definition" of a portscan -- a fixed source port sending traffic to differing ports on the same destination IP.
You could exclude source port 53 to eliminate these, but bear in mind that the black hats are a step ahead of you; a clever UDP port scan will use source port 53, or other common incoming service port (which is typically allowed in a simple firewall ruleset).
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Current thread:
- False positives with UDP Portscan PROTO255 Mike Lieberman (Mar 05)
- Re: False positives with UDP Portscan PROTO255 Jeff Kell (Mar 05)
- RE: False positives with UDP Portscan PROTO255 Mike Lieberman (Mar 05)
- RE: False positives with UDP Portscan PROTO255 Orit Vidas (Mar 08)
- Re: False positives with UDP Portscan PROTO255 Jeremy Hewlett (Mar 09)
- Re: False positives with UDP Portscan PROTO255 Rich Adamson (Mar 05)
- RE: False positives with UDP Portscan PROTO255 Mike Lieberman (Mar 05)
- Re: False positives with UDP Portscan PROTO255 Jeff Kell (Mar 05)