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Half Scan attack

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Half Scan attack - 7.Mar.2002 12:45:00 AM   


Posts: 7
Joined: 21.Feb.2002
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Can anyone explain to me what a half scan attack on the ISA server is

I found my event viewer full of this error.
Post #: 1
RE: Half Scan attack - 7.Mar.2002 1:33:00 AM   


Posts: 37
Joined: 10.Apr.2001
From: Lexington, KY
Status: offline
An IP half-scan attack should be defined in the ISA Server help file, but essentially it is where a computer tries to bypass establishing a connection by starting to send multiple SYN packets while never completing the packet with an ACK.

This type of attack is built-in to Intrusion Detection as a pre-configured alert.


(in reply to argyp)
Post #: 2
RE: Half Scan attack - 7.Mar.2002 9:00:00 AM   


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Joined: 22.Jan.2002
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A half-scan attack consits of a TCP SYN scan. A regular TCP connection is:

Client: SYN
Server: SYN/ACK
Client: ACK

A TCP SYN scan connection is:

Hacker: SYN
Server: SYN/ACK
Client: no response

This is different than a full SYN scan in that the connection is dropped after receiving the returned SYN-ACK packet instead of letting the connection complete. This is turn is somewhat 'stealth' as a full TCP connection never occurs thus identities of rogue parties are not ALWAYS identified.

ISA understands and protects against SYN scans though and IDS systems, like SNORT understand TCP SYN scan traffic.

(in reply to argyp)
Post #: 3
RE: Half Scan attack - 2.Apr.2002 9:02:00 PM   


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Joined: 18.Mar.2002
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If I am receiving several of these all of a sudden should be concerned?

Is ISA protecting me? [Confused]

(in reply to argyp)
Post #: 4
RE: Half Scan attack - 3.Apr.2002 3:51:00 PM   


Posts: 19
Joined: 8.May2001
From: Florida - USA
Status: offline
I used to get half scan attack as well as portscan attack alerts all the time. In order to determine the threat, whenever I receive an alert I will use ping -a to give me the Domain of the computer scanning me. About 80% of the time I will get a response and its almost always some search engine robot probing for web sites. If you have internal users listening to streaming music like launchcast or spinner you can get several alerts as this services probes your firewall for open ports. If the IP address does not answer back with a name then I treat it as suspect and I add the IP to my "Intrusion Rule" (my own setup) to deny them access to prevent possible attacks. Now the scan attacks have slowed tremendously.

[ April 03, 2002, 03:52 PM: Message edited by: jokan7 ]

(in reply to argyp)
Post #: 5

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