[kdepim-users] Snort "TCP session without 3-way handshake" warning

Paul Sobey buddha at the-annexe.net
Thu Apr 10 16:51:50 BST 2014

On Wednesday 09 Apr 2014 10:23:32 Peter Humphrey wrote:
> I'm running KMail 4.11.5 on a Gentoo amd64 box. One of my accounts is a
> POP3  connection to my LAN server. I installed snort on the server
> yesterday, and now I get warnings like this in the snort alert log:
> [**] [129:20:1] TCP session without 3-way handshake [**]
> [Classification: Potentially Bad Traffic] [Priority: 2] 
> 04/09-10:13:39.343914 ->
> TCP TTL:64 TOS:0x10 ID:37404 IpLen:20 DgmLen:104 DF
> ***AP*** Seq: 0xF037D897  Ack: 0x8F8FE3C5  Win: 0x152  TcpLen: 32
> TCP Options (3) => NOP NOP TS: 249552861 325460764 
> The IPs are this box,, and the server,, which is 
> running dovecot 2.2.9 to serve POP3 e-mails.
> Do I need to set something in KMail, or is this a bug - or is snort being
> too  pernickety? I have nearly 5MB of logs from snort already.

The fragment you cite above is to tcp/22 which is ssh, not pop3 (unless you 
have things configured differently than the norm).

Without seeing more it's hard to be sure, but I wonder if your network card is 
doing some sort of tcp offload such that snort doesn't see the entire traffic 
stream? I've seen offload upset packet analysers a few times.

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