RE: [load balancing] Delayed Binding and Half Open Connections in Alteon AD3

From: <jon.hartman [izzat]>
Date: Thu Feb 09 2006 - 11:53:57 EST

We were given the impression that the later versions of AlteonOS use
syn-cookies to prevent SYN floods and that an actual session isn't opened
until the ACK (third step of the handshake) is sent with the correct ACK #
hash. Is that the case?

-----Original Message-----
From: [] On Behalf Of Ken
Sent: Thursday, February 09, 2006 8:53 AM
Subject: Re: [load balancing] Delayed Binding and Half Open Connections in
Alteon AD3


   That counter is historic counter so no need to worry. If one of your
VIP's was under a SYN flood attack using delayed bindings would protect
the real servers as the session is not bound to a real server until the
3-way TCP handshake has been completed. The switch will only wait for a
few seconds for the client to finish the 3-way handshake before it flushes
the connection. (I think it's 2 seconds but it could be 10
seconds) So unless the switch recieves enough SYN's in that period to fill
up the session table you are good.

As for a netstat command, it would not be very useful, but the session
table dump is. /inf/slb/sess Depending on the version of code you are
running you can dump based on CIP, DIP and other criteria or the whole
table with /inf/slb/sess/dump . To find out how to read the session table,
/inf/slb/sess/help will dump out the information on how to read it.



--- "Rivera Alonso, David" <>

> From: "Rivera Alonso, David" <>
> To: "''" <>
> Subject: Delayed Binding and Half Open Connections in Alteon AD3
> Date: Thu, 9 Feb 2006 11:17:31 +0100
> Dear friends,
> I have a question about WebOS's SYN Attack Detection and Protection:
> After you enable Delayed Binding for a certain Virtual Service,
> there's a counter (Half Open Connections) in /stat/slb/layer7/maint
> that tracks the number of Started TCP Handshakes.
> I'd like to know if this is an "historic" or a "current state"
> counter.
> Our counter keeps growing since the activation of the Delayed Binding,
> and we want to know if there can be any risks (memory or cpu excessive
> consumption) associated with this counter.
> If it's just a "historic" counter then we won't be afraid, as we have
> no syslog messages about SYN Attack detection (more than 1000 half
> open connections per second).
> Besides, do you have any deeper information about "where" are these
> Half Open Connections stored until they become an stablished TCP
> connection in memory? And what happens if a real and dangeorus SYN
> Flood arrives to Alteon?
> And last question, is there anything similar to NETSTAT which can tell
> us about TCP sessions and their state (SYN_RECV,
> FIN_WAIT...)
> many thanks for everything and best regards from Spain,
> >
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Received on Thu Feb 9 13:58:31 2006

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