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Cisco FWSM and Random Sequencing
From: Josh Flaherty <Josh.Flaherty () INDSTATE EDU>
Date: Fri, 18 Oct 2013 09:16:13 -0400


We have been having an ongoing issue with the transfer of files large files across firewall contexts (Cisco FWSM).  We 
have identified the problem as the initial sequence number randomization feature of the FWSM being enabled.  The 
feature is causing a miss-alignment TCP sequence numbers that causes some software firewalls to block traffic and stall 
the transfers.

The sequence number randomization is a legacy security feature that is enabled by default.  Most operating systems 
since around 2000 already do ISN randomization so we are discussing disabling the feature on all of our FWSM firewall 

Has anyone else encountered this issue with a FWSM or any other firewalls?  If so was the solution to disable the 
sequence number randomization?

Josh Flaherty
Information Technology Security Officer
Office of Information Technology
Indiana State University

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