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[ScreenOS] Secure FTP traffic gets dropped when reassembly for ALG is enabled on either incoming or outgoing zone

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Article ID: KB31352 KB Last Updated: 28 Sep 2020Version: 2.0
Summary:

The article describes the behavior of FTP ALG for secure FTP traffic when the reassembly for ALG is enabled on either incoming or outgoing zone.

 

Symptoms:

Secure FTP traffic gets dropped with the error, ‘packet dropped, nat xlate reassembly’ in debugs after upgrading the device from 6.3r7 to 6.3r22.

The debugs show the following errors:

****** 03501.0: <zone/ethernet0/1:1> packet received [93]******
ipid = 577(0241), @03c20430
packet passed sanity check.
flow_decap_vector IPv4 process
ethernet0/1:1:172.27.1.1/1256->172.27.10.10/21,6<Root>
existing session found. sess token 16
flow got session.
flow session id 16042
flow_main_body_vector in ifp ethernet0/1:1 out ifp N/A
flow vector index 0x1e3, vector addr 0x4c9b524, orig vector 0x4c9b524
vsd 1 is active
packet dropped, nat xlate reassembly

 

Cause:

This behavior is by design. When the reassembly for FTP ALG is enabled, the firewall tries to check for the reassembly process. But as the ftps packet is encrypted after auth/234, the ALG cannot recognize the packet termination and drops the packet. We need to bypass the reassembly process on both incoming and outgoing zone.

 

Solution:

Disable the reassembly process on both incoming and outgoing zone:

unset zone <zone name> reassembly-for-alg

 

Modification History:

2020-09-28: Article reviewed for accuracy; article valid and relevant; no changes made

 

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