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Duplicate TCP SYN on ASA Firewall for LPD service



On my ASA Firewall I noticed in logs the following warnings:

4    Nov 28 2013    11:31:13    419002    731    515    Duplicate TCP SYN from WAN: to DMZ: with different initial sequence number

6    Nov 28 2013    11:34:26    106015     724     515    Deny TCP (no connection) from to flags FIN ACK  on interface WAN

I created the service policy

class-map WAN-class

        match port tcp eq lpd

      policy-map WAN-policy

        class WAN-class

          set connection conn-max 0 embryonic-conn-max 0 per-client-max 0 per-client-embryonic-max 0 random-sequence-number disable

      service-policy WAN-policy interface WAN

After apply the map I didn't get any duplicate TCP SYN..... but after a couple of hours they had appered.

How to overcome that kind of situation?

Kind Regards


1 Reply 1

Marius Gunnerud
VIP Advisor VIP Advisor
VIP Advisor


Have you identified what machine is sending these SYN packets (ip  I have heard of some applications trying to initiate several connections at a time.  Have you introduced any new machines/PC to the network recently?  How long have you been seeing these messages?

I would first of all protect your network against SYN flood attacks as your network is currently wide open, given the configuration you posted.  The following config will help minimize your exposure to a SYN flood attack.

policy-map WAN-policy

class WAN-class

set connection conn-max 100

set connection embryonic-conn-max 200

set connection per-client-embryonic-max 7

set connection per-client-max 5

set connection random-sequence-number enable

set connection timeout embryonic 0:0:45

The below link goes more in depth on attack mitigation and might be worth you reading.


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