First packet isn't SYN

[ServerBasket]

Hi,

Hope you are well,

One of my source ip traffic is blocking due to this First packet isn't SYN after i kept this effected IP in protected scope.

Pls let me know how to fix this.

Mani.

[PhoneBoy]

It means the gateway never saw the initial TCP SYN for the connection.
This sometimes happens with applications that do not follow the RFCs.
It can happen for other reasons.
See: https://support.checkpoint.com/results/sk/sk31382 

[the_rock]

That error has been around for ages and to really find out why it happens, you need to do the packet captures and see what happens.

Andy

[the_rock]

Foegot to add...to make it easier for captures, check out below site.

Andy

https://tcpdump101.com/#