topic Re: First packet isn't SYN in Firewall and Security Management

First packet isn't SYN

ServerBasket — Fri, 20 Jun 2025 11:25:46 GMT

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.

Re: First packet isn't SYN

PhoneBoy — Mon, 23 Jun 2025 21:33:44 GMT

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

Re: First packet isn't SYN

the_rock — Mon, 23 Jun 2025 22:28:01 GMT

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

Re: First packet isn't SYN

the_rock — Tue, 24 Jun 2025 02:33:26 GMT

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

Andy

https://tcpdump101.com/#