topic Re: "TCP packet out of state: First packet isn't SYN; tcp_flags: SYN-ACK" with Strongswan in General Topics

"TCP packet out of state: First packet isn't SYN; tcp_flags: SYN-ACK" with Strongswan

jcano — Tue, 07 Jul 2020 23:54:23 GMT

Hi, I have a VPN tunnel connection to a CHECKPOINT GATEWAY configured on my UBUNTU server using Strongswan VPN. When trying to connect with protocol SMPP to a server on the VPN provider I receive a timeout, I contacted my VPN provider and they told me they were blocking the SMPP response packages with the following error:

TCP packet out of state: First packet isn't SYN; tcp_flags: SYN-ACK

I would be gratefull if someone could help me sort this error.

Re: "TCP packet out of state: First packet isn't SYN; tcp_flags: SYN-ACK" with Strongswan

PhoneBoy — Wed, 08 Jul 2020 06:11:33 GMT

This means the remote end received a packet for a connection that it doesn't know about.
This can be because the connection was inactive and it timed out on the remote end OR the initial SYN packet was never received through the VPN.
All you can do on your end is configure your application to send keepalive traffic (if possible) or restart the app so it opens a new TCP connection.
You should also confirm the initial three-way handshake is being communicated through the VPN on your end