What means SYN_SENT status in a socket?

Recently we had some problems with a DDoS attack on some machines that execute workers, so I started the task of configuring a firewall (before that, we just removed the machine and created a new one, they are machines without a state).

Using a:

netstat -an

To see the status of the available ports, and well, each port can have several states, for example a socket can be in LISTENING state or it may be in ESTABLISHED, but one of those that I saw and that prevented to execute some tasks was the SYN_SENT.

When you see a socket with this state, it is very likely that it has a problem with the Firewall (in this case block a port), although this state always happens, but should not last longer than a second (depending on the state of the network), but if it is maintained, there is a problem.

When a client initiates the connection to a server, it sends a SYN packet first, if the remote server is available and functioning, the client should receive a SYN + ACK and the client would respond with an ACK and with this a TCP connection is formed.

Use your firewall wisely.