TCP/IP discards SYN packet from client

Resolving The Problem

Here are ten reasons SYN packets (see topic "4.3.1 TCP concept" in TCP/IP Tutorial and Technical Overview) are discarded:

1. If a SYN request is received, but the TCP header has the RESET bit set, this SYN request is discarded if the listening socket is found.
2. If a SYN request is received that contains options that are not valid for security, compartment, or precedence, it is dropped and the connection is RESET.
3. If the listening socket is a Pascal listener that has specified a desired foreign port, and if the connection request does not originate from the indicated foreign port, the connection is dropped.
4. If the application's connection backlog is equal to 0, the SYN is discarded. This is an indication of an application problem; no application should specify a connection backlog of zero.
5. If the application's connection backlog is full, TCP/IP discards the SYN. This is an indication of an application problem; no new connections are established until the application can finish processing those already pending.
6. If the client sending the SYN has a route to the TCP/IP stack with which it is establishing a socket connection, but this stack has no return route, the SYN is discarded. This should not occur.
7. If the stack is unable to allocate storage to represent the connection, the SYN is discarded. This should not occur.
8. If there is an existing connection, the SYN is dropped.
9. If firewall rules prevent a SYN from entering the host, the connection is dropped.
10. If a policy rule indicates the connection request should be denied, the connection is RESET.