Strange problem: Telnet cannot connect to the normal open port of another server

Problem Description:

One server (known as a) opened Port 11211 as a memcache server, but another server (called B) could not call A's memcache and the display could not connect.

Investigation:

1. Try to connect the 11211 port of connection A with Telnet on B first, time out.
   

2. Swap with another machine to telnet A to port 11211, normal.

3. Using B to Telnet to the open port of another machine is also normal.

4. Other ports with B telnet A, such as 22, are not normal.
   

5. With a Telnet b open port, also normal.

So, the problem seems to be a problem with only B telnet a.

Later thought on a grab a packet to see, respectively, with B telnet A and another can telnet to a machine telnet A, observe the two on a grab packet results, found that the results are not the same

B telnet A:

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When other machines telnet A:

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You can see that the Telnet request for B has reached a, but a does not give B the corresponding normal condition, so B will show a timeout.

Although know the problem is where, but still no train of thought, finally suspected of the kernel network-related configuration, that is, the/etc/sysctl.conf configuration, compared to B and other machines of the file, found that B configuration in a few lines of configuration

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Then in B's/etc/sysctl.conf added these configuration, sysctl-p to take effect, and then try to telnet a, even succeeded.

Said so much, although the problem solved, but with my current skill level, or can not precisely know which configuration item played a key role, so still a little depressed.

You have to be knowledgeable!

Strange problem: Telnet cannot connect to the normal open port of another server