C.5.2.11. Communication Errors and Aborted Connections

If connection problems occur such as communication errors or aborted connections, use these sources of information to diagnose problems:

The error log. See Section 5.2.2, "The Error Log".
The general query log. See Section 5.2.3, "The General Query Log".
The Aborted_xxx and Connection_errors_xxx status variables. See Section 5.1.6, "Server Status Variables".
The host cache, which is accessible using the host_cache Performance Schema table. See Section 8.11.5.2, "DNS Lookup Optimization and the Host Cache", and Section 21.9.9.1, "The host_cache Table".

If you start the server with the --log-warnings option, you might find messages like this in your error log:

010301 14:38:23  Aborted connection 854 to db:'users' user: 'josh'

If a client successfully connects but later disconnects improperly or is terminated, the server increments the Aborted_clients status variable, and logs an Aborted connection message to the error log. The cause can be any of the following:

The client program did not call mysql_close() before exiting.
The client had been sleeping more than wait_timeout or interactive_timeout seconds without issuing any requests to the server. See Section 5.1.4, "Server System Variables".
The client program ended abruptly in the middle of a data transfer.

If a client is unable even to connect, the server increments the Aborted_connects status variable. Unsuccessful connection attempts can occur for the following reasons:

A client does not have privileges to connect to a database.
A client uses an incorrect password.
A connection packet does not contain the right information.
It takes more than connect_timeout seconds to get a connect packet. See Section 5.1.4, "Server System Variables".

If these kinds of things happen, it might indicate that someone is trying to break into your server! Messages for these types of problems are logged to the general query log if it is enabled.

Other reasons for problems with aborted clients or aborted connections:

The max_allowed_packet variable value is too small or queries require more memory than you have allocated for mysqld. See Section C.5.2.10, "Packet Too Large".
Use of Ethernet protocol with Linux, both half and full duplex. Many Linux Ethernet drivers have this bug. You should test for this bug by transferring a huge file using FTP between the client and server machines. If a transfer goes in burst-pause-burst-pause mode, you are experiencing a Linux duplex syndrome. Switch the duplex mode for both your network card and hub/switch to either full duplex or to half duplex and test the results to determine the best setting.
A problem with the thread library that causes interrupts on reads.
Badly configured TCP/IP.
Faulty Ethernets, hubs, switches, cables, and so forth. This can be diagnosed properly only by replacing hardware.