How MySQL server listens for a TCP/IP connection

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The MySQL server spawns a separate background thread for each enabled protocol that it supports on the target platform. This single thread per protocol further manages each client connection and helps to deliver a separate thread for each connection request. There can be different supported protocol depending upon the platform like Unix socket on Unix like platform, Named Pipe and Shared Memory on Windows platform.

The MySQL server listens for TCP/IP connections (remote connections) from the client on all platforms.

The MySQL server listens on a single network socket for TCP/IP connections. This socket is bound to a single address, but it is possible for an address to map onto multiple network interfaces. To specify an address, use the --bind-address=addr option at server startup, where addr is an IPv4 or IPv6 address or a host name. If addr is a host name, the server resolves the name to an IP address and binds to that address. If a host name resolves to multiple IP addresses, the server uses the first IPv4 address if there are any, or the first IPv6 address otherwise.

The server treats different types of addresses as follows:
  • If the address is *, the server accepts TCP/IP connections on all server host IPv4 interfaces, and, if the server host supports IPv6, on all IPv6 interfaces. Use this address to permit both IPv4 and IPv6 connections on all server interfaces. This value is the default. 
  • If the address is 0.0.0.0, the server accepts TCP/IP connections on all server host IPv4 interfaces.
  • If the address is ::, the server accepts TCP/IP connections on all server host IPv4 and IPv6 interfaces.
  • If the address is an IPv4-mapped address, the server accepts TCP/IP connections for that address, in either IPv4 or IPv6 format. For example, if the server is bound to ::ffff:127.0.0.1, clients can connect using --host=127.0.0.1 or --host=::ffff:127.0.0.1.
  • If the address is a “regular” IPv4 or IPv6 address (such as 127.0.0.1 or ::1), the server accepts TCP/IP connections only for that IPv4 or IPv6 address.
If binding to the address fails, the server produces an error and does not start.

If you intend to bind the server to a specific address, be sure that the mysql.user system table contains an account with administrative privileges that you can use to connect to that address. Otherwise, you will not be able to shut down the server. For example, if you bind the server to *, you can connect to it using all existing accounts. But if you bind the server to ::1, it accepts connections only on that address. In that case, first make sure that the 'root'@'::1' account is present in the mysql.user table so you can still connect to the server to shut it down.

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