mysql — 客户端与服务端之间的能力谈判

超、凢脫俗 2022-05-21 10:11 201阅读 0赞

# mysql — 客户端与服务端之间的能力谈判 #

我们在了解mysql网络通信协议时，在客户端与服务器在初次连接的时候，服务器会先发送一个握手初始包，其内部字段如图所示：

![70][]

可以看到服务器在向客户端传递自己的信息，比如自己的版本/状态/字符编码/以及后面登录认证需要用到的挑战随机数/还有权能

标志位 我们今天的主题就是权能标志位！ 客户端和服务器使用权能标志位来指示自己支持和想要使用那些功能.

服务端在发送初始化握手包时会将自己支持的功能在16位服务器权能标志位上面对应位置置位. 然后客户端接收到服务器发送过来的

初始化报文之后，会对服务器发送的权能标志进行修改，保留自身所支持的功能，然后将权能标志返回给服务器，从而保证服务器与

客户端通讯的兼容性.

那么这么多标志位的作用都是什么呢？ 了解一下：

**Protocol::CapabilityFlags:**  
  
  
**CLIENT\_LONG\_PASSWORD**  
Use the improved version of Old Password Authentication.  
  
Value  
0x00000001  
  
  
**CLIENT\_FOUND\_ROWS**  
Send found rows instead of affected rows in EOF\_Packet.  
  
Value  
0x00000002  
  
  
**CLIENT\_LONG\_FLAG**  
Longer flags in Protocol::ColumnDefinition320.  
  
Value  
0x00000004  
  
Server  
Supports longer flags.  
  
Client  
Expects longer flags.  
  
**CLIENT\_CONNECT\_WITH\_DB**  
Database (schema) name can be specified on connect in Handshake Response Packet.  
  
Value  
0x00000008  
  
Server  
Supports schema-name in Handshake Response Packet.  
  
Client  
Handshake Response Packet contains a schema-name.  
  
**CLIENT\_NO\_SCHEMA**  
Value  
0x00000010  
  
Server  
Do not permit database.table.column.  
  
**CLIENT\_COMPRESS**  
Compression protocol supported.  
  
Value  
0x00000020  
  
Server  
Supports compression.  
  
Client  
Switches to Compression compressed protocol after successful authentication.  
  
**CLIENT\_ODBC**  
Value  
0x00000040  
  
Special handling of ODBC behavior.  
  
Note  
No special behavior since 3.22.  
  
**CLIENT\_LOCAL\_FILES**  
Can use LOAD DATA LOCAL.  
  
Value  
0x00000080  
  
Server  
Enables the LOCAL INFILE request of LOAD DATA|XML.  
  
Client  
Will handle LOCAL INFILE request.  
  
**CLIENT\_IGNORE\_SPACE**  
Value  
0x00000100  
  
Server  
Parser can ignore spaces before '('.  
  
Client  
Let the parser ignore spaces before '('.  
  
**CLIENT\_PROTOCOL\_41**  
Value  
0x00000200  
  
Server  
Supports the 4.1 protocol.  
  
Client  
Uses the 4.1 protocol.  
  
Note  
this value was CLIENT\_CHANGE\_USER in 3.22, unused in 4.0  
  
**CLIENT\_INTERACTIVE**  
wait\_timeout versus wait\_interactive\_timeout.  
  
Value  
0x00000400  
  
Server  
Supports interactive and noninteractive clients.  
  
Client  
Client is interactive.  
  
See  
mysql\_real\_connect()  
  
  
**CLIENT\_SSL**  
Value  
0x00000800  
  
Server  
Supports SSL.  
  
Client  
Switch to SSL after sending the capability-flags.  
  
**CLIENT\_IGNORE\_SIGPIPE**  
Value  
0x00001000  
  
Client  
Do not issue SIGPIPE if network failures occur (libmysqlclient only).  
  
See  
mysql\_real\_connect()  
  
**CLIENT\_TRANSACTIONS**  
Value  
0x00002000  
  
Server  
Can send status flags in EOF\_Packet.  
  
Client  
Expects status flags in EOF\_Packet.  
  
Note  
This flag is optional in 3.23, but always set by the server since 4.0.  
  
**CLIENT\_RESERVED**  
Unused.  
  
Value  
0x00004000  
  
Note  
Was named CLIENT\_PROTOCOL\_41 in 4.1.0.  
  
**CLIENT\_SECURE\_CONNECTION**  
Value  
0x00008000  
  
Server  
Supports Authentication::Native41.  
  
Client  
Supports Authentication::Native41.  
  
**CLIENT\_MULTI\_STATEMENTS**  
Value  
0x00010000  
  
Server  
Can handle multiple statements per COM\_QUERY and COM\_STMT\_PREPARE.  
  
Client  
May send multiple statements per COM\_QUERY and COM\_STMT\_PREPARE.  
  
Note  
Was named CLIENT\_MULTI\_QUERIES in 4.1.0, renamed later.  
  
Requires  
CLIENT\_PROTOCOL\_41

**CLIENT\_MULTI\_RESULTS**  
Value  
0x00020000  
  
Server  
Can send multiple resultsets for COM\_QUERY.  
  
Client  
Can handle multiple resultsets for COM\_QUERY.  
  
Requires  
CLIENT\_PROTOCOL\_41  
  
  
**CLIENT\_PS\_MULTI\_RESULTS**  
Server  
Can send multiple resultsets for COM\_STMT\_EXECUTE.  
  
Client  
Can handle multiple resultsets for COM\_STMT\_EXECUTE.  
  
Value  
0x00040000  
  
Requires  
CLIENT\_PROTOCOL\_41  
  
**CLIENT\_PLUGIN\_AUTH**  
Value  
0x00080000  
  
Server  
Sends extra data in Initial Handshake Packet and supports the pluggable authentication protocol.  
  
Client  
Supports authentication plugins.  
  
Requires  
CLIENT\_PROTOCOL\_41  
  
**CLIENT\_CONNECT\_ATTRS**  
Value  
0x00100000  
  
Server  
Permits connection attributes in Protocol::HandshakeResponse41.  
  
Client  
Sends connection attributes in Protocol::HandshakeResponse41.  
  
**CLIENT\_PLUGIN\_AUTH\_LENENC\_CLIENT\_DATA**  
Value  
0x00200000  
  
Server  
Understands length-encoded integer for auth response data in Protocol::HandshakeResponse41.  
  
Client  
Length of auth response data in Protocol::HandshakeResponse41 is a length-encoded integer.  
  
Note  
The flag was introduced in 5.6.6, but had the wrong value.  
  
**CLIENT\_CAN\_HANDLE\_EXPIRED\_PASSWORDS**  
Value  
0x00400000  
  
Server  
Announces support for expired password extension.  
  
Client  
Can handle expired passwords.  
  
**CLIENT\_SESSION\_TRACK**  
Value  
0x00800000  
  
Server  
Can set SERVER\_SESSION\_STATE\_CHANGED in the Status Flags and send session-state change data after a OK packet.  
  
Client  
Expects the server to send sesson-state changes after a OK packet.  
  
**CLIENT\_DEPRECATE\_EOF**  
Value  
0x01000000  
  
Server  
Can send OK after a Text Resultset.  
  
Client  
Expects an OK (instead of EOF) after the resultset rows of a Text Resultset.  
  
Background  
To support CLIENT\_SESSION\_TRACK, additional information must be sent after all successful commands. Although the OK packet is extensible, the EOF packet is not due to the overlap of its bytes with the content of the Text Resultset Row.  
  
Therefore, the EOF packet in the Text Resultset is replaced with an OK packet. EOF packets are deprecated as of MySQL 5.7.5.

[70]: /images/20220521/a96008cdf891428eac4f228ae1a7c119.png