MySQL [Warning]: IP address 'xxxx' could not be resolved: Name or service not known

MySQL的error log 出现大量的 DNS反解析错误。

DNS解析是指，将 域名解析成ip地址；

DNS反解析是指，将IP地址反解析成域名；

Version: '5.5.40-log'  socket: '/tmp/mysql.sock'  port:   MySQL Community Server (GPL)
  :: [Warning] IP address 'xxx.xxx.xx.xxx' could not be resolved: Name or service not known
  :: [Warning] IP address 'xxx.xxx.xx.xxx' could not be resolved: Name or service not known 
 ::  [Warning] IP address 'xxx.xxx.xx.xxx' could not be resolved: Name or service not known 
 ::  [Warning] IP address 'xxx.xxx.xx.xxx' could not be resolved: Name or service not known 
 :: [Warning] IP address 'xxx.xxx.xx.xxx' could not be resolved: Name or service not known 
 :: [Warning] IP address 'xxx.xxx.xx.xxx' could not be resolved: Name or service not known 
 ::  [Warning] IP address 'xxx.xxx.xx.xxx' could not be resolved: Name or service not known 
 :: [Warning] IP address 'xxx.xxx.xx.xxx' could not be resolved: Name or service not known 
1 :: [Warning] IP address 'xxx.xxx.xx.xxx' could not be resolved: Temporary failure in name resolution 
 :: [Warning] IP address 'xxx.xxx.xx.xxx' could not be resolved: Temporary failure in name resolution 
 :: [Warning] IP address 'xxx.xxx.xx.xxx' has been resolved to the host name 'xxx.xxx.xx.xxx.broadxxxxx.com.cn', which resembles IPv4-address itself. 
 :: [Warning] IP address 'xxx.xxx.xx.xxx' could not be resolved: Name or service not known 
 :: [Warning] IP address 'xxx.xxx.xx.xxx' could not be resolved: Name or service not known 
 :: [Warning] IP address 'xxx.xxx.xx.xxx' could not be resolved: Name or service not known 

出现该错误的原因：

MySQL 数据库服务器没有配置 /etc/hosts，也没有DNS服务，导致mysqld线程解析IP对应的主机名时，解析失败。

解决方法：

使用--skip-name-resolve 参数禁用DNS的主机名解析功能，禁用该功能后，在MySQL授权表里面，你只能使用IP地址。

所以我们修改MySQL的my.cnf中的参数：

[mysqld]
skip_host_cache
skip-name-resolve=

could not be resolved: Temporary failure in name resolution 的警告，可以使用 skip_host_cache 来搞定；
而 could not be resolved: Name or service not known 的警告，可以通过 skip_name_resolve=1 来搞定。禁止了域名解析，显然就不会出该警告了。

注意：skip_host_cache=1 会报错。直接 skip_host_cache 或者 skip-host-cache 就行了。

修改之后，需要重启。

注意：mysql中的权限表，就不能使用域名了，必须使用IP地址来设置。

msyql文档:

--skip-name-resolve

Do not resolve host names when checking client connections. Use only IP addresses. If you use this option, all Host column values 
in the grant tables must be IP addresses or localhost. See Section 8.12.6.2, “DNS Lookup Optimization and the Host Cache”.

Depending on the network configuration of your system and the Host values for your accounts, clients may need to connect using an 
explicit --host option, such as --host=localhost, --host=127.0.0.1, or --host=::.

An attempt to connect to the host 127.0.0.1 normally resolves to the localhost account. However, this fails if the server is run 
with the --skip-name-resolve option, so make sure that an account exists that can accept a connection. 

For example, to be able to connect as root using --host=127.0.0.1 or --host=::, create these accounts:

CREATE USER 'root'@'127.0.0.1' IDENTIFIED BY 'root-password';
CREATE USER 'root'@'::1' IDENTIFIED BY 'root-password';

--skip-host-cache

Disable use of the internal host cache for faster name-to-IP resolution. In this case, the server performs a DNS lookup every time 
a client connects. See Section 8.12.6.2, “DNS Lookup Optimization and the Host Cache”.

Use of --skip-host-cache is similar to setting the host_cache_size system variable to , but host_cache_size is more flexible 
because it can also be used to resize, enable, or disable the host cache at runtime, not just at server startup.

If you start the server with --skip-host-cache, that does not prevent changes to the value of host_cache_size, but such changes 
have no effect and the cache is not re-enabled even if host_cache_size is set larger than . 

其实，按照上面的介绍，如果我们启用了 skip-name-resolve=1, 那么因为禁止了 ip 和 hostname 之间的解析，那么就没有必要启用 skip-host-cache 了。启用的话，可能会减少一点点内存的占用吧？

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Mysql域名解析(转自：http://www.jb51.net/article/70893.htm)：

当一个新的客户端尝试跟mysqld创建连接时，mysqld产生一个新线程来处理这个请求。新线程会先检查请求建立连接的主机名是否在Mysql的主机名缓冲中，如果不在，线程会尝试去解析请求连接的主机名。

解析的逻辑如下：

a. Mysql线程通过gethostbyaddr()把获取的IP地址解析成主机名，然后通过gethostbyname()把获取的主机名解析成IP地址，保障主机名和IP地址对应关系的准确；

b. 如果操作系统支持使用安全进程的gethostbyaddr_r()和gethostbyname_r() 调用，Mysqld线程可以用它俩来优化主机名解析；

c. 如果操作系统不支持安全线程调用，Mysqld进程先做一个互斥锁，然后调用gethostbyaddr()和gethostbyname()解析主机 名。此时，在第一个进程释放掉主机名缓冲池的主机名之前，其它进程无法再次解析这个主机名; <-------MySQL手册里面在此处说的host name ，意思应该是指同一个IP地址和对应的第一个主机名关系。

在启动mysqld进程是，可以使用 --skip-name-resolve 参数禁用DNS的主机名解析功能，禁用该功能后，在MySQL授权表里面，你只能使用IP地址。

如果你所处环境的DNS非常慢 或者 有很多主机， 你可以通过禁用DNS解析功能--skip-name-resolve 或者 提高 HOST_CACHE_SIZE大小 来提升数据库的响应效率。

禁用主机名缓冲的发方法： 使用--skip-host-cache 参数； 刷新主机名缓冲区： 执行 flush hosts 或者执行mysqladmin flush-hosts；

=============================

启用 skip_name_resolve=1 之后，可能会有新的Warining:

[Warning] 'proxies_priv' entry '@ root@localhost.localdomain' ignored in --skip-name-resolve mode.

解决方法，删除表 mysq.proxies_priv 中值为 localhost.localdomain 的行

参见：http://blog.itpub.net/14184018/viewspace-1061224/