Linux的DNS配置(resolv.conf)导致Unknown Host问题 博客分类： 故障排查

今天接到用户投诉说不定期会收到Unknown Host的错误，怀疑是应用服务器的DNS配置有问题，上应用服务器看了下/etc/resolv.conf，发现配置的第一行是：

 

options attempts:1 timeout:1

 

nameserver 10.10.10.10

 

查询相关配置说明发现DNS这块的逻辑是：

内网的DNS服务器的地址是10.10.10.10，应用服务器到内网的DNS去解析域名，内网DNS会在缓存中查找对应的域名解析，如果缓存中没有找到对应的，就会去上一级DNS域名解析器去查询，这个过程比较复杂，我改天写一篇详细的文章来仔细说明。

目前的配置的意思是应用服务器去获取域名的这个动作只尝试一次，超时时间是一秒。也就是说服务器去DNS服务器查询域名的时候，如果服务器没有在1秒之内收到应答就直接给应用程序报错。当本地DNS缓存没有请求域名的信息的时候，会去外网查询域名，这个过程会比较不稳定。很明显超时时间设置为1秒并且不重试的情况出现解析域名超时的情况会比较常见。下面是这两个参数的详细解释。把这行设置去掉，使用默认设置后，问题消失。

 

options
              Options allows certain internal resolver variables to be modified.  The syntax is

                     options option ...

              where option is one of the following:

              debug  sets RES_DEBUG in _res.options.

              ndots:n
                     sets  a threshold for the number of dots which must appear in a name given to res_query(3) (see resolver(3)) before an initial abso‐
                     lute query will be made.  The default for n is 1, meaning that if there are any dots in a name, the name will be tried first  as  an
                     absolute name before any search list elements are appended to it.  The value for this option is silently capped to 15.

              timeout:n
                     sets  the  amount  of time the resolver will wait for a response from a remote name server before retrying the query via a different
                     name server.  Measured in seconds, the default is RES_TIMEOUT (currently 5, see <resolv.h>).  The value for this option is  silently
                     capped to 30.

              attempts:n
                     sets  the  number of times the resolver will send a query to its name servers before giving up and returning an error to the calling
                     application.  The default is RES_DFLRETRY (currently 2, see <resolv.h>).  The value for this option is silently capped to 5.