Bind 的 forwarding 和 recursion

很少接触bind的各种服务,可是几乎每年都会用到一次两次,每次都得重新搜索bind 的文档,太麻烦了。。下面是在stackoverflow上看到关于bind 的forwarding 和 recursion 的描述,写的很好.

Forwarding: just passes the DNS query to another DNS server (e.g. your ISP’s). Home routers use forwarding to pass DNS queries from your home network’s clients to your ISP’s DNS servers. For example, for foo.example.com, a forwarding DNS server would first check its cache (did it already ask this question before), and if the answer is not in its cache, it would ask its forwarder (your ISP’s DNS server) for the answer, which would respond with either a cached response, or would perform recursion until it figured out the answer.

Recursion: or caching server, the DNS server receiving the query takes it upon itself to figure out the answer to that query by recursively querying authoritative DNS servers for that domain. For example, for foo.example.com, a recursor would first query the root servers for what DNS servers are responsible for the .com TLD, then it would ask those servers for example.com, then it would query the servers for example.com for foo.example.com, finally getting the answer to the original query.

In terms of security, you should separate recursors/forwarders (typically DNS servers used to service a bunch of clients) and authoritative DNS servers (typically these are responsible ONLY for answering queries re: domains that they are authoritative for – these servers will NOT perform recursive queries for anyone).

在centos 7上安装bind 并把它设置为resursor server 最简单了,直接 yum install bind bind-utils就可以了,bind 的默认named.conf 就是一个完整只适用于本机localhost 的 resursor server 设置

centos 7 安装geoipupdate

很多人在安装完centos 7 mini iso 以后会安装bind-utils,bind-utils 包含了许多DNS查询的工具包,其中就有一个dependency GeoIP library

但是这个GeoIP library 是包含在老版的geoipupdate这个程序中的,版本号应该为1.5,但是maxmind 早就放弃了对于老版geoipupdate的支持,因此如果你的程序需要geoipupdate 的支持的话,用centos 7 默认repo 自带的geoipupdate 的话,肯定是不行的了

因此我们肯定是要安装经过重新开发过的只支持新的geoip 数据库格式的geoipupdate

github地址是https://github.com/maxmind/geoipupdate

但是这里又出现了一个问题,如果我们直接使用rpm 的安装包来安装的话,默认的可执行command 会被安装在/usr/bin/geoipupdate, conf 地址会被安装在/etc/GeoIP.conf, 这样会和老版的geoipupdate 发生冲突,但是bind-utils 又依赖于老版的GeoIP 库,不可能卸载掉

这里有一个比较简单的解决办法,就是直接安装linux 的通用安装包,而不是用专门的centos的rpm安装包,这样可执行程序的路径就可以被我们自行选择

bootstrap中的sr-only

最近在看bootstrap4,经常会有sr-only这个类

sr是screen reader的缩写,这个表示是为视力有障碍的人士准备的类.

在前端开发中,有些时候设计图上面会出现仅供正常视觉用户看的元素。比如:导航栏菜单当前页面选中高亮状态,这些状态只有视力正常的人才能正常使用。

而残障人士,弱势或盲人是很难或者根本看不出导航菜单高亮的。他们上网可能借助的是屏幕阅读器,也就是 screen reader(sr),屏幕阅读器需要找到能辨识的文本说明然后“读”出来给用户听。