引用本文请注明出处：Just Do IT (http://www.toplee.com) < Michael Lee @ toplee.com >
前几天服务器更换了电信的ip和dns之后，速度有了很大提高，系统发信的速度也基本达到了正常可以接受的水平，但是如果要应付大量的邮件列表功能，还是感觉有些吃力，于是考虑进一步提高dns解析速度，因为sendmail响应慢很大程度跟dns反向解析解析有关，在网上查了很多资料都提到受sendmail发信慢的困扰，但是都没有看到好的解决办法。
偶然间从freebsd的ports/dns/下看到了djbdns这个包，一查资料发现有个dnscache功能，于是决定一试，没想效果出奇的好，非常值得推荐给大家！
下面是我安装过程，和大家分享！
全部都是使用root用户操作下面的命令！
[coolcode]
#cd /usr/ports/dns/djbdns
#make install clean
#mkdir /var/service
#csh （或者exit退出再登陆，或者运行bash也可以）
#dnscache-conf nobody nobody /var/service/dnscache 127.0.0.1
#vi /etc/rc.conf 里加入 svscan_enable=”YES”
#/usr/local/etc/rc.d/svscan.sh start 启动服务，完成安装
下面检查服务启动情况
#netstat -anl |grep LISTEN
看看里面是否有53端口的监听服务，如果有就ok了
接着修改/etc/reslove.conf文件，把127.0.0.1加入到第一行，如下
nameserver 127.0.0.1
nameserver xx.xx.xx.xx (其他的公网dns）
这时测试一下效果
dig www.sina.com.cn
你会发现可能第一次稍微时间长一点，第二次反应时间都基本是0.000msec或者0.001msec，这就是取到了本地的缓存，效果好得不行！
[/coolcode]
接着测试了sendmail的大量并发发信，结果非常的喜人，完全能满足中等网站的并发业务处理了，朋友们可以自己测试一下：）
djbdns其他的功能我还没有研究到，欢迎大家和我一起学习和交流！
用BIND可以吗？
bind也是可以的，届时设置您的reslove.conf文件，把nameserver第一个设置为您的bind地址，bind相对复杂一些，在缓存方面没有djbdns轻量级和高效。
对外查询是在本地打开随机的UDP端口连接外部的53端口吗？如果是，IPFW的规则要怎么写？
往外连接你还需要限制？ 你可以指定协议UDP到 target的port是53的都开放，具体书写语句看ipfw的man吧。理论上这个规则是默认都会打开的，要不你的机器怎么做dns解析.
怎么才可以清空dnscache的缓存呢？找了好久，没找到办法.
最笨的办法： 删除目录，dnscache-conf重建一下
Thank you!
Thanks for sharing your thoughts on domanda di visto.
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