DNS缓存服务 — NSCD

偶然发现,自己的服务器上运行着一个叫 nscd 的服务。

所谓 nscd(Name Service Cache Daemon),是一种能够缓存 passwd、group、hosts 的本地缓存服务,分别对应三个源 /etc/passwd/etc/hosts/etc/resolv.conf

每个源保存两份缓存,一份是找到记录的,一份是没有找到记录的。每一种缓存都保存有生存时间(TTL)。其作用就是在本当中增加cache ,加快如DNS的解析等的速度。

安装

  • RHEL/CentOS
$ yum -y install nscd

缓存文件路径为/var/db/nscd/

  • Debian/Ubuntu
$ apt-get install nscd

nscd 的缓存文件路径为/var/cache/nscd/

命令

nscd 服务默认是关闭的,通过service nscd start开启。

  • 查看统计信息
$ nscd -g
  • 清除缓存
# 当更改完域名指向后,清除dns缓存
$ nscd -i hosts

  • 关闭服务
$ nscd -K

配置

阿里云的配置如下:

#
# /etc/nscd.conf
#
# An example Name Service Cache config file.  This file is needed by nscd.
#
# Legal entries are:
#
#	logfile			<file>
#	debug-level		<level>
#	threads			<initial #threads to use>
#	max-threads		<maximum #threads to use>
#	server-user             <user to run server as instead of root>
#		server-user is ignored if nscd is started with -S parameters
#       stat-user               <user who is allowed to request statistics>
#	reload-count		unlimited|<number>
#	paranoia		<yes|no>
#	restart-interval	<time in seconds>
#
#       enable-cache		<service> <yes|no>
#	positive-time-to-live	<service> <time in seconds>
#	negative-time-to-live   <service> <time in seconds>
#       suggested-size		<service> <prime number>
#	check-files		<service> <yes|no>
#	persistent		<service> <yes|no>
#	shared			<service> <yes|no>
#	max-db-size		<service> <number bytes>
#	auto-propagate		<service> <yes|no>
#
# Currently supported cache names (services): passwd, group, hosts, services
#

#	logfile			/var/log/nscd.log
#	threads			4
#	max-threads		32
#	server-user		nobody
#	stat-user		somebody
	debug-level		0
#	reload-count		5
	paranoia		no
#	restart-interval	3600

	enable-cache		passwd		yes
	positive-time-to-live	passwd		600
	negative-time-to-live	passwd		20
	suggested-size		passwd		211
	check-files		passwd		yes
	persistent		passwd		yes
	shared			passwd		yes
	max-db-size		passwd		33554432
	auto-propagate		passwd		yes

	enable-cache		group		yes
	positive-time-to-live	group		3600
	negative-time-to-live	group		60
	suggested-size		group		211
	check-files		group		yes
	persistent		group		yes
	shared			group		yes
	max-db-size		group		33554432
	auto-propagate		group		yes

	enable-cache		hosts		yes
	positive-time-to-live	hosts		3600
	negative-time-to-live	hosts		20
	suggested-size		hosts		211
	check-files		hosts		yes
	persistent		hosts		yes
	shared			hosts		yes
	max-db-size		hosts		33554432

	enable-cache		services	yes
	positive-time-to-live	services	28800
	negative-time-to-live	services	20
	suggested-size		services	211
	check-files		services	yes
	persistent		services	yes
	shared			services	yes
	max-db-size		services	33554432

	enable-cache		netgroup	yes
	positive-time-to-live	netgroup	28800
	negative-time-to-live	netgroup	20
	suggested-size		netgroup	211
	check-files		netgroup	yes
	persistent		netgroup	yes
	shared			netgroup	yes
	max-db-size		netgroup	33554432

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