nsswitch.conf与resolv.conf文件介绍
时间：2006-10-08 来源：适兕
查询DNS不是GNU resolver唯一的功能，它还能够查询许多种系统数据库，包括密码文件，组定义文件、NIS等。
先看下man -S 5 nsswitch.conf(man -S的意思是按照某种章节列出、5的意思按文本格式）
NAME
nsswitch.conf - System Databases and Name Service Switch configuration file
系统数据库和名称服务切换配置文件
DESCRIPTION
Various functions in the C Library need to be configured to work correctly in the local environment. Traditionally, this was done
by using files (e.g., ‘/etc/passwd’), but other nameservices (like the Network Information Service (NIS) and the Domain Name Service
(DNS)) became popular, and were hacked into the C library, usually with a fixed search order.
The Linux libc5 with NYS support and the GNU C Library 2.x (libc.so.6) contain a cleaner solution of this problem. It is designed
after a method used by Sun Microsystems in the C library of Solaris 2. We follow their name and call this scheme "Name Service
Switch" (NSS). The sources for the "databases" and their lookup order are specified in the /etc/nsswitch.conf file.
The following databases are available in the NSS:
aliases
Mail aliases, used by sendmail(8). Presently ignored.
ethers Ethernet numbers.
group Groups of users, used by getgrent(3) functions.
hosts Host names and numbers, used by gethostbyname(3) and similar functions.
netgroup
Network wide list of hosts and users, used for access rules. C libraries before glibc 2.1 only support netgroups over NIS.
networks
Network names and numbers, used by getnetent(3) functions.
passwd User passwords, used by getpwent(3) functions.
protocols
Network protocols, used by getprotoent(3) functions.
publickey
Public and secret keys for Secure_RPC used by NFS and NIS+.
rpc Remote procedure call names and numbers, used by getrpcbyname(3) and similar functions.
services
Network services, used by getservent(3) functions.
shadow Shadow user passwords, used by getspnam(3).
An example /etc/nsswitch.conf (namely, the default used when /etc/nsswitch.conf is missing):
passwd: compat
group: compat
shadow: compat
hosts: dns [!UNAVAIL=return] files
networks: nis [NOTFOUND=return] files
ethers: nis [NOTFOUND=return] files
protocols: nis [NOTFOUND=return] files
rpc: nis [NOTFOUND=return] files
services: nis [NOTFOUND=return] files
The first column is the database. The rest of the line specifies how the lookup process works. You can specify the way it works
for each database individually.
The configuration specification for each database can contain two different items:
* The service specification like ‘files’, ‘db’, or ‘nis’.
* The reaction on lookup result like ‘[NOTFOUND=return]’.
For libc5 with NYS, the allowed service specifications are ‘files’, ‘nis’, and ‘nisplus’. For hosts, you could specify ‘dns’ as
extra service, for passwd and group ‘compat’, but not for shadow.
For glibc, you must have a file called /lib/libnss_SERVICE.so.X for every SERVICE you are using. On a standard installation, you
could use ‘files’, ‘db’, ‘nis’, and ‘nisplus’. For hosts, you could specify ‘dns’ as extra service, for passwd, group, and shadow
‘compat’. These services will not be used by libc5 with NYS. The version number X is 1 for glibc 2.0 and 2 for glibc 2.1.
The second item in the specification gives the user much finer control on the lookup process. Action items are placed between two
service names and are written within brackets. The general form is
‘[’ ( ‘!’? STATUS ‘=’ ACTION )+ ‘]’
where
STATUS => success | notfound | unavail | tryagain
ACTION => return | continue
The case of the keywords is insignificant. The STATUS values are the results of a call to a lookup function of a specific service.
They mean:
success
No error occurred and the wanted entry is returned. The default action for this is ‘return’.
notfound
The lookup process works ok but the needed value was not found. The default action is ‘continue’.
unavail
The service is permanently unavailable. This can either mean the needed file is not available, or, for DNS, the server is
not available or does not allow queries. The default action is ‘continue’.
tryagain
The service is temporarily unavailable. This could mean a file is locked or a server currently cannot accept more connec‐
tions. The default action is ‘continue’.
Interaction with +/- syntax (compat mode)
Linux libc5 without NYS does not have the name service switch but does allow the user some policy control. In /etc/passwd you could
have entries of the form +user or +@netgroup (include the specified user from the NIS passwd map), -user or -@netgroup (exclude the
specified user), and + (include every user, except the excluded ones, from the NIS passwd map). Since most people only put a + at
the end of /etc/passwd to include everything from NIS, the switch provides a faster alternative for this case (‘passwd: files nis’)
which doesn’t require the single + entry in /etc/passwd, /etc/group, and /etc/shadow. If this is not sufficient, the NSS ‘compat’
service provides full +/- semantics. By default, the source is ‘nis’, but this may be overridden by specifying ‘nisplus’ as source
for the pseudo-databases passwd_compat, group_compat and shadow_compat. This pseudo-databases are only available in GNU C Library.
FILES
A service named SERVICE is implemented by a shared object library named libnss_SERVICE.so.X that resides in /lib.
/etc/nsswitch.conf configuration file
/lib/libnss_compat.so.X implements ‘compat’ source for glibc2
unavail
The service is permanently unavailable. This can either mean the needed file is not available, or, for DNS, the server is
not available or does not allow queries. The default action is ‘continue’.
tryagain
The service is temporarily unavailable. This could mean a file is locked or a server currently cannot accept more connec‐
tions. The default action is ‘continue’.
Interaction with +/- syntax (compat mode)
Linux libc5 without NYS does not have the name service switch but does allow the user some policy control. In /etc/passwd you could
have entries of the form +user or +@netgroup (include the specified user from the NIS passwd map), -user or -@netgroup (exclude the
specified user), and + (include every user, except the excluded ones, from the NIS passwd map). Since most people only put a + at
the end of /etc/passwd to include everything from NIS, the switch provides a faster alternative for this case (‘passwd: files nis’)
which doesn’t require the single + entry in /etc/passwd, /etc/group, and /etc/shadow. If this is not sufficient, the NSS ‘compat’
service provides full +/- semantics. By default, the source is ‘nis’, but this may be overridden by specifying ‘nisplus’ as source
for the pseudo-databases passwd_compat, group_compat and shadow_compat. This pseudo-databases are only available in GNU C Library.
FILES
A service named SERVICE is implemented by a shared object library named libnss_SERVICE.so.X that resides in /lib.
/etc/nsswitch.conf configuration file
/lib/libnss_compat.so.X implements ‘compat’ source for glibc2
/lib/libnss_db.so.X implements ‘db’ source for glibc2
/lib/libnss_dns.so.X implements ‘dns’ source for glibc2
/lib/libnss_files.so.X implements ‘files’ source for glibc2
/lib/libnss_hesiod.so.X implements ‘hesiod’ source for glibc2
/lib/libnss_nis.so.X implements ‘nis’ source for glibc2
/lib/libnss_nisplus.so.2 implements ‘nisplus’ source for glibc 2.1
NOTES
Within each process that uses nsswitch.conf, the entire file is read only once; if the file is later changed, the process will con‐
tinue using the old configuration.
With Solaris, it isn’t possible to link programs using the NSS Service statically. With Linux, this is no problem.
On a Debian system other mail transport agents may or may not ignore the aliases file. For example, unlike sendmail Exim does not
ignore it.
小生的系统配置范本：
lee@work:/etc$ cat nsswitch.conf
# /etc/nsswitch.conf
#
# Example configuration of GNU Name Service Switch functionality.
# If you have the `glibc-doc' and `info' packages installed, try:
# `info libc "Name Service Switch"' for information about this file.
passwd: compat
group: compat
shadow: compat
hosts: files dns mdns //重要
networks: files ／／网络
protocols: db files
services: db files
ethers: db files
rpc: db files
netgroup: nis
下面我们接着看resolv.conf.这个文件相对就单纯多了，两个选项，nameserver 和search\domain中的任一个（冲突，但不是两个）。
nameserver 指出DNS server 的IP地址。只能是IP地址，若是名称（有人说和“鸡蛋”“鸡”的问题了）
规则：
GNU libc目前只查询优先列举的前三个nameserver,即使多了，也不再继续。
search可视为广义的 domain.这两者可让我们方便的查找本地域的主机！
search与domain 是互斥的，必须择一而用！
先看下man -S 5 nsswitch.conf(man -S的意思是按照某种章节列出、5的意思按文本格式）
NAME
nsswitch.conf - System Databases and Name Service Switch configuration file
系统数据库和名称服务切换配置文件
DESCRIPTION
Various functions in the C Library need to be configured to work correctly in the local environment. Traditionally, this was done
by using files (e.g., ‘/etc/passwd’), but other nameservices (like the Network Information Service (NIS) and the Domain Name Service
(DNS)) became popular, and were hacked into the C library, usually with a fixed search order.
The Linux libc5 with NYS support and the GNU C Library 2.x (libc.so.6) contain a cleaner solution of this problem. It is designed
after a method used by Sun Microsystems in the C library of Solaris 2. We follow their name and call this scheme "Name Service
Switch" (NSS). The sources for the "databases" and their lookup order are specified in the /etc/nsswitch.conf file.
The following databases are available in the NSS:
aliases
Mail aliases, used by sendmail(8). Presently ignored.
ethers Ethernet numbers.
group Groups of users, used by getgrent(3) functions.
hosts Host names and numbers, used by gethostbyname(3) and similar functions.
netgroup
Network wide list of hosts and users, used for access rules. C libraries before glibc 2.1 only support netgroups over NIS.
networks
Network names and numbers, used by getnetent(3) functions.
passwd User passwords, used by getpwent(3) functions.
protocols
Network protocols, used by getprotoent(3) functions.
publickey
Public and secret keys for Secure_RPC used by NFS and NIS+.
rpc Remote procedure call names and numbers, used by getrpcbyname(3) and similar functions.
services
Network services, used by getservent(3) functions.
shadow Shadow user passwords, used by getspnam(3).
An example /etc/nsswitch.conf (namely, the default used when /etc/nsswitch.conf is missing):
passwd: compat
group: compat
shadow: compat
hosts: dns [!UNAVAIL=return] files
networks: nis [NOTFOUND=return] files
ethers: nis [NOTFOUND=return] files
protocols: nis [NOTFOUND=return] files
rpc: nis [NOTFOUND=return] files
services: nis [NOTFOUND=return] files
The first column is the database. The rest of the line specifies how the lookup process works. You can specify the way it works
for each database individually.
The configuration specification for each database can contain two different items:
* The service specification like ‘files’, ‘db’, or ‘nis’.
* The reaction on lookup result like ‘[NOTFOUND=return]’.
For libc5 with NYS, the allowed service specifications are ‘files’, ‘nis’, and ‘nisplus’. For hosts, you could specify ‘dns’ as
extra service, for passwd and group ‘compat’, but not for shadow.
For glibc, you must have a file called /lib/libnss_SERVICE.so.X for every SERVICE you are using. On a standard installation, you
could use ‘files’, ‘db’, ‘nis’, and ‘nisplus’. For hosts, you could specify ‘dns’ as extra service, for passwd, group, and shadow
‘compat’. These services will not be used by libc5 with NYS. The version number X is 1 for glibc 2.0 and 2 for glibc 2.1.
The second item in the specification gives the user much finer control on the lookup process. Action items are placed between two
service names and are written within brackets. The general form is
‘[’ ( ‘!’? STATUS ‘=’ ACTION )+ ‘]’
where
STATUS => success | notfound | unavail | tryagain
ACTION => return | continue
The case of the keywords is insignificant. The STATUS values are the results of a call to a lookup function of a specific service.
They mean:
success
No error occurred and the wanted entry is returned. The default action for this is ‘return’.
notfound
The lookup process works ok but the needed value was not found. The default action is ‘continue’.
unavail
The service is permanently unavailable. This can either mean the needed file is not available, or, for DNS, the server is
not available or does not allow queries. The default action is ‘continue’.
tryagain
The service is temporarily unavailable. This could mean a file is locked or a server currently cannot accept more connec‐
tions. The default action is ‘continue’.
Interaction with +/- syntax (compat mode)
Linux libc5 without NYS does not have the name service switch but does allow the user some policy control. In /etc/passwd you could
have entries of the form +user or +@netgroup (include the specified user from the NIS passwd map), -user or -@netgroup (exclude the
specified user), and + (include every user, except the excluded ones, from the NIS passwd map). Since most people only put a + at
the end of /etc/passwd to include everything from NIS, the switch provides a faster alternative for this case (‘passwd: files nis’)
which doesn’t require the single + entry in /etc/passwd, /etc/group, and /etc/shadow. If this is not sufficient, the NSS ‘compat’
service provides full +/- semantics. By default, the source is ‘nis’, but this may be overridden by specifying ‘nisplus’ as source
for the pseudo-databases passwd_compat, group_compat and shadow_compat. This pseudo-databases are only available in GNU C Library.
FILES
A service named SERVICE is implemented by a shared object library named libnss_SERVICE.so.X that resides in /lib.
/etc/nsswitch.conf configuration file
/lib/libnss_compat.so.X implements ‘compat’ source for glibc2
unavail
The service is permanently unavailable. This can either mean the needed file is not available, or, for DNS, the server is
not available or does not allow queries. The default action is ‘continue’.
tryagain
The service is temporarily unavailable. This could mean a file is locked or a server currently cannot accept more connec‐
tions. The default action is ‘continue’.
Interaction with +/- syntax (compat mode)
Linux libc5 without NYS does not have the name service switch but does allow the user some policy control. In /etc/passwd you could
have entries of the form +user or +@netgroup (include the specified user from the NIS passwd map), -user or -@netgroup (exclude the
specified user), and + (include every user, except the excluded ones, from the NIS passwd map). Since most people only put a + at
the end of /etc/passwd to include everything from NIS, the switch provides a faster alternative for this case (‘passwd: files nis’)
which doesn’t require the single + entry in /etc/passwd, /etc/group, and /etc/shadow. If this is not sufficient, the NSS ‘compat’
service provides full +/- semantics. By default, the source is ‘nis’, but this may be overridden by specifying ‘nisplus’ as source
for the pseudo-databases passwd_compat, group_compat and shadow_compat. This pseudo-databases are only available in GNU C Library.
FILES
A service named SERVICE is implemented by a shared object library named libnss_SERVICE.so.X that resides in /lib.
/etc/nsswitch.conf configuration file
/lib/libnss_compat.so.X implements ‘compat’ source for glibc2
/lib/libnss_db.so.X implements ‘db’ source for glibc2
/lib/libnss_dns.so.X implements ‘dns’ source for glibc2
/lib/libnss_files.so.X implements ‘files’ source for glibc2
/lib/libnss_hesiod.so.X implements ‘hesiod’ source for glibc2
/lib/libnss_nis.so.X implements ‘nis’ source for glibc2
/lib/libnss_nisplus.so.2 implements ‘nisplus’ source for glibc 2.1
NOTES
Within each process that uses nsswitch.conf, the entire file is read only once; if the file is later changed, the process will con‐
tinue using the old configuration.
With Solaris, it isn’t possible to link programs using the NSS Service statically. With Linux, this is no problem.
On a Debian system other mail transport agents may or may not ignore the aliases file. For example, unlike sendmail Exim does not
ignore it.
小生的系统配置范本：
lee@work:/etc$ cat nsswitch.conf
# /etc/nsswitch.conf
#
# Example configuration of GNU Name Service Switch functionality.
# If you have the `glibc-doc' and `info' packages installed, try:
# `info libc "Name Service Switch"' for information about this file.
passwd: compat
group: compat
shadow: compat
hosts: files dns mdns //重要
networks: files ／／网络
protocols: db files
services: db files
ethers: db files
rpc: db files
netgroup: nis
下面我们接着看resolv.conf.这个文件相对就单纯多了，两个选项，nameserver 和search\domain中的任一个（冲突，但不是两个）。
nameserver 指出DNS server 的IP地址。只能是IP地址，若是名称（有人说和“鸡蛋”“鸡”的问题了）
规则：
GNU libc目前只查询优先列举的前三个nameserver,即使多了，也不再继续。
search可视为广义的 domain.这两者可让我们方便的查找本地域的主机！
search与domain 是互斥的，必须择一而用！
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