Name Service Switch

<h2 id="nsswitchconf">nsswitch.conf</h2>
<p>A <a href="/facts/System_administrator/zNhThrh9">system administrator</a> usually configures the operating system's name services using the file /etc/nsswitch.conf. This file lists databases (such as <a href="/facts/%2fetc%2fpasswd/zumE69MQ">passwd</a>, <a href="/facts/Shadow_password/zumE69MQ">shadow</a> and <a href="/facts/Group_(database)/uNjvmMVT">group</a>), and one or more sources for obtaining that information. Examples for sources are <i>files</i> for local files,  <i>ldap</i> for the <a href="/facts/Lightweight_Directory_Access_Protocol/KgpfD8nR">Lightweight Directory Access Protocol</a>, <i>nis</i> for the <a href="/facts/Network_Information_Service/MawdYWjj">Network Information Service</a>, <i>nisplus</i> for <a href="/facts/Nisplus/BL3bjo6V">NIS+</a>, <i>dns</i> for the <a href="/facts/Domain_Name_System/rfxdWFsj">Domain Name System</a> (DNS), and <i>wins</i> for <a href="/facts/Windows_Internet_Name_Service/TMKILlVt">Windows Internet Name Service</a>.
</p><p>The nsswitch.conf file has line entries for each service consisting of a database name in the first field, terminated by a colon, and a list of possible source databases in the second field. 
</p><p>A typical file might look like:
</p>
passwd:     files ldap
shadow:     files
group:      files ldap

hosts:      dns nis files

ethers:     files nis
netmasks:   files nis
networks:   files nis
protocols:  files nis
rpc:        files nis
services:   files nis

automount:  files
aliases:    files

<p>The order of the source databases determines the order the NSS will attempt to look up those sources to resolve queries for the specified service. A bracketed list of criteria may be specified following each source name to govern the conditions under which the NSS will proceed to querying the next source based on the preceding source's response.
</p>
<h2 id="history">History</h2>
<p>Earlier <a href="/facts/Unix-like/YAKrAxzd">Unix-like</a> systems either accessed only local files or had hard-coded rules for accessing files or network-stored databases. <a href="/facts/Ultrix/NWH3Udfj">Ultrix</a> was a notable exception with its nearly identical functionality of the NSS configuration file in /etc/svc.conf.
</p><p><a href="/facts/Sun_Microsystems/VJ9EzEFd">Sun Microsystems</a> first developed the NSS for their <a href="/facts/Solaris_(operating_system)/KPfT98EJ">Solaris</a> operating system.
</p><p>Solaris' compliance with SVR4, which Sun Microsystems and <a href="/facts/AT%2526T/y3Lo9uhk">AT&T</a> Unix System Laboratories jointly developed by merging <a href="/facts/UNIX_System_V/Epm6GEY4">UNIX System V</a>, <a href="/facts/Berkeley_Software_Distribution/XQKcURrh">BSD</a> and <a href="/facts/Xenix/G8sH08Tp">Xenix</a>, required that third parties be able to plug in name service implementations for the <a href="/facts/Transport_layer/UmIGUCeK">transport layer</a> of their choosing (<a href="/facts/OSI_Model/oxN9Xhjz">OSI</a> or <a href="/facts/Internet_Protocol_Suite/KNkxtSpw">IP</a>) without rewriting SVR4-compliant Transport-Independent <a href="/facts/Remote_procedure_call/uMMb9CQw">RPC</a> (TI-RPC) applications or rebuilding the operating system. Sun introduced the <a href="/facts/NIS%252B/BL3bjo6V">NIS+</a> directory service in Solaris to supersede <a href="/facts/Network_Information_Service/MawdYWjj">NIS</a>, which required co-existence of the two directory services within an enterprise to ease migration. 
</p><p>Sun engineers Thomas Maslen and Sanjay Dani were the first to design and implement the Name Service Switch. They fulfilled Solaris requirements with the nsswitch.conf file specification and the implementation choice to load  database access modules as <a href="/facts/Dynamic_loading/mJ9bcYuv">dynamically loaded libraries</a>, which Sun was also the first to introduce.
</p><p>Sun engineers' original design of the configuration file and runtime loading of name service back-end libraries has withstood the test of time as operating systems have evolved and new name services are introduced.  Over the years, programmers ported the NSS configuration file with nearly identical implementations to many other operating systems including <a href="/facts/FreeBSD/RKsFTm7F">FreeBSD</a>, <a href="/facts/NetBSD/PTm5HWls">NetBSD</a>, <a href="/facts/Linux/9FvCKSTq">Linux</a>, <a href="/facts/HP-UX/hIvLDdJF">HP-UX</a>, <a href="/facts/IRIX/vK9rSatM">IRIX</a> and <a href="/facts/IBM_AIX/cRWERua2">AIX</a>. More than two decades after the NSS was invented, <a href="/facts/GNU_libc/AL6IYDkA">GNU libc</a> implements it almost identically.
</p>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/BSD_Authentication/hxvGJd4p">BSD Authentication</a></li>
<li><a href="/facts/Group_(database)/uNjvmMVT">Group (database)</a></li>
<li><a href="/facts/Name_server/pUgmBMNW">Name server</a></li>
<li><a href="/facts/Pluggable_Authentication_Modules/g8lhxyUk">Pluggable Authentication Modules</a></li></ul>

<h2 id="external-links">External links</h2>
<ul><li><a href="https://man.netbsd.org/nsswitch.conf.5">nsswitch.conf(5)</a>: name-service switch configuration file – <a href="/facts/NetBSD/PTm5HWls">NetBSD</a> File Formats <a href="/facts/Man_page/S2yYjy0J">Manual</a></li>
<li><a href="https://www.gnu.org/software/libc/manual/html_node/Name-Service-Switch.html">Name Service Switch implementation in the GNU C Library</a></li>
<li><a href="https://arthurdejong.org/nss-ldapd/">Another NSS module supporting LDAP</a>: nss-ldapd</li>
<li><a href="http://megacz.com/software/libnss-afs.html">NSS module supporting AFS</a>: nss_afs</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>"Name Service Switch (The GNU C Library)". www.gnu.org. Retrieved 2024-03-13. <a href="https://www.gnu.org/software/libc/manual/html_node/Name-Service-Switch.html" target="_blank">https://www.gnu.org/software/libc/manual/html_node/Name-Service-Switch.html</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
</ol>

Name Service Switch open-in-new

Name Service Switch