1NSSWITCH.CONF(5)           Linux Programmer's Manual          NSSWITCH.CONF(5)
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NAME

6       nsswitch.conf - Name Service Switch configuration file
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DESCRIPTION

9       The  Name  Service Switch (NSS) configuration file, /etc/nsswitch.conf,
10       is used by the GNU C Library and certain other applications  to  deter‐
11       mine  the  sources  from  which to obtain name-service information in a
12       range of categories, and in what order.  Each category  of  information
13       is identified by a database name.
14
15       The  file  is plain ASCII text, with columns separated by spaces or tab
16       characters.  The first column specifies the database name.  The remain‐
17       ing columns describe the order of sources to query and a limited set of
18       actions that can be performed by lookup result.
19
20       The following databases are understood by the GNU C Library:
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22       aliases     Mail aliases, used by getaliasent(3) and related functions.
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24       ethers      Ethernet numbers.
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26       group       Groups of users, used by getgrent(3) and related functions.
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28       hosts       Host  names  and  numbers,  used  by  gethostbyname(3)  and
29                   related functions.
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31       initgroups  Supplementary  group  access  list, used by getgrouplist(3)
32                   function.
33
34       netgroup    Network-wide list of  hosts  and  users,  used  for  access
35                   rules.   C  libraries  before glibc 2.1 supported netgroups
36                   only over NIS.
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38       networks    Network names and numbers, used by getnetent(3) and related
39                   functions.
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41       passwd      User passwords, used by getpwent(3) and related functions.
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43       protocols   Network protocols, used by getprotoent(3) and related func‐
44                   tions.
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46       publickey   Public and secret keys for Secure_RPC used by NFS and NIS+.
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48       rpc         Remote procedure call names and numbers, used by  getrpcby‐
49                   name(3) and related functions.
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51       services    Network  services,  used by getservent(3) and related func‐
52                   tions.
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54       shadow      Shadow user passwords,  used  by  getspnam(3)  and  related
55                   functions.
56
57       The  GNU C Library ignores databases with unknown names.  Some applica‐
58       tions use this to implement special handling for their  own  databases.
59       For example, sudo(8) consults the sudoers database.
60
61       Here is an example /etc/nsswitch.conf file:
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63           passwd:         compat
64           group:          compat
65           shadow:         compat
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67           hosts:          dns [!UNAVAIL=return] files
68           networks:       nis [NOTFOUND=return] files
69           ethers:         nis [NOTFOUND=return] files
70           protocols:      nis [NOTFOUND=return] files
71           rpc:            nis [NOTFOUND=return] files
72           services:       nis [NOTFOUND=return] files
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74       The first column is the database name.  The remaining columns specify:
75
76       *  One  or  more service specifications, for example, "files", "db", or
77          "nis".  The order of the services on the line determines  the  order
78          in  which those services will be queried, in turn, until a result is
79          found.
80
81       *  Optional actions to perform if a particular result is obtained  from
82          the preceding service, for example, "[NOTFOUND=return]".
83
84       The service specifications supported on your system depend on the pres‐
85       ence of shared libraries,  and  are  therefore  extensible.   Libraries
86       called  /lib/libnss_SERVICE.so.X  will provide the named SERVICE.  On a
87       standard installation, you can use "files", "db", "nis", and "nisplus".
88       For  the  hosts  database, you can additionally specify "dns".  For the
89       passwd, group, and shadow databases, you can additionally specify "com‐
90       pat" (see Compatibility mode below).  The version number X may be 1 for
91       glibc 2.0, or 2 for glibc 2.1 and later.  On  systems  with  additional
92       libraries  installed,  you  may have access to further services such as
93       "hesiod", "ldap", "winbind" and "wins".
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95       An action may also be specified following a service specification.  The
96       action  modifies the behavior following a result obtained from the pre‐
97       ceding data source.  Action items take the general form:
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99           [STATUS=ACTION]
100           [!STATUS=ACTION]
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102       where
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104           STATUS => success | notfound | unavail | tryagain
105           ACTION => return | continue | merge
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107       The ! negates the test, matching all possible results  except  the  one
108       specified.  The case of the keywords is not significant.
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110       The  STATUS  value is matched against the result of the lookup function
111       called by the preceding service specification, and can be one of:
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113           success     No error occurred and the requested entry is  returned.
114                       The default action for this condition is "return".
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116           notfound    The  lookup  succeeded, but the requested entry was not
117                       found.  The default action for this condition is  "con‐
118                       tinue".
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120           unavail     The  service is permanently unavailable.  This can mean
121                       either that the required file cannot be read,  or,  for
122                       network  services,  that the server is not available or
123                       does not allow queries.  The default  action  for  this
124                       condition is "continue".
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126           tryagain    The  service  is  temporarily  unavailable.  This could
127                       mean a file is locked  or  a  server  currently  cannot
128                       accept  more  connections.  The default action for this
129                       condition is "continue".
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131       The ACTION value can be one of:
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133           return      Return a result now.  Do not call  any  further  lookup
134                       functions.  However, for compatibility reasons, if this
135                       is the selected action for the group database  and  the
136                       notfound  status,  and  the configuration file does not
137                       contain the initgroups line, the next  lookup  function
138                       is always called, without affecting the search result.
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140           continue    Call the next lookup function.
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142           merge       [SUCCESS=merge]  is  used between two database entries.
143                       When a group is located in the first of the  two  group
144                       entries,  processing  will continue on to the next one.
145                       If the group is also found in the next entry  (and  the
146                       group name and GID are an exact match), the member list
147                       of the second entry will be added to the  group  object
148                       to be returned.  Available since glibc 2.24.  Note that
149                       merging will not  be  done  for  getgrent(3)  nor  will
150                       duplicate  members  be  pruned  when they occur in both
151                       entries being merged.
152
153   Compatibility mode (compat)
154       The NSS "compat" service is similar to "files" except that it addition‐
155       ally  permits special entries in corresponding files for granting users
156       or members of netgroups access to the system.   The  following  entries
157       are valid in this mode:
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159           For passwd and shadow databases:
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161               +user       Include   the   specified   user   from   the   NIS
162                           passwd/shadow map.
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164               +@netgroup  Include all users in the given netgroup.
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166               -user       Exclude   the   specified   user   from   the   NIS
167                           passwd/shadow map.
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169               -@netgroup  Exclude all users in the given netgroup.
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171               +           Include  every  user,  except  previously  excluded
172                           ones, from the NIS passwd/shadow map.
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174           For group database:
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176               +group      Include the specified group from the NIS group map.
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178               -group      Exclude the specified group from the NIS group map.
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180               +           Include every  group,  except  previously  excluded
181                           ones, from the NIS group map.
182
183       By default, the source is "nis", but this may be overridden by specify‐
184       ing any NSS service except  "compat"  itself  as  the  source  for  the
185       pseudo-databases passwd_compat, group_compat, and shadow_compat.
186

FILES

188       A service named SERVICE is implemented by a shared object library named
189       libnss_SERVICE.so.X that resides in /lib.
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191           /etc/nsswitch.conf       NSS configuration file.
192           /lib/libnss_compat.so.X  implements "compat" source.
193           /lib/libnss_db.so.X      implements "db" source.
194           /lib/libnss_dns.so.X     implements "dns" source.
195           /lib/libnss_files.so.X   implements "files" source.
196           /lib/libnss_hesiod.so.X  implements "hesiod" source.
197           /lib/libnss_nis.so.X     implements "nis" source.
198           /lib/libnss_nisplus.so.X implements "nisplus" source.
199
200       The following files are read  when  "files"  source  is  specified  for
201       respective databases:
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203           aliases     /etc/aliases
204           ethers      /etc/ethers
205           group       /etc/group
206           hosts       /etc/hosts
207           initgroups  /etc/group
208           netgroup    /etc/netgroup
209           networks    /etc/networks
210           passwd      /etc/passwd
211           protocols   /etc/protocols
212           publickey   /etc/publickey
213           rpc         /etc/rpc
214           services    /etc/services
215           shadow      /etc/shadow
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NOTES

218       Within  each  process  that uses nsswitch.conf, the entire file is read
219       only once.  If the file is later changed,  the  process  will  continue
220       using the old configuration.
221
222       Traditionally,  there was only a single source for service information,
223       often in the form of a single configuration file  (e.g.,  /etc/passwd).
224       However,  as  other name services, such as the Network Information Ser‐
225       vice (NIS) and the Domain Name Service (DNS), became popular, a  method
226       was  needed  that would be more flexible than fixed search orders coded
227       into the C library.  The Name Service Switch mechanism, which was based
228       on  the  mechanism used by Sun Microsystems in the Solaris 2 C library,
229       introduced a cleaner solution to the problem.
230

SEE ALSO

232       getent(1), nss(5)
233

COLOPHON

235       This page is part of release 4.16 of the Linux  man-pages  project.   A
236       description  of  the project, information about reporting bugs, and the
237       latest    version    of    this    page,    can     be     found     at
238       https://www.kernel.org/doc/man-pages/.
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242Linux                             2017-05-03                  NSSWITCH.CONF(5)
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