nsswitch.conf - Name Service Switch configuration file
The Name Service Switch (NSS) configuration file, /etc/nsswitch.conf
used by the GNU C Library and certain other applications to determine the
sources from which to obtain name-service information in a range of
categories, and in what order. Each category of information is identified by a
The file is plain ASCII text, with columns separated by spaces or tab
characters. The first column specifies the database name. The remaining
columns describe the order of sources to query and a limited set of actions
that can be performed by lookup result.
The following databases are understood by the GNU C Library:
- Mail aliases, used by getaliasent(3) and related functions.
- Ethernet numbers.
- Groups of users, used by getgrent(3) and related functions.
- Host names and numbers, used by gethostbyname(3) and related
- Supplementary group access list, used by getgrouplist(3)
- Network-wide list of hosts and users, used for access rules. C libraries
before glibc 2.1 supported netgroups only over NIS.
- Network names and numbers, used by getnetent(3) and related
- User passwords, used by getpwent(3) and related functions.
- Network protocols, used by getprotoent(3) and related
- Public and secret keys for Secure_RPC used by NFS and NIS+.
- Remote procedure call names and numbers, used by getrpcbyname(3)
and related functions.
- Network services, used by getservent(3) and related functions.
- Shadow user passwords, used by getspnam(3) and related
The GNU C Library ignores databases with unknown names. Some applications use
this to implement special handling for their own databases. For example,
(8) consults the sudoers
Here is an example /etc/nsswitch.conf
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 name. The remaining columns specify:
- One or more service specifications, for example, "files",
"db", or "nis". The order of the services on the line
determines the order in which those services will be queried, in turn,
until a result is found.
- Optional actions to perform if a particular result is obtained from the
preceding service, for example, "[NOTFOUND=return]".
The service specifications supported on your system depend on the presence of
shared libraries, and are therefore extensible. Libraries called
will provide the named SERVICE
On a standard installation, you can use "files", "db",
"nis", and "nisplus". For the hosts
can additionally specify "dns". For the passwd
databases, you can additionally specify "compat"
(see Compatibility mode
below). The version number X
may be 1
for glibc 2.0, or 2 for glibc 2.1 and later. On systems with additional
libraries installed, you may have access to further services such as
"hesiod", "ldap", "winbind" and
An action may also be specified following a service specification. The action
modifies the behavior following a result obtained from the preceding data
source. Action items take the general form:
The ! negates the test, matching all possible results except the one specified.
The case of the keywords is not significant.
value is matched against the result of the lookup function
called by the preceding service specification, and can be one of:
- No error occurred and the requested entry is returned. The default action
for this condition is "return".
- The lookup succeeded, but the requested entry was not found. The default
action for this condition is "continue".
- The service is permanently unavailable. This can mean either that the
required file cannot be read, or, for network services, that the server is
not available or does not allow queries. The default action for this
condition is "continue".
- The service is temporarily unavailable. This could mean a file is locked
or a server currently cannot accept more connections. The default action
for this condition is "continue".
value can be one of:
- Return a result now. Do not call any further lookup functions. However,
for compatibility reasons, if this is the selected action for the
group database and the notfound status, and the
configuration file does not contain the initgroups line, the next
lookup function is always called, without affecting the search
- Call the next lookup function.
- [SUCCESS=merge] is used between two database entries. When a group
is located in the first of the two group entries, processing will continue
on to the next one. If the group is also found in the next entry (and the
group name and GID are an exact match), the member list of the second
entry will be added to the group object to be returned. Available since
glibc 2.24. Note that merging will not be done for getgrent(3) nor
will duplicate members be pruned when they occur in both entries being
The NSS "compat" service is similar to "files" except that
it additionally permits special entries in corresponding files for granting
users or members of netgroups access to the system. The following entries are
valid in this mode:
- Include the specified user from the NIS passwd/shadow map.
- Include all users in the given netgroup.
- Exclude the specified user from the NIS passwd/shadow map.
- Exclude all users in the given netgroup.
- Include every user, except previously excluded ones, from the NIS
- Include the specified group from the NIS group map.
- Exclude the specified group from the NIS group map.
- Include every group, except previously excluded ones, from the NIS group
By default, the source is "nis", but this may be overridden by
specifying any NSS service except "compat" itself as the source for
the pseudo-databases passwd_compat
A service named SERVICE
is implemented by a shared object library named
that resides in /lib
- NSS configuration file.
- implements "compat" source.
- implements "db" source.
- implements "dns" source.
- implements "files" source.
- implements "hesiod" source.
- implements "nis" source.
- implements "nisplus" source.
The following files are read when "files" source is specified for
Within each process that uses nsswitch.conf
, the entire file is read only
once. If the file is later changed, the process will continue using the old
Traditionally, there was only a single source for service information, often in
the form of a single configuration file (e.g., /etc/passwd
as other name services, such as the Network Information Service (NIS) and the
Domain Name Service (DNS), became popular, a method was needed that would be
more flexible than fixed search orders coded into the C library. The Name
Service Switch mechanism, which was based on the mechanism used by Sun
Microsystems in the Solaris 2 C library, introduced a cleaner solution to the