.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: GPL-2.0-or-later
.\"
.TH nscd.conf 5 2025-05-17 "Linux man-pages (unreleased)"
.SH NAME
nscd.conf \- name service cache daemon configuration file
.SH DESCRIPTION
The file
.I /etc/nscd.conf
is read from
.BR nscd (8)
at startup.
Each line specifies either an attribute and a value, or an
attribute, service, and a value.
Fields are separated either by SPACE
or TAB characters.
A \[aq]#\[aq] (number sign) indicates the beginning of a
comment; following characters, up to the end of the line,
are not interpreted by nscd.
.P
Valid services are
.IR passwd ,
.IR group ,
.IR hosts ,
.IR services ,
or
.IR netgroup .
.P
.B logfile
.I debug-file-name
.RS
Specifies name of the file to which debug info should be written.
.RE
.P
.B debug\-level
.I value
.RS
Sets the desired debug level.
0 hides debug info.
1 shows general debug info.
2 additionally shows data in cache dumps.
3 (and above) shows all debug info.
The default is 0.
.RE
.P
.B threads
.I number
.RS
This is the initial number of threads that are started to wait for
requests.
At least five threads will always be created.
The number of threads may increase dynamically up to
.B max\-threads
in response to demand from clients,
but never decreases.
.RE
.P
.B max\-threads
.I number
.RS
Specifies the maximum number of threads.
The default is 32.
.RE
.P
.B server\-user
.I user
.RS
If this option is set, nscd will run as this user and not as root.
If a separate cache for every user is used (\-S parameter), this
option is ignored.
.RE
.P
.B stat\-user
.I user
.RS
Specifies the user who is allowed to request statistics.
.RE
.P
.B reload\-count
unlimited |
.I number
.RS
Sets a limit on the number of times a cached entry
gets reloaded without being used
before it gets removed.
The limit can take values ranging from 0 to 254;
values 255 or higher behave the same as
.BR unlimited .
Limit values can be specified in either decimal
or hexadecimal with a "0x" prefix.
The special value
.B unlimited
is case-insensitive.
The default limit is 5.
A limit of 0 turns off the reloading feature.
See NOTES below for further discussion of reloading.
.RE
.P
.B paranoia
.I <yes|no>
.RS
Enabling paranoia mode causes nscd to restart itself periodically.
The default is no.
.RE
.P
.B restart\-interval
.I time
.RS
Sets the restart interval to
.I time
seconds
if periodic restart is enabled by enabling
.B paranoia
mode.
The default is 3600.
.RE
.P
.B enable\-cache
.I service
.I <yes|no>
.RS
Enables or disables the specified
.I service
cache.
The default is no.
.RE
.P
.B positive\-time\-to\-live
.I service
.I value
.RS
Sets the TTL (time-to-live) for positive entries (successful queries)
in the specified cache for
.IR service .
.I Value
is in seconds.
Larger values increase cache hit rates and reduce mean
response times, but increase problems with cache coherence.
Note that for some name services (including specifically DNS)
the TTL returned from the name service is used and
this attribute is ignored.
.RE
.P
.B negative\-time\-to\-live
.I service
.I value
.RS
Sets the TTL (time-to-live) for negative entries (unsuccessful queries)
in the specified cache for
.IR service .
.I Value
is in seconds.
Can result in significant performance improvements if there
are several files owned by UIDs (user IDs) not in system databases (for
example untarring the Linux kernel sources as root); should be kept small
to reduce cache coherency problems.
.RE
.P
.B suggested\-size
.I service
.I value
.RS
This is the internal hash table size,
.I value
should remain a prime number for optimum efficiency.
The default is 211.
.RE
.P
.B check\-files
.I service
.I <yes|no>
.RS
Enables or disables checking the file belonging to the specified
.I service
for changes.
The files are
.IR /etc/passwd ,
.IR /etc/group ,
.IR /etc/hosts ,
.IR /etc/resolv.conf ,
.IR /etc/services ,
and
.IR /etc/netgroup .
The default is yes.
.RE
.P
.B persistent
.I service
.I <yes|no>
.RS
Keep the content of the cache for
.I service
over server restarts; useful when
.B paranoia
mode is set.
The default is no.
.RE
.P
.B shared
.I service
.I <yes|no>
.RS
The memory mapping of the nscd databases for
.I service
is shared with the clients so
that they can directly search in them instead of having to ask the
daemon over the socket each time a lookup is performed.
The default is no.
Note that a cache miss will still result in
asking the daemon over the socket.
.RE
.P
.B max\-db\-size
.I service
.I bytes
.RS
The maximum allowable size, in bytes, of the database files for the
.IR service .
The default is 33554432.
.RE
.P
.B auto\-propagate
.I service
.I <yes|no>
.RS
When set to
.I no
for
.I passwd
or
.I group
service, then the
.I .byname
requests are not added to
.I passwd.byuid
or
.I group.bygid
cache.
This can help with tables containing multiple records for the same ID.
The default is yes.
This option is valid only for services
.I passwd
and
.IR group .
.RE
.SH NOTES
The default values stated in this manual page originate
from the source code of
.BR nscd (8)
and are used if not overridden in the configuration file.
The default values used in the configuration file of
your distribution might differ.
.SS Reloading
.BR nscd (8)
has a feature called reloading,
whose behavior can be surprising.
.P
Reloading is enabled when the
.B reload-count
attribute has a non-zero value.
The default value in the source code enables reloading,
although your distribution may differ.
.P
When reloading is enabled,
positive cached entries (the results of successful queries)
do not simply expire when their TTL is up.
Instead, at the expiry time,
.B nscd
will "reload",
i.e.,
re-issue to the name service the same query that created the cached entry,
to get a new value to cache.
Depending on
.I /etc/nsswitch.conf
this may mean that a DNS, LDAP, or NIS request is made.
If the new query is successful,
reloading will repeat when the new value would expire,
until
.B reload-count
reloads have happened for the entry,
and only then will it actually be removed from the cache.
A request from a client which hits the entry will
reset the reload counter on the entry.
Purging the cache using
.I nscd\~-i
overrides the reload logic and removes the entry.
.P
Reloading has the effect of extending cache entry TTLs
without compromising on cache coherency,
at the cost of additional load on the backing name service.
Whether this is a good idea on your system depends on
details of your applications' behavior,
your name service,
and the effective TTL values of your cache entries.
Note that for some name services
(for example, DNS),
the effective TTL is the value returned from the name service and
.I not
the value of the
.B positive\-time\-to\-live
attribute.
.P
Please consider the following advice carefully:
.IP \[bu] 3
If your application will make a second request for the same name,
after more than 1 TTL but before
.B reload\-count
TTLs,
and is sensitive to the latency of a cache miss,
then reloading may be a good idea for you.
.IP \[bu]
If your name service is configured to return very short TTLs,
and your applications only make requests rarely under normal circumstances,
then reloading may result in additional load on your backing name service
without any benefit to applications,
which is probably a bad idea for you.
.IP \[bu]
If your name service capacity is limited,
reloading may have the surprising effect of
increasing load on your name service instead of reducing it,
and may be a bad idea for you.
.IP \[bu]
Setting
.B reload\-count
to
.B unlimited
is almost never a good idea,
as it will result in a cache that never expires entries
and puts never-ending additional load on the backing name service.
.P
Some distributions have an init script for
.BR nscd (8)
with a
.I reload
command which uses
.I nscd\~-i
to purge the cache.
That use of the word "reload" is entirely different
from the "reloading" described here.
.SH SEE ALSO
.BR nscd (8)
.\" .SH AUTHOR
.\" .B nscd
.\" was written by Thorsten Kukuk and Ulrich Drepper.