nscd.conf(5) — Linux manual page


nscd.conf(5)               File Formats Manual              nscd.conf(5)

NAME         top

       nscd.conf - name service cache daemon configuration file

DESCRIPTION         top

       The file /etc/nscd.conf is read from 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 '#' (number sign) indicates the beginning of a
       comment; following characters, up to the end of the line, are not
       interpreted by nscd.

       Valid services are passwd, group, hosts, services, or netgroup.

       logfile debug-file-name
              Specifies name of the file to which debug info should be

       debug-level value
              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.

       threads number
              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 max-threads in response to demand from clients, but
              never decreases.

       max-threads number
              Specifies the maximum number of threads.  The default is

       server-user user
              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.

       stat-user user
              Specifies the user who is allowed to request statistics.

       reload-count unlimited | number
              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 unlimited.  Limit values can be
              specified in either decimal or hexadecimal with a "0x"
              prefix.  The special value 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.

       paranoia <yes|no>
              Enabling paranoia mode causes nscd to restart itself
              periodically.  The default is no.

       restart-interval time
              Sets the restart interval to time seconds if periodic
              restart is enabled by enabling paranoia mode.  The default
              is 3600.

       enable-cache service <yes|no>
              Enables or disables the specified service cache.  The
              default is no.

       positive-time-to-live service value
              Sets the TTL (time-to-live) for positive entries
              (successful queries) in the specified cache for service.
              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

       negative-time-to-live service value
              Sets the TTL (time-to-live) for negative entries
              (unsuccessful queries) in the specified cache for service.
              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.

       suggested-size service value
              This is the internal hash table size, value should remain
              a prime number for optimum efficiency.  The default is

       check-files service <yes|no>
              Enables or disables checking the file belonging to the
              specified service for changes.  The files are /etc/passwd,
              /etc/group, /etc/hosts, /etc/resolv.conf, /etc/services,
              and /etc/netgroup.  The default is yes.

       persistent service <yes|no>
              Keep the content of the cache for service over server
              restarts; useful when paranoia mode is set.  The default
              is no.

       shared service <yes|no>
              The memory mapping of the nscd databases for 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.

       max-db-size service bytes
              The maximum allowable size, in bytes, of the database
              files for the service.  The default is 33554432.

       auto-propagate service <yes|no>
              When set to no for passwd or group service, then the
              .byname requests are not added to passwd.byuid or
              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 passwd and group.

NOTES         top

       The default values stated in this manual page originate from the
       source code of 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.

       nscd(8) has a feature called reloading, whose behavior can be

       Reloading is enabled when the reload-count attribute has a non-
       zero value.  The default value in the source code enables
       reloading, although your distribution may differ.

       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, 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 /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 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 nscd -i
       overrides the reload logic and removes the entry.

       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 not
       the value of the positive-time-to-live attribute.

       Please consider the following advice carefully:

       •  If your application will make a second request for the same
          name, after more than 1 TTL but before reload-count TTLs, and
          is sensitive to the latency of a cache miss, then reloading
          may be a good idea for you.

       •  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.

       •  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.

       •  Setting reload-count to 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.

       Some distributions have an init script for nscd(8) with a reload
       command which uses nscd -i to purge the cache.  That use of the
       word "reload" is entirely different from the "reloading"
       described here.

SEE ALSO         top


Linux man-pages (unreleased)     (date)                     nscd.conf(5)

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