nscd.conf - Man Page
name service cache daemon configuration file
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 written.
- 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 32.
- 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 ignored.
- 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 211.
- 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.
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 surprising.
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.