Mac OS X: What is Lookupd?

Relatively few processes communicate directly with NetInfo. Yet, many processes need information that resides in NetInfo. How do these processes get that information? They use an intermediary: lookupd.

This article explains what lookupd is and what it does, and gives you some tips on spotting and fixing problems that involve it.
This article has been archived and is no longer updated by Apple.
What Is Lookupd?

Lookupd is a daemon that simplifies the tasks of the Unix-style library routines that need system and network administration information. These routines, such as getpwuid(), gethostbyname(), and getgrent(), are principally part of the C library (also known as libc). They access information like user names, computer addresses, and group IDs.

Lookupd gets information from NetInfo, the DNS, NIS, and the Unix-style system files. (The Unix-style system files are actually accessed as part of consulting NIS.) It uses Mach messages and SunRPCs to provide information to callers, usually libc routines.

Lookupd was created to avoid rewriting all the libc routines any time a new information service is added. If, for example, we wanted client processes to get network administration information from a relational database, we could modify lookupd so it referenced the information in the database. This would be easier than modifying all the appropriate libc routines to give them relational database access.

In addition to providing a centralized data access service, lookupd also caches some information. This caching improves system responsiveness and decreases network traffic.

When searching for information, lookupd consults its information sources in the following order:

1. lookupd's cache
2. The NetInfo domain hierarchy
3. The DNS, if appropriate
4. NIS

This search order applies in all of Apple's releases through Mac OS X Server. To find out about the DNS see Albitz and Liu 1992, and Nemeth, Snyder, and Seebass 1989; to find out about NIS see Nemeth, Snyder, and Seebass 1989, and Stern 1991.


Lookupd Caching

There are various types of lookupd caches, and each type is flushed and refreshed differently. The following sections provide details.

Password entries cache

One of the caches maintained by lookupd holds the information used by the getpwent() library routine. This routine lists the users known to the system (see Computer Systems Research Group 1986 and the system manual pages). lookupd can optionally cache the information it provides to getpwent(). The cache is enabled by default.

lookupd uses a "lazy refresh" on this cache. By default, lookupd loads the cache when it starts, normally at boot time. It then refreshes the cache only if the cache is referenced. At regular intervals, lookupd checks to see whether the cache has been referenced, and reloads it if it has. In addition, if the cache is referenced after a periodic check, lookupd sends the old data to the caller, then refreshes the cache. (See the lookupd(8) system manual page.)

To gain some perspective about this cache, consider that Mac OS X Server uses getpwent() only to complete a partial user name. For example, you can invoke this function in Workspace Manager's Finder by typing ~ followed by a partial user name, and pressing the Escape key. For example, if smarco is a user name, type "~smar", then press Escape. The Finder completes the name for you if the prefix you supplied is unique.

Logged-in user cache

The user information for the currently logged-in user and the root user are cached when someone logs in through loginwindow. lookupd refreshes this cache every 20 minutes and whenever the logged-in user changes his or her password. It clears the cache when the user logs out.

Local host information

The host information--host name and Internet address--for the local computer are cached when lookupd starts. If these change, lookupd must be restarted, typically by rebooting the computer.

Printers

Information regarding available network printers is also cached by lookupd. Each time the printer database is accessed through lookupd, lookupd verifies the validity of the cache by comparing the current checksums of the source databases with the checksums from when the cache was last loaded. If the cache is out of date, lookupd reloads it and sends the new data to the caller. If the cache is current, lookupd just sends the data from the cache.

Unix-Style Groups

The cache for Unix-style groups is maintained and consulted just like that for printers. It's referenced by the getgrent() library routine, but not by getgrnam() or getgrgid().

Other hosts

Lookupd maintains a one-record cache for information about computers other than the local computer. The cache contains the most recently referenced host name and Internet address. lookupd accesses the cache in response to calls to gethostbyname(), but not for calls to gethostbyaddr().

Mount points

The cache for mount points--locations for imported file systems--is like the cache for printers. It's accessed for calls to getmntent().


Looking It Up With Lookupd

Now let's examine what happens when an application makes a call to a library routine that provides system administration information. For this example, assume the application calls gethostbyname() to get information about a particular computer. The remote computer is Tute.EDU and is outside Rhino Aviation's network.

First, the application invokes gethostbyname(). This executes code in libc; the libc code checks to see if NetInfo is running. It is, so the libc code sends a Mach message to lookupd, requesting that a gethostbyname() operation be performed.

When it receives the Mach message, lookupd first checks to see if the request is for information about the local computer. It isn't, so lookupd then checks to see if the last host name referenced was Tute.EDU. If it was, lookupd would return the information from the cache. For this example, though, let's assume some other computer was referenced last.

Mac OS X Server lookupd consults NetInfo to get the information. This results in an NI_LOOKUPREAD SunRPC message to the local NetInfo server--netinfod local. Since the information isn't in NetInfo, this call returns an error and lookupd repeats the process, ascending the NetInfo domain hierarchy. This may require locating an appropriate NetInfo server, and so could require connecting or binding. (See "NetInfo Binding and Connecting.")

Since the information isn't in NetInfo and the application process is requesting machine-related information, eventually lookupd consults the DNS, using the normal resolver library routines. In this example, it finds the information in the DNS. It then returns the host information for Tute.EDU, using a Mach message, to the gethostbyname() libc routine. The libc routine then returns the information to the client application.

If the information hadn't been available from the DNS, then lookupd could have checked NIS, using the normal NIS semantics. At Rhino, though, NIS isn't used, so lookupd always stops with the DNS.


Managing Lookupd

Ordinarily, you may not have to deal with lookupd directly, so you don't need to do anything to manage it. However, if you want to know what lookupd is doing over time, it can log all requests it handles. You can also restart it if it's running into problems or if you need to refresh caches or tallies. The following sections explain how to work with lookupd.

Logging lookupd requests

Beginning in Mac OS X Server, lookupd can log information about requests it receives. You can set the logging option for lookupd in the system startup script /etc/rc. Logging is described fully in the system manual pages under lookupd(8).

For example, the arguments -L file cause lookupd to log information about requests it receives to the specified file. Logged information includes the called procedure, the number of calls to the procedure since lookupd started, the time required to process this request, and the total time consumed by all instances of this type of request. (Times are in microseconds.) When appropriate, the argument to the call is also logged, and the argument is prefixed with an asterisk if the data was retrieved from the cache. Example 1 shows an example of lookupd logging output.

Example 1: Logged information about lookupd

getservbyname Ncalls: 1 Elapsed: 78 Total time: 78
gethostbyname (rhino) Ncalls: 1 Elapsed: 32 Total time: 32
getservbyname (ntp) Ncalls: 2 Elapsed: 108 Total time: 186
getmntent Ncalls: 1 Elapsed: 3384 Total time: 3384
gethostbyname (sabre) Ncalls: 2 Elapsed: 18 Total time: 50
gethostbyname (ranger) Ncalls: 3 Elapsed: 51 Total time: 101
gethostbyname (*ranger) Ncalls: 4 Elapsed: 0 Total time: 101
setpwent Ncalls: 1 Elapsed: 24 Total time: 24
getpwnam (smarco) Ncalls: 5 Elapsed: 28 Total time: 46
setloginuser (672) Ncalls: 1 Elapsed: 64 Total time: 64
getpwnam (*smarco) Ncalls: 6 Elapsed: 1 Total time: 47
getmntent (*) Ncalls: 3 Elapsed: 39 Total time: 3461
gethostbyname (ranger) Ncalls: 5 Elapsed: 38 Total time: 406
getpwnam (*smarco) Ncalls: 7 Elapsed: 0 Total time: 47
getpwuid (*67) Ncalls: 8 Elapsed: 1 Total time: 27
initgroups (smarco) Ncalls: 1 Elapsed: 239 Total time: 239
getgrent Ncalls: 1 Elapsed: 1407 Total time: 1407
getpwuid (*0) Ncalls: 9 Elapsed: 0 Total time: 41
getpwnam (*root) Ncalls: 8 Elapsed: 0 Total time: 353
gethostbyaddr Ncalls: 1 Elapsed: 32 Total time: 32
getpwuid (22) Ncalls: 10 Elapsed: 27 Total time: 68
gethostbyaddr Ncalls: 2 Elapsed: 12 Total time: 44
getservbyport Ncalls: 1 Elapsed: 49 Total time: 49


Note: Since lookupd is invoked by /etc/rc at system boot time, you have to modify /etc/rc and restart the system to enable logging. You can't turn logging on and off dynamically in Mac OS X Server.

Restarting lookupd

Sometimes, you might want to restart lookupd. For example, you might want to force a cache to refresh, change the NetInfo servers being used, or reset the totals reported by the logging feature. You restart a lookupd daemon by sending it a "hang-up" signal, also called a SIGHUP.

To do this, first find the lookupd process ID number, using ps for example. Then, run the following command as root, substituting the process ID number for pid:

kill -HUP pid

This kills and automatically restarts lookupd.

If you were instead to try to restart lookupd by terminating it and rerunning the program, your computer would hang, because the library functions would be unable to contact the new instance. (If you ever find yourself in this state, reboot the computer.)
Published Date: Feb 20, 2012