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finger can return quite a bit of information about a user, including the user’s login status, the contents of the user’s GECOS field in /etc/passwd, and the contents of the user’s ~/.plan and ~/.project files.

If you are connected to the Internet and are running fingerd, anyone in the world can obtain this information. fingerd has enabled some really neat services (such as the Internet white pages), but it has also enabled people to run a variety of scams, such as finding people to cold-call and prospecting for spammable addresses. Some sites have responded to this invasion by turning off fingerd, while others just restrict the amount of information it returns. If you choose to run fingerd, you should install a current version; a security hole in older fingerds was exploited by the Internet worm.

httpd: World Wide Web server

httpd lets your site become a web server. httpd can send text, pictures, and sound to its clients. See Chapter 22, Web Hosting and Internet Servers, for more information about serving up web pages.

28.9 TIME SYNCHRONIZATION DAEMONS

As computers have grown increasingly interdependent, it has become more and more important for them share a consistent idea of time. Synchronized clocks are essential for correlating log file entries in the event of a security breach, and they’re also important for a variety of end-user applications, from joint development of software projects to the processing of financial transactions.

timed: synchronize clocks

There are several different time synchronization systems, and more than one time daemon is named timed. Most systems use essentially the same scheme. One or more machines are designated as time masters. Their clocks are considered authoritative, and they negotiate with each other to agree on the “correct” time. Other machines are slaves; they periodically converse with a master to learn the time and then adjust their internal clocks.

The time between settings of a slave’s clock is short enough that only slight adjustments are usually needed. Slaves use the adjtime system call (if it is available) to smooth the adjustment of the system’s clock and prevent large time leaps backward or forward.8 It is especially harmful to set the clock back suddenly; time should be a monotonically increasing function.

The notion of “correct” time is rather nebulously defined. Some systems poll the network to compute an average time, whereas others declare one master correct by fiat.

xntpd: synchronize clocks even better

xntpd is a daemon that uses the Network Time Protocol defined in RFC1119 to synchronize a number of “peer” clocks to within milliseconds of each other. Servers are arranged in a hierarchal tree, each level of which is called a “stratum.”

xntpd can access a number of reference time standards, such as those provided by WWV and GPS. As a result, xntpd provides a much more accurate way to set the clock on your UNIX machine than does timed; clocks are not only synchronized but are also accurate within a few milliseconds. You can obtain the current version of xntp by anonymous ftp from ftp.udel.edu.

28.10 BOOTING AND CONFIGURATION DAEMONS

In the 1980s, the UNIX world was swept by a wave of diskless workstation mania. These machines booted entirely over the network and performed all their disk operations through a remote filesystem technology such as NFS. As disk prices dropped and speeds increased, interest in diskless workstations quickly faded. They could come back into fashion at any moment, however, like the platform shoes of the 1970s. The two main remnants of the diskless era are a plethora of daemons designed to support diskless systems and the bizarre organization of most vendors’ filesystems.

Although diskless workstations are not very common anymore, their booting protocols have been usurped by other devices. Most manageable network hubs and network printers boot using some combination of the services listed in this section.

bootpd: boot server

When