syslog system is one of the most delightful things about
Unix. Unlike some operating systems that force you to use the limited
range of logs that they condescend to provide, Unix allows you to log
almost anything, at almost any level of detail. While system logging
hooks are provided for the most common Unix resources, administrators
can choose a logging configuration that meets their needs. My networks
usually have a single logging host that handles not only the
FreeBSD boxes, but Cisco routers, switches, and any other
The system logger is actually fairly straightforward.
Programs send log entries to the system logging daemon,
Syslogd compares each submission to the entries in
When it finds a matching entry, it processes the log entry in the
/etc/syslog.conf has two columns. The first is the system
providing the information to be logged. The second is the action to
be taken when a log message matches. The most confusing part is
understanding exactly how to specify a logging information source.
The standard method of specifying a logging source is by
facility and level. A facility is a log entry source, or a program
that sends messages to
syslogd. These facilities are described below.
Anything having to do with user authorization, such as
This is identical to
Messages that are normally printed to the system console can be captured by using the console facility.
Messages from the system scheduler.
This is a catch-all for all system daemons that don't have other explicit handlers.
You can configure your FTP daemon to log its transfers. See
This is for messages from the kernel.
Messages from the printing system
Messages from the mail system
This isn't an actual log from a system; instead, the
Messages from the Internet News daemons.
Messages from Network Time Protocol
Messages from various security systems, such as
Yes, the log service can log to itself. Just don't log when you log logs from the log system, or you'll make yourself dizzy.
The catch-all messages facility. If you don't specify a logging facility for user programs, they'll use this.
Logs from the Unix-to-Unix Copy Protocol. This is a piece of Unix history you'll probably never encounter.
These are reserved for administrator use. Many programs have an option to set a logging facility; choose one of these if at all possible.
Most systems don't log everything their programs send to
syslog; rather, they discard trivial messages and only record the important stuff. One man's trivial is another's vital data. This is where the level comes in.
Also in Big Scary Daemons:
FreeBSD offers eight levels of
syslog importance. You can use these levels to tell
syslog what to record and what to discard. The levels follow, in order from most to least important.
emerg-- System panic. Messages are flashed on every terminal. The system is basically hosed, or at best horribly, horribly unstable.
alert-- This is bad, but not as bad as the
emerglevel. The system can continue to operate, but this error should be attended to immediately.
crit-- These are critical errors, such as hardware problems or serious software issues. If your hard drive has bad blocks, they'll show up as critical errors. You can continue running, if you're brave.
err-- Miscellaneous errors. They're bad, and should be fixed, but aren't going to destroy the system.
warning-- Miscellaneous warnings.
notice-- General information that should be logged, in case you need it, but probably doesn't really require action on your part.
info-- General system information.
debug-- This level is usually only of use to programmers, and occasionally to system admininistrators who are trying to figure out just why some program behaves in this way. Debugging logs can contain whatever information the programmer felt necessary to debug the code; this might include information that will violate this users' privacy.
none-- This special level means "don't log anything from this facility here." It's most commonly used when excluding information from wildcard entries.
Information sources include both a facility and a level, separated by a period. When you specify a level, the system defaults to recording messages of that level or greater. For example, look at this entry from
Messages from the mail system, with a level equal to or above info, are logged to
If you like, you can use wildcards in your information source. To log absolutely all messages from the mail system, you would use this.
To log everything from everywhere, uncomment the
This works, but it's far too full of information to be of any real use; you'll find yourself building complex
grep commands just to find what you want.
Also, this would include all sorts of private information, thanks to the debug level. You probably don't want to record that sort of thing. You can exclude authentication information with the
authpriv facility and the none level. The semicolon allows you to combine entries on a single line.
You can also use comparison operators in
/etc/syslog.conf. The valid operators are
< (less than),
= (equals), and
> (greater than). You might want to have a log for mail traffic, and another for mail debugging.
mail.info /var/log/maillog mail.=debug /var/log/maillog.debug
This way, you don't have to sort through debugging information to learn what your mail server thinks it's doing.
Similarly, you might have a program that wants to log to
local3. You can set this up as such.
You can also use a program name as an information source. If a program supports a facility, use it. If you're out of facilities, however, or if your program simply doesn't support
syslogd, you can use the name.
An entry for a name requires at least two lines. The first is the program name with a leading exclamation point. The second is the logging information. For example, look at the sample entry for logging PPP.
!ppp *.* /var/log/ppp.log
It starts by specifying the program name, and then tells
syslogd to append absolutely everything to a file. You can't be certain a random third-party program will have reasonable logging facilities, so it's safest to record everything.
Finally, we have the log message destination. The most common destination is a log file, specified by full path name, but there are other destinations.
You can send log messages to another host with the
@ symbol. The following example would dump everything your
syslog receives to the logging host on my network.
/etc/syslog.conf on the loghost will be used to send messages to their final destinations.
You can list user names, separated by commas. If they're logged in when the log message arrives, the system will write the message on its terminal.
If you want the messages to be written to all users' terminals, use a destination of "
Finally, if you want another program to handle the logs, you can use a pipe symbol to redirect the messages to that program.
Now that you have logging running, all you have to worry about is your logs eventually filling your disk. We'll cover that next time, when we discuss
Michael W. Lucas
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