In order to effectively manage a web server, it is necessary to get feedback about the activity and performance of the server as well as any problems that may be occuring. The Apache HTTP Server provides very comprehensive and flexible logging capabilities. This document describes how to configure its logging capabilities, and how to understand what the logs contain.
Anyone who can write to the directory where Apache is writing a log file can almost certainly gain access to the uid that the server is started as, which is normally root. Do NOT give people write access to the directory the logs are stored in without being aware of the consequences; see the security tips document for details.
In addition, log files may contain information supplied directly by the client, without escaping. Therefore, it is possible for malicious clients to insert control-characters in the log files, so care must be taken in dealing with raw logs.
| Related Directives ErrorLog LogLevel |
The server error log, whose name and location is set by the ErrorLog directive, is the most important log file. This is the place where Apache httpd will send diagnostic information and record any errors that it encounters in processing requests. It is the first place to look when a problem occurs with starting the server or with the operation of the server, since it will often contain details of what went wrong and how to fix it.
The error log is usually written to a file (typically
error_log on unix systems and
error.log on Windows and OS/2). On unix systems it
is also possible to have the server send errors to
syslog or pipe them to a
program.
The format of the error log is relatively free-form and descriptive. But there is certain information that is contained in most error log entries. For example, here is a typical message.
[Wed Oct 11 14:32:52 2000] [error] [client 127.0.0.1]
client denied by server configuration:
/export/home/live/ap/htdocs/test
The first item in the log entry is the date and time of the message. The second entry lists the severity of the error being reported. The LogLevel directive is used to control the types of errors that are sent to the error log by restricting the severity level. The third entry gives the IP address of the client that generated the error. Beyond that is the message itself, which in this case indicates that the server has been configured to deny the client access. The server reports the file-system path (as opposed to the web path) of the requested document.
A very wide variety of different messages can appear in the
error log. Most look similar to the example above. The error
log will also contain debugging output from CGI scripts. Any
information written to stderr by a CGI script will
be copied directly to the error log.
It is not possible to customize the error log by adding or removing information. However, error log entries dealing with particular requests have corresponding entries in the access log. For example, the above example entry corresponds to an access log entry with status code 403. Since it is possible to customize the access log, you can obtain more information about error conditions using that log file.
During testing, it is often useful to continuously monitor the error log for any problems. On unix systems, you can accomplish this using:
tail -f error_log
| Related Modules mod_log_config |
Related Directives CustomLog LogFormat SetEnvIf |
The server access log records all requests processed by the server. The location and content of the access log are controlled by the CustomLog directive. The LogFormat directive can be used to simplify the selection of the contents of the logs. This section describes how to configure the server to record information in the access log.
Of course, storing the information in the access log is only the start of log management. The next step is to analyze this information to produce useful statistics. Log analysis in general is beyond the scope of this document, and not really part of the job of the web server itself. For more information about this topic, and for applications which perform log analysis, check the Open Directory or Yahoo.
Various versions of Apache httpd have used other modules and
directives to control access logging, including
mod_log_referer, mod_log_agent, and the
TransferLog directive. The CustomLog
directive now subsumes the functionality of all the older
directives.
The format of the access log is highly configurable. The format is specified using a format string that looks much like a C-style printf(1) format string. Some examples are presented in the next sections. For a complete list of the possible contents of the format string, see the mod_log_config documentation.
A typical configuration for the access log might look as follows.
LogFormat "%h %l %u %t \"%r\" %>s %b" common
CustomLog logs/access_log common
This defines the nickname common and
associates it with a particular log format string. The format
string consists of percent directives, each of which tell the
server to log a particular piece of information. Literal
characters may also be placed in the format string and will be
copied directly into the log output. The quote character
(") must be escaped by placing a back-slash before
it to prevent it from being interpreted as the end of the
format string. The format string may also contain the special
control characters "\n" for new-line and
"\t" for tab.
The CustomLog directive sets up a new log file
using the defined nickname. The filename for the
access log is relative to the ServerRoot unless it begins
with a slash.
The above configuration will write log entries in a format known as the Common Log Format (CLF). This standard format can be produced by many different web servers and read by many log analysis programs. The log file entries produced in CLF will look something like this:
127.0.0.1 - frank [10/Oct/2000:13:55:36 -0700] "GET
/apache_pb.gif HTTP/1.0" 200 2326
Each part of this log entry is described below.
127.0.0.1 (%h)On, then the server will try to determine
the hostname and log it in place of the IP address. However,
this configuration is not recommended since it can
significantly slow the server. Instead, it is best to use a
log post-processor such as logresolve to determine
the hostnames. The IP address reported here is not
necessarily the address of the machine at which the user is
sitting. If a proxy server exists between the user and the
server, this address will be the address of the proxy, rather
than the originating machine.- (%l)identd on the clients
machine. This information is highly unreliable and should
almost never be used except on tightly controlled internal
networks. Apache httpd will not even attempt to determine
this information unless IdentityCheck is set
to On.frank (%u)REMOTE_USER environment variable. If the status
code for the request (see below) is 401, then this value
should not be trusted because the user is not yet
authenticated. If the document is not password protected,
this entry will be "-" just like the previous
one.[10/Oct/2000:13:55:36 -0700]
(%t)
[day/month/year:hour:minute:second zone]
day = 2*digit
month = 3*letter
year = 4*digit
hour = 2*digit
minute = 2*digit
second = 2*digit
zone = (`+' | `-') 4*digit
It is possible to have the time displayed in another format
by specifying %{format}t in the log format
string, where format is as in
strftime(3) from the C standard library.
"GET /apache_pb.gif HTTP/1.0"
(\"%r\")GET. Second, the client requested the resource
/apache_pb.gif, and third, the client used the
protocol HTTP/1.0. It is also possible to log
one or more parts of the request line independently. For
example, the format string "%m %U%q %H" will log
the method, path, query-string, and protocol, resulting in
exactly the same output as "%r".200 (%>s)2326 (%b)-". To log "0" for no content, use
%B instead.Another commonly used format string is called the Combined Log Format. It can be used as follows.
LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\"
\"%{User-agent}i\"" combined
CustomLog log/acces_log combined
This format is exactly the same as the Common Log Format,
with the addition of two more fields. Each of the additional
fields uses the percent-directive
%{header}i, where header can be
any HTTP request header. The access log under this format will
look like:
127.0.0.1 - frank [10/Oct/2000:13:55:36 -0700] "GET
/apache_pb.gif HTTP/1.0" 200 2326
"http://www.example.com/start.html" "Mozilla/4.08 [en]
(Win98; I ;Nav)"
The additional fields are:
"http://www.example.com/start.html"
(\"%{Referer}i\")/apache_pb.gif)."Mozilla/4.08 [en] (Win98; I ;Nav)"
(\"%{User-agent}i\")Multiple access logs can be created simply by specifying
multiple CustomLog directives in the configuration
file. For example, the following directives will create three
access logs. The first contains the basic CLF information,
while the second and third contain referer and browser
information. The last two CustomLog lines show how
to mimic the effects of the ReferLog and
AgentLog directives.
LogFormat "%h %l %u %t \"%r\" %>s %b" common
CustomLog logs/access_log common
CustomLog logs/referer_log "%{Referer}i -> %U"
CustomLog logs/agent_log "%{User-agent}i"
This example also shows that it is not necessary to define a
nickname with the LogFormat directive. Instead,
the log format can be specified directly in the
CustomLog directive.
There are times when it is convenient to exclude certain
entries from the access logs based on characteristics of the
client request. This is easily accomplished with the help of environment variables. First, an
environment variable must be set to indicate that the request
meets certain conditions. This is usually accomplished with SetEnvIf. Then the
env= clause of the CustomLog
directive is used to include or exclude requests where the
environment variable is set. Some examples:
# Mark requests from the loop-back interface
SetEnvIf Remote_Addr "127\.0\.0\.1" dontlog
# Mark requests for the robots.txt file
SetEnvIf Request_URI "^/robots\.txt$" dontlog
# Log what remains
CustomLog logs/access_log common env=!dontlog
As another example, consider logging requests from english-speakers to one log file, and non-english speakers to a different log file.
SetEnvIf Accept-Language "en" english
CustomLog logs/english_log common env=english
CustomLog logs/non_english_log common env=!english
Although we have just shown that conditional logging is very powerful and flexibly, it is not the only way to control the contents of the logs. Log files are more useful when they contain a complete record of server activity. It is often easier to simply post-process the log files to remove requests that you do not want to consider.
On even a moderately busy server, the quantity of information stored in the log files is very large. The access log file typically grows 1 MB or more per 10,000 requests. It will consequently be necessary to periodically rotate the log files by moving or deleting the existing logs. This cannot be done while the server is running, because Apache will continue writing to the old log file as long as it holds the file open. Instead, the server must be restarted after the log files are moved or deleted so that it will open new log files.
By using a graceful restart, the server can be instructed to open new log files without losing any existing or pending connections from clients. However, in order to accomplish this, the server must continue to write to the old log files while it finishes serving old requests. It is therefore necessary to wait for some time after the restart before doing any processing on the log files. A typical scenario that simply rotates the logs and compresses the old logs to save space is:
mv access_log access_log.old
mv error_log error_log.old
apachectl graceful
sleep 600
gzip access_log.old error_log.old
Another way to perform log rotation is using piped logs as discussed in the next section.
Apache httpd is capable of writing error and access log
files through a pipe to another process, rather than directly
to a file. This capability dramatically increases the
flexibility of logging, without adding code to the main server.
In order to write logs to a pipe, simply replace the filename
with the pipe character "|", followed by the name
of the executable which should accept log entries on its
standard input. Apache will start the piped-log process when
the server starts, and will restart it if it crashes while the
server is running. (This last feature is why we can refer to
this technique as "reliable piped logging".)
Piped log processes are spawned by the parent Apache httpd process, and inherit the userid of that process. This means that piped log programs usually run as root. It is therefore very important to keep the programs simple and secure.
Some simple examples using piped logs:
# compressed logs
CustomLog "|/usr/bin/gzip -c >>
/var/log/access_log.gz" common
# almost-real-time name resolution
CustomLog "|/usr/local/apache/bin/logresolve >>
/var/log/access_log" common
Notice that quotes are used to enclose the entire command that will be called for the pipe. Although these examples are for the access log, the same technique can be used for the error log.
One important use of piped logs is to allow log rotation without having to restart the server. The Apache HTTP Server includes a simple program called rotatelogs for this purpose. For example, to rotate the logs every 24 hours, you can use:
CustomLog "|/usr/local/apache/bin/rotatelogs
/var/log/access_log 86400" common
A similar, but much more flexible log rotation program called cronolog is available at an external site.
As with conditional logging, piped logs are a very powerful tool, but they should not be used where a simpler solution like off-line post-processing is available.
When running a server with many virtual
hosts, there are several options for dealing with log
files. First, it is possible to use logs exactly as in a
single-host server. Simply by placing the logging directives
outside the <VirtualHost> sections in the
main server context, it is possible to log all requests in the
same access log and error log. This technique does not allow
for easy collection of statistics on individual virtual
hosts.
If CustomLog or ErrorLog
directives are placed inside a <VirtualHost>
section, all requests or errors for that virtual host will be
logged only to the specified file. Any virtual host which does
not have logging directives will still have its requests sent
to the main server logs. This technique is very useful for a
small number of virtual hosts, but if the number of hosts is
very large, it can be complicated to manage. In addition, it
can often create problems with insufficient file
descriptors.
For the access log, there is a very good compromise. By adding information on the virtual host to the log format string, it is possible to log all hosts to the same log, and later split the log into individual files. For example, consider the following directives.
LogFormat "%v %l %u %t \"%r\" %>s %b"
comonvhost
CustomLog logs/access_log comonvhost
The %v is used to log the name of the virtual
host that is serving the request. Then a program like split-logfile can be used to
post-process the access log in order to split it into one file
per virtual host.
Unfortunately, no similar technique is available for the error log, so you must choose between mixing all virtual hosts in the same error log and using one error log per virtual host.
| Related Modules mod_cgi mod_rewrite |
Related Directives PidFile RewriteLog RewriteLogLevel ScriptLog ScriptLogLength ScriptLogBuffer |
On startup, Apache httpd saves the process id of the parent
httpd process to the file logs/httpd.pid. This
filename can be changed with the PidFile directive. The
process-id is for use by the administrator in restarting and
terminating the daemon by sending signals to the parent
process; on Windows, use the -k command line option instead.
For more information see the Stopping
and Restarting page.
In order to aid in debugging, the ScriptLog directive allows you to record the input to and output from CGI scripts. This should only be used in testing - not for live servers. More information is available in the mod_cgi documentation.
When using the powerful and complex features of mod_rewrite, it is almost always necessary to use the RewriteLog to help in debugging. This log file produces a detailed analysis of how the rewriting engine transforms requests. The level of detail is controlled by the RewriteLogLevel directive.