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Apache HTTP Server
Log Files
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.
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
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)
- This is the IP address of the client (remote host) which made the request to the server.
If HostnameLookups
is set to
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)
- The "hyphen" in the output indicates that the requested piece of information
is not available. In this case, the information that is not available is the RFC 1413
identity of the client determined by
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)
- This is the userid of the person requesting the document as determined by HTTP
authentication. The same value is typically provided to CGI scripts in the
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)
- The time that the server finished processing the request. The format is:
[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\")
- The request line from the client is given in double quotes. The request line contains a
great deal of useful information. First, the method used by the client is
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)
- This is the status code that the server sends back to the client. This information is
very valuable, because it reveals whether the request resulted in a successful response
(codes beginning in 2), a redirection (codes beginning in 3), an error caused by the
client (codes beginning in 4), or an error in the server (codes beginning in 5). The full
list of possible status codes can be found in the HTTP specification
(RFC2616 section 10).
2326 (%b)
- The last entry indicates the size of the object returned to the client, not including
the response headers. If no content was returned to the client, this value will be "
-".
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\")
- The "Referer" (sic) HTTP request header. This gives the site that the client
reports having been referred from. (This should be the page that links to or includes
/apache_pb.gif).
"Mozilla/4.08 [en] (Win98; I ;Nav)" (\"%{User-agent}i\")
- The User-Agent HTTP request header. This is the identifying information that the client
browser reports about itself.
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.
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.
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.
Apache HTTP Server
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