Logging the Action (Apache: The Definitive Guide)

11.5.1. ErrorLog

ErrorLog filename|syslog[:facility] 
Default: ErrorLog logs/error_log
Server config, virtual host

The ErrorLog directive sets the name of the file to which the server will log any errors it encounters. If the filename does not begin with a slash ("/"), it is assumed to be relative to the server root.

If the filename begins with a pipe ("|"), it is assumed to be a command to spawn a file to handle the error log.

Apache 1.3 and above: Using syslog instead of a filename enables logging via syslogd(8) if the system supports it. The default is to use syslog facility local7, but you can override this by using the syslog:facility syntax, where facility can be one of the names usually documented in syslog(1).

Your security could be compromised if the directory where log files are stored is writable by anyone other than the user who starts the server.

11.5.2. TransferLog

TransferLog [ file | '|' command ]
Default: none
Server config, virtual host

TransferLog specifies the file in which to store the log of accesses to the site. If it is not explicitly included in the Config file, no log will be generated.

file: A filename relative to the server root (if it doesn't start with a slash), or an absolute path (if it does).
command

11.5.3. LogFormat

LogFormat format_string [nickname]
Default: "%h %l %u %t \"%r\" %s %b"
Server config, virtual host

LogFormat sets the information to be included in the log file and the way in which it is written. The default format is the Common Log Format (CLF), which is expected by off-the-shelf log analyzers such as wusage (http://www.boutell.com/ ) or ANALOG, so if you want to use one of them, leave this directive alone.[57] The CLF format is:

[57]Actually, some log analyzers support some extra information in the log file, but you need to read the analyzer's documentation for details.

host ident authuser date request status bytes

host: Domain name of the client or its IP number.
ident: If IdentityCheck is enabled and the client machine runs identd, then this is the identity information reported by the client.
authuser: If the request was for a password-protected document, then this is the user ID.
date: The date and time of the request, in the following format: [day/month/year:hour:minute:second tzoffset].
request: Request line from client, in double quotes.
status: Three-digit status code returned to the client.
bytes: The number of bytes returned, excluding headers.

The log format can be customized using a format_string. The commands in it have the format %[condition]key_letter ; the condition need not be present. If it is, and the specified condition is not met, the output will be a "-". The key_letter s are as follows:

b: Bytes sent.
{env_name}e: The value of the environment variable env_name.
f: The filename being served.
a: Remote IP address
h: Remote host.
{header_name}i: Contents of header_name: header line(s) in the request sent from the client.
l: Remote log name (from identd, if supplied).
{note_name}n: The value of a note. A note is a named entry in a table used internally in Apache for passing information between modules.
{header_name}o: The contents of the header_name header line(s) in the reply.
P: The PID of the child Apache handling the request.
p: The server port.
r: First line of request.
s: Status: for requests that were internally redirected, this is the status of the original request.
>s: Status of the last request.
t: Time, in common log time format.
U: The URL requested.
u: Remote user (from auth ; this may be bogus if return status [ %s ] is 401).
v: The server virtual host.

The format string can have ordinary text of your choice in it in addition to the % directives.

11.5.4. CustomLog

LogFormat file|pipe format|nickname
Server config, virtual host

The first argument is the filename to which log records should be written. This is used exactly like the argument to TransferLog; that is, it is either a full path, relative to the current server root, or a pipe to a program.

The format argument specifies a format for each line of the log file. The options available for the format are exactly the same as for the argument of the LogFormat directive. If the format includes any spaces (which it will do in almost all cases), it should be enclosed in double quotes.

Instead of an actual format string, you can use a format nickname defined with the LogFormat directive.

11.5.5. site.authent -- Another Example

site.authent is set up with two virtual hosts, one for customers and one for salespeople, and each has its own logs in ... /logs/customers and ... /logs/salesmen. We can follow that scheme and apply one LogFormat to both, or each can have its own logs with its own LogFormats inside the <VirtualHost> directives. They can also have common log files, set up by moving ErrorLog and TransferLog outside the <VirtualHost> sections, with different LogFormats within the sections to distinguish the entries. In this last case, the LogFormat files could look like this:

<VirtualHost www.butterthlies.com>
LogFormat "Customer:..."
...
</VirtualHost>

<VirtualHost sales.butterthlies.com>
LogFormat "Sales:..."
...
</VirtualHost>

Let's experiment with a format for customers, leaving everything else the same:

<VirtualHost www.butterthlies.com>
LogFormat "customers: host %h, logname %l, user %u, time %t, request %r
    status %s, bytes %b,"
...

We have inserted the words host, logname, and so on, to make it clear in the file what is doing what. In real life you probably wouldn't want to clutter the file up in this way because you would look at it regularly and remember what was what, or, more likely, process the logs with a program that would know the format. Logging on to www.butterthlies.com and going to summer catalog produces this log file:

customers: host 192.168.123.1, logname unknown, user -, time [07/Nov/
    1996:14:28:46 +0000], request GET / HTTP/1.0, status 200,bytes -
customers: host 192.168.123.1, logname unknown, user -, time [07/Nov/
    1996:14:28:49 +0000], request GET /hen.jpg HTTP/1.0, status 200,
    bytes 12291,
customers: host 192.168.123.1, logname unknown, user -, time [07/Nov
    /1996:14:29:04 +0000], request GET /tree.jpg HTTP/1.0, status 200,
    bytes 11532,
customers: host 192.168.123.1, logname unknown, user -, time [07/Nov/
    1996:14:29:19 +0000], request GET /bath.jpg HTTP/1.0, status 200,
    bytes 5880,

This is not too difficult to follow. Notice that while we have logname unknown, the user is "-", the usual report for an unknown value. This is because customers do not have to give an ID; the same log for salespeople, who do, would have a value here.

We can improve things by inserting lists of conditions based on the error codes after the % and before the command letter. The error codes are defined in the HTTP/1.0 specification:

200 OK
302 Found
304 Not Modified
400 Bad Request
401 Unauthorized
403 Forbidden
404 Not found
500 Server error
503 Out of resources
501 Not Implemented
502 Bad Gateway

The list from HTTP/1.1 is as follows:

100  Continue
101  Switching Protocols
200  OK
201  Created
202  Accepted
203  Non-Authoritative Information
204  No Content
205  Reset Content 
206  Partial Content
300  Multiple Choices
301  Moved Permanently
302  Moved Temporarily
303  See Other
304  Not Modified
305  Use Proxy
400  Bad Request
401  Unauthorized
402  Payment Required
403  Forbidden
404  Not Found
405  Method Not Allowed
406  Not Acceptable
407  Proxy Authentication Required
408  Request Time-out
409  Conflict
410  Gone
411  Length Required
412  Precondition Failed
413  Request Entity Too Large
414  Request-URI Too Large
415  Unsupported Media Type
500  Internal Server Error
501  Not Implemented
502  Bad Gateway
503  Service Unavailable
504  Gateway Time-out
505  HTTP Version not supported

You can use "!" before a code to mean "if not." !200 means "log this if the response was not OK." Let's put this in salesmen:

<VirtualHost sales.butterthlies.com>
LogFormat "sales: host %!200h, logname %!200l, user %u, time %t, request %r,
    status %s,bytes %b,"
...

An attempt to log in as fred with the password don't know produces the following entry:

sales: host 192.168.123.1, logname unknown, user fred, time [19/Aug/
    1996:07:58:04 +0000], request GET HTTP/1.0, status 401, bytes -

However, if it had been the infamous Bill with the password theft, we would see:

host -, logname -, user bill, ...

because we asked for host and logname to be logged only if the request was not OK. We can combine more than one condition, so that if we only want to know about security problems on sales, we could log usernames only if they failed to authenticate:

LogFormat "sales: bad user: %400,401,403u"

We can also extract data from the HTTP headers in both directions:

%[condition]{user-agent}i

prints the user agent (i.e., the software the client is running) if condition is met. The old way of doing this was AgentLog logfile and ReferLog logfile.


11.4. Server Info		12. Extra Modules

11.5. Logging the Action

11.5.1. ErrorLog

11.5.2. TransferLog

11.5.3. LogFormat

11.5.4. CustomLog

11.5.5. site.authent -- Another Example