The Webalizer - A log file analysis program  -- DNS information

The webalizer now has the ability to perform reverse DNS lookups.  This
document attempts to explain how it works and some things that you should
be aware of when using the DNS lookup features.

Note: The Reverse DNS feature may be enabled or disabled at compile
      time.  It is enabled by using the -DUSE_DNS compiler switch, or
      by specifing '--enable-dns' when "configure' is run.  DNS lookups
      are disabled by default.


How it works
------------

DNS lookups are made against a DNS cache file containing IP addresses
and resolved names.  If the IP address is not found in the cache file,
it will be left as an IP address.  In order for this to happen, a
cache file MUST be specified when the Webalizer is run, either using
the '-D' command line switch, or a "DNSCache" configuration file
keyword.  If no cache file is specified, no attempts to perform DNS
lookups will be done. The cache file can be made in two different ways.

1) You can have the Webalizer pre-process the specified log file at
   run-time, creating the cache file before processing the log file
   normally.  This is done by setting the number of DNS Children
   processes to run, either by using the '-N' command line switch or
   the "DNSChildren" configuration keyword.  This will cause the
   Webalizer to spawn the specified number of processes which will
   be used to do reverse DNS lookups.. generally, a larger number
   of processes will result in faster resolution of the log, however
   if set too high may cause overall system degredation.  A setting
   of between 5 and 20 should be acceptable, and there is a maximum
   limit of 100.   If used, a cache filename MUST be specified also,
   using either the '-D' command line switch, or the "DNSCache"
   configuration keyword.  Using this method, normal processing will
   continue only after all IP addresses have been processed, and the
   cache file is created/updated.

2) You can pre-process the log file as a standalone process, creating
   the cache file that will be used later by the Webalizer.  This is
   done by running the Webalizer with a name of 'webazolver' (ie: the
   name 'webazolver' is a symbolic link to 'webalizer') and specifing
   the cache filename (either with '-D' or DNSCache).   If the number
   of child processes is not given, the default of 5 will be used. In
   this mode, the log will be read and processed, creating a DNS cache
   file or updating an existing one, and the program will then exit
   without any further processing.


Run-time DNS cache file creation/update
---------------------------------------

The creation/update of a DNS cache file at run-time occurs as follows:

1) The log file is read, creating a list of all IP addresses that are
   not already cached and need to be resolved.

2) The specified number of children processes are forked, and are used
   to perform DNS lookups.

3) Each IP address is given, one at a time, to the next available child
   process until all IP addresses have been processed.  Each child will
   update the cache file when a name is found.

4) Once all IP addresses have been processed and the cache file updated,
   the Webalizer will process the log normally.  Each record it finds
   that has an unresolved IP address will be looked up in the cache file
   to see if a hostname is available (ie: was previously found).

Because there may be a significant amount of time between the inital
unresolved IP list and normal processing, the Webalizer should not be
run against live log files (ie: a log file that is activly being written
to by a server), otherwise there may be additional records present that
were not resolved.


Stand-Alone DNS cache file creation/update
------------------------------------------

The creation/update of the DNS cache file, when run in stand-alone mode,
occurs as follows:

1) The log file is read, creating a list of all IP addresses that are
   not already cached and need to be resolved.

2) The specified number of children processes are forked, and are used
   to perform DNS lookups.  If the number of processes was not specified,
   the default of 5 will be used.

3) Each IP address is given, one at a time, to the next available child
   process until all IP addresses have been processed.  Each child will
   update the cache file when a name is found.

4) Once all IP addresses have been processed and the cache file updated,
   the program will terminate without any further processing.


Larger sites may prefer to use a stand-alone process to create the DNS
cache file, and then run the Webalizer against the cache file.  This
allows a single cache file to be used for many virtual hosts, and reduces
the processing needed if many sites are being processed.  The Webalizer
can be used in stand alone mode by running it as 'webazolver'.  When
run in this fashion, it will only create the cache file and then exit
without any further processing.  A cache filename MUST be specified,
however unlike when running the Webalizer normally, the number of child
processes does not have to be given (will default to 5).  All normal
configuration and command line options are recognized, however, many
of them will simply be ignored.. this allows the use of a standard
configuration file for both normal use and stand alone use.


Examples:
---------

webalizer -c test.conf -N 10 -D dns_cache.db /var/log/my_www_log

   This will use the configuration file 'test.conf' to obtain normal
   configuration options such as hostname and output directory.. it
   will then either create or update the file 'dns_cache.db' in the
   default output directory (using 10 child processes) based on the
   IP addresses it finds in the log /var/lib/my_www_log, and then
   process that log file normally.


webalizer -o out -D dns_cache.db /var/log/my_www_log

  This will process the log file /var/log/my_www_log, resolving IP
  addresses from the cache file 'dns_cache.db' found in the default
  output directory "out".  The cache file must be present as it will
  not be created with this command.


for i in /var/log/*/access_log; do
  webazolver -N 20 -D /var/lib/dns_cache.db $i
done

  The above is an example of how to run through multiple log files
  creating a single DNS cache file.. this might be typically used on
  a larger site that has many virtual hosts, all keeping their log
  files in a seperate directory.  It will process each access_log it
  finds in /var/log/* and create a cache file (var/lib/dns_cache.db).
  This cache file can then be used to process the logs normally with
  with the Webalizer.


for i in /etc/webalizer/*.conf; do webalizer -c $i -D /etc/cache.db; done

  This will process each configuration file found in /etc/webalizer,
  using the DNS cache file /etc/cache.db.  This will also typically be
  used on a larger site with multiple hosts..  Each configration file
  will specify a site specific log file, hostname, output directory, etc.
  The cache file used will typically be created using a command similar
  to the one previous to this example.


Considerations
--------------

Processing of live log files is discouraged, as the chances of log records
being written between the time of DNS resolution and normal processing will
cause problems.

Cached DNS addresses have a TTL (time to live) of 3 days.  This may be
changed at compile time by editing the dns_resolv.h header file and
changing the value for DNS_CACHE_TTL.

There is an absolute maximum of 100 child processes that may be created,
however the actual number of children should be significantly less than
the maximum.. typical usage should be between 5 and 20.

If you are using STDIN for the input stream (log file) and have run-time
DNS cache file creation/update enabled.. the program will exit after the
cache file has been created/updated and no output will be produced.  If
you must use STDIN for the input log, you will need to process the stream
twice, once to create/update the cache file, and again to produce the
reports.

Special thanks to Henning P. Schmiedehausen <hps@tanstaafl.de> for the
original dns-resolver code he submitted,  which was the basis for this
implementation.

