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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.
Another Note: DNS lookups will not work under Windows yet, see the
README.WIN file for more information.
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.
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