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 for the original dns-resolver code he submitted, which was the basis for this implementation.