mod_perl guide: mod_perl Configuration

Table of Contents:

Server Configuration
Apache Configuration
mod_perl Configuration
The Startup File
Apache Configuration in Perl
Validating the Configuration Syntax
Enabling Remote Server Configuration Reports
Publishing Port Numbers other than 80
Configuring Apache + mod_perl with mod_macro
General Pitfalls
Configuration Security Concerns
Apache Restarts Twice On Start
Knowing the proxy_pass'ed Connection Type
Adding Custom Configuration Directives

The next step after building and installing your new mod_perl enabled Apache server is to configure the server. There are two separate parts to configure: Apache and mod_perl. Each has its own set of directives.

To configure your mod_perl enabled Apache server, the only file that you should need to edit is httpd.conf. By default, httpd.conf is put into the conf directory under the server root directory. The default server root is /usr/local/apache/ on many UNIX platforms, but within reason it can be any directory you choose. If you are new to Apache and mod_perl, you will probably find it helpful to keep to the directory layouts we use in this Guide if you can.

Apache versions 1.3.4 and later are distributed with the configuration directives in a single file -- httpd.conf. This Guide uses the same approach in its examples. Prior to version 1.3.4, the default Apache installation used three configuration files -- httpd.conf, srm.conf, and access.conf. If you wish you can still use all three files, by setting the AccessConfig and ResourceConfig directives in httpd.conf. You will also see later on that we use other files, for example perl.conf and startup.pl. This is just for our convenience, you could still do everything in httpd.conf if you wished.

Apache configuration can be confusing. To minimize the number of things that can go wrong, it can be a good idea first to configure Apache itself without mod_perl. This will give you the confidence that it works and maybe that you have some idea how to configure it.

There is a warning in the httpd.conf distributed with Apache about simply editing httpd.conf and running the server, without understanding all the implications. This is another warning. Modifying the config file and adding new directives can introduce security problems, and have performance implications.

The Apache distribution comes with an extensive configuration manual, and in addition each section of the distributed configuration file includes helpful comments explaining how every directive should be configured and what the defaults values are.

If you haven't moved Apache's directories around, the installation program will have configured everything for you. You can just start the server and test it. To start the server use the apachectl utility which comes bundled with the Apache distribution. It resides in the same directory as httpd, the Apache server itself. Execute:

Now you can test the server, for example by accessing http://localhost from a browser running on the same host.

For a basic setup there are just a few things to configure. If you have moved any directories you have to update them in httpd.conf. There are many of them, here are just a couple of examples:

You might want to change the user and group names the server will run under. Note that if started as the root user (which is generally the case), the parent process will continue to run as root, but its children will run as the user and group you have specified. For example:

There are many other directives that you might need to configure as well. In addition to directives which take a single value there are whole sections of the configuration (such as the <Directory> and <Location> sections) which apply only to certain areas of your Web space. As mentioned earlier you will find them all in httpd.conf.

If there is a file with the name .htaccess in any directory, Apache scans it for further configuration directives which it then applies only to that directory (and its subdirectories). The name .htaccess is confusing because it can contain any configuration directives, not just those related to access to resources. You will not be surprised to find that a configuration directive can change the names of the files used in this way.

I'll explain just the basics of the <Directory>, <Location> and <Files> sections. Remember that there is more to know and the rest of the information is available in the Apache documentation. The information I'll present here is just what is important for understanding the mod_perl configuration sections.

Apache considers directories and files on your machine all to be resources. For each resource you can determine a particular behaviour which will apply to every request for information from that particular resource.

Obviously the directives in <Directory> sections apply to specific directories on your host machine, and those in <Files> sections apply only to specific files (actually groups of files with names which have something in common). In addition to these sections, Apache has the concept of a <Location>, which is also just a resource. <Location> sections apply to specific URIs. Locations are based at the document root, directories are based at the filesystem root. For example, if you have the default server directory layout where the server root is /usr/local/apache and the document root is /usr/local/apache/htdocs then static files in the directory /usr/local/apache/htdocs/pub are in the location /pub.

It is up to you to decide which directories on your host machine are mapped to which locations. You should be careful how you do it, because the security of your server may be at stake.

Locations do not necessarily have to refer to existing physical directories, but may refer to virtual resources which the server creates for the duration of a single browser request. As you will see, this is often the case for a mod_perl server.

When a browser asks for a resource from your server, Apache determines from its configuration whether or not to serve the request, whether to pass the request to another server, what (if any) authorization is required for access to the resource, and how to reply. For any given resource, the various sections in your configuration may provide conflicting information. For example you may have a <Directory> section which tells Apache that authorization is required for access to the resource but you may have a <Files> section which says that it is not. It is not always obvious which directive takes precedence in these cases. This can be a trap for the unwary.

When configuring the server, it's important to understand the order in which the rules of each section apply to requests. The order of merging is:

Apart from <Directory>, each group is processed in the order that it appears in the configuration files. <Directory> (group 1 above) is processed in the order shortest directory component to longest. If multiple <Directory> sections apply to the same directory then they are processed in the configuration file order.

Sections inside <VirtualHost> sections are applied as if you were running several independent servers. The directives inside <VirtualHost> sections do not interact with each other. They are applied after first processing any sections outside the virtual host definition. This allows virtual host configurations to override the main server configuration.

Let's say that you want all files, except for a few of the files in a specific directory and below, to be handled in the same way. For example if you want all the files in /home/http/docs to be served as plain files, but any files with ending .html and .txt to be processed by the content handler of your Apache::MyFilter module.

  <Directory /home/httpd/docs>
    <FilesMatch "\.(html|txt)$">
      SetHandler perl-script
      PerlHandler Apache::MyFilter
    </FilesMatch>
  </Directory>

Thus it is possible to embed sections inside sections to create subgroups which have their own distinct behavior. Alternatively you could use a <Files> section inside an .htaccess file.

Note that you can't put <Files> or <FilesMatch> sections inside a <Location> section, but you can put them inside a <Directory> section.

Normally, if multiple Options directives apply to a directory, then the most specific one is taken complete; the options are not merged.

However if all the options on the Options directive are preceded by a + or - symbol, the options are merged. Any options preceded by + are added to the options currently in force, and any options preceded by - are removed.

  <Directory /home/httpd/docs>
    Options Indexes FollowSymLinks
  </Directory>
  <Directory /home/httpd/docs/shtml>
    Options Includes
  </Directory>

then only Includes will be set for the /home/httpd/docs/shtml directory. However if the second Options directive uses the + and - symbols:

  <Directory /home/httpd/docs>
    Options Indexes FollowSymLinks
  </Directory>
  <Directory /home/httpd/docs/shtml>
    Options +Includes -Indexes
  </Directory>

then the options FollowSymLinks and Includes are set for the /home/httpd/docs/shtml directory.

When you have tested that the Apache server works on your machine, it's time to configure mod_perl. Some of the configuration directives are already familiar to you, but mod_perl introduces a few new ones.

It can be a good idea to keep all the mod_perl related configuration at the end of the configuration file, after the native Apache configuration directives.

To ease maintenance and to simplify multiple server installations, the Apache/mod_perl configuration system allows you several alternative ways to keep your configration directives in separate places. The Include directive in httpd.conf allow you to include the contents of other files, just as if the information were all contained in httpd.conf. This is a feature of Apache itself. For example if you want all your mod_perl configuration to be placed in a separate file mod_perl.conf you can do that by adding to httpd.conf this directive:

mod_perl adds two further directives: <Perl> sections allow you to execute Perl code from within any configuration file at server startup time, and as you will see later, a file containing any Perl program can be executed (also at server startup time) simply by mentioning its name in a PerlRequire or PerlModule directve.

The ScriptAlias and Alias directives provide a mapping of a URI to a file system directory. The directive:

will map all requests starting with /foo onto the files starting with /home/httpd/foo/. So when Apache gets a request http://www.example.com/foo/test.pl the server will remap this into the file test.pl in the directory /home/httpd/foo/.

In addition ScriptAlias assigns all the requests that match the URI (i.e. /cgi-bin) to be executed under mod_cgi.

  Alias /cgi-bin /home/httpd/cgi-bin
  <Location /cgi-bin>
    SetHandler cgi-script
  </Location>

where latter directive invokes mod_cgi. You shouldn't use the ScriptAlias directive unless you want the request to be processed under mod_cgi. Therefore when you configure mod_perl sections use Alias instead.

Under mod_perl the Alias directive will be followed by two further directives. The first is the SetHandler perl-script directive, which tells Apache to invoke mod_perl to run the script. The second directive (for example PerlHandler) tells mod_perl which handler (Perl module) the script should be run under, and hence for which phase of the request. Refer to the section Perl*Handlers for more information about handlers for the various request phases.

When you have decided which methods to use to run your scripts and where you will keep them, you can add the configuration directive(s) to httpd.conf. They will look like those below, but they will of course reflect the locations of your scripts in your file-system and the decisions you have made about how to run the scripts:

In the examples above all the requests issued for URIs starting with /cgi-bin will be served from the directory /home/httpd/cgi-bin/, and starting with /perl from the directory /home/httpd/perl/.

  # Typical for plain cgi scripts:
  ScriptAlias /cgi-bin/  /home/httpd/perl/
    
  # Typical for Apache::Registry scripts:
  Alias       /perl/     /home/httpd/perl/
    
  # Typical for Apache::PerlRun scripts:
  Alias       /cgi-perl/ /home/httpd/perl/

In the examples above we have mapped the three different URIs (http://www.example.com/perl/test.pl http://www.example.com/cgi-bin/test.pl and http://www.example.com/cgi-perl/test.pl) all to the same file /home/httpd/perl/test.pl. This means that we can have all our CGI scripts located at the same place in the file-system, and call the script in any of three ways simply by changing one component of the URI (cgi-bin|perl|cgi-perl).

This technique makes it easy to migrate your scripts to mod_perl. If your script does not seem to be working while running under mod_perl, then in most cases you can easily call the script in straight mod_cgi mode or under Apache::PerlRun without making any script changes. Simply change the URL you use to invoke it.

Although in the configuration above we have configured all three Aliases to point to the same directory within our file system, you can of course have them point to different directories if you prefer.

You should remember that it is undesirable to run scripts in plain mod_cgi mode from a mod_perl-enabled server--the resource consumption is too high. It is better to run these on a plain Apache server. See Standalone mod_perl Enabled Apache Server.

The <Location> section assigns a number of rules which the server should follow when the request's URI matches the Location. Just as it is the widely accepted convention to use /cgi-bin for your mod_cgi scripts, it is conventional to use /perl as the base URI of the perl scripts which you are running under mod_perl. Let's review the following very widely used <Location> section:

  Alias /perl/ /home/httpd/perl/
  PerlModule Apache::Registry
  <Location /perl>
    SetHandler perl-script
    PerlHandler Apache::Registry
    Options ExecCGI
    allow from all
    PerlSendHeader On
  </Location>

This configuration causes all requests for URIs starting with /perl to be handled by the mod_perl Apache module with the handler from the Apache::Registry Perl module. Let's review the directives inside the <Location> section in the example:

Remember the Alias from the above section? We use the same Alias here; if you were to use a <Location> that does not have the same Alias, the server would fail to locate the script in the file system. You need the Alias setting only if the code that should be executed is located in the file. So Alias just provides the URI to filepath translation rule.

Sometimes there is no script to be executed. Instead there is some module whose method is being executed, similar to /perl-status, where the code is stored in an Apache module. In such cases we don't need Alias settings for those <Location>s.

Here we tell Apache to use the Apache::Registry Perl module for the actual content generation.

The Options directive accepts various parameters (options), one of which is ExecCGI. This tells the server that the file is a program and should be executed, instead of just being displayed like a static file (like HTML file). If you omit this option then the script will either be rendered as plain text or else it will trigger a Save-As dialog, depending on the client's configuration.

This directive is used to set access control based on domain. The above settings allow clients from any domain to run the script.

PerlSendHeader On tells the server to send an HTTP headers to the browser on every script invocation. You will want to turn this off for nph (non-parsed-headers) scripts.

The PerlSendHeader On setting invokes the Apache's ap_send_http_header() method after parsing the headers generated by the script. It is only meant for emulation of mod_cgi behavior with regard to headers.

To send the HTTP headers it's always better either to use the $r->send_http_header method using the Apache Perl API or to use the $q->header method from the CGI.pm module.

Note that sometimes you will have to preload the module before using it in the <Location> section. In the case of Apache::Registry the configuration will look like this:

  PerlModule Apache::Registry
  <Location /perl>
    SetHandler perl-script
    PerlHandler Apache::Registry
    Options ExecCGI
    allow from all
    PerlSendHeader On
  </Location>

No changes are required to the /cgi-bin location (mod_cgi), since it has nothing to do with mod_perl.

Here is another very similar example, this time using Apache::PerlRun (For more information see Apache::PerlRun):

  <Location /cgi-perl>
    SetHandler perl-script
    PerlHandler Apache::PerlRun
    Options ExecCGI
    allow from all
    PerlSendHeader On
  </Location>

The only difference from the Apache::Registry configuration is the argument of the PerlHandler directive, where Apache::Registry has been replaced with Apache::PerlRun.

If you want to remove a mod_perl handler setting from a location beneath a location where the handler was set (i.e. /foo/bar), all you have to do is to reset it, like this:

Now, all the requests starting with /foo/bar would be served by Apache's default handler.

As we saw earlier, a module should be loaded before it is used. PerlModule and PerlRequire are the two mod_perl directives which are used to load modules and code. They are almost equivalent to Perl's use() and require() functions respectively and called from the Apache configuration file. You can pass one or more module names as arguments to PerlModule:

Generally the modules are preloaded from the startup script, which is usually called startup.pl. This is a file containing plain Perl code which is executed through the PerlRequire directive. For example:

A PerlRequire file name can be absolute or relative to ServerRoot or a path in @INC.

As with any file with Perl code that gets use()'d or require()'d, it must return a true value. To ensure that this happens don't forget to add 1; at the end of startup.pl.

After you have compiled and installed mod_perl, your Apache mod_perl configuration directives tell Apache to invoke the module mod_perl as the handler for some request which it receives. Although it could in fact handle all the phases of the request loop, usually it does not. You tell mod_perl which phases it is to handle (and so which to leave to other modules, or to the default Apache routines) by putting Perl*Handler directives in the configuration files.

Because you need the Perl interpreter to be present for your Perl script to do any processing at all, there is a slight difference between the way that you configure Perl and C handlers to handle parts of the request loop. Ordinarily a C module is written, compiled and configured to hook into a specific phase of the request loop. For a Perl handler you compile mod_perl itself to hook into the appropriate phases, as if it were to handle the phases itself. Then you put Perl*Handler directives in your configuration file to tell mod_perl that it is to pass the responsibility for handling that part of the request phase to your Perl module.

mod_perl is an Apache module written in C. As most programmers will only need to handle the response phase, in the default compilation most of the Perl*Handlers are disabled. When you configure the Makefile.PL file for its compilation, you must specify whether or not you will want to handle parts of the request loop other than the usual content generation phase. If so you need to specify which parts. See the ``Callback Hooks'' section for how to do this.

Apache specifies about eleven phases of the request loop, namely (and in order of processing): Post-Read-Request, URI Translation, Header Parsing, Access Control, Authentication, Authorization, MIME type checking, FixUp, Response (also known as the Content handling phase), Logging and finally Cleanup. These are the stages of a request where the Apache API allows a module to step in and do something. There is a dedicated Perl*Handler for each of these stages plus a couple of others which don't correspond to parts of the request loop.

We call them Perl*Handler directives because the names of the many mod_perl handler directives for the various phases of the request loop all follow the same format. The * in Perl*Handler is a placeholder to be replaced by something which identifies the phase to be handled. For example PerlLogHandler is a Perl Handler which (fairly obviously) handles the logging phase.

The slight exception is PerlHandler, which you can think of as PerlResponseHandler. It is the content generation handler and so it is probably the one that you will use most frequently.

Note that it is mod_perl which recognizes these directives, and not Apache. They are mod_perl directives, and an ordinary Apache does not recognize them. If you get error messages about these directives being "perhaps mis-spelled" it is a sure sign that the appropriate part of mod_perl (or the entire mod_perl module!) is not present in your copy of Apache executable.

The full list of Perl*Handlers follows. They are in the order that they are processed by Apache and mod_perl:

    PerlChildInitHandler
    PerlPostReadRequestHandler
    PerlInitHandler
    PerlTransHandler
    PerlHeaderParserHandler
    PerlAccessHandler
    PerlAuthenHandler
    PerlAuthzHandler
    PerlTypeHandler
    PerlFixupHandler
    PerlHandler
    PerlLogHandler
    PerlCleanupHandler
    PerlChildExitHandler
    PerlDispatchHandler
    PerlRestartHandler

PerlChildInitHandler and PerlChildExitHandler do not refer to parts of the request loop, they are to allow your modules to initialize data structures and to clean up at the child process start-up and shutdown respectively, for example by allocating and deallocating memory.

All <Location>, <Directory> and <Files> sections contain a physical path specification. Like PerlChildInitHandler and PerlChildExitHandler, the directives PerlPostReadRequestHandler and PerlTransHandler cannot be used in these sections, nor in .htaccess files, because it is not until the end of the Translation Handler (PerlTransHandler) phase that the path translation is completed and a physical path is known.

PerlInitHandler changes its behaviour depending upon where it is used. In any case it is the first handler to be invoked in serving a request. If found outside any <Location>, <Directory> or <Files> section (at the top level), it is an alias for PerlPostReadRequestHandler. When inside any such section it is an alias for PerlHeaderParserHandler.

Starting from PerlHeaderParserHandler the requested URI has been mapped to a physical server pathname, and thus it can be used to match a <Location>, <Directory> or <Files> configuration section, or to look in a .htaccess file if such a file exists in the specified directory in the translated path.

PerlDispatchHandler and PerlRestartHandler do not correspond to parts of the Apache API, but allow you to fine-tune the mod_perl API.

The Apache documentation will tell you all about these stages and what your modules can do. By default, most of these hooks are disabled at compile time, see the``Callback Hooks'' section for information on enabling them.

By default the mod_perl API expects a subroutine called handler() to handle the request in the registered Perl*Handler module. Thus if your module implements this subroutine, you can register the handler with mod_perl like this:

Replace Perl*Handler with the name of a specific handler from the list given above. mod_perl will preload the specified module for you. Please note that this approach will not preload the module at startup. To make sure it gets loaded you have three options: you can explicitly preload it with the PerlModule directive:

If you decide to give the handler routine a name other than handler, for example my_handler, you must preload the module and explicitly give the name of the handler subroutine:

If a module needs to know which handler is currently being run, it can find out with the current_callback method. This method is most useful to PerlDispatchHandlers which wish to take action for certain phases only.

With the mod_perl stacked handlers mechanism, during any stage of a request it is possible for more than one Perl*Handler to be defined and run.

Perl*Handler directives (in your configuration files) can define any number of subroutines. For example:

With the method Apache->push_handlers(), callbacks (handlers) can be added to a stack at runtime by mod_perl scripts.

Apache->push_handlers() takes the callback hook name as its first argument and a subroutine name or reference as its second.

  use Apache::Constants qw(:common);
  sub my_logger {
    #some code here
    return OK;
  }
  Apache->push_handlers("PerlLogHandler", \&my_logger);

  use Apache::Constants qw(:common);
  $r->push_handlers("PerlLogHandler", sub {
      print STDERR "__ANON__ called\n";
      return OK;
  });

All handlers will be called unless a handler returns a status other than OK or DECLINED.

CGI.pm maintains a global object for its plain function interface. Since the object is global, it does not go out of scope, DESTROY is never called. CGI->new can call:

This function will be called during the final stage of a request, refreshing CGI.pm's globals before the next request comes in.

Apache::DCELogin establishes a DCE login context which must exist for the lifetime of a request, so the DCE::Login object is stored in a global variable. Without stacked handlers, users must set

in the configuration files to destroy the context. This is not ``user-friendly''. Now, Apache::DCELogin::handler can call:

Persistent database connection modules such as Apache::DBI could push a PerlCleanupHandler handler that iterates over %Connected, refreshing connections or just checking that connections have not gone stale. Remember, by the time we get to PerlCleanupHandler, the client has what it wants and has gone away, so we can spend as much time as we want here without slowing down response time to the client (although the process itself is unavailable for serving new requests before the operation is completed).

PerlTransHandlers (e.g. Apache::MsqlProxy) may decide, based on the URI or some arbitrary condition, whether or not to handle a request. Without stacked handlers, users must configure it themselves:

PerlHandler is never actually invoked unless translate() sees that the request is a proxy request ($r->proxyreq). If it is a proxy request, translate() sets $r->handler("perl-script"), and only then will PerlHandler handle the request. Now users do not have to specify PerlHandler Apache::MsqlProxy, the translate() function can set it with push_handlers().

Imagine that you want to include footers, headers, etc., piecing together a document, without using SSI. The following example shows how to implement it. First we prepare the code as follows:

  Test/Compose.pm
  ---------------
  package Test::Compose;
  use Apache::Constants qw(:common);
  
  sub header {
     my $r = shift;
     $r->content_type("text/plain");
     $r->send_http_header;
     $r->print("header text\n");
     return OK;
  }
  sub body   { shift->print("body text\n")   ; return OK}
  sub footer { shift->print("footer text\n") ; return OK}
  1;
  __END__

  # in httpd.conf or perl.conf
  PerlModule Test::Compose
  <Location /foo>
     SetHandler "perl-script"
     PerlHandler Test::Compose::header Test::Compose::body Test::Compose::footer   
   </Location>

Parsing the output of another PerlHandler? This is a little more tricky, but consider:

  <Location /foo>
    SetHandler "perl-script"
    PerlHandler OutputParser SomeApp
  </Location>
  
  <Location /bar>
    SetHandler "perl-script"
    PerlHandler OutputParser AnotherApp
  </Location>

Now, OutputParser goes first, but it untie()'s *STDOUT and re-tie()'s it to its own package like so:

  package OutputParser;
 
  sub handler {
      my $r = shift;
      untie *STDOUT;
      tie *STDOUT => 'OutputParser', $r;
   }
   
  sub TIEHANDLE {
      my($class, $r) = @_;
      bless { r => $r}, $class;
  }
  
  sub PRINT {
      my $self = shift;   
      for (@_) {
          #do whatever you want to $_ for example:
          $self->{r}->print($_ . "[insert stuff]");
      }
  }
  
  1;
  __END__

If you want to test whether your running mod_perl Apache can stack handlers, the method Apache->can_stack_handlers will return TRUE if mod_perl was configured with PERL_STACKED_HANDLERS=1, and FALSE otherwise.

If a Perl*Handler is prototyped with $$, this handler will be invoked as a method. For example:

  package My;
  @ISA = qw(BaseClass);
   
  sub handler ($$) {
      my($class, $r) = @_;
      ...;
  }
   
  package BaseClass;
   
  sub method ($$) {
      my($class, $r) = @_;
      ...;
  }
  
  1;
  __END__

To reload PerlRequire, PerlModule and other use()'d modules, and to flush the Apache::Registry cache on server restart, add to httpd.conf:

Starting from mod_perl version 1.22 PerlFreshRestart is ignored when mod_perl is compiled as a DSO. But it almost doesn't matter, since mod_perl as a DSO will do a full tear-down (perl_destruct()). So it's still a FreshRestart, just fresher than static (non-DSO) mod_perl :)

PerlPassEnv passes, PerlSetEnv sets and passes ENVironment variables to your scripts. You can access them in your scripts through %ENV (e.g. $ENV{"key"}).

Regarding the setting of PerlPassEnv PERL5LIB in httpd.conf: if you turn on taint checks (PerlTaintCheck On), $ENV{PERL5LIB} will be ignored (unset). See the 'Switches -w, -T' section.

PerlSetVar is very similar to PerlSetEnv, but you extract it with another method.

All values are treated as strings, so you will get a stringified reference to a hash as a value (something which will look like ``HASH(0x87a5108)''). This cannot be turned back into a reference and therefore into the original hash upon retrieval.

However you can use the PerlAddVar directive to push more values into the variable, emulating arrays. For example:

Make sure that you use an even number of elements if you store the retrieved values in a hash.

While the Apache's SetEnv and mod_perl's PerlSetEnv apparently perform the same thing, the former doesn't happen until the fixup phase, the latter happens as soon as possible, so those variables are available before then, e.g. in PerlAuthenHandler for $ENV{ORACLE_HOME} (or another environment variable that you need in these early request processing stages).

For PerlWarn and PerlTaintCheck directives see the 'Switches -w, -T' section.

MinSpareServers, MaxSpareServers, StartServers and MaxClients are standard Apache configuration directives that control the number of servers that will be launched at server startup and kept alive during the server's operation.

MaxRequestsPerChild lets you specify the maximum number of requests which each child will be allowed to serve. When a process has served MaxRequestsPerChild requests the parent kills it and replaces it with a new one. There may also be other reasons why a child is killed, so it does not mean that each child will in fact serve this many requests, only that it will not be allowed to serve more than that number.

At server startup, before child processes are spawned to receive incoming requests, there is more that can be done than just preloading files. You might want to register code that will initialize a database connection for each child when it is forked, tie read-only dbm files, etc.

The startup.pl file is an ideal place to put the code that should be executed when the server starts. Once you have prepared the code, load it in httpd.conf before the rest of the mod_perl configuration directives like this:

I must stress that all the code that is run at server initialization time is run with root priveleges if you are executing it as the root user (which you have to do unless you choose to run the server on an unprivileged port, above 1024). This means that anyone who has write access to a script or module that is loaded by PerlModule or PerlRequire effectively has root access to the system. You might want to take a look at the new and experimental PerlOpmask directive and PERL_OPMASK_DEFAULT compile time option to try to disable some of the more dangerous operations.

Since the startup file is a file written in plain Perl, one can validate its syntax with:

  startup.pl
  ----------
  use strict;
  
  # Extend @INC if needed
  use lib qw(/dir/foo /dir/bar);
  
  # Make sure we are in a sane environment.
  $ENV{MOD_PERL} or die "not running under mod_perl!";
   
  # For things in the "/perl" URL
  use Apache::Registry;          
   
  # Load Perl modules of your choice here
  # This code is interpreted *once* when the server starts
  use LWP::UserAgent ();
  use Apache::DBI ();
  use DBI ();
  
  # Tell me more about warnings
  use Carp ();
  $SIG{__WARN__} = \&Carp::cluck;
  
  # Load CGI.pm and call its compile() method to precompile 
  # (but not to import) its autoloaded methods. 
  use CGI ();
  CGI->compile(':all');
  
  # Initialize the database connections for each child
  Apache::DBI->connect_on_init
  ("DBI:mysql:database=test;host=localhost",
   "user","password",
   {
    PrintError => 1, # warn() on errors
    RaiseError => 0, # don't die on error
    AutoCommit => 1, # commit executes immediately
   }
  );

This pragma is worth using in every script longer than half a dozen lines. It will save a lot of time and debugging later on.

The only chance to permanently modify @INC before the server is started is with this command. Later the running code can modify @INC just for the moment it require()'s some file, and then @INC's value gets reset to what it was originally.

A sanity check, if Apache/mod_perl wasn't properly built, the above code will abort the server startup.

Preload the modules that get used by our Perl code serving the requests. Unless you need the symbols (variables and subroutines) exported by the modules you preload to accomplish something within the startup file, don't import them, since it's just a waste of startup time. Instead use the empty list () to tell the import() function not to import anything.

This is a useful snippet to enable extended warnings logged in the error_log file. In addition to basic warnings, a trace of calls is added. This makes the tracking of the potential problem a much easier task, since you know who called whom. For example, with normal warnings you might see:

  Use of uninitialized value at
      /usr/lib/perl5/site_perl/5.005/Apache/DBI.pm  line 110.

but you have no idea where it was called from. When we use Carp as shown above we might see:

  Use of uninitialized value at
            /usr/lib/perl5/site_perl/5.005/Apache/DBI.pm line 110.
      Apache::DBI::connect(undef, 'mydb::localhost', 'user',
         'passwd', 'HASH(0x87a5108)') called at
            /usr/lib/perl5/site_perl/5.005/i386-linux/DBI.pm line 382
      DBI::connect('DBI', 'DBI:mysql:mydb::localhost', 'user',
         'passwd', 'HASH(0x8375e4c)') called at
            /usr/lib/perl5/site_perl/5.005/Apache/DBI.pm line 36
      Apache::DBI::__ANON__('Apache=SCALAR(0x87a50c0)') called at 
            PerlChildInitHandler subroutine 
            `Apache::DBI::__ANON__' line 0
      eval {...} called at PerlChildInitHandler subroutine 
            `Apache::DBI::__ANON__' line 0

we clearly see that the warning was triggered by eval()'uating the Apache::DBI::__ANON__ which called DBI::connect (with the arguments that we see as well), which in turn called the Apache::DBI::connect method. Now we know where to look for our problem.

Some modules create their subroutines at run time to improve their load time. This helps when the module includes many subroutines, but only a few are actually used. CGI.pm falls into this category. Since with mod_perl the module is loaded only once, it might be a good idea to precompile all or a part of its methods.

CGI.pm's compile() method performs this task. Notice that this is a proprietary function of this module, other modules can implement this feature or not and use this or some other name for this functionality. As with all modules we preload in the startup file, we don't import symbols from them as they will be lost when they go out of the file's scope.

Note that starting with $CGI::VERSION 2.46, the recommended method to precompile the code in CGI.pm is:

Every module loaded at server startup will be shared among the server children, saving a lot of RAM on your machine. Usually I put most of the code I develop into modules and preload them.

You can even preload your CGI script with Apache::RegistryLoader (See Preload Perl modules at server startup) and you can get the children to preopen their database connections with Apache::DBI.

Some people wonder why you need to duplicate the use() clause in the startup file and in the script itself. The confusion arises due to misunderstanding the use() function. use() normally performs two operations, namely require() and import(), called within a BEGIN block. See the section ``use()'' for a detailed explanation of the use(), require() and import() functions.

In the startup file we don't want to import any symbols since they will be lost when we leave the scope of the startup file anyway, i.e. they won't be visible to any of the child processes which run our mod_perl scripts. Instead we want to preload the module in the startup file and then import any symbols that we actually need in each script individually.

Normally when we write use MyModule;, use() will both load the module and import its symbols; so for the startup file we write

use
MyModule ();

and the empty parentheses will ensure that the module is loaded but that no symbols are imported. Then in the actual mod_perl script we write use() in the standard way, e.g. use MyModule;. Since the module has already been preloaded, the only action taken is to import the symbols. For example in the startup file you write:

since you probably don't need any symbols to be imported there. But in your code you would probably write:

For example, if you have use()'d Apache::Constants in the startup file, it does not mean you can have the following handler:

If you want to use the function interface without exporting the symbols, use fully qualified function names, e.g. CGI::param. The same rule applies to variables, you can import variables and you can access them by their full name. e g. $My::Module::bar. When you use the object oriented (method) interface you don't need to export the method symbols.

Technically, you aren't required to supply the use() statement in your (handler?) code if it was already loaded during server startup (i.e. by 'PerlRequire startup.pl'). When writing your code, however, you should not assume the module code has been preloaded. In the future, you or someone else will revist this code and will not understand how it is possible to use a module's methods without first loading the module itself.

Read the Exporter and perlmod manpages for more information about import().

With <Perl>...</Perl> sections, it is possible to configure your server entirely in Perl.

<Perl> sections can contain any and as much Perl code as you wish. These sections are compiled into a special package whose symbol table mod_perl can then walk and grind the names and values of Perl variables/structures through the Apache core configuration gears. Most of the configuration directives can be represented as scalars ($scalar) or lists (@list). A @list inside these sections is simply converted into a space delimited string for you. Here is an example:

   httpd.conf
  ------------
  <Perl>
  @PerlModule = qw(Mail::Send Devel::Peek);
  
  #run the server as whoever starts it
  $User  = getpwuid($>) || $>;
  $Group = getgrgid($)) || $); 
  
  $ServerAdmin = $User;
  
  </Perl>

Block sections such as <Location>..</Location> are represented in a %Location hash, e.g.:

  <Perl>
  
  $Location{"/~dougm/"} = {
    AuthUserFile => '/tmp/htpasswd',
    AuthType => 'Basic',
    AuthName => 'test',
    DirectoryIndex => [qw(index.html index.htm)],
    Limit => {
      METHODS => 'GET POST',
      require => 'user dougm',
    },
  };
  
  </Perl>

If an Apache directive can take two or three arguments you may push strings (the lowest number of arguments will be shifted off the @list) or use an array reference to handle any number greater than the minimum for that directive:

  push @Redirect, "/foo", "http://www.foo.com/";;
  
  push @Redirect, "/imdb", "http://www.imdb.com/";;
  
  push @Redirect, [qw(temp "/here" "http://www.there.com";)];

To pass all environment variables to the children with a single configuration directive, rather than listing each one via PassEnv or PerlPassEnv, a <Perl> section could read in a file and:

These are somewhat simple examples, but they should give you the basic idea. You can mix in any Perl code you desire. See eg/httpd.conf.pl and eg/perl_sections.txt in the mod_perl distribution for more examples.

Assume that you have a cluster of machines with similar configurations and only small distinctions between them: ideally you would want to maintain a single configuration file, but because the configurations aren't exactly the same (e.g. the ServerName directive) it's not quite that simple.

<Perl> sections come to rescue. Now you have a single configuration file and the full power of Perl to tweak the local configuration. For example to solve the problem of the ServerName directive you might have this <Perl> section:

For example if you want to allow personal directories on all machines except the ones whose names start with secure:

  <Perl>
  $ServerName = `hostname`;
  if ( $ServerName !~ /^secure/) {
    $UserDir = "public.html";
  } else {
    $UserDir = "DISABLED";
  }
  </Perl>

Behind the scenes, mod_perl defines a package called Apache::ReadConfig. Here it keeps all the variables that you define inside the <Perl> sections. Therefore it's not necessarily to configure the server within the <Perl> sections. Actually what you can do is to write the Perl code to configure the server just like you'd do in the <Perl> sections, but instead place it into a separate file that should be called during the configuration parsing with either PerlModule or PerlRequire directives, or from within the startup file. All you have to do is to declare the package Apache::ReadConfig within this file. Using the last example:

  apache_config.pl
  ---------------- 
  package Apache::ReadConfig;
  
  $ServerName = `hostname`;
  if ( $ServerName !~ /^secure/) {
    $UserDir = "public.html";
  } else {
    $UserDir = "DISABLED";
  }
  
  1;

To enable <Perl> sections you should build mod_perl with perl Makefile.PL PERL_SECTIONS=1 [ ... ].

Be careful when you declare package names inside <Perl> sections, for example this code has a problem:

  <Perl>  
    package My::Trans;
    use Apache::Constants qw(:common);
    sub handler{ OK }
    
    $PerlTransHandler = "My::Trans";
  </Perl>

When you put code inside a <Perl> section, by default it actually goes into the Apache::ReadConfig package, which is already declared for you. This means that the PerlTransHandler we have tried to define above is actually undefined. If you define a different package name within a <Perl> section you must make sure to close the scope of that package and return to the Apache::ReadConfig package when you want to define the configuration directives, like this:

  <Perl>  
    package My::Trans;
    use Apache::Constants qw(:common);
    sub handler{ OK }
    
    package Apache::ReadConfig;  
    $PerlTransHandler = "My::Trans";
  </Perl>

This section shows how to check and dump the configuration you have made with the help of <Perl> sections in httpd.conf.

To check the <Perl> section syntax outside of httpd, we make it look like a Perl script:

In a running httpd you can see how you have configured the <Perl> sections through the URI /perl-status, by choosing Perl Section Configuration from the menu. In order to make this item show up in the menu you should set $Apache::Server::SaveConfig to a true value. When you do that the Apache::ReadConfig namespace (in which the configuration data is stored) will not be flushed, making configuration data available to Perl modules at request time.

At request time, the value of $DocumentRoot can be accessed with the fully qualified name $Apache::ReadConfig::DocumentRoot.

  <Perl>
  use Apache::PerlSections();
  ...
  # Configuration Perl code here
  ...
  print STDERR Apache::PerlSections->dump();
  </Perl>

If the Perl code doesn't compile, the server won't start. If the generated Apache config is invalid, <Perl> sections have always just logged an error and carried on, since there might be globals in the section that are not intended for the config.

The variable $Apache::Server::StrictPerlSections has been added in mod_perl version 1.22. If you set this variable to a true value, for example

then mod_perl will not tolerate invalid Apache configuration syntax and will croak (die) if this is the case. At the time of writing the default value is 0.

If you compile mod_perl with PERL_TRACE=1 and set the environment variable MOD_PERL_TRACE then you should see some useful diagnostics when mod_perl is processing <Perl> sections.

apachectl configtest tests the configuration file without starting the server. You can safely validate the configuration file on your production server, if you run this test before you restart the server with apachectl restart. Of course it is not 100% perfect, but it will reveal any syntax errors you might have made while editing the file.

'apachectl configtest' is the same as 'httpd -t' and it doesn't just parse the code in startup.pl, it actually executes it. <Perl> configuration has always started Perl during the configuration read, and Perl{Require,Module} do so as well.

Of course we assume that the code that gets called during this test cannot cause any harm to your running production environment. The following hint shows how to prevent the code in the startup script and <Perl> from being executed during the syntax check, if that's what you want.

If you want your startup code to get control over the -t (configtest) server launch, start the server configuration test with:

and, if for example you want to prevent your startup code from being executed, at the top of the code add:

The nifty mod_info module displays the complete server configuration in your browser. In order to use it you have compile it in or, if the server was compiled with DSO mode enabled, load it as an object. Then just uncomment the ready-prepared section in the httpd.conf file:

  <Location /server-info>
    SetHandler server-info
    Order deny,allow
    Deny from all
    Allow from www.example.com
  </Location>

If you are using a two-server setup, with a mod_perl server listening on a high port, it is advised that you do not publish the number of the high port number in URLs. Rather use a proxying rewrite rule in the non-mod_perl server:

  RewriteEngine      On
  RewriteLogLevel    0
  RewriteRule       ^/perl/(.*) http://localhost:8080/perl/$1 [P]
  ProxyPassReverse   /          http://localhost/

I was told one problem with publishing high port numbers is that IE 4.x has a bug when re-posting data to a non-port-80 URL. It drops the port designator, and uses port 80 anyway.

Another reason is that firewalls probably will have the high port closed, therefore users behind the firewalls will be unable to reach your service, running on the blocked port.

mod_macro is an Apache module written by Fabien Coelho that lets you define and use macros in the Apache configuration file.

mod_macro can be really useful when you have many virtual hosts, and where each virtual host has a number of scripts/modules, most of them with a moderately complex configuration setup.

  # set up a registry script
  <Macro registry>
  SetHandler "perl-script"
  PerlHandler Apache::Registry
  Options +ExecCGI
  </Macro>

  # example
  Alias /stuff /usr/www/scripts/stuff
  <Location /stuff>
  Use registry
  </Location>

If your registry scripts are all located in the same directory, and your aliasing rules consistent, you can use this macro:

  # set up a registry script for a specific location
  <Macro registry $location $script>
  Alias /$location /home/httpd/perl/scripts/$script
  <Location /$location>
  SetHandler "perl-script"
  PerlHandler Apache::Registry
  Options +ExecCGI
  </Location>
  </Macro>

If you're using content handlers packaged as modules, you can use the following macro:

  # set up a mod_perl content handler module
  <Macro modperl $module>
  SetHandler "perl-script"
  Options +ExecCGI
  PerlHandler $module
  </Macro>
  
  #examples
  <Location /perl-status>
  PerlSetVar StatusPeek On
  PerlSetVar StatusGraph On
  PerlSetVar StatusDumper On
  Use modperl Apache::Status
  </Location>

The following macro sets up a Location for use with HTML::Embperl. Here we define all ``.html'' files to be processed by Embperl.

  <Macro embperl>
  SetHandler "perl-script"
  Options +ExecCGI
  PerlHandler HTML::Embperl
  PerlSetEnv EMBPERL_FILESMATCH \.html$
  </Macro>
  
  # examples
  <Location /mrtg>
  Use embperl
  </Location>

Macros are also very useful for things that tend to be verbose, such as setting up Basic Authentication:

  # Sets up Basic Authentication
  <Macro BasicAuth $realm $group>
  Order deny,allow
  Satisfy any
  AuthType Basic
  AuthName $realm
  AuthGroupFile /usr/www/auth/groups
  AuthUserFile /usr/www/auth/users
  Require group $group
  Deny from all
  </Macro>
  
  # example of use
  <Location /stats>
  Use BasicAuth WebStats Admin
  </Location>

Finally, here is a complete example that uses macros to set up simple virtual hosts. It uses the BasicAuth macro defined previously (yes, macros can be nested!).

  <Macro vhost $ip $domain $docroot $admingroup>
  <VirtualHost $ip>
  ServerAdmin webmaster@$domain
  DocumentRoot /usr/www/htdocs/$docroot
  ServerName www.$domain
  <Location /stats>
  Use BasicAuth Stats-$domain $admingroup
  </Location>
  </VirtualHost>
  </Macro>
  
  # define some virtual hosts
  Use vhost 10.1.1.1 example.com example example-admin
  Use vhost 10.1.1.2 example.net examplenet examplenet-admin

mod_macro is also useful in a non vhost setting. Some sites for example have lots of scripts which people use to view various statistics, email settings and etc. It is much easier to read things like:

The actual macros for the last example are left as an exercise to reader. These can be easily constructed based on the examples presented in this section.

Check your configuration files and make sure that the ExecCGI is turned on in your configurations.

  <Location /perl>
    SetHandler perl-script
    PerlHandler Apache::Registry
    Options ExecCGI
    allow from all
    PerlSendHeader On
  </Location>

Did you put PerlSendHeader On in the configuration part of the <Location foo></Location>.

No. Any virtual host will be able to see the routines from a startup.pl loaded for any other virtual host.

You can use PerlSetEnv PERL5LIB ... or a PerlFixupHandler with the lib pragma (use lib qw(...)).

This has been a bug before, last fixed in 1.15_01, i.e. if you are running 1.15, that could be the problem. You should set this variable in a startup file (which you load with PerlRequire in httpd.conf):

But, as we know sometimes a bug turns out to be a feature. If the same script is running for more than one Virtual host on the same machine, this can be a waste, right? Set it to 0 in a startup script if you want to turn it off and have this bug as a feature. (Only makes sense if you are sure that there will be no other scripts with the same path/name). It also saves you some memory as well.

The problem was reported by users who declared mod_perl configuration inside a <Directory> section for all files matching *.pl. The problem went away after placing the directives in a <Files> section.

The mod_alias and mod_rewrite are both Trans handlers in the normal case. So in the setup where both are used, if mod_alias runs first and matches it will return OK and mod_rewrite won't see the request.

The opposite can happen as well, where mod_rewrite rules apply but the Alias directives are completely ignored.

The behavior is not random, but depends on the Apache modules loading order. Apache modules are being executed in reverse order, i.e. module that was Added first will be executed last.

The solution is not to mix mod_rewrite and mod_alias. mod_rewrite does everything mod_alias does--except for ScriptAlias which is not really relevant to mod_perl anyway. Don't rely on the module ordering, but use explicitely disjoint URL namespaces for Alias and Rewrite. In other words any URL regexp that can potentially match a Rewrite rule should not be used in an Alias, and vice versa. Given that mod_rewrite can easily do what mod_alias does, it's no problem.

Here is one of the examples where Alias is replaced with RedirectMatch. This is a snippet of configuration at the light non-mod_perl Apache server:

  RewriteEngine     on
  RewriteLogLevel   0
  RewriteRule       ^/(perl.*)$    http://127.0.0.1:8045/$1  [P,L]
  RewriteRule       ^/(mail.*)$    http://127.0.0.1:8045/$1  [P,L]
  NoCache           *
  ProxyPassReverse  / http://www.example.com/
  
  RedirectMatch permanent ^/$      /pages/index
  RedirectMatch permanent ^/foo$   /pages/bar

This configuration works fine because any URI that matches one of the redirects will never match one of the rewrite rules.

In the above setup we proxy requests starting with /perl or /mail to the mod_perl server, forbid proxy requests to the external sites, and make sure that the proxied requests will use the http://www.example.com/ as their URL on the way back to the client.

  RewriteEngine     on
  RewriteLogLevel   0
  RewriteMap        lowercase int:tolower
  RewriteRule       ^/(perl.*)$  http://127.0.0.1:8042/$1   [P,L]
  RewriteRule       ^/$           /pages/welcome.htm        [R=301,L]
  RewriteRule       ^(.*)$        ${lowercase:$1}
  NoCache           *
  ProxyPassReverse  /  http://www.example.com/

If we omit the rewrite rule that matches ^/$, and instead use a redirect, it will never be called, because the URL is still matched by the last rule ^(.*)$. This is a somewhat contrived example because that last regex could be rewritten as ^(/.+)$ and all would be well.

It is better not to advertise the port that mod_perl server uses to the outside world, for it creates a potential security risk by revealing which module(s) and/or OS you are running your web server on.

The more chances for bugs, the more chance that some of those bugs may involve security.

We never were completely sure why the default of the ServerToken directive in Apache is Full rather than Minimal. Seems like you would only make it Full if you are debugging. Probably the reason for using the ServerToken Full is for a show-off, so NetCraft (http://netcraft.com) and other similar survey services will count more Apache servers, which is good for all of us, but you really want to reveal as little information as possible to the potential crackers.

Another approach is to modify httpd sources to reveal no unwanted information, so all responses will return an empty or phony Server: field.

From the other point of view, security by obscurity is a lack of security. Any determined cracker will eventually figure out what version of Apache run and what third party modules you have built in.

You can see what information is revealed by your server, by telneting to it and issuing some request. For example:

  % telnet localhost 8080
  Trying 127.0.0.1
  Connected to localhost
  Escape character is '^]'.
  HEAD / HTTP1.0
  
  HTTP/1.1 200 OK
  Date: Sun, 16 Apr 2000 11:06:25 GMT
  Server: Apache/1.3.12 (Unix) mod_perl/1.22 mod_ssl/2.6.2 OpenSSL/0.9.5
  [more lines snipped]

So as you see that a lot of information is revealed and a Full ServerToken has been used.

When the server is restarted, the configuration and module initialization phases are called twice in total before the children are forked. The second restart is done in order to ensure that future restarts will work correctly, by making sure that all modules can survive a restart (SIGHUP). This is very important if you restart a production server.

You can control what code will be executed on the start or restart by checking the value of $Apache::Server::Starting and $Apache::Server::ReStarting respectively. The former variable is true when the server is starting and the latter is true when it's restarting.

  <Perl>
  print STDERR "Server is Starting\n"   if $Apache::Server::Starting;
  print STDERR "Server is ReStarting\n" if $Apache::Server::ReStarting;
  </Perl>

The startup.pl file and similar loaded via PerlModule or PerlRequire are compiled only once. Because once the module is compiled it enters the special %INC hash. When Apache restarts--Perl checks whether the module or script in question is already registered in %INC and won't try to compile it again.

So the only code that you might need to protect from running on restart is the one in the <Perl> sections. But since one usually uses the <Perl> sections mainly for on the fly configuration creation, there shouldn't be a reason why it'd be undesirable to run the code more than once.

Let's say that you have a frontend server running mod_ssl, mod_rewrite and mod_proxy. You want to make sure that your user is using a secure connection for some specific actions like login information submission. You don't want to let the user login unless the request was submitted through a secure port.

Since you have to proxy_pass the request between front and backend servers, you cannot know where the connection has come from. Neither is using the HTTP headers reliable.

A possible solution for this problem is to have the the mod_perl server listen on two different ports (e.g. 8000 and 8001) and have the mod_rewrite proxy rule in the regular server redirect to port 8000 and the mod_rewrite proxy rule in the SSL virtual host redirect to port 8001. In the mod_perl server just check the PORT variable to tell if the connection is secure.

This is covered in the Eagle Book in a great detail. This is just a simple example, showing how to add your own Configuration directives.

  Makefile.PL
  -----------
  package Apache::TestDirective;
  
  use ExtUtils::MakeMaker;
  
  use Apache::ExtUtils qw(command_table);
  use Apache::src ();
  
  my @directives = (
                  {   name        =>  'Directive4',
                      errmsg      =>  'Anything',
                      args_how    =>  'RAW_ARGS',
                      req_override=>  'OR_ALL',
                  },
                 );
    
  command_table(\@directives);
  
  WriteMakefile(
    'NAME'      => 'Apache::TestDirective',
    'VERSION_FROM' => 'TestDirective.pm',
    'INC'       => Apache::src->new->inc,
  );

  TestDirective.pm
  ----------------
  package Apache::TestDirective;
  
  use strict;
  use Apache::ModuleConfig ();
  use DynaLoader ();

  if($ENV{MOD_PERL}) {
    no strict;
    $VERSION = '0.01';
    @ISA = qw(DynaLoader);
     __PACKAGE__->bootstrap($VERSION); #command table, etc.
  }
  
  sub Directive4 {
    warn "Directive4 @_\n";
  }
  
  1;
  __END__

  Directive4 Apache::TestDirective=HASH(0x83379d0)
  Apache::CmdParms=SCALAR(0x862b80c) hi

  % grep Directive4 t/logs/error_log 
  Directive4 Apache::TestDirective=HASH(0x83119dc)
  Apache::CmdParms=SCALAR(0x8326878) hi

If it didn't work as expected try building mod_perl with PERL_TRACE=1, then do:

before starting the server. Now you should get some useful diagnostics. [ TOC ]