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       perlfaq9 -  (2003/01/31 17:36:57 )



       What is the correct form of response from a CGI script?

       (Alan Flavell <> answers...)

       The Common Gateway Interface (CGI) specifies a software interface
       between a program ("CGI script") and a web server (HTTPD). It is not
       specific to Perl, and has its own FAQs and tutorials, and usenet group,

       The original CGI specification is at:

       Current best-practice RFC draft at: http://CGI-Spec.Golux.Com/

       Other relevant documentation listed in:

       These Perl FAQs very selectively cover some CGI issues. However, Perl
       programmers are strongly advised to use the module, to take care
       of the details for them.

       The similarity between CGI response headers (defined in the CGI
       specification) and HTTP response headers (defined in the HTTP
       specification, RFC2616) is intentional, but can sometimes be confusing.

       The CGI specification defines two kinds of script: the "Parsed Header"
       script, and the "Non Parsed Header" (NPH) script. Check your server
       documentation to see what it supports. "Parsed Header" scripts are
       simpler in various respects. The CGI specification allows any of the
       usual newline representations in the CGI response (it's the server's
       job to create an accurate HTTP response based on it). So "\n" written
       in text mode is technically correct, and recommended. NPH scripts are
       more tricky: they must put out a complete and accurate set of HTTP
       transaction response headers; the HTTP specification calls for records
       to be terminated with carriage-return and line-feed, i.e ASCII \015\012
       written in binary mode.

       Using gives excellent platform independence, including EBCDIC
       systems. selects an appropriate newline representation
       ($CGI::CRLF) and sets binmode as appropriate.

        CGI  (500 Server Error)

        "Troubleshooting Perl CGI scripts" guide,


        FAQ  post comp.infosystems.www.authoring.cgi HTTP  HTML  CGI Perl CGI
       post comp.lang.perl.misc

        FAQ CGI Meta FAQ



       Use the CGI::Carp module.  It replaces "warn" and "die", plus the
       normal Carp modules "carp", "croak", and "confess" functions with more
       verbose and safer versions.  It still sends them to the normal server
       error log.

           use CGI::Carp;
           warn "This is a complaint";
           die "But this one is serious";

       The following use of CGI::Carp also redirects errors to a file of your
       choice, placed in a BEGIN block to catch compile-time warnings as well:

           BEGIN {
               use CGI::Carp qw(carpout);
               open(LOG, ">>/var/local/cgi-logs/mycgi-log")
                   or die "Unable to append to mycgi-log: $!\n";

       You can even arrange for fatal errors to go back to the client browser,
       which is nice for your own debugging, but might confuse the end user.

           use CGI::Carp qw(fatalsToBrowser);
           die "Bad error here";

       Even if the error happens before you get the HTTP header out, the
       module will try to take care of this to avoid the dreaded server 500
       errors.  Normal warnings still go out to the server error log (or
       wherever you've sent them with "carpout") with the application name and
       date stamp prepended.


        HTML::Parse CPAN Web libwww-perl  HTML::FormatText HTML

        "s/<.*?>//g" quote HTML comment <  entities "&lt;"

           #!/usr/bin/perl -p0777

        striphtml .

       Here are some tricky cases that you should think about when picking a

           <IMG SRC = "foo.gif" ALT = "A > B">

           <IMG SRC = "foo.gif"
                ALT = "A > B">

           <!-- <A comment> -->

           <script>if (a<b && a>c)</script>

           <# Just data #>

           <![INCLUDE CDATA [ >>>>>>>>>>>> ]]>

       If HTML comments include other tags, those solutions would also break
       on text like this:

           <!-- This section commented out.
               <B>You can't see me!</B>


        HTML  URL "HTML::SimpleLinkExtor"  URL  "HTML::LinkExtor"
       "HTML::Parser".  "HTML::SimpleLinkExtor"

       You can use URI::Find to extract URLs from an arbitrary text document.

       Less complete solutions involving regular expressions can save you a
       lot of processing time if you know that the input is simple.  One
       solution from Tom Christiansen runs 100 times faster than most module
       based approaches but only extracts URLs from anchors where the first
       attribute is HREF and there are no other attributes.

               #!/usr/bin/perl -n00
               # qxurl -
               print "$2\n" while m{
                   < \s*
                     A \s+ HREF \s* = \s* (["']) (.*?) \1
                   \s* >

       In this case, download means to use the file upload feature of HTML
       forms.  You allow the web surfer to specify a file to send to your web
       server.  To you it looks like a download, and to the user it looks like
       an upload.  No matter what you call it, you do it with what's known as
       multipart/form-data encoding.  The module (which comes with Perl
       as part of the Standard Library) supports this in the
       start_multipart_form() method, which isn't the same as the startform()

       See the section in the documentation on file uploads for code
       examples and details.

        HTML ?

        <SELECT>  <OPTION> CPAN widget

        HTML ?

        lynx HTML

           $html_code = `lynx -source $url`;
           $text_data = `lynx -dump $url`;

        CPAN libwww-perl (LWP) proxies lynx

           # simplest version
           use LWP::Simple;
           $content = get($URL);

           # or print HTML from a URL
           use LWP::Simple;
           getprint "";

           # or print ASCII from HTML from a URL
           # also need HTML-Tree package from CPAN
           use LWP::Simple;
           use HTML::Parser;
           use HTML::FormatText;
           my ($html, $ascii);
           $html = get("");
           defined $html
               or die "Can't fetch HTML from";
           $ascii = HTML::FormatText->new->format(parse_html($html));
           print $ascii;

        HTML ?

       If you're submitting values using the GET method, create a URL and
       encode the form using the "query_form" method:

           use LWP::Simple;
           use URI::URL;

           my $url = url('');
           $url->query_form(module => 'DB_File', readme => 1);
           $content = get($url);

       If you're using the POST method, create your own user agent and encode
       the content appropriately.

           use HTTP::Request::Common qw(POST);
           use LWP::UserAgent;

           $ua = LWP::UserAgent->new();
           my $req = POST '',
                          [ module => 'DB_File', readme => 1 ];
           $content = $ua->request($req)->as_string;

        web  %-encoding?

       If you are writing a CGI script, you should be using the module
       that comes with perl, or some other equivalent module.  The CGI module
       automatically decodes queries for you, and provides an escape()
       function to handle encoding.

       The best source of detailed information on URI encoding is RFC 2396.
       Basically, the following substitutions do it:

           s/([^\w()'*~!.-])/sprintf '%%%02x', ord $1/eg;   # encode

           s/%([A-Fa-f\d]{2})/chr hex $1/eg;            # decode

       However, you should only apply them to individual URI components, not
       the entire URI, otherwise you'll lose information and generally mess
       things up.  If that didn't explain it, don't worry.  Just go read
       section 2 of the RFC, it's probably the best explanation there is.

       RFC 2396 also contains a lot of other useful information, including a
       regexp for breaking any arbitrary URI into components (Appendix B).

       Specify the complete URL of the destination (even if it is on the same
       server). This is one of the two different kinds of CGI "Location:"
       responses which are defined in the CGI specification for a Parsed
       Headers script. The other kind (an absolute URLpath) is resolved
       internally to the server without any HTTP redirection. The CGI
       specifications do not allow relative URLs in either case.

       Use of is strongly recommended.  This example shows redirection
       with a complete URL. This redirection is handled by the web browser.

             use CGI qw/:standard/;

             my $url = '';
             print redirect($url);

       This example shows a redirection with an absolute URLpath.  This
       redirection is handled by the local web server.

             my $url = '/CPAN/index.html';
             print redirect($url);

       But if coded directly, it could be as follows (the final "\n" is shown
       separately, for clarity), using either a complete URL or an absolute

             print "Location: $url\n";   # CGI response header
             print "\n";                 # end of headers

        web  web ---apache  iPlanet  IIS  web

        Perl  .htpasswd  .htgroup ?

       HTTPD::UserAdmin  HTTPD::GroupAdmin  dbmBerkeley DB DBI (drivers)
       HTTPD::UserAdmin`Basic'  `Digest'

           use HTTPD::UserAdmin ();
                 ->new(DB => "/foo/.htpasswd")
                 ->add($username => $password);


        CGI Meta FAQ


        perlfunc  "split"

           $/ = '';
           $header = <MSG>;
           $header =~ s/\n\s+/ /g;      #
           %head = ( UNIX_FROM_LINE, split /^([-\w]+):\s*/m, $header );

        Received Received  CPAN Mail::Header  MailTools


       don't deal with GET/POST combinations where query fields are in more
       than one place.  They don't deal with keywords in the query string.

       In short, they're bad hacks.  Resist them at all costs.  Please do not
       be tempted to reinvent the wheel.  Instead, use the or (available from CPAN), or if you're trapped in the module-
       free land of perl1 .. perl4, you might look into (available
       from ).

       Make sure you know whether to use a GET or a POST in your form.  GETs
       should only be used for something that doesn't update the server.
       Otherwise you can get mangled databases and repeated feedback mail
       messages.  The fancy word for this is ``idempotency''.  This simply
       means that there should be no difference between making a GET request
       for a particular URL once or multiple times.  This is because the HTTP
       protocol definition says that a GET request may be cached by the
       browser, or server, or an intervening proxy.  POST requests cannot be
       cached, because each request is independent and matters.  Typically,
       POST requests change or depend on state on the server (query or update
       a database, send mail, or purchase a computer).

        email   RFC-822

       You can use the Email::Valid or RFC::RFC822::Address which check the
       format of the address, although they cannot actually tell you if it is
       a deliverable address (i.e. that mail to the address will not bounce).
       Modules like Mail::CheckUser and Mail::EXPN try to interact with the
       domain name system or particular mail servers to learn even more, but
       their methods do not work everywhere---especially for security
       conscious administrators.

        "/^[\w.-]+\@(?:[\w-]+\.)+\w+$/"  email ,
       script RFC comments, Bill Clinton postmaster DNS script

       Our best advice for verifying a person's mail address is to have them
       enter their address twice, just as you normally do to change a
       password.  This usually weeds out typos.  If both versions match, send
       mail to that address with a personal message that looks somewhat like:


           Please confirm the mail address you gave us Wed May  6 09:38:41
           MDT 1998 by replying to this message.  Include the string
           "Rumpelstiltskin" in that reply, but spelled in reverse; that is,
           start with "Nik...".  Once this is done, your confirmed address will
           be entered into our records.

       If you get the message back and they've followed your directions, you
       can be reasonably assured that it's real.

       A related strategy that's less open to forgery is to give them a PIN
       (personal ID number).  Record the address and PIN (best that it be a
       random one) for later processing.  In the mail you send, ask them to
       include the PIN in their reply.  But if it bounces, or the message is
       included via a ``vacation'' script, it'll be there anyway.  So it's
       best to ask them to mail back a slight alteration of the PIN, such as
       with the characters reversed, one added or subtracted to each digit,

        MIME/BASE64 ?

       MIME-tools CPAN BASE64

           use MIME::Base64;
           $decoded = decode_base64($encoded);

       The MIME-Tools package (available from CPAN) supports extraction with
       decoding of BASE64 encoded attachments and content directly from email

        unpack() ``u''

           tr#A-Za-z0-9+/##cd;                   # remove non-base64 chars
           tr#A-Za-z0-9+/# -_#;                  # convert to uuencoded format
           $len = pack("c", 32 + 0.75*length);   # compute length byte
           print unpack("u", $len . $_);         # uudecode and print

       On systems that support getpwuid, the $< variable, and the
       Sys::Hostname module (which is part of the standard perl distribution),
       you can probably try using something like this:

           use Sys::Hostname;
           $address = sprintf('%s@%s', scalar getpwuid($<), hostname);

       Company policies on mail address can mean that this generates addresses
       that the company's mail system will not accept, so you should ask for
       users' mail addresses when this matters.  Furthermore, not all systems
       on which Perl runs are so forthcoming with this information as is Unix.

       The Mail::Util module from CPAN (part of the MailTools package)
       provides a mailaddress() function that tries to guess the mail address
       of the user.  It makes a more intelligent guess than the code above,
       using information given when the module was installed, but it could
       still be incorrect.  Again, the best way is often just to ask the user.

       Use the "sendmail" program directly:

           open(SENDMAIL, "|/usr/lib/sendmail -oi -t -odq")
                               or die "Can't fork for sendmail: $!\n";
           print SENDMAIL <<"EOF";
           From: User Originating Mail <me\@host>
           To: Final Destination <you\@otherhost>
           Subject: A relevant subject line

           Body of the message goes here after the blank line
           in as many lines as you like.
           close(SENDMAIL)     or warn "sendmail didn't close nicely";

       The -oi option prevents sendmail from interpreting a line consisting of
       a single dot as "end of message".  The -t option says to use the
       headers to decide who to send the message to, and -odq says to put the
       message into the queue.  This last option means your message won't be
       immediately delivered, so leave it out if you want immediate delivery.

       Alternate, less convenient approaches include calling mail (sometimes
       called mailx) directly or simply opening up port 25 have having an
       intimate conversation between just you and the remote SMTP daemon,
       probably sendmail.

       Or you might be able use the CPAN module Mail::Mailer:

           use Mail::Mailer;

           $mailer = Mail::Mailer->new();
           $mailer->open({ From    => $from_address,
                           To      => $to_address,
                           Subject => $subject,
               or die "Can't open: $!\n";
           print $mailer $body;

       The Mail::Internet module uses Net::SMTP which is less Unix-centric
       than Mail::Mailer, but less reliable.  Avoid raw SMTP commands.  There
       are many reasons to use a mail transport agent like sendmail.  These
       include queuing, MX records, and security.


       This answer is extracted directly from the MIME::Lite documentation.
       Create a multipart message (i.e., one with attachments).

           use MIME::Lite;

           ### Create a new multipart message:
           $msg = MIME::Lite->new(
                        From    =>'',
                        To      =>'',
                        Cc      =>',',
                        Subject =>'A message with 2 parts...',
                        Type    =>'multipart/mixed'

           ### Add parts (each "attach" has same arguments as "new"):
           $msg->attach(Type     =>'TEXT',
                        Data     =>"Here's the GIF file you wanted"
           $msg->attach(Type     =>'image/gif',
                        Path     =>'aaa000123.gif',
                        Filename =>'logo.gif'

           $text = $msg->as_string;

       MIME::Lite also includes a method for sending these things.


       This defaults to using sendmail but can be customized to use SMTP via

       While you could use the Mail::Folder module from CPAN (part of the
       MailFolder package) or the Mail::Internet module from CPAN (part of the
       MailTools package), often a module is overkill.  Here's a mail sorter.


           my(@msgs, @sub);
           my $msgno = -1;
           $/ = '';                    # paragraph reads
           while (<>) {
               if (/^From /m) {
                   $sub[++$msgno] = lc($1) || '';
               $msgs[$msgno] .= $_;
           for my $i (sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msgs)) {
               print $msgs[$i];

       Or more succinctly,

           #!/usr/bin/perl -n00
           # bysub2 - awkish sort-by-subject
           BEGIN { $msgno = -1 }
           $sub[++$msgno] = (/^Subject:\s*(?:Re:\s*)*(.*)/mi)[0] if /^From/m;
           $msg[$msgno] .= $_;
           END { print @msg[ sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msg) ] }


        code `hostname`

       Sys::Hostname perl gethostbyname() IP DNS

           use Socket;
           use Sys::Hostname;
           my $host = hostname();
           my $addr = inet_ntoa(scalar gethostbyname($host || 'localhost'));

        Unix  DNS /etc/resolv.conf  resolv.conf

       (Perl Unix)

        Net::NNTP News::NNTPClient CPAN

           perl -MNews::NNTPClient
             -e 'print News::NNTPClient->new->list("newsgroups")'

       / FTP ?

       LWP::Simple CPAN Net::FTP CPAN

        RPC ?

        DCE::RPC  () DCE-Perl  ( CPAN ) rpcgen  CPAN/authors/id/JAKE/  RPC


       Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.  All
       rights reserved.

       This documentation is free; you can redistribute it and/or modify it
       under the same terms as Perl itself.

       Irrespective of its distribution, all code examples in this file are
       hereby placed into the public domain.  You are permitted and encouraged
       to use this code in your own programs for fun or for profit as you see
       fit.  A simple comment in the code giving credit would be courteous but
       is not required.