perlfaq9


DESCRIPTION
       This section deals with questions related to networking, the internet,
       and a few on the web.

   What is the correct form of response from a CGI script?
       (Alan Flavell <flavell+www@a5.ph.gla.ac.uk> 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,
       comp.infosystems.www.authoring.cgi

       The CGI specification is outlined in an informational RFC:
       http://www.ietf.org/rfc/rfc3875

       These Perl FAQs very selectively cover some CGI issues. However, Perl
       programmers are strongly advised to use the "CGI.pm" 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 "CGI.pm" gives excellent platform independence, including EBCDIC
       systems. "CGI.pm" selects an appropriate newline representation
       ($CGI::CRLF) and sets binmode as appropriate.

   My CGI script runs from the command line but not the browser. (500 Server
       Error)
       (contributed by brian d foy)

       There are many things that might be wrong with your CGI program, and
       only some of them might be related to Perl. Try going through the
       troubleshooting guide on Perlmonks:

               http://www.perlmonks.org/?node_id=380424

   How can I get better error messages from a CGI program?
       Use the "CGI::Carp" module.  It replaces "warn" and "die", plus the
       normal "Carp" module's "carp", "croak", and "confess" functions with
       more verbose and safer versions.  It still sends them to the normal
                       open(LOG, ">>/var/local/cgi-logs/mycgi-log")
                               or die "Unable to append to mycgi-log: $!\n";
                       carpout(*LOG);
               }

       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.

   How do I remove HTML from a string?
       The most correct way (albeit not the fastest) is to use "HTML::Parser"
       from CPAN.  Another mostly correct way is to use "HTML::FormatText"
       which not only removes HTML but also attempts to do a little simple
       formatting of the resulting plain text.

       Many folks attempt a simple-minded regular expression approach, like
       "s/<.*?>//g", but that fails in many cases because the tags may
       continue over line breaks, they may contain quoted angle-brackets, or
       HTML comments may be present.  Plus, folks forget to convert
       entities--like "&lt;" for example.

       Here's one "simple-minded" approach, that works for most files:

               #!/usr/bin/perl -p0777
               s/<(?:[^>'"]*|(['"]).*?\g1)*>//gs

       If you want a more complete solution, see the 3-stage striphtml program
       in http://www.cpan.org/authors/Tom_Christiansen/scripts/striphtml.gz .

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

               <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
       example for something specifically suited to your needs.

       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 - tchrist@perl.com
               print "$2\n" while m{
                       < \s*
                         A \s+ HREF \s* = \s* (["']) (.*?) \g1
                       \s* >
               }gsix;

   How do I download a file from the user's machine?  How do I open a file on
       another machine?
       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 "CGI.pm" 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()" method.

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

   How do I make an HTML pop-up menu with Perl?
       (contributed by brian d foy)

       The "CGI.pm" module (which comes with Perl) has functions to create the
       HTML form widgets. See the "CGI.pm" documentation for more examples.

               use CGI qw/:standard/;
               print header,
                       start_html('Favorite Animals'),

                       start_form,
                               "What's your favorite animal? ",
                       popup_menu(
                               -name   => 'animal',
                               -values => [ qw( Llama Alpaca Camel Ram ) ]
                               ),
                       submit,

                       end_form,
                       end_html;

               use LWP::Simple qw(getstore);

               getstore( "http://www.example.com/index.html", "foo.html" );

       If you need to do something more complicated, you can use
       "LWP::UserAgent" module to create your own user-agent (e.g. browser) to
       get the job done. If you want to simulate an interactive web browser,
       you can use the "WWW::Mechanize" module.

   How do I automate an HTML form submission?
       If you are doing something complex, such as moving through many pages
       and forms or a web site, you can use "WWW::Mechanize".  See its
       documentation for all the details.

       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('http://www.perl.com/cgi-bin/cpan_mod');
               $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 'http://www.perl.com/cgi-bin/cpan_mod',
                                          [ module => 'DB_File', readme => 1 ];
               $content = $ua->request($req)->as_string;

   How do I decode or create those %-encodings on the web?
       (contributed by brian d foy)

       Those "%" encodings handle reserved characters in URIs, as described in
       RFC 2396, Section 2. This encoding replaces the reserved character with
       the hexadecimal representation of the character's number from the US-
       ASCII table. For instance, a colon, ":", becomes %3A.

       In CGI scripts, you don't have to worry about decoding URIs if you are
       using "CGI.pm". You shouldn't have to process the URI yourself, either
       on the way in or the way out.

       If you have to encode a string yourself, remember that you should never
       try to encode an already-composed URI. You need to escape the
       components separately then put them together. To encode a string, you
       can use the "URI::Escape" module. The "uri_escape" function returns the
       escaped string:

               my $original = "Colon : Hash # Percent %";

       reserved characters with their encodings. A global substitution is one
       way to do it:

               # encode
               $string =~ s/([^^A-Za-z0-9\-_.!~*'()])/ sprintf "%%%0x", ord $1 /eg;

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

   How do I redirect to another page?
       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 "CGI.pm" is strongly recommended.  This example shows
       redirection with a complete URL. This redirection is handled by the web
       browser.

               use CGI qw/:standard/;

               my $url = 'http://www.cpan.org/';
               print redirect($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
       URLpath.

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

   How do I put a password on my web pages?
       To enable authentication for your web server, you need to configure
       your web server.  The configuration is different for different sorts of
       web servers--apache does it differently from iPlanet which does it
       differently from IIS.  Check your web server documentation for the
       details for your particular server.

   How do I edit my .htpasswd and .htgroup files with Perl?
       The "HTTPD::UserAdmin" and "HTTPD::GroupAdmin" modules provide a
       consistent OO interface to these files, regardless of how they're
       stored.  Databases may be text, dbm, Berkeley DB or any database with a
       DBI compatible driver.  "HTTPD::UserAdmin" supports files used by the
       "Basic" and "Digest" authentication schemes.  Here's an example:

               use HTTPD::UserAdmin ();

       SQL injection or other sorts of attacks (and you should want to), you
       have to not trust any data that enter your program.

       The perlsec documentation has general advice about data security.  If
       you are using the "DBI" module, use placeholder to fill in data.  If
       you are running external programs with "system" or "exec", use the list
       forms. There are many other precautions that you should take, too many
       to list here, and most of them fall under the category of not using any
       data that you don't intend to use. Trust no one.

   How do I parse a mail header?
       For a quick-and-dirty solution, try this solution derived from "split"
       in perlfunc:

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

       That solution doesn't do well if, for example, you're trying to
       maintain all the Received lines.  A more complete approach is to use
       the "Mail::Header" module from CPAN (part of the "MailTools" package).

   How do I decode a CGI form?
       (contributed by brian d foy)

       Use the "CGI.pm" module that comes with Perl.  It's quick, it's easy,
       and it actually does quite a bit of work to ensure things happen
       correctly.  It handles GET, POST, and HEAD requests, multipart forms,
       multivalued fields, query string and message body combinations, and
       many other things you probably don't want to think about.

       It doesn't get much easier: the "CGI.pm" module automatically parses
       the input and makes each value available through the "param()"
       function.

               use CGI qw(:standard);

               my $total = param( 'price' ) + param( 'shipping' );

               my @items = param( 'item' ); # multiple values, same field name

       If you want an object-oriented approach, "CGI.pm" can do that too.

               use CGI;

               my $cgi = CGI->new();

               my $total = $cgi->param( 'price' ) + $cgi->param( 'shipping' );

               my @items = $cgi->param( 'item' );

       You might also try "CGI::Minimal" which is a lightweight version of the
       same thing.  Other CGI::* modules on CPAN might work better for you,

       b) How do I verify that an email address targets a valid recipient?

       Without sending mail to the address and seeing whether there's a human
       on the other end to answer you, you cannot fully answer part b, but
       either the "Email::Valid" or the "RFC::RFC822::Address" module will do
       both part a and part b as far as you can in real-time.

       If you want to just check part a to see that the address is valid
       according to the mail header standard with a simple regular expression,
       you can have problems, because there are deliverable addresses that
       aren't RFC-2822 (the latest mail header standard) compliant, and
       addresses that aren't deliverable which, are compliant.  However,  the
       following will match valid RFC-2822 addresses that do not have
       comments, folding whitespace, or any other obsolete or non-essential
       elements.  This just matches the address itself:

               my $atom       = qr{[a-zA-Z0-9_!#\$\%&'*+/=?\^`{}~|\-]+};
               my $dot_atom   = qr{$atom(?:\.$atom)*};
               my $quoted     = qr{"(?:\\[^\r\n]|[^\\"])*"};
               my $local      = qr{(?:$dot_atom|$quoted)};
               my $quotedpair = qr{\\[\x00-\x09\x0B-\x0c\x0e-\x7e]};
               my $domain_lit = qr{\[(?:$quotedpair|[\x21-\x5a\x5e-\x7e])*\]};
               my $domain     = qr{(?:$dot_atom|$domain_lit)};
               my $addr_spec  = qr{$local\@$domain};

       Just match an address against "/^${addr_spec}$/" to see if it follows
       the RFC2822 specification.  However, because it is impossible to be
       sure that such a correctly formed address is actually the correct way
       to reach a particular person or even has a mailbox associated with it,
       you must be very careful about how you use this.

       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. 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, etc.

   How do I decode a MIME/BASE64 string?
       The "MIME-Base64" package (available from CPAN) handles this as well as
       the MIME/QP encoding.  Decoding BASE64 becomes as simple as:

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


   How do I return the user's mail address?
       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.

   How do I send mail?
       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.
               EOF
               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,

       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    =>'me@myhost.com',
                                        To      =>'you@yourhost.com',
                                        Cc      =>'some@other.com, some@more.com',
                                        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.

               $msg->send;

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

   How do I read mail?
       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.

               #!/usr/bin/perl

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


       The "Net::Domain" module, which is part of the standard distribution
       starting in perl5.7.3, can get you the fully qualified domain name
       (FQDN), the host name, or the domain name.

               use Net::Domain qw(hostname hostfqdn hostdomain);

               my $host = hostfqdn();

       The "Sys::Hostname" module, included in the standard distribution since
       perl5.6, can also get the hostname.

               use Sys::Hostname;

               $host = hostname();

       To get the IP address, you can use the "gethostbyname" built-in
       function to turn the name into a number. To turn that number into the
       dotted octet form (a.b.c.d) that most people expect, use the
       "inet_ntoa" function from the "Socket" module, which also comes with
       perl.

               use Socket;

               my $address = inet_ntoa(
                       scalar gethostbyname( $host || 'localhost' )
                       );

   How do I fetch a news article or the active newsgroups?
       Use the "Net::NNTP" or "News::NNTPClient" modules, both available from
       CPAN.  This can make tasks like fetching the newsgroup list as simple
       as

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

   How do I fetch/put an FTP file?
       (contributed by brian d foy)

       The "LWP" family of modules (available on CPAN as the libwww-perl
       distribution) can work with FTP just like it can with many other
       protocols. "LWP::Simple" makes it quite easy to fetch a file:

               use LWP::Simple;

               my $data = get( 'ftp://some.ftp.site/some/file.txt' );

       If you want more direct or low-level control of the FTP process, you
       can use the "Net::FTP" module (in the Standard Library since Perl 5.8).
       It's documentation has examples showing you just how to do that.

   How can I do RPC in Perl?
       (contributed by brian d foy)

       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.



perl v5.14.2                      2011-09-26                       PERLFAQ9(1)
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