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NAME
       perlfaq9 - Networking

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

       Other relevant documentation listed in:
       http://www.perl.org/CGI_MetaFAQ.html

       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)
       Several things could be wrong.  You can go through the
       "Troubleshooting Perl CGI scripts" guide at



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               http://www.perl.org/troubleshooting_CGI.html

       If, after that, you can demonstrate that you've read the
       FAQs and that your problem isn't something simple that can
       be easily answered, you'll probably receive a courteous
       and useful reply to your question if you post it on
       comp.infosystems.www.authoring.cgi (if it's something to
       do with HTTP or the CGI protocols).  Questions that appear
       to be Perl questions but are really CGI ones that are
       posted to comp.lang.perl.misc are not so well received.

       The useful FAQs, related documents, and troubleshooting
       guides are listed in the CGI Meta FAQ:

               http://www.perl.org/CGI_MetaFAQ.html

   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 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";
                       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



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       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 comment 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/<(?:[^>'"]*|(['"]).*?\1)*>//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 on text like this:

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

   How do I extract URLs?
       You can easily extract all sorts of URLs from HTML with
       "HTML::SimpleLinkExtor" which handles anchors, images,
       objects, frames, and many other tags that can contain a
       URL.  If you need anything more complex, you can create
       your own subclass of "HTML::LinkExtor" or "HTML::Parser".
       You might even use "HTML::SimpleLinkExtor" as an example
       for something specifically suited to your needs.




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       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* (["']) (.*?) \1
                       \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;



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   How do I fetch an HTML file?
       (contributed by brian d foy)

       Use the libwww-perl distribution. The "LWP::Simple" module
       can fetch web resources and give their content back to you
       as a string:

               use LWP::Simple qw(get);

               my $html = get( "http://www.example.com/index.html" );

       It can also store the resource directly in a file:

               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



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       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 the
       "URI::Escape" module. The "uri_escape" function returns
       the escaped string:

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

               my $escaped = uri_escape( $original )

               print "$string\n"; # 'Colon%20%3A%20Hash%20%23%20Percent%20%25%20'

       To decode the string, use the "uri_unescape" function:

               my $unescaped = uri_unescape( $escaped );

               print $unescaped; # back to original

       If you wanted to do it yourself, you simply need to
       replace the 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/;




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

   How do I make sure users can't enter values into a form that
       cause my CGI script to do bad things?
       See the security references listed in the CGI Meta FAQ

               http://www.perl.org/CGI_MetaFAQ.html

   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



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       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 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, too.

       Many people try to write their own decoder (or copy one
       from another program) and then run into one of the many
       "gotchas" of the task.  It's much easier and less hassle
       to use CGI.pm.

   How do I check a valid mail address?
       (partly contributed by Aaron Sherman)

       This isn't as simple a question as it sounds.  There are
       two parts:

       a) How do I verify that an email address is correctly
       formatted?

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



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       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



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       as simple as:

               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 messages.

       If the string to decode is short (less than 84 bytes long)
       a more direct approach is to use the unpack() function's
       "u" format after minor transliterations:

               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

   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.



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               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,
                                               Subject => $subject,
                                         })
                       or die "Can't open: $!\n";
               print $mailer $body;
               $mailer->close();

       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.

   How do I use MIME to make an attachment to a mail message?
       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"



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                                        );
               $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];
               }

       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) ] }

   How do I find out my hostname, domainname, or IP address?
       gethostbyname, Socket, Net::Domain, Sys::Hostname"
       (contributed by brian d foy)

       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



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       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?
       LWP::Simple (available from CPAN) can fetch but not put.
       Net::FTP (also available from CPAN) is more complex but
       can put as well as fetch.

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

       Use one of the RPC modules you can find on CPAN (
       http://search.cpan.org/search?query=RPC&mode=all ).

REVISION
       Revision: $Revision: 1.10 $

       Date: $Date: 2009/10/12 18:24:47 $

       See perlfaq for source control details and availability.

AUTHOR AND COPYRIGHT
       Copyright (c) 1997-2009 Tom Christiansen, Nathan
       Torkington, and other authors as noted. All rights



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PERLFAQ9(1)      Perl Programmers Reference Guide     PERLFAQ9(1)


       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.














































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