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



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

               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 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.

       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;

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

       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/;

               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 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, 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?

       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);

       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.
               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"
                                        );
               $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 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 ).

AUTHOR AND COPYRIGHT
       Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and other authors as noted. 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.



perl v5.12.5                                     2012-11-03                                      PERLFAQ9(1)