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

       perlfaq9 - Networking

       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 <> 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 CGI specification is outlined in an informational RFC:

       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.

   My CGI script runs from the command line but not the browser.  (500 Server
       Several things could be wrong.  You can go through the "Troubleshooting
       Perl CGI scripts" guide at

       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:

   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";

       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 "<" for example.

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

	       #!/usr/bin/perl -p0777

       If you want a more complete solution, see the 3-stage striphtml program
       in .

       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>

   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

       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* >

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

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

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


   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( "" );

       It can also store the resource directly in a file:

	       use LWP::Simple qw(getstore);

	       getstore( "", "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('');
	       $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;

   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 "". 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 "$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;

	       $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 "" is strongly recommended.	 This example shows
       redirection with a complete URL. This redirection is handled by the web

	       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

   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 ();
		 ->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

   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 "" 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 "" module automatically parses
       the input and makes each value available through the "param()"

	       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, "" 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,

       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 "".

   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

       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

   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.
	       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.

   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	=>'',
					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

   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


	       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) ] }

   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

	       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

	       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

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

       Use one of the RPC modules you can find on CPAN ( ).

       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 v5.12.2			  2010-09-06			   PERLFAQ9(1)
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