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

       perlfaq9 - Networking ($Revision: 8539 $)

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

       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 cor-
       rect, and recommended. NPH scripts are more tricky: they must put out a complete and accu-
       rate 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

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

       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


       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 use-
       ful 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 ques-
       tions but are really CGI ones that are posted to comp.lang.perl.misc are not so well

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


       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 "&lt;" 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
       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 han-
       dles 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* >

       How do I download a file from the user's machine?  How do I open a file on another

       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'),

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


       How do I fetch an HTML file?

       One approach, if you have the lynx text-based HTML browser installed on your system, is

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

       The libwww-perl (LWP) modules from CPAN provide a more powerful way to do this.	They
       don't require lynx, but like lynx, can still work through proxies:

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

	   # or print HTML from a URL
	   use LWP::Simple;
	   getprint "http://www.linpro.no/lwp/";

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

       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 appro-

	   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?

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

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

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

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

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

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

       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 ();
		 ->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

       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 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

       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

       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 $domain_lit = qr{\[(?:\\\S|[\x21-\x5a\x5e-\x7e])*\]};
	   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 specifi-
       cation.	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 mes-
       sage 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 encod-
       ing.  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 mat-
       ters.  Furthermore, not all systems on which Perl runs are so forthcoming with this infor-
       mation 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.
	   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	=>'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.


       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.


	   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 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: 8539 $

       Date: $Date: 2007-01-11 00:07:14 +0100 (Thu, 11 Jan 2007) $

       See perlfaq for source control details and availability.

       Copyright (c) 1997-2007 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 v5.8.9				    2007-11-17				      PERLFAQ9(1)
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