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perlfaq9 (1)


perlfaq9 - Networking


Please see following description for synopsis


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

  My CGI script runs from the command line but not the browser.
     (500 Server Error)

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     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

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  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

             #!/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 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

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     "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 -
             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'),


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                             "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"

  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

     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.

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             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

             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

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     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 browser.

             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 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 things?

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     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()" 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, "" 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.

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     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

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

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

     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

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     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

     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

             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.

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     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

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     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

             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


     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) || '';

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                     $msgs[$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

             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

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             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 ( ).

     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.

     See attributes(5) for descriptions of the following

     |Availability   | runtime/perl-512 |
     |Stability      | Uncommitted      |
     This software was built from source available at  The original
     community source was downloaded from

     Further information about this software can be found on the
     open source community website at

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