MirBSD manpage: perlfaq9(1)

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 <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,

     The CGI specification is outlined in an informational RFC:

     Other relevant documentation listed in:

     These Perl FAQs very selectively cover some CGI issues. How-
     ever, 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 some-
     times 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 sup-
     ports. "Parsed Header" scripts are simpler in various
     respects. The CGI specification allows any of the usual new-
     line 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 recom-
     mended. 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 ter-
     minated with carriage-return and line-feed, i.e ASCII
     \015\012 written in binary mode.

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

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     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
     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";

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     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

     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:

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         <!-- 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 - 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 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 down-
     load, 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 a pop-up menu in HTML?

     Use the <SELECT> and <OPTION> tags.  The CGI.pm module
     (available from CPAN) supports this widget, as well as many

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     others, including some that it cleverly synthesizes on its

     How do I fetch an HTML file?

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

         $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

     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);

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     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?

     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 following 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 com-
     ponents, not the entire URI, otherwise you'll lose informa-
     tion 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 is.

     RFC 2396 also contains a lot of other useful information,
     including a regexp for breaking any arbitrary URI into com-
     ponents (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/;

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           my $url = 'http://www.cpan.org/';
           print redirect($url);

     This example shows a redirection with an absolute URLpath.
     This redirection is handled by the local web server.

           my $url = '/CPAN/index.html';
           print redirect($url);

     But if coded directly, it could be as follows (the final
     "\n" is shown separately, for clarity), using either a com-
     plete 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 dif-
     ferently 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" authentica-
     tion 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:

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         $/ = '';
         $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

             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 ver-
     sion 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

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     How do I check a valid mail address?

     You can't, at least, not in real time.  Bummer, eh?

     Without sending mail to the address and seeing whether
     there's a human on the other end to answer you, you cannot
     determine whether a mail address is valid.  Even if you
     apply the mail header standard, you can have problems,
     because there are deliverable addresses that aren't RFC-822
     (the mail header standard) compliant, and addresses that
     aren't deliverable which are compliant.

     You can use the Email::Valid or RFC::RFC822::Address which
     check the format of the address, although they cannot actu-
     ally tell you if it is a deliverable address (i.e. that mail
     to the address will not bounce).  Modules like
     Mail::CheckUser and Mail::EXPN try to interact with the
     domain name system or particular mail servers to learn even
     more, but their methods do not work everywhere---especially
     for security conscious administrators.

     Many are tempted to try to eliminate many frequently-invalid
     mail addresses with a simple regex, such as
     "/^[\w.-]+\@(?:[\w-]+\.)+\w+$/".  It's a very bad idea.
     However, this also throws out many valid ones, and says
     nothing about potential deliverability, so it is not sug-
     gested.  Instead, see
     , which actually checks against the full RFC spec (except
     for nested comments), looks for addresses you may not wish
     to accept mail to (say, Bill Clinton or your postmaster),
     and then makes sure that the hostname given can be looked up
     in the DNS MX records.  It's not fast, but it works for what
     it tries to do.

     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 that looks somewhat like:

         Dear someuser@host.com,

         Please confirm the mail address you gave us Wed May  6 09:38:41
         MDT 1998 by replying to this message.  Include the string
         "Rumpelstiltskin" in that reply, but spelled in reverse; that is,
         start with "Nik...".  Once this is done, your confirmed address will
         be entered into our records.

     If you get the message back and they've followed your direc-
     tions, you can be reasonably assured that it's real.

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     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 "vaca-
     tion" 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 gen-
     erates 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 pack-
     age) 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

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     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 tran-
     sport agent like sendmail.  These include queuing, MX
     records, and security.

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

     This answer is extracted directly from the MIME::Lite docu-
     mentation. Create a multipart message (i.e., one with

         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.


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         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" (contri-
     buted by brian d foy)

     The Net::Domain module, which is part of the standard dis-
     tribution starting in perl5.7.3, can get you the fully qual-
     ified 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 distri-
     bution 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;

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         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 avail-
     able from CPAN. This can make tasks like fetching the news-
     group 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 ).
     Copyright (c) 1997-2006 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 per-
     mitted 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

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