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perlfaq9




DESCRIPTION

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

       What is the correct form of response from a CGI script?

       (Alan Flavell <flavell+www@a5.ph.gla.ac.uk> answers...)

       The Common Gateway Interface (CGI) specifies a software
       interface between a program ("CGI script") and a web
       server (HTTPD). It is not specific to Perl, and has its
       own FAQs and tutorials, and usenet group, comp.infosys­
       tems.www.authoring.cgi

       The original CGI specification is at:
       http://hoohoo.ncsa.uiuc.edu/cgi/

       Current best-practice RFC draft at:
       http://CGI-Spec.Golux.Com/

       Other relevant documentation listed in:
       http://www.perl.org/CGI_MetaFAQ.html

       These Perl FAQs very selectively cover some CGI issues.
       However, Perl programmers are strongly advised to use the
       CGI.pm module, to take care of the details for them.

       The similarity between CGI response headers (defined in
       the CGI specification) and HTTP response headers (defined
       in the HTTP specification, RFC2616) is intentional, but
       can sometimes be confusing.

       The CGI specification defines two kinds of script: the
       "Parsed Header" script, and the "Non Parsed Header" (NPH)
       script. Check your server documentation to see what it
       supports. "Parsed Header" scripts are simpler in various
       respects. The CGI specification allows any of the usual
       newline representations in the CGI response (it's the
       server's job to create an accurate HTTP response based on
       it). So "\n" written in text mode is technically correct,
       and recommended. NPH scripts are more tricky: they must
       put out a complete and accurate set of HTTP transaction
       response headers; the HTTP specification calls for records
       to be terminated with carriage-return and line-feed, i.e
       ASCII \015\012 written in binary mode.

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

       to be Perl questions but are really CGI ones that are
       posted to comp.lang.perl.misc are not so well received.

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

               http://www.perl.org/CGI_MetaFAQ.html

       How can I get better error messages from a CGI program?

       Use the CGI::Carp module.  It replaces "warn" and "die",
       plus the normal Carp modules "carp", "croak", and "con­
       fess" 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 com­
       pile-time warnings as well:

           BEGIN {
               use CGI::Carp qw(carpout);
               open(LOG, ">>/var/local/cgi-logs/mycgi-log")
                   or die "Unable to append to mycgi-log: $!\n";
               carpout(*LOG);
           }

       You can even arrange for fatal errors to go back to the
       client browser, which is nice for your own debugging, but
       might confuse the end user.

           use CGI::Carp qw(fatalsToBrowser);
           die "Bad error here";

       Even if the error happens before you get the HTTP header
       out, the module will try to take care of this to avoid the
       dreaded server 500 errors.  Normal warnings still go out
       to the server error log (or wherever you've sent them with
       "carpout") with the application name and date stamp
       prepended.

       How do I remove HTML from a string?

       The most correct way (albeit not the fastest) is to use
       HTML::Parser from CPAN.  Another mostly correct way is to
       use HTML::FormatText which not only removes HTML but also
       attempts to do a little simple formatting of the resulting
       plain text.

       striphtml program in
       http://www.cpan.org/authors/Tom_Chris­
       tiansen/scripts/striphtml.gz .

       Here are some tricky cases that you should think about
       when picking a solution:

           <IMG SRC = "foo.gif" ALT = "A > B">

           <IMG SRC = "foo.gif"
                ALT = "A > B">

           <!-- <A comment> -->

           <script>if (a<b && a>c)</script>

           <# Just data #>

           <![INCLUDE CDATA [ >>>>>>>>>>>> ]]>

       If HTML comments include other tags, those solutions would
       also break on text like this:

           <!-- This section commented out.
               <B>You can't see me!</B>
           -->

       How do I extract URLs?

       You can easily extract all sorts of URLs from HTML with
       "HTML::SimpleLinkExtor" which handles anchors, images,
       objects, frames, and many other tags that can contain a
       URL.  If you need anything more complex, you can create
       your own subclass of "HTML::LinkExtor" or "HTML::Parser".
       You might even use "HTML::SimpleLinkExtor" as an example
       for something specifically suited to your needs.

       You can use URI::Find to extract URLs from an arbitrary
       text document.

       Less complete solutions involving regular expressions can
       save you a lot of processing time if you know that the
       input is simple.  One solution from Tom Christiansen runs
       100 times faster than most module based approaches but
       only extracts URLs from anchors where the first attribute
       is HREF and there are no other attributes.

               #!/usr/bin/perl -n00
               # qxurl - tchrist@perl.com
               print "$2\n" while m{
                   < \s*
                     A \s+ HREF \s* = \s* (["']) (.*?) \1

       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 others, including some that it cleverly synthesizes
       on its own.

       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'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;


       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 equiva­
       lent 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

       However, you should only apply them to individual URI com­
       ponents, not the entire URI, otherwise you'll lose infor­
       mation 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
       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 specifi­
       cations 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 han­
       dled 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
       Check your web server documentation for the details for
       your particular server.

       How do I edit my .htpasswd and .htgroup files with Perl?

       The HTTPD::UserAdmin and HTTPD::GroupAdmin modules provide
       a consistent OO interface to these files, regardless of
       how they're stored.  Databases may be text, dbm, Berkeley
       DB or any database with a DBI compatible driver.
       HTTPD::UserAdmin supports files used by the `Basic' and
       `Digest' authentication schemes.  Here's an example:

           use HTTPD::UserAdmin ();
           HTTPD::UserAdmin
                 ->new(DB => "/foo/.htpasswd")
                 ->add($username => $password);

       How do I make sure users can't enter values into a form
       that cause my CGI script to do bad things?

       See the security references listed in the CGI Meta FAQ

               http://www.perl.org/CGI_MetaFAQ.html

       How do I parse a mail header?

       For a quick-and-dirty solution, try this solution derived
       from "split" in perlfunc:

           $/ = '';
           $header = <MSG>;
           $header =~ s/\n\s+/ /g;      # merge continuation lines
           %head = ( UNIX_FROM_LINE, split /^([-\w]+):\s*/m, $header );

       That solution doesn't do well if, for example, you're try­
       ing 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?

       You use a standard module, probably CGI.pm.  Under no cir­
       cumstances should you attempt to do so by hand!

       You'll see a lot of CGI programs that blindly read from
       STDIN the number of bytes equal to CONTENT_LENGTH for
       POSTs, or grab QUERY_STRING for decoding GETs.  These pro­
       grams are very poorly written.  They only work sometimes.
       They typically forget to check the return value of the
       read() system call, which is a cardinal sin.  They don't
       handle HEAD requests.  They don't handle multipart forms
       used for file uploads.  They don't deal with GET/POST com­
       word for this is ``idempotency''.  This simply means that
       there should be no difference between making a GET request
       for a particular URL once or multiple times.  This is
       because the HTTP protocol definition says that a GET
       request may be cached by the browser, or server, or an
       intervening proxy.  POST requests cannot be cached,
       because each request is independent and matters.  Typi­
       cally, POST requests change or depend on state on the
       server (query or update a database, send mail, or purchase
       a computer).

       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 hand to answer you, you can­
       not 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
       actually 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 every­
       where---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
       http://www.cpan.org/authors/Tom_Chris­
       tiansen/scripts/ckaddr.gz , 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 nor­
       mally 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:

       In the mail you send, ask them to include the PIN in their
       reply.  But if it bounces, or the message is included via
       a ``vacation'' script, it'll be there anyway.  So it's
       best to ask them to mail back a slight alteration of the
       PIN, such as with the characters reversed, one added or
       subtracted to each digit, etc.

       How do I decode a MIME/BASE64 string?

       The MIME-Base64 package (available from CPAN) handles this
       as well as the MIME/QP encoding.  Decoding BASE64 becomes
       as simple as:

           use MIME::Base64;
           $decoded = decode_base64($encoded);

       The MIME-Tools package (available from CPAN) supports
       extraction with decoding of BASE64 encoded attachments and
       content directly from email messages.

       If the string to decode is short (less than 84 bytes long)
       a more direct approach is to use the unpack() function's
       "u" format after minor transliterations:

           tr#A-Za-z0-9+/##cd;                   # remove non-base64 chars
           tr#A-Za-z0-9+/# -_#;                  # convert to uuencoded format
           $len = pack("c", 32 + 0.75*length);   # compute length byte
           print unpack("u", $len . $_);         # uudecode and print

       How do I return the user's mail address?

       On systems that support getpwuid, the $< variable, and the
       Sys::Hostname module (which is part of the standard perl
       distribution), you can probably try using something like
       this:

           use Sys::Hostname;
           $address = sprintf('%s@%s', scalar getpwuid($<), hostname);

       Company policies on mail address can mean that this gener­
       ates addresses that the company's mail system will not
       accept, so you should ask for users' mail addresses when
       this matters.  Furthermore, not all systems on which Perl
       runs are so forthcoming with this information as is Unix.

       The Mail::Util module from CPAN (part of the MailTools
       package) provides a mailaddress() function that tries to
       guess the mail address of the user.  It makes a more
       intelligent guess than the code above, using information
       given when the module was installed, but it could still be
       incorrect.  Again, the best way is often just to ask the
       user.
           Body of the message goes here after the blank line
           in as many lines as you like.
           EOF
           close(SENDMAIL)     or warn "sendmail didn't close nicely";

       The -oi option prevents sendmail from interpreting a line
       consisting of a single dot as "end of message".  The -t
       option says to use the headers to decide who to send the
       message to, and -odq says to put the message into the
       queue.  This last option means your message won't be imme­
       diately delivered, so leave it out if you want immediate
       delivery.

       Alternate, less convenient approaches include calling mail
       (sometimes called mailx) directly or simply opening up
       port 25 have having an intimate conversation between just
       you and the remote SMTP daemon, probably sendmail.

       Or you might be able use the CPAN module Mail::Mailer:

           use Mail::Mailer;

           $mailer = Mail::Mailer->new();
           $mailer->open({ From    => $from_address,
                           To      => $to_address,
                           Subject => $subject,
                         })
               or die "Can't open: $!\n";
           print $mailer $body;
           $mailer->close();

       The Mail::Internet module uses Net::SMTP which is less
       Unix-centric than Mail::Mailer, but less reliable.  Avoid
       raw SMTP commands.  There are many reasons to use a mail
       transport agent like sendmail.  These include queuing, MX
       records, and security.

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

       This answer is extracted directly from the MIME::Lite doc­
       umentation.  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'
                        );

           $text = $msg->as_string;

       MIME::Lite also includes a method for sending these
       things.

           $msg->send;

       This defaults to using sendmail but can be customized to
       use SMTP via Net::SMTP.

       How do I read mail?

       While you could use the Mail::Folder module from CPAN
       (part of the MailFolder package) or the Mail::Internet
       module from CPAN (part of the MailTools package), often a
       module is overkill.  Here's a mail sorter.

           #!/usr/bin/perl

           my(@msgs, @sub);
           my $msgno = -1;
           $/ = '';                    # paragraph reads
           while (<>) {
               if (/^From /m) {
                   /^Subject:\s*(?:Re:\s*)*(.*)/mi;
                   $sub[++$msgno] = lc($1) || '';
               }
               $msgs[$msgno] .= $_;
           }
           for my $i (sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msgs)) {
               print $msgs[$i];
           }

       Or more succinctly,

           #!/usr/bin/perl -n00
           # bysub2 - awkish sort-by-subject
           BEGIN { $msgno = -1 }
           $sub[++$msgno] = (/^Subject:\s*(?:Re:\s*)*(.*)/mi)[0] if /^From/m;
           $msg[$msgno] .= $_;
           END { print @msg[ sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msg) ] }

       How do I find out my hostname/domainname/IP address?

       The normal way to find your own hostname is to call the
       `hostname` program.  While sometimes expedient, this has
       some problems, such as not knowing whether you've got the
       canonical name or not.  It's one of those tradeoffs of
       convenience versus portability.

       Of course, this assumes several things about your
       resolv.conf configuration, including that it exists.

       (We still need a good DNS domain name-learning method for
       non-Unix systems.)

       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
       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?

       A DCE::RPC module is being developed (but is not yet
       available) and will be released as part of the DCE-Perl
       package (available from CPAN).  The rpcgen suite, avail­
       able from CPAN/authors/id/JAKE/, is an RPC stub generator
       and includes an RPC::ONC module.


AUTHOR AND COPYRIGHT

       Copyright (c) 1997-2002 Tom Christiansen and Nathan Tork­
       ington.  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 v5.8.1                 2003-09-02                PERLFAQ9(1)

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