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


NAME
       perlfaq9 - Networking ($Revision: 1.2 $, $Date: 1999/04/29
       22:52:12 $)



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

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

       If you can demonstrate that you've read the following 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, HTML, or the CGI protocols).  Questions that
       appear to be Perl questions but are really CGI ones that
       are posted to comp.lang.perl.misc may not be so well
       received.

       The useful FAQs and related documents are:

           CGI FAQ
               http://www.webthing.com/tutorials/cgifaq.html

           Web FAQ
               http://www.boutell.com/faq/

           WWW Security FAQ
               http://www.w3.org/Security/Faq/

           HTTP Spec
               http://www.w3.org/pub/WWW/Protocols/HTTP/

           HTML Spec
               http://www.w3.org/TR/REC-html40/
               http://www.w3.org/pub/WWW/MarkUp/

           CGI Spec
               http://www.w3.org/CGI/

           CGI Security FAQ
               http://www.go2net.com/people/paulp/cgi-security/safe-cgi.txt


       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.





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           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";
               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::Parse from CPAN (part of the HTML-Tree package on
       CPAN).

       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
           s/<(?:[^>'"]*|(['"]).*?\1)*>//gs

       If you want a more complete solution, see the 3-stage
       striphtml program in
       http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/striphtml.gz
       .

       Here are some tricky cases that you should think about



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       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?

       A quick but imperfect approach is

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

       This version does not adjust relative URLs, understand
       alternate bases, deal with HTML comments, deal with HREF
       and NAME attributes in the same tag, or accept URLs
       themselves as arguments.  It also runs about 100x faster
       than a more "complete" solution using the LWP suite of
       modules, such as the
       http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/xurl.gz
       program.

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

       In the context of an HTML form, you can use what's known
       as multipart/form-data encoding.  The CGI.pm module
       (available from CPAN) supports this in the
       start_multipart_form() method, which isn't the same as the
       startform() method.





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       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 work through proxies, and
       don't require lynx:

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

           # or print HTML from a URL
           use LWP::Simple;
           getprint "http://www.sn.no/libwww-perl/";

           # or print ASCII from HTML from a URL
           # also need HTML-Tree package from CPAN
           use LWP::Simple;
           use HTML::Parse;
           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;

           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 appropriately.



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           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?

       Here's an example of decoding:

           $string = "http://altavista.digital.com/cgi-bin/query?pg=q&what=news&fmt=.&q=%2Bcgi-bin+%2Bperl.exe";
           $string =~ s/%([a-fA-F0-9]{2})/chr(hex($1))/ge;

       Encoding is a bit harder, because you can't just blindly
       change all the non-alphanumeric characters (\W) into their
       hex escapes.  It's important that characters with special
       meaning like / and ?  not be translated.  Probably the
       easiest way to get this right is to avoid reinventing the
       wheel and just use the URI::Escape module, which is part
       of the libwww-perl package (LWP) available from CPAN.

       How do I redirect to another page?

       Instead of sending back a Content-Type as the headers of
       your reply, send back a Location: header.  Officially this
       should be a URI: header, so the CGI.pm module (available
       from CPAN) sends back both:

           Location: http://www.domain.com/newpage
           URI: http://www.domain.com/newpage

       Note that relative URLs in these headers can cause strange
       effects because of "optimizations" that servers do.

           $url = "http://www.perl.com/CPAN/";
           print "Location: $url\n\n";
           exit;

       To be correct to the spec, each of those "\n" should
       really each be "\015\012", but unless you're stuck on
       MacOS, you probably won't notice.

       How do I put a password on my web pages?

       That depends.  You'll need to read the documentation for
       your web server, or perhaps check some of the other FAQs
       referenced above.







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       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, Berkley
       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?

       Read the CGI security FAQ, at http://www-
       genome.wi.mit.edu/WWW/faqs/www-security-faq.html, and the
       Perl/CGI FAQ at
       http://www.perl.com/CPAN/doc/FAQs/cgi/perl-cgi-faq.html.

       In brief: use tainting (see the perlsec manpage), which
       makes sure that data from outside your script (eg, CGI
       parameters) are never used in eval or system calls.  In
       addition to tainting, never use the single-argument form
       of system() or exec().  Instead, supply the command and
       arguments as a list, which prevents shell globbing.

       How do I parse a mail header?

       For a quick-and-dirty solution, try this solution derived
       from page 222 of the 2nd edition of "Programming Perl":

           $/ = '';
           $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?

       You use a standard module, probably CGI.pm.  Under no
       circumstances 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
       programs are very poorly written.  They only work



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       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 combinations where query fields are in more
       than one place.  They don't deal with keywords in the
       query string.

       In short, they're bad hacks.  Resist them at all costs.
       Please do not be tempted to reinvent the wheel.  Instead,
       use the CGI.pm or CGI_Lite.pm (available from CPAN), or if
       you're trapped in the module-free land of perl1 .. perl4,
       you might look into cgi-lib.pl (available from http://cgi-
       lib.stanford.edu/cgi-lib/ ).

       Make sure you know whether to use a GET or a POST in your
       form.  GETs should only be used for something that doesn't
       update the server.  Otherwise you can get mangled
       databases and repeated feedback mail messages.  The fancy
       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.
       Typically, 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
       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.

       Many are tempted to try to eliminate many frequently-
       invalid mail addresses with a simple regexp, 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 is not
       suggested.  Instead, see
       http://www.perl.com/CPAN/authors/Tom_Christiansen/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



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       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
       directions, you can be reasonably assured that it's real.

       A related strategy that's less open to forgery is to give
       them a PIN (personal ID number).  Record the address and
       PIN (best that it be a random one) for later processing.
       In the mail you send, ask them to include the PIN in their
       reply.  But if it bounces, or the message is included via
       a ``vacation'' script, it'll be there anyway.  So it's
       best to ask them to mail back a slight alteration of the
       PIN, such as with the characters reversed, one added or
       subtracted to each digit, etc.

       How do I decode a MIME/BASE64 string?

       The MIME-tools package (available from CPAN) handles this
       and a lot more.  Decoding BASE64 becomes as simple as:

           use MIME::base64;
           $decoded = decode_base64($encoded);

       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:





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

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

       The Mail::Util module from CPAN (part of the MailTools
       package) provides a mailaddress() function that tries to
       guess the mail address of the user.  It makes a more
       intelligent guess than the code above, using information
       given when the module was installed, but it could still be
       incorrect.  Again, the best way is often just to ask the
       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.
           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
       immediately delivered, so leave it out if you want
       immediate delivery.

       Or 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();



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       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 queueing, MX
       records, and security.

       How do I read mail?

       Use the Mail::Folder module from CPAN (part of the
       MailFolder package) or the Mail::Internet module from CPAN
       (also part of the MailTools package).

          # sending mail
           use Mail::Internet;
           use Mail::Header;
           # say which mail host to use
           $ENV{SMTPHOSTS} = 'mail.frii.com';
           # create headers
           $header = new Mail::Header;
           $header->add('From', 'gnat@frii.com');
           $header->add('Subject', 'Testing');
           $header->add('To', 'gnat@frii.com');
           # create body
           $body = 'This is a test, ignore';
           # create mail object
           $mail = new Mail::Internet(undef, Header => $header, Body => \[$body]);
           # send it
           $mail->smtpsend or die;

       Often a module is overkill, though.  Here's a mail sorter.

           #!/usr/bin/perl
           # bysub1 - simple sort by subject
           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,









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           #!/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.

       The Sys::Hostname module (part of the standard perl
       distribution) will give you the hostname after which you
       can find out the IP address (assuming you have working
       DNS) with a gethostbyname() call.

           use Socket;
           use Sys::Hostname;
           my $host = hostname();
           my $addr = inet_ntoa(scalar gethostbyname($host || 'localhost'));

       Probably the simplest way to learn your DNS domain name is
       to grok it out of /etc/resolv.conf, at least under Unix.
       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
       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?

       A DCE::RPC module is being developed (but is not yet
       available), and will be released as part of the DCE-Perl



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       package (available from CPAN).  The rpcgen suite,
       available from CPAN/authors/id/JAKE/, is an RPC stub
       generator and includes an RPC::ONC module.

AUTHOR AND COPYRIGHT
       Copyright (c) 1997-1999 Tom Christiansen and Nathan
       Torkington.  All rights reserved.

       When included as part of the Standard Version of Perl, or
       as part of its complete documentation whether printed or
       otherwise, this work may be distributed only under the
       terms of Perl's Artistic Licence.  Any distribution of
       this file or derivatives thereof outside of that package
       require that special arrangements be made with copyright
       holder.

       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.



































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