perlfaq9

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NAME

       perlfaq9 - Networking ($Revision: 1.17 $, $Date:
       1997/04/24 22:44:29 $)


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.  Can you help me fix it?

       Sure, but you probably can't afford our contracting rates
       :-)

       Seriously, 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 are:

           http://www.perl.com/perl/faq/idiots-guide.html
           http://www3.pair.com/webthing/docs/cgi/faqs/cgifaq.shtml
           http://www.perl.com/perl/faq/perl-cgi-faq.html
           http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html
           http://www.boutell.com/faq/


       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 libwww-perl
       distribution, which is a must-have module for all web
       hackers).

       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
       .

       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.

       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:


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

           # print ASCII from HTML from a URL
           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 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-alphanumunder character (\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.

       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.

       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 an email 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?

       A lot of people are tempted to code this up themselves, so
       you've probably all seen a lot of code involving
       $ENV{CONTENT_LENGTH} and $ENV{QUERY_STRING}.  It's true
       that this can work, but there are also a lot of versions
       of this floating around that are quite simply broken!

       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://www.bio.cam.ac.uk/web/form.html).
       How do I check a valid email address?

       You can't.

       Without sending mail to the address and seeing whether it
       bounces (and even then you face the halting problem), you
       cannot determine whether an email address is valid.  Even
       if you apply the email 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 email addresses with a simple regexp, such as
       /^[\w.-]+\@([\w.-]\.)+\w+$/.  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 email to (say, Bill Clinton or your postmaster),
       and then makes sure that the hostname given can be looked
       up in DNS.  It's not fast, but it works.

       Here's an alternative strategy used by many CGI script
       authors: Check the email address with a simple regexp
       (such as the one above).  If the regexp matched the
       address, accept the address.  If the regexp didn't match
       the address, request confirmation from the user that the
       email address they entered was correct.

       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 email 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;
           $address = sprintf('%s@%s', getpwuid($<), hostname);

       Company policies on email address can mean that this
       generates addresses that the company's email system will
       not accept, so you should ask for users' email 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/read mail?

       Sending mail: the Mail::Mailer module from CPAN (part of
       the MailTools package) is UNIX-centric, while
       Mail::Internet uses Net::SMTP which is not UNIX-centric.
       Reading 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;


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

       A lot of code has historically cavalierly called the
       `hostname` program.  While sometimes expedient, this isn't
       very portable.  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 $addr = inet_ntoa(scalar(gethostbyname($name)) || '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
       package (available from CPAN).  No ONC::RPC module is
       known.


AUTHOR AND COPYRIGHT

       Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
       All rights reserved.  See the perlfaq manpage for
       distribution information.
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