path: root/pod/perlfaq9.pod

=head1 NAME

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

=head1 DESCRIPTION

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

=head2 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/

=head2 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
C<s/E<lt>.*?E<gt>//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 C<&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
.

=head2 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.

=head2 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
B<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.

=head2 How do I make a pop-up menu in HTML?

Use the B<E<lt>SELECTE<gt>> and B<E<lt>OPTIONE<gt>> 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.

=head2 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;

=head2 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 (C<\W>) into their hex escapes.
It's important that characters with special meaning like C</> and C<?>
I<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.

=head2 How do I redirect to another page?

Instead of sending back a C<Content-Type> as the headers of your
reply, send back a C<Location:> header.  Officially this should be a
C<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.

=head2 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.

=head2 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);

=head2 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 L<perlsec>), which makes sure that data
from outside your script (eg, CGI parameters) are never used in
C<eval> or C<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.

=head2 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).

=head2 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 C<$ENV{CONTENT_LENGTH}> and
C<$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).

=head2 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
C</^[\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.

=head2 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

=head2 How do I return the user's email address?

On systems that support getpwuid, the $E<lt> 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.

=head2 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;

=head2 How do I find out my hostname/domainname/IP address?

A lot of code has historically cavalierly called the C<`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.)

=head2 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")'

=head2 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.

=head2 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.

=head1 AUTHOR AND COPYRIGHT

Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
All rights reserved.  See L<perlfaq> for distribution information.