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