path: root/pod/perlfaq9.pod

=head1 NAME

perlfaq9 - Networking ($Revision: 1.26 $, $Date: 1999/05/23 16:08:30 $)

=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.   (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

=head2 How can I get better error messages from a CGI program?

Use the CGI::Carp module.  It replaces C<warn> and C<die>, plus the
normal Carp modules C<carp>, C<croak>, and C<confess> 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 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 C<carpout>) with the application name and date
stamp prepended.

=head2 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.

Many folks attempt a simple-minded regular expression approach, like
C<< 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 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
.

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>
    -->

=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, understand extra qualifiers like TARGET, 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<< <SELECT> >> and B<< <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.

=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 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;

=head2 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 C<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.

    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;

=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 characters that are not letters, digits or underscores (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,
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.

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

To target a particular frame in a frameset, include the "Window-target:"
in the header.

    print <<EOF;
    Location: http://www.domain.com/newpage
    Window-target: <FrameName>

    EOF

To be correct to the spec, each of those virtual newlines should really be
physical C<"\015\012"> sequences by the time you hit the client browser.
Except for NPH scripts, though, that local newline should get translated
by your server into standard form, so you shouldn't have a problem
here, even if you are stuck on MacOS.  Everybody else probably won't
even notice.

=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 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).

=head2 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
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).

=head2 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 regex, such as
C</^[\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 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.

=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 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 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.

=head2 How do I send mail?

Use the C<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 B<-oi> option prevents sendmail from interpreting a line consisting
of a single dot as "end of message".  The B<-t> option says to use the
headers to decide who to send the message to, and B<-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.

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 queueing, MX records, and security.

=head2 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 (also part
of the MailTools package), 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,

    #!/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) ] }

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

The normal way to find your own hostname is to call the C<`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.)

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

=head1 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 License.
Any distribution of this file or derivatives thereof I<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.