Doc patches: assorted minor nits

[perl5.git] / pod / perlfaq9.pod

=head1 NAME

perlfaq9 - Networking ($Revision: 1.9 $, $Date: 2002/04/07 18:46:13 $)

=head1 DESCRIPTION

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

=head2 What is the correct form of response from a CGI script?

(Alan Flavell <flavell+www@a5.ph.gla.ac.uk> answers...)

The Common Gateway Interface (CGI) specifies a software interface between 
a program ("CGI script") and a web server (HTTPD). It is not specific 
to Perl, and has its own FAQs and tutorials, and usenet group, 
comp.infosystems.www.authoring.cgi 

The original CGI specification is at: http://hoohoo.ncsa.uiuc.edu/cgi/

Current best-practice RFC draft at: http://CGI-Spec.Golux.Com/

Other relevant documentation listed in: http://www.perl.org/CGI_MetaFAQ.html

These Perl FAQs very selectively cover some CGI issues. However, Perl 
programmers are strongly advised to use the CGI.pm module, to take care
of the details for them. 

The similarity between CGI response headers (defined in the CGI
specification) and HTTP response headers (defined in the HTTP
specification, RFC2616) is intentional, but can sometimes be confusing.

The CGI specification defines two kinds of script: the "Parsed Header"
script, and the "Non Parsed Header" (NPH) script. Check your server
documentation to see what it supports. "Parsed Header" scripts are
simpler in various respects. The CGI specification allows any of the
usual newline representations in the CGI response (it's the server's
job to create an accurate HTTP response based on it). So "\n" written in
text mode is technically correct, and recommended. NPH scripts are more
tricky: they must put out a complete and accurate set of HTTP
transaction response headers; the HTTP specification calls for records
to be terminated with carriage-return and line-feed, i.e ASCII \015\012
written in binary mode.

Using CGI.pm gives excellent platform independence, including EBCDIC
systems. CGI.pm selects an appropriate newline representation
($CGI::CRLF) and sets binmode as appropriate.

=head2 My CGI script runs from the command line but not the browser.  (500 Server Error)

Several things could be wrong.  You can go through the "Troubleshooting
Perl CGI scripts" guide at

        http://www.perl.org/troubleshooting_CGI.html

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

The useful FAQs, related documents, and troubleshooting guides are 
listed in the CGI Meta FAQ:

        http://www.perl.org/CGI_MetaFAQ.html


=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.cpan.org/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?

You can easily extract all sorts of URLs from HTML with
C<HTML::SimpleLinkExtor> which handles anchors, images, objects,
frames, and many other tags that can contain a URL.  If you need 
anything more complex, you can create your own subclass of 
C<HTML::LinkExtor> or C<HTML::Parser>.  You might even use 
C<HTML::SimpleLinkExtor> as an example for something specifically
suited to your needs.

Less complete solutions involving regular expressions can save 
you a lot of processing time if you know that the input is simple.  One
solution from Tom Christiansen runs 100 times faster than most
module based approaches but only extracts URLs from anchors where the first
attribute is HREF and there are no other attributes. 

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


=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?


If you are writing a CGI script, you should be using the CGI.pm module
that comes with perl, or some other equivalent module.  The CGI module
automatically decodes queries for you, and provides an escape()
function to handle encoding.


The best source of detailed information on URI encoding is RFC 2396.
Basically, the following substitutions do it:

    s/([^\w()'*~!.-])/sprintf '%%%02x', ord $1/eg;   # encode

    s/%([A-Fa-f\d]{2})/chr hex $1/eg;            # decode

However, you should only apply them to individual URI components, not
the entire URI, otherwise you'll lose information and generally mess
things up.  If that didn't explain it, don't worry.  Just go read
section 2 of the RFC, it's probably the best explanation there is.

RFC 2396 also contains a lot of other useful information, including a
regexp for breaking any arbitrary URI into components (Appendix B).

=head2 How do I redirect to another page?

Specify the complete URL of the destination (even if it is on the same
server). This is one of the two different kinds of CGI "Location:"
responses which are defined in the CGI specification for a Parsed Headers
script. The other kind (an absolute URLpath) is resolved internally to
the server without any HTTP redirection. The CGI specifications do not
allow relative URLs in either case.

Use of CGI.pm is strongly recommended.  This example shows redirection
with a complete URL. This redirection is handled by the web browser.

      use CGI qw/:standard/;

      my $url = 'http://www.cpan.org/';
      print redirect($url);


This example shows a redirection with an absolute URLpath.  This
redirection is handled by the local web server.

      my $url = '/CPAN/index.html';
      print redirect($url);


But if coded directly, it could be as follows (the final "\n" is 
shown separately, for clarity), using either a complete URL or
an absolute URLpath. 

      print "Location: $url\n";   # CGI response header
      print "\n";                 # end of headers


=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, Berkeley 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?

See the security references listed in the CGI Meta FAQ

        http://www.perl.org/CGI_MetaFAQ.html

=head2 How do I parse a mail header?

For a quick-and-dirty solution, try this solution derived
from L<perlfunc/split>:

    $/ = '';
    $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 it is not suggested.  Instead, see
http://www.cpan.org/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-Base64 package (available from CPAN) handles this as well as
the MIME/QP encoding.  Decoding BASE64 becomes as simple as:

    use MIME::Base64;
    $decoded = decode_base64($encoded);

The MIME-Tools package (available from CPAN) supports extraction with
decoding of BASE64 encoded attachments and content directly from email
messages.

If the string to decode is short (less than 84 bytes long)
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 queuing, MX records, and security.

=head2 How do I use MIME to make an attachment to a mail message?

This answer is extracted directly from the MIME::Lite documentation.
Create a multipart message (i.e., one with attachments).

    use MIME::Lite;

    ### Create a new multipart message:
    $msg = MIME::Lite->new(
                 From    =>'me@myhost.com',
                 To      =>'you@yourhost.com',
                 Cc      =>'some@other.com, some@more.com',
                 Subject =>'A message with 2 parts...',
                 Type    =>'multipart/mixed'
                 );

    ### Add parts (each "attach" has same arguments as "new"):
    $msg->attach(Type     =>'TEXT',
                 Data     =>"Here's the GIF file you wanted"
                 );
    $msg->attach(Type     =>'image/gif',
                 Path     =>'aaa000123.gif',
                 Filename =>'logo.gif'
                 );

    $text = $msg->as_string;

MIME::Lite also includes a method for sending these things.

    $msg->send;

This defaults to using L<sendmail(1)> but can be customized to use
SMTP via L<Net::SMTP>.

=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.  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) 1997-2002 Tom Christiansen and Nathan Torkington.
All rights reserved.

This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.

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.