=head1 NAME
-perlfaq9 - Networking ($Revision: 1.17 $, $Date: 1997/04/24 22:44:29 $)
+perlfaq9 - Networking ($Revision: 1.20 $, $Date: 1998/06/22 18:31:09 $)
=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?
+=head2 My CGI script runs from the command line but not the browser. (500 Server Error)
-Sure, but you probably can't afford our contracting rates :-)
+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.
-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 and related documents are:
-The useful FAQs are:
+ CGI FAQ
+ http://www.webthing.com/page.cgi/cgifaq
- 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/
+ 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?
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
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
+ # simplest version
+ use LWP::Simple;
+ $content = get($URL);
+
+ # or print HTML from a URL
use LWP::Simple;
getprint "http://www.sn.no/libwww-perl/";
- # print ASCII from HTML from a URL
+ # or print ASCII from HTML from a URL
use LWP::Simple;
use HTML::Parse;
use HTML::FormatText;
$ascii = HTML::FormatText->new->format(parse_html($html));
print $ascii;
-=head2 how do I decode or create those %-encodings on the web?
+=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:
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: $url\n\n";
+ exit;
+
+To be correct to the spec, each of those C<"\n">
+should really each be C<"\015\012">, but unless you're
+stuck on MacOS, you probably won't notice.
+
=head2 How do I put a password on my web pages?
That depends. You'll need to read the documentation for your web
=head2 How do I decode a CGI form?
-A lot of people are tempted to code this up themselves, so you've
-probably all seen a lot of code involving C<$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).
+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://www.bio.cam.ac.uk/web/form.html).
+
+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.
+You can't, at least, not in real time. Bummer, eh?
-Without sending mail to the address and seeing whether it bounces (and
-even then you face the halting problem), you cannot determine whether
-an 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.
+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 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
+Many are tempted to try to eliminate many frequently-invalid
+mail addresses with a simple regexp, 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 DNS. It's not fast, but it works.
+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.
-Here's an alternative strategy used by many CGI script authors: Check
-the mail 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
-mail address they entered was correct.
+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?
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?
+=head2 How do I send 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
+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, 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.
+
+Or 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?
+
+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).
# send it
$mail->smtpsend or die;
+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?
-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 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
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