3 perlfaq9 - Networking ($Revision: 1.26 $, $Date: 1999/05/23 16:08:30 $)
7 This section deals with questions related to networking, the internet,
10 =head2 My CGI script runs from the command line but not the browser. (500 Server Error)
12 If you can demonstrate that you've read the following FAQs and that
13 your problem isn't something simple that can be easily answered, you'll
14 probably receive a courteous and useful reply to your question if you
15 post it on comp.infosystems.www.authoring.cgi (if it's something to do
16 with HTTP, HTML, or the CGI protocols). Questions that appear to be Perl
17 questions but are really CGI ones that are posted to comp.lang.perl.misc
18 may not be so well received.
20 The useful FAQs and related documents are:
23 http://www.webthing.com/page.cgi/cgifaq
26 http://www.boutell.com/faq/
29 http://www.w3.org/Security/Faq/
32 http://www.w3.org/pub/WWW/Protocols/HTTP/
35 http://www.w3.org/TR/REC-html40/
36 http://www.w3.org/pub/WWW/MarkUp/
39 http://www.w3.org/CGI/
42 http://www.go2net.com/people/paulp/cgi-security/safe-cgi.txt
44 =head2 How can I get better error messages from a CGI program?
46 Use the CGI::Carp module. It replaces C<warn> and C<die>, plus the
47 normal Carp modules C<carp>, C<croak>, and C<confess> functions with
48 more verbose and safer versions. It still sends them to the normal
52 warn "This is a complaint";
53 die "But this one is serious";
55 The following use of CGI::Carp also redirects errors to a file of your choice,
56 placed in a BEGIN block to catch compile-time warnings as well:
59 use CGI::Carp qw(carpout);
60 open(LOG, ">>/var/local/cgi-logs/mycgi-log")
61 or die "Unable to append to mycgi-log: $!\n";
65 You can even arrange for fatal errors to go back to the client browser,
66 which is nice for your own debugging, but might confuse the end user.
68 use CGI::Carp qw(fatalsToBrowser);
71 Even if the error happens before you get the HTTP header out, the module
72 will try to take care of this to avoid the dreaded server 500 errors.
73 Normal warnings still go out to the server error log (or wherever
74 you've sent them with C<carpout>) with the application name and date
77 =head2 How do I remove HTML from a string?
79 The most correct way (albeit not the fastest) is to use HTML::Parse
80 from CPAN (part of the HTML-Tree package on CPAN).
82 Many folks attempt a simple-minded regular expression approach, like
83 C<s/E<lt>.*?E<gt>//g>, but that fails in many cases because the tags
84 may continue over line breaks, they may contain quoted angle-brackets,
85 or HTML comment may be present. Plus folks forget to convert
86 entities, like C<<> for example.
88 Here's one "simple-minded" approach, that works for most files:
90 #!/usr/bin/perl -p0777
91 s/<(?:[^>'"]*|(['"]).*?\1)*>//gs
93 If you want a more complete solution, see the 3-stage striphtml
95 http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/striphtml.gz
98 Here are some tricky cases that you should think about when picking
101 <IMG SRC = "foo.gif" ALT = "A > B">
108 <script>if (a<b && a>c)</script>
112 <![INCLUDE CDATA [ >>>>>>>>>>>> ]]>
114 If HTML comments include other tags, those solutions would also break
117 <!-- This section commented out.
118 <B>You can't see me!</B>
121 =head2 How do I extract URLs?
123 A quick but imperfect approach is
126 # qxurl - tchrist@perl.com
127 print "$2\n" while m{
129 A \s+ HREF \s* = \s* (["']) (.*?) \1
133 This version does not adjust relative URLs, understand alternate
134 bases, deal with HTML comments, deal with HREF and NAME attributes
135 in the same tag, understand extra qualifiers like TARGET, or accept
136 URLs themselves as arguments. It also runs about 100x faster than a
137 more "complete" solution using the LWP suite of modules, such as the
138 http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/xurl.gz program.
140 =head2 How do I download a file from the user's machine? How do I open a file on another machine?
142 In the context of an HTML form, you can use what's known as
143 B<multipart/form-data> encoding. The CGI.pm module (available from
144 CPAN) supports this in the start_multipart_form() method, which isn't
145 the same as the startform() method.
147 =head2 How do I make a pop-up menu in HTML?
149 Use the B<E<lt>SELECTE<gt>> and B<E<lt>OPTIONE<gt>> tags. The CGI.pm
150 module (available from CPAN) supports this widget, as well as many
151 others, including some that it cleverly synthesizes on its own.
153 =head2 How do I fetch an HTML file?
155 One approach, if you have the lynx text-based HTML browser installed
156 on your system, is this:
158 $html_code = `lynx -source $url`;
159 $text_data = `lynx -dump $url`;
161 The libwww-perl (LWP) modules from CPAN provide a more powerful way
162 to do this. They don't require lynx, but like lynx, can still work
167 $content = get($URL);
169 # or print HTML from a URL
171 getprint "http://www.sn.no/libwww-perl/";
173 # or print ASCII from HTML from a URL
174 # also need HTML-Tree package from CPAN
177 use HTML::FormatText;
179 $html = get("http://www.perl.com/");
181 or die "Can't fetch HTML from http://www.perl.com/";
182 $ascii = HTML::FormatText->new->format(parse_html($html));
185 =head2 How do I automate an HTML form submission?
187 If you're submitting values using the GET method, create a URL and encode
188 the form using the C<query_form> method:
193 my $url = url('http://www.perl.com/cgi-bin/cpan_mod');
194 $url->query_form(module => 'DB_File', readme => 1);
195 $content = get($url);
197 If you're using the POST method, create your own user agent and encode
198 the content appropriately.
200 use HTTP::Request::Common qw(POST);
203 $ua = LWP::UserAgent->new();
204 my $req = POST 'http://www.perl.com/cgi-bin/cpan_mod',
205 [ module => 'DB_File', readme => 1 ];
206 $content = $ua->request($req)->as_string;
208 =head2 How do I decode or create those %-encodings on the web?
210 Here's an example of decoding:
212 $string = "http://altavista.digital.com/cgi-bin/query?pg=q&what=news&fmt=.&q=%2Bcgi-bin+%2Bperl.exe";
213 $string =~ s/%([a-fA-F0-9]{2})/chr(hex($1))/ge;
215 Encoding is a bit harder, because you can't just blindly change
216 all the non-alphanumunder character (C<\W>) into their hex escapes.
217 It's important that characters with special meaning like C</> and C<?>
218 I<not> be translated. Probably the easiest way to get this right is
219 to avoid reinventing the wheel and just use the URI::Escape module,
220 which is part of the libwww-perl package (LWP) available from CPAN.
222 =head2 How do I redirect to another page?
224 Instead of sending back a C<Content-Type> as the headers of your
225 reply, send back a C<Location:> header. Officially this should be a
226 C<URI:> header, so the CGI.pm module (available from CPAN) sends back
229 Location: http://www.domain.com/newpage
230 URI: http://www.domain.com/newpage
232 Note that relative URLs in these headers can cause strange effects
233 because of "optimizations" that servers do.
235 $url = "http://www.perl.com/CPAN/";
236 print "Location: $url\n\n";
239 To target a particular frame in a frameset, include the "Window-target:"
243 Location: http://www.domain.com/newpage
244 Window-target: <FrameName>
248 To be correct to the spec, each of those virtual newlines should really be
249 physical C<"\015\012"> sequences by the time you hit the client browser.
250 Except for NPH scripts, though, that local newline should get translated
251 by your server into standard form, so you shouldn't have a problem
252 here, even if you are stuck on MacOS. Everybody else probably won't
255 =head2 How do I put a password on my web pages?
257 That depends. You'll need to read the documentation for your web
258 server, or perhaps check some of the other FAQs referenced above.
260 =head2 How do I edit my .htpasswd and .htgroup files with Perl?
262 The HTTPD::UserAdmin and HTTPD::GroupAdmin modules provide a
263 consistent OO interface to these files, regardless of how they're
264 stored. Databases may be text, dbm, Berkley DB or any database with a
265 DBI compatible driver. HTTPD::UserAdmin supports files used by the
266 `Basic' and `Digest' authentication schemes. Here's an example:
268 use HTTPD::UserAdmin ();
270 ->new(DB => "/foo/.htpasswd")
271 ->add($username => $password);
273 =head2 How do I make sure users can't enter values into a form that cause my CGI script to do bad things?
275 Read the CGI security FAQ, at
276 http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html, and the
278 http://www.perl.com/CPAN/doc/FAQs/cgi/perl-cgi-faq.html.
280 In brief: use tainting (see L<perlsec>), which makes sure that data
281 from outside your script (eg, CGI parameters) are never used in
282 C<eval> or C<system> calls. In addition to tainting, never use the
283 single-argument form of system() or exec(). Instead, supply the
284 command and arguments as a list, which prevents shell globbing.
286 =head2 How do I parse a mail header?
288 For a quick-and-dirty solution, try this solution derived
289 from page 222 of the 2nd edition of "Programming Perl":
293 $header =~ s/\n\s+/ /g; # merge continuation lines
294 %head = ( UNIX_FROM_LINE, split /^([-\w]+):\s*/m, $header );
296 That solution doesn't do well if, for example, you're trying to
297 maintain all the Received lines. A more complete approach is to use
298 the Mail::Header module from CPAN (part of the MailTools package).
300 =head2 How do I decode a CGI form?
302 You use a standard module, probably CGI.pm. Under no circumstances
303 should you attempt to do so by hand!
305 You'll see a lot of CGI programs that blindly read from STDIN the number
306 of bytes equal to CONTENT_LENGTH for POSTs, or grab QUERY_STRING for
307 decoding GETs. These programs are very poorly written. They only work
308 sometimes. They typically forget to check the return value of the read()
309 system call, which is a cardinal sin. They don't handle HEAD requests.
310 They don't handle multipart forms used for file uploads. They don't deal
311 with GET/POST combinations where query fields are in more than one place.
312 They don't deal with keywords in the query string.
314 In short, they're bad hacks. Resist them at all costs. Please do not be
315 tempted to reinvent the wheel. Instead, use the CGI.pm or CGI_Lite.pm
316 (available from CPAN), or if you're trapped in the module-free land
317 of perl1 .. perl4, you might look into cgi-lib.pl (available from
318 http://cgi-lib.stanford.edu/cgi-lib/ ).
320 Make sure you know whether to use a GET or a POST in your form.
321 GETs should only be used for something that doesn't update the server.
322 Otherwise you can get mangled databases and repeated feedback mail
323 messages. The fancy word for this is ``idempotency''. This simply
324 means that there should be no difference between making a GET request
325 for a particular URL once or multiple times. This is because the
326 HTTP protocol definition says that a GET request may be cached by the
327 browser, or server, or an intervening proxy. POST requests cannot be
328 cached, because each request is independent and matters. Typically,
329 POST requests change or depend on state on the server (query or update
330 a database, send mail, or purchase a computer).
332 =head2 How do I check a valid mail address?
334 You can't, at least, not in real time. Bummer, eh?
336 Without sending mail to the address and seeing whether there's a human
337 on the other hand to answer you, you cannot determine whether a mail
338 address is valid. Even if you apply the mail header standard, you
339 can have problems, because there are deliverable addresses that aren't
340 RFC-822 (the mail header standard) compliant, and addresses that aren't
341 deliverable which are compliant.
343 Many are tempted to try to eliminate many frequently-invalid
344 mail addresses with a simple regex, such as
345 C</^[\w.-]+\@([\w.-]\.)+\w+$/>. It's a very bad idea. However,
346 this also throws out many valid ones, and says nothing about
347 potential deliverability, so is not suggested. Instead, see
348 http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/ckaddr.gz ,
349 which actually checks against the full RFC spec (except for nested
350 comments), looks for addresses you may not wish to accept mail to
351 (say, Bill Clinton or your postmaster), and then makes sure that the
352 hostname given can be looked up in the DNS MX records. It's not fast,
353 but it works for what it tries to do.
355 Our best advice for verifying a person's mail address is to have them
356 enter their address twice, just as you normally do to change a password.
357 This usually weeds out typos. If both versions match, send
358 mail to that address with a personal message that looks somewhat like:
360 Dear someuser@host.com,
362 Please confirm the mail address you gave us Wed May 6 09:38:41
363 MDT 1998 by replying to this message. Include the string
364 "Rumpelstiltskin" in that reply, but spelled in reverse; that is,
365 start with "Nik...". Once this is done, your confirmed address will
366 be entered into our records.
368 If you get the message back and they've followed your directions,
369 you can be reasonably assured that it's real.
371 A related strategy that's less open to forgery is to give them a PIN
372 (personal ID number). Record the address and PIN (best that it be a
373 random one) for later processing. In the mail you send, ask them to
374 include the PIN in their reply. But if it bounces, or the message is
375 included via a ``vacation'' script, it'll be there anyway. So it's
376 best to ask them to mail back a slight alteration of the PIN, such as
377 with the characters reversed, one added or subtracted to each digit, etc.
379 =head2 How do I decode a MIME/BASE64 string?
381 The MIME-tools package (available from CPAN) handles this and a lot
382 more. Decoding BASE64 becomes as simple as:
385 $decoded = decode_base64($encoded);
387 A more direct approach is to use the unpack() function's "u"
388 format after minor transliterations:
390 tr#A-Za-z0-9+/##cd; # remove non-base64 chars
391 tr#A-Za-z0-9+/# -_#; # convert to uuencoded format
392 $len = pack("c", 32 + 0.75*length); # compute length byte
393 print unpack("u", $len . $_); # uudecode and print
395 =head2 How do I return the user's mail address?
397 On systems that support getpwuid, the $E<lt> variable and the
398 Sys::Hostname module (which is part of the standard perl distribution),
399 you can probably try using something like this:
402 $address = sprintf('%s@%s', getpwuid($<), hostname);
404 Company policies on mail address can mean that this generates addresses
405 that the company's mail system will not accept, so you should ask for
406 users' mail addresses when this matters. Furthermore, not all systems
407 on which Perl runs are so forthcoming with this information as is Unix.
409 The Mail::Util module from CPAN (part of the MailTools package) provides a
410 mailaddress() function that tries to guess the mail address of the user.
411 It makes a more intelligent guess than the code above, using information
412 given when the module was installed, but it could still be incorrect.
413 Again, the best way is often just to ask the user.
415 =head2 How do I send mail?
417 Use the C<sendmail> program directly:
419 open(SENDMAIL, "|/usr/lib/sendmail -oi -t -odq")
420 or die "Can't fork for sendmail: $!\n";
421 print SENDMAIL <<"EOF";
422 From: User Originating Mail <me\@host>
423 To: Final Destination <you\@otherhost>
424 Subject: A relevant subject line
426 Body of the message goes here after the blank line
427 in as many lines as you like.
429 close(SENDMAIL) or warn "sendmail didn't close nicely";
431 The B<-oi> option prevents sendmail from interpreting a line consisting
432 of a single dot as "end of message". The B<-t> option says to use the
433 headers to decide who to send the message to, and B<-odq> says to put
434 the message into the queue. This last option means your message won't
435 be immediately delivered, so leave it out if you want immediate
438 Alternate, less convenient approaches include calling mail (sometimes
439 called mailx) directly or simply opening up port 25 have having an
440 intimate conversation between just you and the remote SMTP daemon,
443 Or you might be able use the CPAN module Mail::Mailer:
447 $mailer = Mail::Mailer->new();
448 $mailer->open({ From => $from_address,
452 or die "Can't open: $!\n";
456 The Mail::Internet module uses Net::SMTP which is less Unix-centric than
457 Mail::Mailer, but less reliable. Avoid raw SMTP commands. There
458 are many reasons to use a mail transport agent like sendmail. These
459 include queueing, MX records, and security.
461 =head2 How do I read mail?
463 While you could use the Mail::Folder module from CPAN (part of the
464 MailFolder package) or the Mail::Internet module from CPAN (also part
465 of the MailTools package), often a module is overkill, though. Here's a
469 # bysub1 - simple sort by subject
472 $/ = ''; # paragraph reads
475 /^Subject:\s*(?:Re:\s*)*(.*)/mi;
476 $sub[++$msgno] = lc($1) || '';
480 for my $i (sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msgs)) {
487 # bysub2 - awkish sort-by-subject
488 BEGIN { $msgno = -1 }
489 $sub[++$msgno] = (/^Subject:\s*(?:Re:\s*)*(.*)/mi)[0] if /^From/m;
491 END { print @msg[ sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msg) ] }
493 =head2 How do I find out my hostname/domainname/IP address?
495 The normal way to find your own hostname is to call the C<`hostname`>
496 program. While sometimes expedient, this has some problems, such as
497 not knowing whether you've got the canonical name or not. It's one of
498 those tradeoffs of convenience versus portability.
500 The Sys::Hostname module (part of the standard perl distribution) will
501 give you the hostname after which you can find out the IP address
502 (assuming you have working DNS) with a gethostbyname() call.
506 my $host = hostname();
507 my $addr = inet_ntoa(scalar gethostbyname($host || 'localhost'));
509 Probably the simplest way to learn your DNS domain name is to grok
510 it out of /etc/resolv.conf, at least under Unix. Of course, this
511 assumes several things about your resolv.conf configuration, including
514 (We still need a good DNS domain name-learning method for non-Unix
517 =head2 How do I fetch a news article or the active newsgroups?
519 Use the Net::NNTP or News::NNTPClient modules, both available from CPAN.
520 This can make tasks like fetching the newsgroup list as simple as:
522 perl -MNews::NNTPClient
523 -e 'print News::NNTPClient->new->list("newsgroups")'
525 =head2 How do I fetch/put an FTP file?
527 LWP::Simple (available from CPAN) can fetch but not put. Net::FTP (also
528 available from CPAN) is more complex but can put as well as fetch.
530 =head2 How can I do RPC in Perl?
532 A DCE::RPC module is being developed (but is not yet available), and
533 will be released as part of the DCE-Perl package (available from
534 CPAN). The rpcgen suite, available from CPAN/authors/id/JAKE/, is
535 an RPC stub generator and includes an RPC::ONC module.
537 =head1 AUTHOR AND COPYRIGHT
539 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
542 When included as part of the Standard Version of Perl, or as part of
543 its complete documentation whether printed or otherwise, this work
544 may be distributed only under the terms of Perl's Artistic License.
545 Any distribution of this file or derivatives thereof I<outside>
546 of that package require that special arrangements be made with
549 Irrespective of its distribution, all code examples in this file
550 are hereby placed into the public domain. You are permitted and
551 encouraged to use this code in your own programs for fun
552 or for profit as you see fit. A simple comment in the code giving
553 credit would be courteous but is not required.