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/tutorials/cgifaq.html
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::Parser
80 from CPAN. Another mostly correct
81 way is to use HTML::FormatText which not only removes HTML but also
82 attempts to do a little simple formatting of the resulting plain text.
84 Many folks attempt a simple-minded regular expression approach, like
85 C<< s/<.*?>//g >>, but that fails in many cases because the tags
86 may continue over line breaks, they may contain quoted angle-brackets,
87 or HTML comment may be present. Plus, folks forget to convert
88 entities--like C<<> for example.
90 Here's one "simple-minded" approach, that works for most files:
92 #!/usr/bin/perl -p0777
93 s/<(?:[^>'"]*|(['"]).*?\1)*>//gs
95 If you want a more complete solution, see the 3-stage striphtml
97 http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/striphtml.gz
100 Here are some tricky cases that you should think about when picking
103 <IMG SRC = "foo.gif" ALT = "A > B">
110 <script>if (a<b && a>c)</script>
114 <![INCLUDE CDATA [ >>>>>>>>>>>> ]]>
116 If HTML comments include other tags, those solutions would also break
119 <!-- This section commented out.
120 <B>You can't see me!</B>
123 =head2 How do I extract URLs?
125 A quick but imperfect approach is
128 # qxurl - tchrist@perl.com
129 print "$2\n" while m{
131 A \s+ HREF \s* = \s* (["']) (.*?) \1
135 This version does not adjust relative URLs, understand alternate
136 bases, deal with HTML comments, deal with HREF and NAME attributes
137 in the same tag, understand extra qualifiers like TARGET, or accept
138 URLs themselves as arguments. It also runs about 100x faster than a
139 more "complete" solution using the LWP suite of modules, such as the
140 http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/xurl.gz program.
142 =head2 How do I download a file from the user's machine? How do I open a file on another machine?
144 In the context of an HTML form, you can use what's known as
145 B<multipart/form-data> encoding. The CGI.pm module (available from
146 CPAN) supports this in the start_multipart_form() method, which isn't
147 the same as the startform() method.
149 =head2 How do I make a pop-up menu in HTML?
151 Use the B<< <SELECT> >> and B<< <OPTION> >> tags. The CGI.pm
152 module (available from CPAN) supports this widget, as well as many
153 others, including some that it cleverly synthesizes on its own.
155 =head2 How do I fetch an HTML file?
157 One approach, if you have the lynx text-based HTML browser installed
158 on your system, is this:
160 $html_code = `lynx -source $url`;
161 $text_data = `lynx -dump $url`;
163 The libwww-perl (LWP) modules from CPAN provide a more powerful way
164 to do this. They don't require lynx, but like lynx, can still work
169 $content = get($URL);
171 # or print HTML from a URL
173 getprint "http://www.linpro.no/lwp/";
175 # or print ASCII from HTML from a URL
176 # also need HTML-Tree package from CPAN
179 use HTML::FormatText;
181 $html = get("http://www.perl.com/");
183 or die "Can't fetch HTML from http://www.perl.com/";
184 $ascii = HTML::FormatText->new->format(parse_html($html));
187 =head2 How do I automate an HTML form submission?
189 If you're submitting values using the GET method, create a URL and encode
190 the form using the C<query_form> method:
195 my $url = url('http://www.perl.com/cgi-bin/cpan_mod');
196 $url->query_form(module => 'DB_File', readme => 1);
197 $content = get($url);
199 If you're using the POST method, create your own user agent and encode
200 the content appropriately.
202 use HTTP::Request::Common qw(POST);
205 $ua = LWP::UserAgent->new();
206 my $req = POST 'http://www.perl.com/cgi-bin/cpan_mod',
207 [ module => 'DB_File', readme => 1 ];
208 $content = $ua->request($req)->as_string;
210 =head2 How do I decode or create those %-encodings on the web?
212 Here's an example of decoding:
214 $string = "http://altavista.digital.com/cgi-bin/query?pg=q&what=news&fmt=.&q=%2Bcgi-bin+%2Bperl.exe";
215 $string =~ s/%([a-fA-F0-9]{2})/chr(hex($1))/ge;
217 Encoding is a bit harder, because you can't just blindly change
218 all characters that are not letters, digits or underscores (C<\W>)
219 into their hex escapes.
220 It's important that characters with special meaning like C</> and C<?>
221 I<not> be translated. Probably the easiest way to get this right is
222 to avoid reinventing the wheel and just use the URI::Escape module,
225 =head2 How do I redirect to another page?
227 According to RFC 2616, "Hypertext Transfer Protocol -- HTTP/1.1", the
228 preferred method is to send a C<Location:> header instead of a
229 C<Content-Type:> header:
231 Location: http://www.domain.com/newpage
233 Note that relative URLs in these headers can cause strange effects
234 because of "optimizations" that servers do.
236 $url = "http://www.perl.com/CPAN/";
237 print "Location: $url\n\n";
240 To target a particular frame in a frameset, include the "Window-target:"
244 Location: http://www.domain.com/newpage
245 Window-target: <FrameName>
249 To be correct to the spec, each of those virtual newlines should
250 really be physical C<"\015\012"> sequences by the time your message is
251 received by the client browser. Except for NPH scripts, though, that
252 local newline should get translated by your server into standard form,
253 so you shouldn't have a problem here, even if you are stuck on MacOS.
254 Everybody else probably won't even notice.
256 =head2 How do I put a password on my web pages?
258 That depends. You'll need to read the documentation for your web
259 server, or perhaps check some of the other FAQs referenced above.
261 =head2 How do I edit my .htpasswd and .htgroup files with Perl?
263 The HTTPD::UserAdmin and HTTPD::GroupAdmin modules provide a
264 consistent OO interface to these files, regardless of how they're
265 stored. Databases may be text, dbm, Berkley DB or any database with a
266 DBI compatible driver. HTTPD::UserAdmin supports files used by the
267 `Basic' and `Digest' authentication schemes. Here's an example:
269 use HTTPD::UserAdmin ();
271 ->new(DB => "/foo/.htpasswd")
272 ->add($username => $password);
274 =head2 How do I make sure users can't enter values into a form that cause my CGI script to do bad things?
276 Read the CGI security FAQ, at
277 http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html , and the
279 http://www.perl.com/CPAN/doc/FAQs/cgi/perl-cgi-faq.html .
281 In brief: use tainting (see L<perlsec>), which makes sure that data
282 from outside your script (eg, CGI parameters) are never used in
283 C<eval> or C<system> calls. In addition to tainting, never use the
284 single-argument form of system() or exec(). Instead, supply the
285 command and arguments as a list, which prevents shell globbing.
287 =head2 How do I parse a mail header?
289 For a quick-and-dirty solution, try this solution derived
290 from L<perlfunc/split>:
294 $header =~ s/\n\s+/ /g; # merge continuation lines
295 %head = ( UNIX_FROM_LINE, split /^([-\w]+):\s*/m, $header );
297 That solution doesn't do well if, for example, you're trying to
298 maintain all the Received lines. A more complete approach is to use
299 the Mail::Header module from CPAN (part of the MailTools package).
301 =head2 How do I decode a CGI form?
303 You use a standard module, probably CGI.pm. Under no circumstances
304 should you attempt to do so by hand!
306 You'll see a lot of CGI programs that blindly read from STDIN the number
307 of bytes equal to CONTENT_LENGTH for POSTs, or grab QUERY_STRING for
308 decoding GETs. These programs are very poorly written. They only work
309 sometimes. They typically forget to check the return value of the read()
310 system call, which is a cardinal sin. They don't handle HEAD requests.
311 They don't handle multipart forms used for file uploads. They don't deal
312 with GET/POST combinations where query fields are in more than one place.
313 They don't deal with keywords in the query string.
315 In short, they're bad hacks. Resist them at all costs. Please do not be
316 tempted to reinvent the wheel. Instead, use the CGI.pm or CGI_Lite.pm
317 (available from CPAN), or if you're trapped in the module-free land
318 of perl1 .. perl4, you might look into cgi-lib.pl (available from
319 http://cgi-lib.stanford.edu/cgi-lib/ ).
321 Make sure you know whether to use a GET or a POST in your form.
322 GETs should only be used for something that doesn't update the server.
323 Otherwise you can get mangled databases and repeated feedback mail
324 messages. The fancy word for this is ``idempotency''. This simply
325 means that there should be no difference between making a GET request
326 for a particular URL once or multiple times. This is because the
327 HTTP protocol definition says that a GET request may be cached by the
328 browser, or server, or an intervening proxy. POST requests cannot be
329 cached, because each request is independent and matters. Typically,
330 POST requests change or depend on state on the server (query or update
331 a database, send mail, or purchase a computer).
333 =head2 How do I check a valid mail address?
335 You can't, at least, not in real time. Bummer, eh?
337 Without sending mail to the address and seeing whether there's a human
338 on the other hand to answer you, you cannot determine whether a mail
339 address is valid. Even if you apply the mail header standard, you
340 can have problems, because there are deliverable addresses that aren't
341 RFC-822 (the mail header standard) compliant, and addresses that aren't
342 deliverable which are compliant.
344 Many are tempted to try to eliminate many frequently-invalid
345 mail addresses with a simple regex, such as
346 C</^[\w.-]+\@([\w.-]\.)+\w+$/>. It's a very bad idea. However,
347 this also throws out many valid ones, and says nothing about
348 potential deliverability, so is not suggested. Instead, see
349 http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/ckaddr.gz ,
350 which actually checks against the full RFC spec (except for nested
351 comments), looks for addresses you may not wish to accept mail to
352 (say, Bill Clinton or your postmaster), and then makes sure that the
353 hostname given can be looked up in the DNS MX records. It's not fast,
354 but it works for what it tries to do.
356 Our best advice for verifying a person's mail address is to have them
357 enter their address twice, just as you normally do to change a password.
358 This usually weeds out typos. If both versions match, send
359 mail to that address with a personal message that looks somewhat like:
361 Dear someuser@host.com,
363 Please confirm the mail address you gave us Wed May 6 09:38:41
364 MDT 1998 by replying to this message. Include the string
365 "Rumpelstiltskin" in that reply, but spelled in reverse; that is,
366 start with "Nik...". Once this is done, your confirmed address will
367 be entered into our records.
369 If you get the message back and they've followed your directions,
370 you can be reasonably assured that it's real.
372 A related strategy that's less open to forgery is to give them a PIN
373 (personal ID number). Record the address and PIN (best that it be a
374 random one) for later processing. In the mail you send, ask them to
375 include the PIN in their reply. But if it bounces, or the message is
376 included via a ``vacation'' script, it'll be there anyway. So it's
377 best to ask them to mail back a slight alteration of the PIN, such as
378 with the characters reversed, one added or subtracted to each digit, etc.
380 =head2 How do I decode a MIME/BASE64 string?
382 The MIME-tools package (available from CPAN) handles this and a lot
383 more. Decoding BASE64 becomes as simple as:
386 $decoded = decode_base64($encoded);
388 A more direct approach is to use the unpack() function's "u"
389 format after minor transliterations:
391 tr#A-Za-z0-9+/##cd; # remove non-base64 chars
392 tr#A-Za-z0-9+/# -_#; # convert to uuencoded format
393 $len = pack("c", 32 + 0.75*length); # compute length byte
394 print unpack("u", $len . $_); # uudecode and print
396 =head2 How do I return the user's mail address?
398 On systems that support getpwuid, the $< variable, and the
399 Sys::Hostname module (which is part of the standard perl distribution),
400 you can probably try using something like this:
403 $address = sprintf('%s@%s', scalar getpwuid($<), hostname);
405 Company policies on mail address can mean that this generates addresses
406 that the company's mail system will not accept, so you should ask for
407 users' mail addresses when this matters. Furthermore, not all systems
408 on which Perl runs are so forthcoming with this information as is Unix.
410 The Mail::Util module from CPAN (part of the MailTools package) provides a
411 mailaddress() function that tries to guess the mail address of the user.
412 It makes a more intelligent guess than the code above, using information
413 given when the module was installed, but it could still be incorrect.
414 Again, the best way is often just to ask the user.
416 =head2 How do I send mail?
418 Use the C<sendmail> program directly:
420 open(SENDMAIL, "|/usr/lib/sendmail -oi -t -odq")
421 or die "Can't fork for sendmail: $!\n";
422 print SENDMAIL <<"EOF";
423 From: User Originating Mail <me\@host>
424 To: Final Destination <you\@otherhost>
425 Subject: A relevant subject line
427 Body of the message goes here after the blank line
428 in as many lines as you like.
430 close(SENDMAIL) or warn "sendmail didn't close nicely";
432 The B<-oi> option prevents sendmail from interpreting a line consisting
433 of a single dot as "end of message". The B<-t> option says to use the
434 headers to decide who to send the message to, and B<-odq> says to put
435 the message into the queue. This last option means your message won't
436 be immediately delivered, so leave it out if you want immediate
439 Alternate, less convenient approaches include calling mail (sometimes
440 called mailx) directly or simply opening up port 25 have having an
441 intimate conversation between just you and the remote SMTP daemon,
444 Or you might be able use the CPAN module Mail::Mailer:
448 $mailer = Mail::Mailer->new();
449 $mailer->open({ From => $from_address,
453 or die "Can't open: $!\n";
457 The Mail::Internet module uses Net::SMTP which is less Unix-centric than
458 Mail::Mailer, but less reliable. Avoid raw SMTP commands. There
459 are many reasons to use a mail transport agent like sendmail. These
460 include queueing, MX records, and security.
462 =head2 How do I read mail?
464 While you could use the Mail::Folder module from CPAN (part of the
465 MailFolder package) or the Mail::Internet module from CPAN (also part
466 of the MailTools package), often a module is overkill. Here's a
470 # bysub1 - simple sort by subject
473 $/ = ''; # paragraph reads
476 /^Subject:\s*(?:Re:\s*)*(.*)/mi;
477 $sub[++$msgno] = lc($1) || '';
481 for my $i (sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msgs)) {
488 # bysub2 - awkish sort-by-subject
489 BEGIN { $msgno = -1 }
490 $sub[++$msgno] = (/^Subject:\s*(?:Re:\s*)*(.*)/mi)[0] if /^From/m;
492 END { print @msg[ sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msg) ] }
494 =head2 How do I find out my hostname/domainname/IP address?
496 The normal way to find your own hostname is to call the C<`hostname`>
497 program. While sometimes expedient, this has some problems, such as
498 not knowing whether you've got the canonical name or not. It's one of
499 those tradeoffs of convenience versus portability.
501 The Sys::Hostname module (part of the standard perl distribution) will
502 give you the hostname after which you can find out the IP address
503 (assuming you have working DNS) with a gethostbyname() call.
507 my $host = hostname();
508 my $addr = inet_ntoa(scalar gethostbyname($host || 'localhost'));
510 Probably the simplest way to learn your DNS domain name is to grok
511 it out of /etc/resolv.conf, at least under Unix. Of course, this
512 assumes several things about your resolv.conf configuration, including
515 (We still need a good DNS domain name-learning method for non-Unix
518 =head2 How do I fetch a news article or the active newsgroups?
520 Use the Net::NNTP or News::NNTPClient modules, both available from CPAN.
521 This can make tasks like fetching the newsgroup list as simple as
523 perl -MNews::NNTPClient
524 -e 'print News::NNTPClient->new->list("newsgroups")'
526 =head2 How do I fetch/put an FTP file?
528 LWP::Simple (available from CPAN) can fetch but not put. Net::FTP (also
529 available from CPAN) is more complex but can put as well as fetch.
531 =head2 How can I do RPC in Perl?
533 A DCE::RPC module is being developed (but is not yet available) and
534 will be released as part of the DCE-Perl package (available from
535 CPAN). The rpcgen suite, available from CPAN/authors/id/JAKE/, is
536 an RPC stub generator and includes an RPC::ONC module.
538 =head1 AUTHOR AND COPYRIGHT
540 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
543 When included as part of the Standard Version of Perl, or as part of
544 its complete documentation whether printed or otherwise, this work
545 may be distributed only under the terms of Perl's Artistic License.
546 Any distribution of this file or derivatives thereof I<outside>
547 of that package require that special arrangements be made with
550 Irrespective of its distribution, all code examples in this file
551 are hereby placed into the public domain. You are permitted and
552 encouraged to use this code in your own programs for fun
553 or for profit as you see fit. A simple comment in the code giving
554 credit would be courteous but is not required.