3 perlfaq9 - Networking ($Revision: 3606 $)
7 This section deals with questions related to networking, the internet,
10 =head2 What is the correct form of response from a CGI script?
12 (Alan Flavell <flavell+www@a5.ph.gla.ac.uk> answers...)
14 The Common Gateway Interface (CGI) specifies a software interface between
15 a program ("CGI script") and a web server (HTTPD). It is not specific
16 to Perl, and has its own FAQs and tutorials, and usenet group,
17 comp.infosystems.www.authoring.cgi
19 The CGI specification is outlined in an informational RFC:
20 http://www.ietf.org/rfc/rfc3875
22 Other relevant documentation listed in: http://www.perl.org/CGI_MetaFAQ.html
24 These Perl FAQs very selectively cover some CGI issues. However, Perl
25 programmers are strongly advised to use the CGI.pm module, to take care
26 of the details for them.
28 The similarity between CGI response headers (defined in the CGI
29 specification) and HTTP response headers (defined in the HTTP
30 specification, RFC2616) is intentional, but can sometimes be confusing.
32 The CGI specification defines two kinds of script: the "Parsed Header"
33 script, and the "Non Parsed Header" (NPH) script. Check your server
34 documentation to see what it supports. "Parsed Header" scripts are
35 simpler in various respects. The CGI specification allows any of the
36 usual newline representations in the CGI response (it's the server's
37 job to create an accurate HTTP response based on it). So "\n" written in
38 text mode is technically correct, and recommended. NPH scripts are more
39 tricky: they must put out a complete and accurate set of HTTP
40 transaction response headers; the HTTP specification calls for records
41 to be terminated with carriage-return and line-feed, i.e ASCII \015\012
42 written in binary mode.
44 Using CGI.pm gives excellent platform independence, including EBCDIC
45 systems. CGI.pm selects an appropriate newline representation
46 ($CGI::CRLF) and sets binmode as appropriate.
48 =head2 My CGI script runs from the command line but not the browser. (500 Server Error)
50 Several things could be wrong. You can go through the "Troubleshooting
51 Perl CGI scripts" guide at
53 http://www.perl.org/troubleshooting_CGI.html
55 If, after that, you can demonstrate that you've read the FAQs and that
56 your problem isn't something simple that can be easily answered, you'll
57 probably receive a courteous and useful reply to your question if you
58 post it on comp.infosystems.www.authoring.cgi (if it's something to do
59 with HTTP or the CGI protocols). Questions that appear to be Perl
60 questions but are really CGI ones that are posted to comp.lang.perl.misc
61 are not so well received.
63 The useful FAQs, related documents, and troubleshooting guides are
64 listed in the CGI Meta FAQ:
66 http://www.perl.org/CGI_MetaFAQ.html
69 =head2 How can I get better error messages from a CGI program?
71 Use the CGI::Carp module. It replaces C<warn> and C<die>, plus the
72 normal Carp modules C<carp>, C<croak>, and C<confess> functions with
73 more verbose and safer versions. It still sends them to the normal
77 warn "This is a complaint";
78 die "But this one is serious";
80 The following use of CGI::Carp also redirects errors to a file of your choice,
81 placed in a BEGIN block to catch compile-time warnings as well:
84 use CGI::Carp qw(carpout);
85 open(LOG, ">>/var/local/cgi-logs/mycgi-log")
86 or die "Unable to append to mycgi-log: $!\n";
90 You can even arrange for fatal errors to go back to the client browser,
91 which is nice for your own debugging, but might confuse the end user.
93 use CGI::Carp qw(fatalsToBrowser);
96 Even if the error happens before you get the HTTP header out, the module
97 will try to take care of this to avoid the dreaded server 500 errors.
98 Normal warnings still go out to the server error log (or wherever
99 you've sent them with C<carpout>) with the application name and date
102 =head2 How do I remove HTML from a string?
104 The most correct way (albeit not the fastest) is to use HTML::Parser
105 from CPAN. Another mostly correct
106 way is to use HTML::FormatText which not only removes HTML but also
107 attempts to do a little simple formatting of the resulting plain text.
109 Many folks attempt a simple-minded regular expression approach, like
110 C<< s/<.*?>//g >>, but that fails in many cases because the tags
111 may continue over line breaks, they may contain quoted angle-brackets,
112 or HTML comment may be present. Plus, folks forget to convert
113 entities--like C<<> for example.
115 Here's one "simple-minded" approach, that works for most files:
117 #!/usr/bin/perl -p0777
118 s/<(?:[^>'"]*|(['"]).*?\1)*>//gs
120 If you want a more complete solution, see the 3-stage striphtml
122 http://www.cpan.org/authors/Tom_Christiansen/scripts/striphtml.gz
125 Here are some tricky cases that you should think about when picking
128 <IMG SRC = "foo.gif" ALT = "A > B">
135 <script>if (a<b && a>c)</script>
139 <![INCLUDE CDATA [ >>>>>>>>>>>> ]]>
141 If HTML comments include other tags, those solutions would also break
144 <!-- This section commented out.
145 <B>You can't see me!</B>
148 =head2 How do I extract URLs?
150 You can easily extract all sorts of URLs from HTML with
151 C<HTML::SimpleLinkExtor> which handles anchors, images, objects,
152 frames, and many other tags that can contain a URL. If you need
153 anything more complex, you can create your own subclass of
154 C<HTML::LinkExtor> or C<HTML::Parser>. You might even use
155 C<HTML::SimpleLinkExtor> as an example for something specifically
156 suited to your needs.
158 You can use URI::Find to extract URLs from an arbitrary text document.
160 Less complete solutions involving regular expressions can save
161 you a lot of processing time if you know that the input is simple. One
162 solution from Tom Christiansen runs 100 times faster than most
163 module based approaches but only extracts URLs from anchors where the first
164 attribute is HREF and there are no other attributes.
167 # qxurl - tchrist@perl.com
168 print "$2\n" while m{
170 A \s+ HREF \s* = \s* (["']) (.*?) \1
175 =head2 How do I download a file from the user's machine? How do I open a file on another machine?
177 In this case, download means to use the file upload feature of HTML
178 forms. You allow the web surfer to specify a file to send to your web
179 server. To you it looks like a download, and to the user it looks
180 like an upload. No matter what you call it, you do it with what's
181 known as B<multipart/form-data> encoding. The CGI.pm module (which
182 comes with Perl as part of the Standard Library) supports this in the
183 start_multipart_form() method, which isn't the same as the startform()
186 See the section in the CGI.pm documentation on file uploads for code
187 examples and details.
189 =head2 How do I make a pop-up menu in HTML?
191 Use the B<< <SELECT> >> and B<< <OPTION> >> tags. The CGI.pm
192 module (available from CPAN) supports this widget, as well as many
193 others, including some that it cleverly synthesizes on its own.
195 =head2 How do I fetch an HTML file?
197 One approach, if you have the lynx text-based HTML browser installed
198 on your system, is this:
200 $html_code = `lynx -source $url`;
201 $text_data = `lynx -dump $url`;
203 The libwww-perl (LWP) modules from CPAN provide a more powerful way
204 to do this. They don't require lynx, but like lynx, can still work
209 $content = get($URL);
211 # or print HTML from a URL
213 getprint "http://www.linpro.no/lwp/";
215 # or print ASCII from HTML from a URL
216 # also need HTML-Tree package from CPAN
219 use HTML::FormatText;
221 $html = get("http://www.perl.com/");
223 or die "Can't fetch HTML from http://www.perl.com/";
224 $ascii = HTML::FormatText->new->format(parse_html($html));
227 =head2 How do I automate an HTML form submission?
229 If you are doing something complex, such as moving through many pages
230 and forms or a web site, you can use C<WWW::Mechanize>. See its
231 documentation for all the details.
233 If you're submitting values using the GET method, create a URL and encode
234 the form using the C<query_form> method:
239 my $url = url('http://www.perl.com/cgi-bin/cpan_mod');
240 $url->query_form(module => 'DB_File', readme => 1);
241 $content = get($url);
243 If you're using the POST method, create your own user agent and encode
244 the content appropriately.
246 use HTTP::Request::Common qw(POST);
249 $ua = LWP::UserAgent->new();
250 my $req = POST 'http://www.perl.com/cgi-bin/cpan_mod',
251 [ module => 'DB_File', readme => 1 ];
252 $content = $ua->request($req)->as_string;
254 =head2 How do I decode or create those %-encodings on the web?
256 If you are writing a CGI script, you should be using the CGI.pm module
257 that comes with perl, or some other equivalent module. The CGI module
258 automatically decodes queries for you, and provides an escape()
259 function to handle encoding.
261 The best source of detailed information on URI encoding is RFC 2396.
262 Basically, the following substitutions do it:
264 s/([^\w()'*~!.-])/sprintf '%%%02x', ord $1/eg; # encode
266 s/%([A-Fa-f\d]{2})/chr hex $1/eg; # decode
267 s/%([[:xdigit:]]{2})/chr hex $1/eg; # same thing
269 However, you should only apply them to individual URI components, not
270 the entire URI, otherwise you'll lose information and generally mess
271 things up. If that didn't explain it, don't worry. Just go read
272 section 2 of the RFC, it's probably the best explanation there is.
274 RFC 2396 also contains a lot of other useful information, including a
275 regexp for breaking any arbitrary URI into components (Appendix B).
277 =head2 How do I redirect to another page?
279 Specify the complete URL of the destination (even if it is on the same
280 server). This is one of the two different kinds of CGI "Location:"
281 responses which are defined in the CGI specification for a Parsed Headers
282 script. The other kind (an absolute URLpath) is resolved internally to
283 the server without any HTTP redirection. The CGI specifications do not
284 allow relative URLs in either case.
286 Use of CGI.pm is strongly recommended. This example shows redirection
287 with a complete URL. This redirection is handled by the web browser.
289 use CGI qw/:standard/;
291 my $url = 'http://www.cpan.org/';
292 print redirect($url);
295 This example shows a redirection with an absolute URLpath. This
296 redirection is handled by the local web server.
298 my $url = '/CPAN/index.html';
299 print redirect($url);
302 But if coded directly, it could be as follows (the final "\n" is
303 shown separately, for clarity), using either a complete URL or
306 print "Location: $url\n"; # CGI response header
307 print "\n"; # end of headers
310 =head2 How do I put a password on my web pages?
312 To enable authentication for your web server, you need to configure
313 your web server. The configuration is different for different sorts
314 of web servers---apache does it differently from iPlanet which does
315 it differently from IIS. Check your web server documentation for
316 the details for your particular server.
318 =head2 How do I edit my .htpasswd and .htgroup files with Perl?
320 The HTTPD::UserAdmin and HTTPD::GroupAdmin modules provide a
321 consistent OO interface to these files, regardless of how they're
322 stored. Databases may be text, dbm, Berkeley DB or any database with
323 a DBI compatible driver. HTTPD::UserAdmin supports files used by the
324 "Basic" and "Digest" authentication schemes. Here's an example:
326 use HTTPD::UserAdmin ();
328 ->new(DB => "/foo/.htpasswd")
329 ->add($username => $password);
331 =head2 How do I make sure users can't enter values into a form that cause my CGI script to do bad things?
333 See the security references listed in the CGI Meta FAQ
335 http://www.perl.org/CGI_MetaFAQ.html
337 =head2 How do I parse a mail header?
339 For a quick-and-dirty solution, try this solution derived
340 from L<perlfunc/split>:
344 $header =~ s/\n\s+/ /g; # merge continuation lines
345 %head = ( UNIX_FROM_LINE, split /^([-\w]+):\s*/m, $header );
347 That solution doesn't do well if, for example, you're trying to
348 maintain all the Received lines. A more complete approach is to use
349 the Mail::Header module from CPAN (part of the MailTools package).
351 =head2 How do I decode a CGI form?
353 (contributed by brian d foy)
355 Use the CGI.pm module that comes with Perl. It's quick,
356 it's easy, and it actually does quite a bit of work to
357 ensure things happen correctly. It handles GET, POST, and
358 HEAD requests, multipart forms, multivalued fields, query
359 string and message body combinations, and many other things
360 you probably don't want to think about.
362 It doesn't get much easier: the CGI module automatically
363 parses the input and makes each value available through the
366 use CGI qw(:standard);
368 my $total = param( 'price' ) + param( 'shipping' );
370 my @items = param( 'item' ); # multiple values, same field name
372 If you want an object-oriented approach, CGI.pm can do that too.
376 my $cgi = CGI->new();
378 my $total = $cgi->param( 'price' ) + $cgi->param( 'shipping' );
380 my @items = $cgi->param( 'item' );
382 You might also try CGI::Minimal which is a lightweight version
383 of the same thing. Other CGI::* modules on CPAN might work better
386 Many people try to write their own decoder (or copy one from
387 another program) and then run into one of the many "gotchas"
388 of the task. It's much easier and less hassle to use CGI.pm.
390 =head2 How do I check a valid mail address?
392 You can't, at least, not in real time. Bummer, eh?
394 Without sending mail to the address and seeing whether there's a human
395 on the other end to answer you, you cannot determine whether a mail
396 address is valid. Even if you apply the mail header standard, you
397 can have problems, because there are deliverable addresses that aren't
398 RFC-822 (the mail header standard) compliant, and addresses that aren't
399 deliverable which are compliant.
401 You can use the Email::Valid or RFC::RFC822::Address which check
402 the format of the address, although they cannot actually tell you
403 if it is a deliverable address (i.e. that mail to the address
404 will not bounce). Modules like Mail::CheckUser and Mail::EXPN
405 try to interact with the domain name system or particular
406 mail servers to learn even more, but their methods do not
407 work everywhere---especially for security conscious administrators.
409 Many are tempted to try to eliminate many frequently-invalid
410 mail addresses with a simple regex, such as
411 C</^[\w.-]+\@(?:[\w-]+\.)+\w+$/>. It's a very bad idea. However,
412 this also throws out many valid ones, and says nothing about
413 potential deliverability, so it is not suggested. Instead, see
414 http://www.cpan.org/authors/Tom_Christiansen/scripts/ckaddr.gz ,
415 which actually checks against the full RFC spec (except for nested
416 comments), looks for addresses you may not wish to accept mail to
417 (say, Bill Clinton or your postmaster), and then makes sure that the
418 hostname given can be looked up in the DNS MX records. It's not fast,
419 but it works for what it tries to do.
421 Our best advice for verifying a person's mail address is to have them
422 enter their address twice, just as you normally do to change a password.
423 This usually weeds out typos. If both versions match, send
424 mail to that address with a personal message that looks somewhat like:
426 Dear someuser@host.com,
428 Please confirm the mail address you gave us Wed May 6 09:38:41
429 MDT 1998 by replying to this message. Include the string
430 "Rumpelstiltskin" in that reply, but spelled in reverse; that is,
431 start with "Nik...". Once this is done, your confirmed address will
432 be entered into our records.
434 If you get the message back and they've followed your directions,
435 you can be reasonably assured that it's real.
437 A related strategy that's less open to forgery is to give them a PIN
438 (personal ID number). Record the address and PIN (best that it be a
439 random one) for later processing. In the mail you send, ask them to
440 include the PIN in their reply. But if it bounces, or the message is
441 included via a "vacation" script, it'll be there anyway. So it's
442 best to ask them to mail back a slight alteration of the PIN, such as
443 with the characters reversed, one added or subtracted to each digit, etc.
445 =head2 How do I decode a MIME/BASE64 string?
447 The MIME-Base64 package (available from CPAN) handles this as well as
448 the MIME/QP encoding. Decoding BASE64 becomes as simple as:
451 $decoded = decode_base64($encoded);
453 The MIME-Tools package (available from CPAN) supports extraction with
454 decoding of BASE64 encoded attachments and content directly from email
457 If the string to decode is short (less than 84 bytes long)
458 a more direct approach is to use the unpack() function's "u"
459 format after minor transliterations:
461 tr#A-Za-z0-9+/##cd; # remove non-base64 chars
462 tr#A-Za-z0-9+/# -_#; # convert to uuencoded format
463 $len = pack("c", 32 + 0.75*length); # compute length byte
464 print unpack("u", $len . $_); # uudecode and print
466 =head2 How do I return the user's mail address?
468 On systems that support getpwuid, the $< variable, and the
469 Sys::Hostname module (which is part of the standard perl distribution),
470 you can probably try using something like this:
473 $address = sprintf('%s@%s', scalar getpwuid($<), hostname);
475 Company policies on mail address can mean that this generates addresses
476 that the company's mail system will not accept, so you should ask for
477 users' mail addresses when this matters. Furthermore, not all systems
478 on which Perl runs are so forthcoming with this information as is Unix.
480 The Mail::Util module from CPAN (part of the MailTools package) provides a
481 mailaddress() function that tries to guess the mail address of the user.
482 It makes a more intelligent guess than the code above, using information
483 given when the module was installed, but it could still be incorrect.
484 Again, the best way is often just to ask the user.
486 =head2 How do I send mail?
488 Use the C<sendmail> program directly:
490 open(SENDMAIL, "|/usr/lib/sendmail -oi -t -odq")
491 or die "Can't fork for sendmail: $!\n";
492 print SENDMAIL <<"EOF";
493 From: User Originating Mail <me\@host>
494 To: Final Destination <you\@otherhost>
495 Subject: A relevant subject line
497 Body of the message goes here after the blank line
498 in as many lines as you like.
500 close(SENDMAIL) or warn "sendmail didn't close nicely";
502 The B<-oi> option prevents sendmail from interpreting a line consisting
503 of a single dot as "end of message". The B<-t> option says to use the
504 headers to decide who to send the message to, and B<-odq> says to put
505 the message into the queue. This last option means your message won't
506 be immediately delivered, so leave it out if you want immediate
509 Alternate, less convenient approaches include calling mail (sometimes
510 called mailx) directly or simply opening up port 25 have having an
511 intimate conversation between just you and the remote SMTP daemon,
514 Or you might be able use the CPAN module Mail::Mailer:
518 $mailer = Mail::Mailer->new();
519 $mailer->open({ From => $from_address,
523 or die "Can't open: $!\n";
527 The Mail::Internet module uses Net::SMTP which is less Unix-centric than
528 Mail::Mailer, but less reliable. Avoid raw SMTP commands. There
529 are many reasons to use a mail transport agent like sendmail. These
530 include queuing, MX records, and security.
532 =head2 How do I use MIME to make an attachment to a mail message?
534 This answer is extracted directly from the MIME::Lite documentation.
535 Create a multipart message (i.e., one with attachments).
539 ### Create a new multipart message:
540 $msg = MIME::Lite->new(
541 From =>'me@myhost.com',
542 To =>'you@yourhost.com',
543 Cc =>'some@other.com, some@more.com',
544 Subject =>'A message with 2 parts...',
545 Type =>'multipart/mixed'
548 ### Add parts (each "attach" has same arguments as "new"):
549 $msg->attach(Type =>'TEXT',
550 Data =>"Here's the GIF file you wanted"
552 $msg->attach(Type =>'image/gif',
553 Path =>'aaa000123.gif',
554 Filename =>'logo.gif'
557 $text = $msg->as_string;
559 MIME::Lite also includes a method for sending these things.
563 This defaults to using L<sendmail> but can be customized to use
564 SMTP via L<Net::SMTP>.
566 =head2 How do I read mail?
568 While you could use the Mail::Folder module from CPAN (part of the
569 MailFolder package) or the Mail::Internet module from CPAN (part
570 of the MailTools package), often a module is overkill. Here's a
577 $/ = ''; # paragraph reads
580 /^Subject:\s*(?:Re:\s*)*(.*)/mi;
581 $sub[++$msgno] = lc($1) || '';
585 for my $i (sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msgs)) {
592 # bysub2 - awkish sort-by-subject
593 BEGIN { $msgno = -1 }
594 $sub[++$msgno] = (/^Subject:\s*(?:Re:\s*)*(.*)/mi)[0] if /^From/m;
596 END { print @msg[ sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msg) ] }
598 =head2 How do I find out my hostname, domainname, or IP address?
599 X<hostname, domainname, IP address, host, domain, hostfqdn, inet_ntoa,
600 gethostbyname, Socket, Net::Domain, Sys::Hostname>
602 (contributed by brian d foy)
604 The Net::Domain module, which is part of the standard distribution starting
605 in perl5.7.3, can get you the fully qualified domain name (FQDN), the host
606 name, or the domain name.
608 use Net::Domain qw(hostname hostfqdn hostdomain);
610 my $host = hostfqdn();
612 The C<Sys::Hostname> module, included in the standard distribution since
613 perl5.6, can also get the hostname.
619 To get the IP address, you can use the C<gethostbyname> built-in function
620 to turn the name into a number. To turn that number into the dotted octet
621 form (a.b.c.d) that most people expect, use the C<inet_ntoa> function
622 from the <Socket> module, which also comes with perl.
626 my $address = inet_ntoa(
627 scalar gethostbyname( $host || 'localhost' )
630 =head2 How do I fetch a news article or the active newsgroups?
632 Use the Net::NNTP or News::NNTPClient modules, both available from CPAN.
633 This can make tasks like fetching the newsgroup list as simple as
635 perl -MNews::NNTPClient
636 -e 'print News::NNTPClient->new->list("newsgroups")'
638 =head2 How do I fetch/put an FTP file?
640 LWP::Simple (available from CPAN) can fetch but not put. Net::FTP (also
641 available from CPAN) is more complex but can put as well as fetch.
643 =head2 How can I do RPC in Perl?
645 (Contributed by brian d foy)
647 Use one of the RPC modules you can find on CPAN (
648 http://search.cpan.org/search?query=RPC&mode=all ).
652 Revision: $Revision: 3606 $
654 Date: $Date: 2006-03-06 12:05:47 +0100 (lun, 06 mar 2006) $
656 See L<perlfaq> for source control details and availability.
658 =head1 AUTHOR AND COPYRIGHT
660 Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
661 other authors as noted. All rights reserved.
663 This documentation is free; you can redistribute it and/or modify it
664 under the same terms as Perl itself.
666 Irrespective of its distribution, all code examples in this file
667 are hereby placed into the public domain. You are permitted and
668 encouraged to use this code in your own programs for fun
669 or for profit as you see fit. A simple comment in the code giving
670 credit would be courteous but is not required.