3 perlfaq9 - Networking ($Revision: 7875 $)
7 This section deals with questions related to networking, the internet,
10 =head2 What is the correct form of response from a CGI script?
12 (Alan Flavell <flavell+www@a5.ph.gla.ac.uk> answers...)
14 The Common Gateway Interface (CGI) specifies a software interface between
15 a program ("CGI script") and a web server (HTTPD). It is not specific
16 to Perl, and has its own FAQs and tutorials, and usenet group,
17 comp.infosystems.www.authoring.cgi
19 The CGI specification is outlined in an informational RFC:
20 http://www.ietf.org/rfc/rfc3875
22 Other relevant documentation listed in: http://www.perl.org/CGI_MetaFAQ.html
24 These Perl FAQs very selectively cover some CGI issues. However, Perl
25 programmers are strongly advised to use the CGI.pm module, to take care
26 of the details for them.
28 The similarity between CGI response headers (defined in the CGI
29 specification) and HTTP response headers (defined in the HTTP
30 specification, RFC2616) is intentional, but can sometimes be confusing.
32 The CGI specification defines two kinds of script: the "Parsed Header"
33 script, and the "Non Parsed Header" (NPH) script. Check your server
34 documentation to see what it supports. "Parsed Header" scripts are
35 simpler in various respects. The CGI specification allows any of the
36 usual newline representations in the CGI response (it's the server's
37 job to create an accurate HTTP response based on it). So "\n" written in
38 text mode is technically correct, and recommended. NPH scripts are more
39 tricky: they must put out a complete and accurate set of HTTP
40 transaction response headers; the HTTP specification calls for records
41 to be terminated with carriage-return and line-feed, i.e ASCII \015\012
42 written in binary mode.
44 Using CGI.pm gives excellent platform independence, including EBCDIC
45 systems. CGI.pm selects an appropriate newline representation
46 ($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 an HTML pop-up menu with Perl?
191 (contributed by brian d foy)
193 The CGI.pm module (which comes with Perl) has functions to create
194 the HTML form widgets. See the CGI.pm documentation for more
197 use CGI qw/:standard/;
199 start_html('Favorite Animals'),
202 "What's your favorite animal? ",
205 -values => [ qw( Llama Alpaca Camel Ram ) ]
213 =head2 How do I fetch an HTML file?
215 One approach, if you have the lynx text-based HTML browser installed
216 on your system, is this:
218 $html_code = `lynx -source $url`;
219 $text_data = `lynx -dump $url`;
221 The libwww-perl (LWP) modules from CPAN provide a more powerful way
222 to do this. They don't require lynx, but like lynx, can still work
227 $content = get($URL);
229 # or print HTML from a URL
231 getprint "http://www.linpro.no/lwp/";
233 # or print ASCII from HTML from a URL
234 # also need HTML-Tree package from CPAN
237 use HTML::FormatText;
239 $html = get("http://www.perl.com/");
241 or die "Can't fetch HTML from http://www.perl.com/";
242 $ascii = HTML::FormatText->new->format(parse_html($html));
245 =head2 How do I automate an HTML form submission?
247 If you are doing something complex, such as moving through many pages
248 and forms or a web site, you can use C<WWW::Mechanize>. See its
249 documentation for all the details.
251 If you're submitting values using the GET method, create a URL and encode
252 the form using the C<query_form> method:
257 my $url = url('http://www.perl.com/cgi-bin/cpan_mod');
258 $url->query_form(module => 'DB_File', readme => 1);
259 $content = get($url);
261 If you're using the POST method, create your own user agent and encode
262 the content appropriately.
264 use HTTP::Request::Common qw(POST);
267 $ua = LWP::UserAgent->new();
268 my $req = POST 'http://www.perl.com/cgi-bin/cpan_mod',
269 [ module => 'DB_File', readme => 1 ];
270 $content = $ua->request($req)->as_string;
272 =head2 How do I decode or create those %-encodings on the web?
274 If you are writing a CGI script, you should be using the CGI.pm module
275 that comes with perl, or some other equivalent module. The CGI module
276 automatically decodes queries for you, and provides an escape()
277 function to handle encoding.
279 The best source of detailed information on URI encoding is RFC 2396.
280 Basically, the following substitutions do it:
282 s/([^\w()'*~!.-])/sprintf '%%%02x', ord $1/eg; # encode
284 s/%([A-Fa-f\d]{2})/chr hex $1/eg; # decode
285 s/%([[:xdigit:]]{2})/chr hex $1/eg; # same thing
287 However, you should only apply them to individual URI components, not
288 the entire URI, otherwise you'll lose information and generally mess
289 things up. If that didn't explain it, don't worry. Just go read
290 section 2 of the RFC, it's probably the best explanation there is.
292 RFC 2396 also contains a lot of other useful information, including a
293 regexp for breaking any arbitrary URI into components (Appendix B).
295 =head2 How do I redirect to another page?
297 Specify the complete URL of the destination (even if it is on the same
298 server). This is one of the two different kinds of CGI "Location:"
299 responses which are defined in the CGI specification for a Parsed Headers
300 script. The other kind (an absolute URLpath) is resolved internally to
301 the server without any HTTP redirection. The CGI specifications do not
302 allow relative URLs in either case.
304 Use of CGI.pm is strongly recommended. This example shows redirection
305 with a complete URL. This redirection is handled by the web browser.
307 use CGI qw/:standard/;
309 my $url = 'http://www.cpan.org/';
310 print redirect($url);
313 This example shows a redirection with an absolute URLpath. This
314 redirection is handled by the local web server.
316 my $url = '/CPAN/index.html';
317 print redirect($url);
320 But if coded directly, it could be as follows (the final "\n" is
321 shown separately, for clarity), using either a complete URL or
324 print "Location: $url\n"; # CGI response header
325 print "\n"; # end of headers
328 =head2 How do I put a password on my web pages?
330 To enable authentication for your web server, you need to configure
331 your web server. The configuration is different for different sorts
332 of web servers--apache does it differently from iPlanet which does
333 it differently from IIS. Check your web server documentation for
334 the details for your particular server.
336 =head2 How do I edit my .htpasswd and .htgroup files with Perl?
338 The HTTPD::UserAdmin and HTTPD::GroupAdmin modules provide a
339 consistent OO interface to these files, regardless of how they're
340 stored. Databases may be text, dbm, Berkeley DB or any database with
341 a DBI compatible driver. HTTPD::UserAdmin supports files used by the
342 "Basic" and "Digest" authentication schemes. Here's an example:
344 use HTTPD::UserAdmin ();
346 ->new(DB => "/foo/.htpasswd")
347 ->add($username => $password);
349 =head2 How do I make sure users can't enter values into a form that cause my CGI script to do bad things?
351 See the security references listed in the CGI Meta FAQ
353 http://www.perl.org/CGI_MetaFAQ.html
355 =head2 How do I parse a mail header?
357 For a quick-and-dirty solution, try this solution derived
358 from L<perlfunc/split>:
362 $header =~ s/\n\s+/ /g; # merge continuation lines
363 %head = ( UNIX_FROM_LINE, split /^([-\w]+):\s*/m, $header );
365 That solution doesn't do well if, for example, you're trying to
366 maintain all the Received lines. A more complete approach is to use
367 the Mail::Header module from CPAN (part of the MailTools package).
369 =head2 How do I decode a CGI form?
371 (contributed by brian d foy)
373 Use the CGI.pm module that comes with Perl. It's quick,
374 it's easy, and it actually does quite a bit of work to
375 ensure things happen correctly. It handles GET, POST, and
376 HEAD requests, multipart forms, multivalued fields, query
377 string and message body combinations, and many other things
378 you probably don't want to think about.
380 It doesn't get much easier: the CGI module automatically
381 parses the input and makes each value available through the
384 use CGI qw(:standard);
386 my $total = param( 'price' ) + param( 'shipping' );
388 my @items = param( 'item' ); # multiple values, same field name
390 If you want an object-oriented approach, CGI.pm can do that too.
394 my $cgi = CGI->new();
396 my $total = $cgi->param( 'price' ) + $cgi->param( 'shipping' );
398 my @items = $cgi->param( 'item' );
400 You might also try CGI::Minimal which is a lightweight version
401 of the same thing. Other CGI::* modules on CPAN might work better
404 Many people try to write their own decoder (or copy one from
405 another program) and then run into one of the many "gotchas"
406 of the task. It's much easier and less hassle to use CGI.pm.
408 =head2 How do I check a valid mail address?
410 You can't, at least, not in real time. Bummer, eh?
412 Without sending mail to the address and seeing whether there's a human
413 on the other end to answer you, you cannot determine whether a mail
414 address is valid. Even if you apply the mail header standard, you
415 can have problems, because there are deliverable addresses that aren't
416 RFC-822 (the mail header standard) compliant, and addresses that aren't
417 deliverable which are compliant.
419 You can use the Email::Valid or RFC::RFC822::Address which check
420 the format of the address, although they cannot actually tell you
421 if it is a deliverable address (i.e. that mail to the address
422 will not bounce). Modules like Mail::CheckUser and Mail::EXPN
423 try to interact with the domain name system or particular
424 mail servers to learn even more, but their methods do not
425 work everywhere--especially for security conscious administrators.
427 Many are tempted to try to eliminate many frequently-invalid
428 mail addresses with a simple regex, such as
429 C</^[\w.-]+\@(?:[\w-]+\.)+\w+$/>. It's a very bad idea. However,
430 this also throws out many valid ones, and says nothing about
431 potential deliverability, so it is not suggested. Instead, see
432 http://www.cpan.org/authors/Tom_Christiansen/scripts/ckaddr.gz ,
433 which actually checks against the full RFC spec (except for nested
434 comments), looks for addresses you may not wish to accept mail to
435 (say, Bill Clinton or your postmaster), and then makes sure that the
436 hostname given can be looked up in the DNS MX records. It's not fast,
437 but it works for what it tries to do.
439 Our best advice for verifying a person's mail address is to have them
440 enter their address twice, just as you normally do to change a password.
441 This usually weeds out typos. If both versions match, send
442 mail to that address with a personal message that looks somewhat like:
444 Dear someuser@host.com,
446 Please confirm the mail address you gave us Wed May 6 09:38:41
447 MDT 1998 by replying to this message. Include the string
448 "Rumpelstiltskin" in that reply, but spelled in reverse; that is,
449 start with "Nik...". Once this is done, your confirmed address will
450 be entered into our records.
452 If you get the message back and they've followed your directions,
453 you can be reasonably assured that it's real.
455 A related strategy that's less open to forgery is to give them a PIN
456 (personal ID number). Record the address and PIN (best that it be a
457 random one) for later processing. In the mail you send, ask them to
458 include the PIN in their reply. But if it bounces, or the message is
459 included via a "vacation" script, it'll be there anyway. So it's
460 best to ask them to mail back a slight alteration of the PIN, such as
461 with the characters reversed, one added or subtracted to each digit, etc.
463 =head2 How do I decode a MIME/BASE64 string?
465 The MIME-Base64 package (available from CPAN) handles this as well as
466 the MIME/QP encoding. Decoding BASE64 becomes as simple as:
469 $decoded = decode_base64($encoded);
471 The MIME-Tools package (available from CPAN) supports extraction with
472 decoding of BASE64 encoded attachments and content directly from email
475 If the string to decode is short (less than 84 bytes long)
476 a more direct approach is to use the unpack() function's "u"
477 format after minor transliterations:
479 tr#A-Za-z0-9+/##cd; # remove non-base64 chars
480 tr#A-Za-z0-9+/# -_#; # convert to uuencoded format
481 $len = pack("c", 32 + 0.75*length); # compute length byte
482 print unpack("u", $len . $_); # uudecode and print
484 =head2 How do I return the user's mail address?
486 On systems that support getpwuid, the $< variable, and the
487 Sys::Hostname module (which is part of the standard perl distribution),
488 you can probably try using something like this:
491 $address = sprintf('%s@%s', scalar getpwuid($<), hostname);
493 Company policies on mail address can mean that this generates addresses
494 that the company's mail system will not accept, so you should ask for
495 users' mail addresses when this matters. Furthermore, not all systems
496 on which Perl runs are so forthcoming with this information as is Unix.
498 The Mail::Util module from CPAN (part of the MailTools package) provides a
499 mailaddress() function that tries to guess the mail address of the user.
500 It makes a more intelligent guess than the code above, using information
501 given when the module was installed, but it could still be incorrect.
502 Again, the best way is often just to ask the user.
504 =head2 How do I send mail?
506 Use the C<sendmail> program directly:
508 open(SENDMAIL, "|/usr/lib/sendmail -oi -t -odq")
509 or die "Can't fork for sendmail: $!\n";
510 print SENDMAIL <<"EOF";
511 From: User Originating Mail <me\@host>
512 To: Final Destination <you\@otherhost>
513 Subject: A relevant subject line
515 Body of the message goes here after the blank line
516 in as many lines as you like.
518 close(SENDMAIL) or warn "sendmail didn't close nicely";
520 The B<-oi> option prevents sendmail from interpreting a line consisting
521 of a single dot as "end of message". The B<-t> option says to use the
522 headers to decide who to send the message to, and B<-odq> says to put
523 the message into the queue. This last option means your message won't
524 be immediately delivered, so leave it out if you want immediate
527 Alternate, less convenient approaches include calling mail (sometimes
528 called mailx) directly or simply opening up port 25 have having an
529 intimate conversation between just you and the remote SMTP daemon,
532 Or you might be able use the CPAN module Mail::Mailer:
536 $mailer = Mail::Mailer->new();
537 $mailer->open({ From => $from_address,
541 or die "Can't open: $!\n";
545 The Mail::Internet module uses Net::SMTP which is less Unix-centric than
546 Mail::Mailer, but less reliable. Avoid raw SMTP commands. There
547 are many reasons to use a mail transport agent like sendmail. These
548 include queuing, MX records, and security.
550 =head2 How do I use MIME to make an attachment to a mail message?
552 This answer is extracted directly from the MIME::Lite documentation.
553 Create a multipart message (i.e., one with attachments).
557 ### Create a new multipart message:
558 $msg = MIME::Lite->new(
559 From =>'me@myhost.com',
560 To =>'you@yourhost.com',
561 Cc =>'some@other.com, some@more.com',
562 Subject =>'A message with 2 parts...',
563 Type =>'multipart/mixed'
566 ### Add parts (each "attach" has same arguments as "new"):
567 $msg->attach(Type =>'TEXT',
568 Data =>"Here's the GIF file you wanted"
570 $msg->attach(Type =>'image/gif',
571 Path =>'aaa000123.gif',
572 Filename =>'logo.gif'
575 $text = $msg->as_string;
577 MIME::Lite also includes a method for sending these things.
581 This defaults to using L<sendmail> but can be customized to use
582 SMTP via L<Net::SMTP>.
584 =head2 How do I read mail?
586 While you could use the Mail::Folder module from CPAN (part of the
587 MailFolder package) or the Mail::Internet module from CPAN (part
588 of the MailTools package), often a module is overkill. Here's a
595 $/ = ''; # paragraph reads
598 /^Subject:\s*(?:Re:\s*)*(.*)/mi;
599 $sub[++$msgno] = lc($1) || '';
603 for my $i (sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msgs)) {
610 # bysub2 - awkish sort-by-subject
611 BEGIN { $msgno = -1 }
612 $sub[++$msgno] = (/^Subject:\s*(?:Re:\s*)*(.*)/mi)[0] if /^From/m;
614 END { print @msg[ sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msg) ] }
616 =head2 How do I find out my hostname, domainname, or IP address?
617 X<hostname, domainname, IP address, host, domain, hostfqdn, inet_ntoa,
618 gethostbyname, Socket, Net::Domain, Sys::Hostname>
620 (contributed by brian d foy)
622 The Net::Domain module, which is part of the standard distribution starting
623 in perl5.7.3, can get you the fully qualified domain name (FQDN), the host
624 name, or the domain name.
626 use Net::Domain qw(hostname hostfqdn hostdomain);
628 my $host = hostfqdn();
630 The C<Sys::Hostname> module, included in the standard distribution since
631 perl5.6, can also get the hostname.
637 To get the IP address, you can use the C<gethostbyname> built-in function
638 to turn the name into a number. To turn that number into the dotted octet
639 form (a.b.c.d) that most people expect, use the C<inet_ntoa> function
640 from the <Socket> module, which also comes with perl.
644 my $address = inet_ntoa(
645 scalar gethostbyname( $host || 'localhost' )
648 =head2 How do I fetch a news article or the active newsgroups?
650 Use the Net::NNTP or News::NNTPClient modules, both available from CPAN.
651 This can make tasks like fetching the newsgroup list as simple as
653 perl -MNews::NNTPClient
654 -e 'print News::NNTPClient->new->list("newsgroups")'
656 =head2 How do I fetch/put an FTP file?
658 LWP::Simple (available from CPAN) can fetch but not put. Net::FTP (also
659 available from CPAN) is more complex but can put as well as fetch.
661 =head2 How can I do RPC in Perl?
663 (Contributed by brian d foy)
665 Use one of the RPC modules you can find on CPAN (
666 http://search.cpan.org/search?query=RPC&mode=all ).
670 Revision: $Revision: 7875 $
672 Date: $Date: 2006-10-04 22:39:26 +0200 (mer, 04 oct 2006) $
674 See L<perlfaq> for source control details and availability.
676 =head1 AUTHOR AND COPYRIGHT
678 Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
679 other authors as noted. All rights reserved.
681 This documentation is free; you can redistribute it and/or modify it
682 under the same terms as Perl itself.
684 Irrespective of its distribution, all code examples in this file
685 are hereby placed into the public domain. You are permitted and
686 encouraged to use this code in your own programs for fun
687 or for profit as you see fit. A simple comment in the code giving
688 credit would be courteous but is not required.