3 perlfaq9 - Networking ($Revision: 1.24 $, $Date: 2005/10/13 19:43:13 $)
7 This section deals with questions related to networking, the internet,
10 =head2 What is the correct form of response from a CGI script?
12 (Alan Flavell <flavell+www@a5.ph.gla.ac.uk> answers...)
14 The Common Gateway Interface (CGI) specifies a software interface between
15 a program ("CGI script") and a web server (HTTPD). It is not specific
16 to Perl, and has its own FAQs and tutorials, and usenet group,
17 comp.infosystems.www.authoring.cgi
19 The CGI specification is outlined in an informational RFC:
20 http://www.ietf.org/rfc/rfc3875
22 Other relevant documentation listed in: http://www.perl.org/CGI_MetaFAQ.html
24 These Perl FAQs very selectively cover some CGI issues. However, Perl
25 programmers are strongly advised to use the CGI.pm module, to take care
26 of the details for them.
28 The similarity between CGI response headers (defined in the CGI
29 specification) and HTTP response headers (defined in the HTTP
30 specification, RFC2616) is intentional, but can sometimes be confusing.
32 The CGI specification defines two kinds of script: the "Parsed Header"
33 script, and the "Non Parsed Header" (NPH) script. Check your server
34 documentation to see what it supports. "Parsed Header" scripts are
35 simpler in various respects. The CGI specification allows any of the
36 usual newline representations in the CGI response (it's the server's
37 job to create an accurate HTTP response based on it). So "\n" written in
38 text mode is technically correct, and recommended. NPH scripts are more
39 tricky: they must put out a complete and accurate set of HTTP
40 transaction response headers; the HTTP specification calls for records
41 to be terminated with carriage-return and line-feed, i.e ASCII \015\012
42 written in binary mode.
44 Using CGI.pm gives excellent platform independence, including EBCDIC
45 systems. CGI.pm selects an appropriate newline representation
46 ($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?
257 If you are writing a CGI script, you should be using the CGI.pm module
258 that comes with perl, or some other equivalent module. The CGI module
259 automatically decodes queries for you, and provides an escape()
260 function to handle encoding.
263 The best source of detailed information on URI encoding is RFC 2396.
264 Basically, the following substitutions do it:
266 s/([^\w()'*~!.-])/sprintf '%%%02x', ord $1/eg; # encode
268 s/%([A-Fa-f\d]{2})/chr hex $1/eg; # decode
270 However, you should only apply them to individual URI components, not
271 the entire URI, otherwise you'll lose information and generally mess
272 things up. If that didn't explain it, don't worry. Just go read
273 section 2 of the RFC, it's probably the best explanation there is.
275 RFC 2396 also contains a lot of other useful information, including a
276 regexp for breaking any arbitrary URI into components (Appendix B).
278 =head2 How do I redirect to another page?
280 Specify the complete URL of the destination (even if it is on the same
281 server). This is one of the two different kinds of CGI "Location:"
282 responses which are defined in the CGI specification for a Parsed Headers
283 script. The other kind (an absolute URLpath) is resolved internally to
284 the server without any HTTP redirection. The CGI specifications do not
285 allow relative URLs in either case.
287 Use of CGI.pm is strongly recommended. This example shows redirection
288 with a complete URL. This redirection is handled by the web browser.
290 use CGI qw/:standard/;
292 my $url = 'http://www.cpan.org/';
293 print redirect($url);
296 This example shows a redirection with an absolute URLpath. This
297 redirection is handled by the local web server.
299 my $url = '/CPAN/index.html';
300 print redirect($url);
303 But if coded directly, it could be as follows (the final "\n" is
304 shown separately, for clarity), using either a complete URL or
307 print "Location: $url\n"; # CGI response header
308 print "\n"; # end of headers
311 =head2 How do I put a password on my web pages?
313 To enable authentication for your web server, you need to configure
314 your web server. The configuration is different for different sorts
315 of web servers---apache does it differently from iPlanet which does
316 it differently from IIS. Check your web server documentation for
317 the details for your particular server.
319 =head2 How do I edit my .htpasswd and .htgroup files with Perl?
321 The HTTPD::UserAdmin and HTTPD::GroupAdmin modules provide a
322 consistent OO interface to these files, regardless of how they're
323 stored. Databases may be text, dbm, Berkeley DB or any database with
324 a DBI compatible driver. HTTPD::UserAdmin supports files used by the
325 "Basic" and "Digest" authentication schemes. Here's an example:
327 use HTTPD::UserAdmin ();
329 ->new(DB => "/foo/.htpasswd")
330 ->add($username => $password);
332 =head2 How do I make sure users can't enter values into a form that cause my CGI script to do bad things?
334 See the security references listed in the CGI Meta FAQ
336 http://www.perl.org/CGI_MetaFAQ.html
338 =head2 How do I parse a mail header?
340 For a quick-and-dirty solution, try this solution derived
341 from L<perlfunc/split>:
345 $header =~ s/\n\s+/ /g; # merge continuation lines
346 %head = ( UNIX_FROM_LINE, split /^([-\w]+):\s*/m, $header );
348 That solution doesn't do well if, for example, you're trying to
349 maintain all the Received lines. A more complete approach is to use
350 the Mail::Header module from CPAN (part of the MailTools package).
352 =head2 How do I decode a CGI form?
354 (contributed by brian d foy)
356 Use the CGI.pm module that comes with Perl. It's quick,
357 it's easy, and it actually does quite a bit of work to
358 ensure things happen correctly. It handles GET, POST, and
359 HEAD requests, multipart forms, multivalued fields, query
360 string and message body combinations, and many other things
361 you probably don't want to think about.
363 It doesn't get much easier: the CGI module automatically
364 parses the input and makes each value available through the
367 use CGI qw(:standard);
369 my $total = param( 'price' ) + param( 'shipping' );
371 my @items = param( 'item' ); # multiple values, same field name
373 If you want an object-oriented approach, CGI.pm can do that too.
377 my $cgi = CGI->new();
379 my $total = $cgi->param( 'price' ) + $cgi->param( 'shipping' );
381 my @items = $cgi->param( 'item' );
383 You might also try CGI::Minimal which is a lightweight version
384 of the same thing. Other CGI::* modules on CPAN might work better
387 Many people try to write their own decoder (or copy one from
388 another program) and then run into one of the many "gotchas"
389 of the task. It's much easier and less hassle to use CGI.pm.
391 =head2 How do I check a valid mail address?
393 You can't, at least, not in real time. Bummer, eh?
395 Without sending mail to the address and seeing whether there's a human
396 on the other end to answer you, you cannot determine whether a mail
397 address is valid. Even if you apply the mail header standard, you
398 can have problems, because there are deliverable addresses that aren't
399 RFC-822 (the mail header standard) compliant, and addresses that aren't
400 deliverable which are compliant.
402 You can use the Email::Valid or RFC::RFC822::Address which check
403 the format of the address, although they cannot actually tell you
404 if it is a deliverable address (i.e. that mail to the address
405 will not bounce). Modules like Mail::CheckUser and Mail::EXPN
406 try to interact with the domain name system or particular
407 mail servers to learn even more, but their methods do not
408 work everywhere---especially for security conscious administrators.
410 Many are tempted to try to eliminate many frequently-invalid
411 mail addresses with a simple regex, such as
412 C</^[\w.-]+\@(?:[\w-]+\.)+\w+$/>. It's a very bad idea. However,
413 this also throws out many valid ones, and says nothing about
414 potential deliverability, so it is not suggested. Instead, see
415 http://www.cpan.org/authors/Tom_Christiansen/scripts/ckaddr.gz ,
416 which actually checks against the full RFC spec (except for nested
417 comments), looks for addresses you may not wish to accept mail to
418 (say, Bill Clinton or your postmaster), and then makes sure that the
419 hostname given can be looked up in the DNS MX records. It's not fast,
420 but it works for what it tries to do.
422 Our best advice for verifying a person's mail address is to have them
423 enter their address twice, just as you normally do to change a password.
424 This usually weeds out typos. If both versions match, send
425 mail to that address with a personal message that looks somewhat like:
427 Dear someuser@host.com,
429 Please confirm the mail address you gave us Wed May 6 09:38:41
430 MDT 1998 by replying to this message. Include the string
431 "Rumpelstiltskin" in that reply, but spelled in reverse; that is,
432 start with "Nik...". Once this is done, your confirmed address will
433 be entered into our records.
435 If you get the message back and they've followed your directions,
436 you can be reasonably assured that it's real.
438 A related strategy that's less open to forgery is to give them a PIN
439 (personal ID number). Record the address and PIN (best that it be a
440 random one) for later processing. In the mail you send, ask them to
441 include the PIN in their reply. But if it bounces, or the message is
442 included via a "vacation" script, it'll be there anyway. So it's
443 best to ask them to mail back a slight alteration of the PIN, such as
444 with the characters reversed, one added or subtracted to each digit, etc.
446 =head2 How do I decode a MIME/BASE64 string?
448 The MIME-Base64 package (available from CPAN) handles this as well as
449 the MIME/QP encoding. Decoding BASE64 becomes as simple as:
452 $decoded = decode_base64($encoded);
454 The MIME-Tools package (available from CPAN) supports extraction with
455 decoding of BASE64 encoded attachments and content directly from email
458 If the string to decode is short (less than 84 bytes long)
459 a more direct approach is to use the unpack() function's "u"
460 format after minor transliterations:
462 tr#A-Za-z0-9+/##cd; # remove non-base64 chars
463 tr#A-Za-z0-9+/# -_#; # convert to uuencoded format
464 $len = pack("c", 32 + 0.75*length); # compute length byte
465 print unpack("u", $len . $_); # uudecode and print
467 =head2 How do I return the user's mail address?
469 On systems that support getpwuid, the $< variable, and the
470 Sys::Hostname module (which is part of the standard perl distribution),
471 you can probably try using something like this:
474 $address = sprintf('%s@%s', scalar getpwuid($<), hostname);
476 Company policies on mail address can mean that this generates addresses
477 that the company's mail system will not accept, so you should ask for
478 users' mail addresses when this matters. Furthermore, not all systems
479 on which Perl runs are so forthcoming with this information as is Unix.
481 The Mail::Util module from CPAN (part of the MailTools package) provides a
482 mailaddress() function that tries to guess the mail address of the user.
483 It makes a more intelligent guess than the code above, using information
484 given when the module was installed, but it could still be incorrect.
485 Again, the best way is often just to ask the user.
487 =head2 How do I send mail?
489 Use the C<sendmail> program directly:
491 open(SENDMAIL, "|/usr/lib/sendmail -oi -t -odq")
492 or die "Can't fork for sendmail: $!\n";
493 print SENDMAIL <<"EOF";
494 From: User Originating Mail <me\@host>
495 To: Final Destination <you\@otherhost>
496 Subject: A relevant subject line
498 Body of the message goes here after the blank line
499 in as many lines as you like.
501 close(SENDMAIL) or warn "sendmail didn't close nicely";
503 The B<-oi> option prevents sendmail from interpreting a line consisting
504 of a single dot as "end of message". The B<-t> option says to use the
505 headers to decide who to send the message to, and B<-odq> says to put
506 the message into the queue. This last option means your message won't
507 be immediately delivered, so leave it out if you want immediate
510 Alternate, less convenient approaches include calling mail (sometimes
511 called mailx) directly or simply opening up port 25 have having an
512 intimate conversation between just you and the remote SMTP daemon,
515 Or you might be able use the CPAN module Mail::Mailer:
519 $mailer = Mail::Mailer->new();
520 $mailer->open({ From => $from_address,
524 or die "Can't open: $!\n";
528 The Mail::Internet module uses Net::SMTP which is less Unix-centric than
529 Mail::Mailer, but less reliable. Avoid raw SMTP commands. There
530 are many reasons to use a mail transport agent like sendmail. These
531 include queuing, MX records, and security.
533 =head2 How do I use MIME to make an attachment to a mail message?
535 This answer is extracted directly from the MIME::Lite documentation.
536 Create a multipart message (i.e., one with attachments).
540 ### Create a new multipart message:
541 $msg = MIME::Lite->new(
542 From =>'me@myhost.com',
543 To =>'you@yourhost.com',
544 Cc =>'some@other.com, some@more.com',
545 Subject =>'A message with 2 parts...',
546 Type =>'multipart/mixed'
549 ### Add parts (each "attach" has same arguments as "new"):
550 $msg->attach(Type =>'TEXT',
551 Data =>"Here's the GIF file you wanted"
553 $msg->attach(Type =>'image/gif',
554 Path =>'aaa000123.gif',
555 Filename =>'logo.gif'
558 $text = $msg->as_string;
560 MIME::Lite also includes a method for sending these things.
564 This defaults to using L<sendmail> but can be customized to use
565 SMTP via L<Net::SMTP>.
567 =head2 How do I read mail?
569 While you could use the Mail::Folder module from CPAN (part of the
570 MailFolder package) or the Mail::Internet module from CPAN (part
571 of the MailTools package), often a module is overkill. Here's a
578 $/ = ''; # paragraph reads
581 /^Subject:\s*(?:Re:\s*)*(.*)/mi;
582 $sub[++$msgno] = lc($1) || '';
586 for my $i (sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msgs)) {
593 # bysub2 - awkish sort-by-subject
594 BEGIN { $msgno = -1 }
595 $sub[++$msgno] = (/^Subject:\s*(?:Re:\s*)*(.*)/mi)[0] if /^From/m;
597 END { print @msg[ sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msg) ] }
599 =head2 How do I find out my hostname/domainname/IP address?
601 The normal way to find your own hostname is to call the C<`hostname`>
602 program. While sometimes expedient, this has some problems, such as
603 not knowing whether you've got the canonical name or not. It's one of
604 those tradeoffs of convenience versus portability.
606 The Sys::Hostname module (part of the standard perl distribution) will
607 give you the hostname after which you can find out the IP address
608 (assuming you have working DNS) with a gethostbyname() call.
612 my $host = hostname();
613 my $addr = inet_ntoa(scalar gethostbyname($host || 'localhost'));
615 Probably the simplest way to learn your DNS domain name is to grok
616 it out of /etc/resolv.conf, at least under Unix. Of course, this
617 assumes several things about your resolv.conf configuration, including
620 (We still need a good DNS domain name-learning method for non-Unix
623 =head2 How do I fetch a news article or the active newsgroups?
625 Use the Net::NNTP or News::NNTPClient modules, both available from CPAN.
626 This can make tasks like fetching the newsgroup list as simple as
628 perl -MNews::NNTPClient
629 -e 'print News::NNTPClient->new->list("newsgroups")'
631 =head2 How do I fetch/put an FTP file?
633 LWP::Simple (available from CPAN) can fetch but not put. Net::FTP (also
634 available from CPAN) is more complex but can put as well as fetch.
636 =head2 How can I do RPC in Perl?
638 (Contributed by brian d foy)
640 Use one of the RPC modules you can find on CPAN (
641 http://search.cpan.org/search?query=RPC&mode=all ).
643 =head1 AUTHOR AND COPYRIGHT
645 Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and
646 other authors as noted. All rights reserved.
648 This documentation is free; you can redistribute it and/or modify it
649 under the same terms as Perl itself.
651 Irrespective of its distribution, all code examples in this file
652 are hereby placed into the public domain. You are permitted and
653 encouraged to use this code in your own programs for fun
654 or for profit as you see fit. A simple comment in the code giving
655 credit would be courteous but is not required.