3 perlfaq9 - Networking ($Revision: 1.4 $, $Date: 2001/10/31 23:54:56 $)
7 This section deals with questions related to networking, the internet,
10 =head2 What is the correct form of response from a CGI script?
12 (Alan Flavell <flavell+www@a5.ph.gla.ac.uk> answers...)
14 The Common Gateway Interface (CGI) specifies a software interface between
15 a program ("CGI script") and a web server (HTTPD). It is not specific
16 to Perl, and has its own FAQs and tutorials, and usenet group,
17 comp.infosystems.www.authoring.cgi
19 The original CGI specification is at: http://hoohoo.ncsa.uiuc.edu/cgi/
21 Current best-practice RFC draft at: http://CGI-Spec.Golux.Com/
23 Other relevant documentation listed in: http://www.perl.org/CGI_MetaFAQ.html
25 These Perl FAQs very selectively cover some CGI issues. However, Perl
26 programmers are strongly advised to use the CGI.pm module, to take care
27 of the details for them.
29 The similarity between CGI response headers (defined in the CGI
30 specification) and HTTP response headers (defined in the HTTP
31 specification, RFC2616) is intentional, but can sometimes be confusing.
33 The CGI specification defines two kinds of script: the "Parsed Header"
34 script, and the "Non Parsed Header" (NPH) script. Check your server
35 documentation to see what it supports. "Parsed Header" scripts are
36 simpler in various respects. The CGI specification allows any of the
37 usual newline representations in the CGI response (it's the server's
38 job to create an accurate HTTP response based on it). So "\n" written in
39 text mode is technically correct, and recommended. NPH scripts are more
40 tricky: they must put out a complete and accurate set of HTTP
41 transaction response headers; the HTTP specification calls for records
42 to be terminated with carriage-return and line-feed, i.e ASCII \015\012
43 written in binary mode.
45 Using CGI.pm gives excellent platform independence, including EBCDIC
46 systems. CGI.pm selects an appropriate newline representation
47 ($CGI::CRLF) and sets binmode as appropriate.
49 =head2 My CGI script runs from the command line but not the browser. (500 Server Error)
51 If you can demonstrate that you've read the FAQs and that
52 your problem isn't something simple that can be easily answered, you'll
53 probably receive a courteous and useful reply to your question if you
54 post it on comp.infosystems.www.authoring.cgi (if it's something to do
55 with HTTP or the CGI protocols). Questions that appear to be Perl
56 questions but are really CGI ones that are posted to comp.lang.perl.misc
57 are not so well received.
59 The useful FAQs, related documents, and troubleshooting guides are
60 listed in the CGI Meta FAQ:
62 http://www.perl.org/CGI_MetaFAQ.html
65 =head2 How can I get better error messages from a CGI program?
67 Use the CGI::Carp module. It replaces C<warn> and C<die>, plus the
68 normal Carp modules C<carp>, C<croak>, and C<confess> functions with
69 more verbose and safer versions. It still sends them to the normal
73 warn "This is a complaint";
74 die "But this one is serious";
76 The following use of CGI::Carp also redirects errors to a file of your choice,
77 placed in a BEGIN block to catch compile-time warnings as well:
80 use CGI::Carp qw(carpout);
81 open(LOG, ">>/var/local/cgi-logs/mycgi-log")
82 or die "Unable to append to mycgi-log: $!\n";
86 You can even arrange for fatal errors to go back to the client browser,
87 which is nice for your own debugging, but might confuse the end user.
89 use CGI::Carp qw(fatalsToBrowser);
92 Even if the error happens before you get the HTTP header out, the module
93 will try to take care of this to avoid the dreaded server 500 errors.
94 Normal warnings still go out to the server error log (or wherever
95 you've sent them with C<carpout>) with the application name and date
98 =head2 How do I remove HTML from a string?
100 The most correct way (albeit not the fastest) is to use HTML::Parser
101 from CPAN. Another mostly correct
102 way is to use HTML::FormatText which not only removes HTML but also
103 attempts to do a little simple formatting of the resulting plain text.
105 Many folks attempt a simple-minded regular expression approach, like
106 C<< s/<.*?>//g >>, but that fails in many cases because the tags
107 may continue over line breaks, they may contain quoted angle-brackets,
108 or HTML comment may be present. Plus, folks forget to convert
109 entities--like C<<> for example.
111 Here's one "simple-minded" approach, that works for most files:
113 #!/usr/bin/perl -p0777
114 s/<(?:[^>'"]*|(['"]).*?\1)*>//gs
116 If you want a more complete solution, see the 3-stage striphtml
118 http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/striphtml.gz
121 Here are some tricky cases that you should think about when picking
124 <IMG SRC = "foo.gif" ALT = "A > B">
131 <script>if (a<b && a>c)</script>
135 <![INCLUDE CDATA [ >>>>>>>>>>>> ]]>
137 If HTML comments include other tags, those solutions would also break
140 <!-- This section commented out.
141 <B>You can't see me!</B>
144 =head2 How do I extract URLs?
146 You can easily extract all sorts of URLs from HTML with
147 C<HTML::SimpleLinkExtor> which handles anchors, images, objects,
148 frames, and many other tags that can contain a URL. If you need
149 anything more complex, you can create your own subclass of
150 C<HTML::LinkExtor> or C<HTML::Parser>. You might even use
151 C<HTML::SimpleLinkExtor> as an example for something specifically
152 suited to your needs.
154 Less complete solutions involving regular expressions can save
155 you a lot of processing time if you know that the input is simple. One
156 solution from Tom Christiansen runs 100 times faster than most
157 module based approaches but only extracts URLs from anchors where the first
158 attribute is HREF and there are no other attributes.
161 # qxurl - tchrist@perl.com
162 print "$2\n" while m{
164 A \s+ HREF \s* = \s* (["']) (.*?) \1
169 =head2 How do I download a file from the user's machine? How do I open a file on another machine?
171 In the context of an HTML form, you can use what's known as
172 B<multipart/form-data> encoding. The CGI.pm module (available from
173 CPAN) supports this in the start_multipart_form() method, which isn't
174 the same as the startform() method.
176 =head2 How do I make a pop-up menu in HTML?
178 Use the B<< <SELECT> >> and B<< <OPTION> >> tags. The CGI.pm
179 module (available from CPAN) supports this widget, as well as many
180 others, including some that it cleverly synthesizes on its own.
182 =head2 How do I fetch an HTML file?
184 One approach, if you have the lynx text-based HTML browser installed
185 on your system, is this:
187 $html_code = `lynx -source $url`;
188 $text_data = `lynx -dump $url`;
190 The libwww-perl (LWP) modules from CPAN provide a more powerful way
191 to do this. They don't require lynx, but like lynx, can still work
196 $content = get($URL);
198 # or print HTML from a URL
200 getprint "http://www.linpro.no/lwp/";
202 # or print ASCII from HTML from a URL
203 # also need HTML-Tree package from CPAN
206 use HTML::FormatText;
208 $html = get("http://www.perl.com/");
210 or die "Can't fetch HTML from http://www.perl.com/";
211 $ascii = HTML::FormatText->new->format(parse_html($html));
214 =head2 How do I automate an HTML form submission?
216 If you're submitting values using the GET method, create a URL and encode
217 the form using the C<query_form> method:
222 my $url = url('http://www.perl.com/cgi-bin/cpan_mod');
223 $url->query_form(module => 'DB_File', readme => 1);
224 $content = get($url);
226 If you're using the POST method, create your own user agent and encode
227 the content appropriately.
229 use HTTP::Request::Common qw(POST);
232 $ua = LWP::UserAgent->new();
233 my $req = POST 'http://www.perl.com/cgi-bin/cpan_mod',
234 [ module => 'DB_File', readme => 1 ];
235 $content = $ua->request($req)->as_string;
237 =head2 How do I decode or create those %-encodings on the web?
240 If you are writing a CGI script, you should be using the CGI.pm module
241 that comes with perl, or some other equivalent module. The CGI module
242 automatically decodes queries for you, and provides an escape()
243 function to handle encoding.
246 The best source of detailed information on URI encoding is RFC 2396.
247 Basically, the following substitutions do it:
249 s/([^\w()'*~!.-])/sprintf '%%%02x', $1/eg; # encode
251 s/%([A-Fa-f\d]{2})/chr hex $1/eg; # decode
253 However, you should only apply them to individual URI components, not
254 the entire URI, otherwise you'll lose information and generally mess
255 things up. If that didn't explain it, don't worry. Just go read
256 section 2 of the RFC, it's probably the best explanation there is.
258 RFC 2396 also contains a lot of other useful information, including a
259 regexp for breaking any arbitrary URI into components (Appendix B).
261 =head2 How do I redirect to another page?
263 Specify the complete URL of the destination (even if it is on the same
264 server). This is one of the two different kinds of CGI "Location:"
265 responses which are defined in the CGI specification for a Parsed Headers
266 script. The other kind (an absolute URLpath) is resolved internally to
267 the server without any HTTP redirection. The CGI specifications do not
268 allow relative URLs in either case.
270 Use of CGI.pm is strongly recommended. This example shows redirection
271 with a complete URL. This redirection is handled by the web browser.
273 use CGI qw/:standard/;
275 my $url = 'http://www.perl.com/CPAN/';
276 print redirect($url);
279 This example shows a redirection with an absolute URLpath. This
280 redirection is handled by the local web server.
282 my $url = '/CPAN/index.html';
283 print redirect($url);
286 But if coded directly, it could be as follows (the final "\n" is
287 shown separately, for clarity), using either a complete URL or
290 print "Location: $url\n"; # CGI response header
291 print "\n"; # end of headers
294 =head2 How do I put a password on my web pages?
296 That depends. You'll need to read the documentation for your web
297 server, or perhaps check some of the other FAQs referenced above.
299 =head2 How do I edit my .htpasswd and .htgroup files with Perl?
301 The HTTPD::UserAdmin and HTTPD::GroupAdmin modules provide a
302 consistent OO interface to these files, regardless of how they're
303 stored. Databases may be text, dbm, Berkeley DB or any database with
304 a DBI compatible driver. HTTPD::UserAdmin supports files used by the
305 `Basic' and `Digest' authentication schemes. Here's an example:
307 use HTTPD::UserAdmin ();
309 ->new(DB => "/foo/.htpasswd")
310 ->add($username => $password);
312 =head2 How do I make sure users can't enter values into a form that cause my CGI script to do bad things?
314 See the security references listed in the CGI Meta FAQ
316 http://www.perl.org/CGI_MetaFAQ.html
318 =head2 How do I parse a mail header?
320 For a quick-and-dirty solution, try this solution derived
321 from L<perlfunc/split>:
325 $header =~ s/\n\s+/ /g; # merge continuation lines
326 %head = ( UNIX_FROM_LINE, split /^([-\w]+):\s*/m, $header );
328 That solution doesn't do well if, for example, you're trying to
329 maintain all the Received lines. A more complete approach is to use
330 the Mail::Header module from CPAN (part of the MailTools package).
332 =head2 How do I decode a CGI form?
334 You use a standard module, probably CGI.pm. Under no circumstances
335 should you attempt to do so by hand!
337 You'll see a lot of CGI programs that blindly read from STDIN the number
338 of bytes equal to CONTENT_LENGTH for POSTs, or grab QUERY_STRING for
339 decoding GETs. These programs are very poorly written. They only work
340 sometimes. They typically forget to check the return value of the read()
341 system call, which is a cardinal sin. They don't handle HEAD requests.
342 They don't handle multipart forms used for file uploads. They don't deal
343 with GET/POST combinations where query fields are in more than one place.
344 They don't deal with keywords in the query string.
346 In short, they're bad hacks. Resist them at all costs. Please do not be
347 tempted to reinvent the wheel. Instead, use the CGI.pm or CGI_Lite.pm
348 (available from CPAN), or if you're trapped in the module-free land
349 of perl1 .. perl4, you might look into cgi-lib.pl (available from
350 http://cgi-lib.stanford.edu/cgi-lib/ ).
352 Make sure you know whether to use a GET or a POST in your form.
353 GETs should only be used for something that doesn't update the server.
354 Otherwise you can get mangled databases and repeated feedback mail
355 messages. The fancy word for this is ``idempotency''. This simply
356 means that there should be no difference between making a GET request
357 for a particular URL once or multiple times. This is because the
358 HTTP protocol definition says that a GET request may be cached by the
359 browser, or server, or an intervening proxy. POST requests cannot be
360 cached, because each request is independent and matters. Typically,
361 POST requests change or depend on state on the server (query or update
362 a database, send mail, or purchase a computer).
364 =head2 How do I check a valid mail address?
366 You can't, at least, not in real time. Bummer, eh?
368 Without sending mail to the address and seeing whether there's a human
369 on the other hand to answer you, you cannot determine whether a mail
370 address is valid. Even if you apply the mail header standard, you
371 can have problems, because there are deliverable addresses that aren't
372 RFC-822 (the mail header standard) compliant, and addresses that aren't
373 deliverable which are compliant.
375 Many are tempted to try to eliminate many frequently-invalid
376 mail addresses with a simple regex, such as
377 C</^[\w.-]+\@(?:[\w-]+\.)+\w+$/>. It's a very bad idea. However,
378 this also throws out many valid ones, and says nothing about
379 potential deliverability, so it is not suggested. Instead, see
380 http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/ckaddr.gz,
381 which actually checks against the full RFC spec (except for nested
382 comments), looks for addresses you may not wish to accept mail to
383 (say, Bill Clinton or your postmaster), and then makes sure that the
384 hostname given can be looked up in the DNS MX records. It's not fast,
385 but it works for what it tries to do.
387 Our best advice for verifying a person's mail address is to have them
388 enter their address twice, just as you normally do to change a password.
389 This usually weeds out typos. If both versions match, send
390 mail to that address with a personal message that looks somewhat like:
392 Dear someuser@host.com,
394 Please confirm the mail address you gave us Wed May 6 09:38:41
395 MDT 1998 by replying to this message. Include the string
396 "Rumpelstiltskin" in that reply, but spelled in reverse; that is,
397 start with "Nik...". Once this is done, your confirmed address will
398 be entered into our records.
400 If you get the message back and they've followed your directions,
401 you can be reasonably assured that it's real.
403 A related strategy that's less open to forgery is to give them a PIN
404 (personal ID number). Record the address and PIN (best that it be a
405 random one) for later processing. In the mail you send, ask them to
406 include the PIN in their reply. But if it bounces, or the message is
407 included via a ``vacation'' script, it'll be there anyway. So it's
408 best to ask them to mail back a slight alteration of the PIN, such as
409 with the characters reversed, one added or subtracted to each digit, etc.
411 =head2 How do I decode a MIME/BASE64 string?
413 The MIME-Base64 package (available from CPAN) handles this as well as
414 the MIME/QP encoding. Decoding BASE64 becomes as simple as:
417 $decoded = decode_base64($encoded);
419 The MIME-Tools package (available from CPAN) supports extraction with
420 decoding of BASE64 encoded attachments and content directly from email
423 If the string to decode is short (less than 84 bytes long)
424 a more direct approach is to use the unpack() function's "u"
425 format after minor transliterations:
427 tr#A-Za-z0-9+/##cd; # remove non-base64 chars
428 tr#A-Za-z0-9+/# -_#; # convert to uuencoded format
429 $len = pack("c", 32 + 0.75*length); # compute length byte
430 print unpack("u", $len . $_); # uudecode and print
432 =head2 How do I return the user's mail address?
434 On systems that support getpwuid, the $< variable, and the
435 Sys::Hostname module (which is part of the standard perl distribution),
436 you can probably try using something like this:
439 $address = sprintf('%s@%s', scalar getpwuid($<), hostname);
441 Company policies on mail address can mean that this generates addresses
442 that the company's mail system will not accept, so you should ask for
443 users' mail addresses when this matters. Furthermore, not all systems
444 on which Perl runs are so forthcoming with this information as is Unix.
446 The Mail::Util module from CPAN (part of the MailTools package) provides a
447 mailaddress() function that tries to guess the mail address of the user.
448 It makes a more intelligent guess than the code above, using information
449 given when the module was installed, but it could still be incorrect.
450 Again, the best way is often just to ask the user.
452 =head2 How do I send mail?
454 Use the C<sendmail> program directly:
456 open(SENDMAIL, "|/usr/lib/sendmail -oi -t -odq")
457 or die "Can't fork for sendmail: $!\n";
458 print SENDMAIL <<"EOF";
459 From: User Originating Mail <me\@host>
460 To: Final Destination <you\@otherhost>
461 Subject: A relevant subject line
463 Body of the message goes here after the blank line
464 in as many lines as you like.
466 close(SENDMAIL) or warn "sendmail didn't close nicely";
468 The B<-oi> option prevents sendmail from interpreting a line consisting
469 of a single dot as "end of message". The B<-t> option says to use the
470 headers to decide who to send the message to, and B<-odq> says to put
471 the message into the queue. This last option means your message won't
472 be immediately delivered, so leave it out if you want immediate
475 Alternate, less convenient approaches include calling mail (sometimes
476 called mailx) directly or simply opening up port 25 have having an
477 intimate conversation between just you and the remote SMTP daemon,
480 Or you might be able use the CPAN module Mail::Mailer:
484 $mailer = Mail::Mailer->new();
485 $mailer->open({ From => $from_address,
489 or die "Can't open: $!\n";
493 The Mail::Internet module uses Net::SMTP which is less Unix-centric than
494 Mail::Mailer, but less reliable. Avoid raw SMTP commands. There
495 are many reasons to use a mail transport agent like sendmail. These
496 include queuing, MX records, and security.
498 =head2 How do I use MIME to make an attachment to a mail message?
500 This answer is extracted directly from the MIME::Lite documentation.
501 Create a multipart message (i.e., one with attachments).
505 ### Create a new multipart message:
506 $msg = MIME::Lite->new(
507 From =>'me@myhost.com',
508 To =>'you@yourhost.com',
509 Cc =>'some@other.com, some@more.com',
510 Subject =>'A message with 2 parts...',
511 Type =>'multipart/mixed'
514 ### Add parts (each "attach" has same arguments as "new"):
515 $msg->attach(Type =>'TEXT',
516 Data =>"Here's the GIF file you wanted"
518 $msg->attach(Type =>'image/gif',
519 Path =>'aaa000123.gif',
520 Filename =>'logo.gif'
523 $text = $msg->as_string;
525 MIME::Lite also includes a method for sending these things.
529 This defaults to using L<sendmail(1)> but can be customized to use
530 SMTP via L<Net::SMTP>.
532 =head2 How do I read mail?
534 While you could use the Mail::Folder module from CPAN (part of the
535 MailFolder package) or the Mail::Internet module from CPAN (also part
536 of the MailTools package), often a module is overkill. Here's a
540 # bysub1 - simple sort by subject
543 $/ = ''; # paragraph reads
546 /^Subject:\s*(?:Re:\s*)*(.*)/mi;
547 $sub[++$msgno] = lc($1) || '';
551 for my $i (sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msgs)) {
558 # bysub2 - awkish sort-by-subject
559 BEGIN { $msgno = -1 }
560 $sub[++$msgno] = (/^Subject:\s*(?:Re:\s*)*(.*)/mi)[0] if /^From/m;
562 END { print @msg[ sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msg) ] }
564 =head2 How do I find out my hostname/domainname/IP address?
566 The normal way to find your own hostname is to call the C<`hostname`>
567 program. While sometimes expedient, this has some problems, such as
568 not knowing whether you've got the canonical name or not. It's one of
569 those tradeoffs of convenience versus portability.
571 The Sys::Hostname module (part of the standard perl distribution) will
572 give you the hostname after which you can find out the IP address
573 (assuming you have working DNS) with a gethostbyname() call.
577 my $host = hostname();
578 my $addr = inet_ntoa(scalar gethostbyname($host || 'localhost'));
580 Probably the simplest way to learn your DNS domain name is to grok
581 it out of /etc/resolv.conf, at least under Unix. Of course, this
582 assumes several things about your resolv.conf configuration, including
585 (We still need a good DNS domain name-learning method for non-Unix
588 =head2 How do I fetch a news article or the active newsgroups?
590 Use the Net::NNTP or News::NNTPClient modules, both available from CPAN.
591 This can make tasks like fetching the newsgroup list as simple as
593 perl -MNews::NNTPClient
594 -e 'print News::NNTPClient->new->list("newsgroups")'
596 =head2 How do I fetch/put an FTP file?
598 LWP::Simple (available from CPAN) can fetch but not put. Net::FTP (also
599 available from CPAN) is more complex but can put as well as fetch.
601 =head2 How can I do RPC in Perl?
603 A DCE::RPC module is being developed (but is not yet available) and
604 will be released as part of the DCE-Perl package (available from
605 CPAN). The rpcgen suite, available from CPAN/authors/id/JAKE/, is
606 an RPC stub generator and includes an RPC::ONC module.
608 =head1 AUTHOR AND COPYRIGHT
610 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
613 This documentation is free; you can redistribute it and/or modify it
614 under the same terms as Perl itself.
616 Irrespective of its distribution, all code examples in this file
617 are hereby placed into the public domain. You are permitted and
618 encouraged to use this code in your own programs for fun
619 or for profit as you see fit. A simple comment in the code giving
620 credit would be courteous but is not required.