3 perlfaq9 - Networking ($Revision: 1.3 $, $Date: 2001/10/16 13:27:22 $)
7 This section deals with questions related to networking, the internet,
10 =head2 What is the correct form of response from a CGI script?
12 (Alan Flavell <flavell+www@a5.ph.gla.ac.uk> answers...)
14 The Common Gateway Interface (CGI) specifies a software interface between
15 a program ("CGI script") and a web server (HTTPD). It is not specific
16 to Perl, and has its own FAQs and tutorials, and usenet group,
17 comp.infosystems.www.authoring.cgi
19 The original CGI specification is at: http://hoohoo.ncsa.uiuc.edu/cgi/
21 Current best-practice RFC draft at: http://CGI-Spec.Golux.Com/
23 Other relevant documentation listed in: http://www.perl.org/CGI_MetaFAQ.html
25 These Perl FAQs very selectively cover some CGI issues. However, Perl
26 programmers are strongly advised to use the CGI.pm module, to take care
27 of the details for them.
29 The similarity between CGI response headers (defined in the CGI
30 specification) and HTTP response headers (defined in the HTTP
31 specification, RFC2616) is intentional, but can sometimes be confusing.
33 The CGI specification defines two kinds of script: the "Parsed Header"
34 script, and the "Non Parsed Header" (NPH) script. Check your server
35 documentation to see what it supports. "Parsed Header" scripts are
36 simpler in various respects. The CGI specification allows any of the
37 usual newline representations in the CGI response (it's the server's
38 job to create an accurate HTTP response based on it). So "\n" written in
39 text mode is technically correct, and recommended. NPH scripts are more
40 tricky: they must put out a complete and accurate set of HTTP
41 transaction response headers; the HTTP specification calls for records
42 to be terminated with carriage-return and line-feed, i.e ASCII \015\012
43 written in binary mode.
45 Using CGI.pm gives excellent platform independence, including EBCDIC
46 systems. CGI.pm selects an appropriate newline representation
47 ($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 A quick but imperfect approach is
149 # qxurl - tchrist@perl.com
150 print "$2\n" while m{
152 A \s+ HREF \s* = \s* (["']) (.*?) \1
156 This version does not adjust relative URLs, understand alternate
157 bases, deal with HTML comments, deal with HREF and NAME attributes
158 in the same tag, understand extra qualifiers like TARGET, or accept
159 URLs themselves as arguments. It also runs about 100x faster than a
160 more "complete" solution using the LWP suite of modules, such as the
161 http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/xurl.gz program.
163 =head2 How do I download a file from the user's machine? How do I open a file on another machine?
165 In the context of an HTML form, you can use what's known as
166 B<multipart/form-data> encoding. The CGI.pm module (available from
167 CPAN) supports this in the start_multipart_form() method, which isn't
168 the same as the startform() method.
170 =head2 How do I make a pop-up menu in HTML?
172 Use the B<< <SELECT> >> and B<< <OPTION> >> tags. The CGI.pm
173 module (available from CPAN) supports this widget, as well as many
174 others, including some that it cleverly synthesizes on its own.
176 =head2 How do I fetch an HTML file?
178 One approach, if you have the lynx text-based HTML browser installed
179 on your system, is this:
181 $html_code = `lynx -source $url`;
182 $text_data = `lynx -dump $url`;
184 The libwww-perl (LWP) modules from CPAN provide a more powerful way
185 to do this. They don't require lynx, but like lynx, can still work
190 $content = get($URL);
192 # or print HTML from a URL
194 getprint "http://www.linpro.no/lwp/";
196 # or print ASCII from HTML from a URL
197 # also need HTML-Tree package from CPAN
200 use HTML::FormatText;
202 $html = get("http://www.perl.com/");
204 or die "Can't fetch HTML from http://www.perl.com/";
205 $ascii = HTML::FormatText->new->format(parse_html($html));
208 =head2 How do I automate an HTML form submission?
210 If you're submitting values using the GET method, create a URL and encode
211 the form using the C<query_form> method:
216 my $url = url('http://www.perl.com/cgi-bin/cpan_mod');
217 $url->query_form(module => 'DB_File', readme => 1);
218 $content = get($url);
220 If you're using the POST method, create your own user agent and encode
221 the content appropriately.
223 use HTTP::Request::Common qw(POST);
226 $ua = LWP::UserAgent->new();
227 my $req = POST 'http://www.perl.com/cgi-bin/cpan_mod',
228 [ module => 'DB_File', readme => 1 ];
229 $content = $ua->request($req)->as_string;
231 =head2 How do I decode or create those %-encodings on the web?
234 If you are writing a CGI script, you should be using the CGI.pm module
235 that comes with perl, or some other equivalent module. The CGI module
236 automatically decodes queries for you, and provides an escape()
237 function to handle encoding.
240 The best source of detailed information on URI encoding is RFC 2396.
241 Basically, the following substitutions do it:
243 s/([^\w()'*~!.-])/sprintf '%%%02x', $1/eg; # encode
245 s/%([A-Fa-f\d]{2})/chr hex $1/eg; # decode
247 However, you should only apply them to individual URI components, not
248 the entire URI, otherwise you'll lose information and generally mess
249 things up. If that didn't explain it, don't worry. Just go read
250 section 2 of the RFC, it's probably the best explanation there is.
252 RFC 2396 also contains a lot of other useful information, including a
253 regexp for breaking any arbitrary URI into components (Appendix B).
255 =head2 How do I redirect to another page?
257 Specify the complete URL of the destination (even if it is on the same
258 server). This is one of the two different kinds of CGI "Location:"
259 responses which are defined in the CGI specification for a Parsed Headers
260 script. The other kind (an absolute URLpath) is resolved internally to
261 the server without any HTTP redirection. The CGI specifications do not
262 allow relative URLs in either case.
264 Use of CGI.pm is strongly recommended. This example shows redirection
265 with a complete URL. This redirection is handled by the web browser.
267 use CGI qw/:standard/;
269 my $url = 'http://www.perl.com/CPAN/';
270 print redirect($url);
273 This example shows a redirection with an absolute URLpath. This
274 redirection is handled by the local web server.
276 my $url = '/CPAN/index.html';
277 print redirect($url);
280 But if coded directly, it could be as follows (the final "\n" is
281 shown separately, for clarity), using either a complete URL or
284 print "Location: $url\n"; # CGI response header
285 print "\n"; # end of headers
288 =head2 How do I put a password on my web pages?
290 That depends. You'll need to read the documentation for your web
291 server, or perhaps check some of the other FAQs referenced above.
293 =head2 How do I edit my .htpasswd and .htgroup files with Perl?
295 The HTTPD::UserAdmin and HTTPD::GroupAdmin modules provide a
296 consistent OO interface to these files, regardless of how they're
297 stored. Databases may be text, dbm, Berkeley DB or any database with
298 a DBI compatible driver. HTTPD::UserAdmin supports files used by the
299 `Basic' and `Digest' authentication schemes. Here's an example:
301 use HTTPD::UserAdmin ();
303 ->new(DB => "/foo/.htpasswd")
304 ->add($username => $password);
306 =head2 How do I make sure users can't enter values into a form that cause my CGI script to do bad things?
308 See the security references listed in the CGI Meta FAQ
310 http://www.perl.org/CGI_MetaFAQ.html
312 =head2 How do I parse a mail header?
314 For a quick-and-dirty solution, try this solution derived
315 from L<perlfunc/split>:
319 $header =~ s/\n\s+/ /g; # merge continuation lines
320 %head = ( UNIX_FROM_LINE, split /^([-\w]+):\s*/m, $header );
322 That solution doesn't do well if, for example, you're trying to
323 maintain all the Received lines. A more complete approach is to use
324 the Mail::Header module from CPAN (part of the MailTools package).
326 =head2 How do I decode a CGI form?
328 You use a standard module, probably CGI.pm. Under no circumstances
329 should you attempt to do so by hand!
331 You'll see a lot of CGI programs that blindly read from STDIN the number
332 of bytes equal to CONTENT_LENGTH for POSTs, or grab QUERY_STRING for
333 decoding GETs. These programs are very poorly written. They only work
334 sometimes. They typically forget to check the return value of the read()
335 system call, which is a cardinal sin. They don't handle HEAD requests.
336 They don't handle multipart forms used for file uploads. They don't deal
337 with GET/POST combinations where query fields are in more than one place.
338 They don't deal with keywords in the query string.
340 In short, they're bad hacks. Resist them at all costs. Please do not be
341 tempted to reinvent the wheel. Instead, use the CGI.pm or CGI_Lite.pm
342 (available from CPAN), or if you're trapped in the module-free land
343 of perl1 .. perl4, you might look into cgi-lib.pl (available from
344 http://cgi-lib.stanford.edu/cgi-lib/ ).
346 Make sure you know whether to use a GET or a POST in your form.
347 GETs should only be used for something that doesn't update the server.
348 Otherwise you can get mangled databases and repeated feedback mail
349 messages. The fancy word for this is ``idempotency''. This simply
350 means that there should be no difference between making a GET request
351 for a particular URL once or multiple times. This is because the
352 HTTP protocol definition says that a GET request may be cached by the
353 browser, or server, or an intervening proxy. POST requests cannot be
354 cached, because each request is independent and matters. Typically,
355 POST requests change or depend on state on the server (query or update
356 a database, send mail, or purchase a computer).
358 =head2 How do I check a valid mail address?
360 You can't, at least, not in real time. Bummer, eh?
362 Without sending mail to the address and seeing whether there's a human
363 on the other hand to answer you, you cannot determine whether a mail
364 address is valid. Even if you apply the mail header standard, you
365 can have problems, because there are deliverable addresses that aren't
366 RFC-822 (the mail header standard) compliant, and addresses that aren't
367 deliverable which are compliant.
369 Many are tempted to try to eliminate many frequently-invalid
370 mail addresses with a simple regex, such as
371 C</^[\w.-]+\@(?:[\w-]+\.)+\w+$/>. It's a very bad idea. However,
372 this also throws out many valid ones, and says nothing about
373 potential deliverability, so it is not suggested. Instead, see
374 http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/ckaddr.gz,
375 which actually checks against the full RFC spec (except for nested
376 comments), looks for addresses you may not wish to accept mail to
377 (say, Bill Clinton or your postmaster), and then makes sure that the
378 hostname given can be looked up in the DNS MX records. It's not fast,
379 but it works for what it tries to do.
381 Our best advice for verifying a person's mail address is to have them
382 enter their address twice, just as you normally do to change a password.
383 This usually weeds out typos. If both versions match, send
384 mail to that address with a personal message that looks somewhat like:
386 Dear someuser@host.com,
388 Please confirm the mail address you gave us Wed May 6 09:38:41
389 MDT 1998 by replying to this message. Include the string
390 "Rumpelstiltskin" in that reply, but spelled in reverse; that is,
391 start with "Nik...". Once this is done, your confirmed address will
392 be entered into our records.
394 If you get the message back and they've followed your directions,
395 you can be reasonably assured that it's real.
397 A related strategy that's less open to forgery is to give them a PIN
398 (personal ID number). Record the address and PIN (best that it be a
399 random one) for later processing. In the mail you send, ask them to
400 include the PIN in their reply. But if it bounces, or the message is
401 included via a ``vacation'' script, it'll be there anyway. So it's
402 best to ask them to mail back a slight alteration of the PIN, such as
403 with the characters reversed, one added or subtracted to each digit, etc.
405 =head2 How do I decode a MIME/BASE64 string?
407 The MIME-Base64 package (available from CPAN) handles this as well as
408 the MIME/QP encoding. Decoding BASE64 becomes as simple as:
411 $decoded = decode_base64($encoded);
413 The MIME-Tools package (available from CPAN) supports extraction with
414 decoding of BASE64 encoded attachments and content directly from email
417 If the string to decode is short (less than 84 bytes long)
418 a more direct approach is to use the unpack() function's "u"
419 format after minor transliterations:
421 tr#A-Za-z0-9+/##cd; # remove non-base64 chars
422 tr#A-Za-z0-9+/# -_#; # convert to uuencoded format
423 $len = pack("c", 32 + 0.75*length); # compute length byte
424 print unpack("u", $len . $_); # uudecode and print
426 =head2 How do I return the user's mail address?
428 On systems that support getpwuid, the $< variable, and the
429 Sys::Hostname module (which is part of the standard perl distribution),
430 you can probably try using something like this:
433 $address = sprintf('%s@%s', scalar getpwuid($<), hostname);
435 Company policies on mail address can mean that this generates addresses
436 that the company's mail system will not accept, so you should ask for
437 users' mail addresses when this matters. Furthermore, not all systems
438 on which Perl runs are so forthcoming with this information as is Unix.
440 The Mail::Util module from CPAN (part of the MailTools package) provides a
441 mailaddress() function that tries to guess the mail address of the user.
442 It makes a more intelligent guess than the code above, using information
443 given when the module was installed, but it could still be incorrect.
444 Again, the best way is often just to ask the user.
446 =head2 How do I send mail?
448 Use the C<sendmail> program directly:
450 open(SENDMAIL, "|/usr/lib/sendmail -oi -t -odq")
451 or die "Can't fork for sendmail: $!\n";
452 print SENDMAIL <<"EOF";
453 From: User Originating Mail <me\@host>
454 To: Final Destination <you\@otherhost>
455 Subject: A relevant subject line
457 Body of the message goes here after the blank line
458 in as many lines as you like.
460 close(SENDMAIL) or warn "sendmail didn't close nicely";
462 The B<-oi> option prevents sendmail from interpreting a line consisting
463 of a single dot as "end of message". The B<-t> option says to use the
464 headers to decide who to send the message to, and B<-odq> says to put
465 the message into the queue. This last option means your message won't
466 be immediately delivered, so leave it out if you want immediate
469 Alternate, less convenient approaches include calling mail (sometimes
470 called mailx) directly or simply opening up port 25 have having an
471 intimate conversation between just you and the remote SMTP daemon,
474 Or you might be able use the CPAN module Mail::Mailer:
478 $mailer = Mail::Mailer->new();
479 $mailer->open({ From => $from_address,
483 or die "Can't open: $!\n";
487 The Mail::Internet module uses Net::SMTP which is less Unix-centric than
488 Mail::Mailer, but less reliable. Avoid raw SMTP commands. There
489 are many reasons to use a mail transport agent like sendmail. These
490 include queuing, MX records, and security.
492 =head2 How do I use MIME to make an attachment to a mail message?
494 This answer is extracted directly from the MIME::Lite documentation.
495 Create a multipart message (i.e., one with attachments).
499 ### Create a new multipart message:
500 $msg = MIME::Lite->new(
501 From =>'me@myhost.com',
502 To =>'you@yourhost.com',
503 Cc =>'some@other.com, some@more.com',
504 Subject =>'A message with 2 parts...',
505 Type =>'multipart/mixed'
508 ### Add parts (each "attach" has same arguments as "new"):
509 $msg->attach(Type =>'TEXT',
510 Data =>"Here's the GIF file you wanted"
512 $msg->attach(Type =>'image/gif',
513 Path =>'aaa000123.gif',
514 Filename =>'logo.gif'
517 $text = $msg->as_string;
519 MIME::Lite also includes a method for sending these things.
523 This defaults to using L<sendmail(1)> but can be customized to use
524 SMTP via L<Net::SMTP>.
526 =head2 How do I read mail?
528 While you could use the Mail::Folder module from CPAN (part of the
529 MailFolder package) or the Mail::Internet module from CPAN (also part
530 of the MailTools package), often a module is overkill. Here's a
534 # bysub1 - simple sort by subject
537 $/ = ''; # paragraph reads
540 /^Subject:\s*(?:Re:\s*)*(.*)/mi;
541 $sub[++$msgno] = lc($1) || '';
545 for my $i (sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msgs)) {
552 # bysub2 - awkish sort-by-subject
553 BEGIN { $msgno = -1 }
554 $sub[++$msgno] = (/^Subject:\s*(?:Re:\s*)*(.*)/mi)[0] if /^From/m;
556 END { print @msg[ sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msg) ] }
558 =head2 How do I find out my hostname/domainname/IP address?
560 The normal way to find your own hostname is to call the C<`hostname`>
561 program. While sometimes expedient, this has some problems, such as
562 not knowing whether you've got the canonical name or not. It's one of
563 those tradeoffs of convenience versus portability.
565 The Sys::Hostname module (part of the standard perl distribution) will
566 give you the hostname after which you can find out the IP address
567 (assuming you have working DNS) with a gethostbyname() call.
571 my $host = hostname();
572 my $addr = inet_ntoa(scalar gethostbyname($host || 'localhost'));
574 Probably the simplest way to learn your DNS domain name is to grok
575 it out of /etc/resolv.conf, at least under Unix. Of course, this
576 assumes several things about your resolv.conf configuration, including
579 (We still need a good DNS domain name-learning method for non-Unix
582 =head2 How do I fetch a news article or the active newsgroups?
584 Use the Net::NNTP or News::NNTPClient modules, both available from CPAN.
585 This can make tasks like fetching the newsgroup list as simple as
587 perl -MNews::NNTPClient
588 -e 'print News::NNTPClient->new->list("newsgroups")'
590 =head2 How do I fetch/put an FTP file?
592 LWP::Simple (available from CPAN) can fetch but not put. Net::FTP (also
593 available from CPAN) is more complex but can put as well as fetch.
595 =head2 How can I do RPC in Perl?
597 A DCE::RPC module is being developed (but is not yet available) and
598 will be released as part of the DCE-Perl package (available from
599 CPAN). The rpcgen suite, available from CPAN/authors/id/JAKE/, is
600 an RPC stub generator and includes an RPC::ONC module.
602 =head1 AUTHOR AND COPYRIGHT
604 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
607 This documentation is free; you can redistribute it and/or modify it
608 under the same terms as Perl itself.
610 Irrespective of its distribution, all code examples in this file
611 are hereby placed into the public domain. You are permitted and
612 encouraged to use this code in your own programs for fun
613 or for profit as you see fit. A simple comment in the code giving
614 credit would be courteous but is not required.