=head1 NAME
-perlfaq9 - Networking ($Revision: 1.26 $, $Date: 1999/05/23 16:08:30 $)
+perlfaq9 - Networking ($Revision: 1.15 $, $Date: 2003/01/31 17:36:57 $)
=head1 DESCRIPTION
This section deals with questions related to networking, the internet,
and a few on the web.
-=head2 My CGI script runs from the command line but not the browser. (500 Server Error)
+=head2 What is the correct form of response from a CGI script?
-If you can demonstrate that you've read the following FAQs and that
-your problem isn't something simple that can be easily answered, you'll
-probably receive a courteous and useful reply to your question if you
-post it on comp.infosystems.www.authoring.cgi (if it's something to do
-with HTTP, HTML, or the CGI protocols). Questions that appear to be Perl
-questions but are really CGI ones that are posted to comp.lang.perl.misc
-may not be so well received.
+(Alan Flavell <flavell+www@a5.ph.gla.ac.uk> answers...)
+
+The Common Gateway Interface (CGI) specifies a software interface between
+a program ("CGI script") and a web server (HTTPD). It is not specific
+to Perl, and has its own FAQs and tutorials, and usenet group,
+comp.infosystems.www.authoring.cgi
+
+The original CGI specification is at: http://hoohoo.ncsa.uiuc.edu/cgi/
-The useful FAQs and related documents are:
+Current best-practice RFC draft at: http://CGI-Spec.Golux.Com/
- CGI FAQ
- http://www.webthing.com/tutorials/cgifaq.html
+Other relevant documentation listed in: http://www.perl.org/CGI_MetaFAQ.html
- Web FAQ
- http://www.boutell.com/faq/
+These Perl FAQs very selectively cover some CGI issues. However, Perl
+programmers are strongly advised to use the CGI.pm module, to take care
+of the details for them.
- WWW Security FAQ
- http://www.w3.org/Security/Faq/
+The similarity between CGI response headers (defined in the CGI
+specification) and HTTP response headers (defined in the HTTP
+specification, RFC2616) is intentional, but can sometimes be confusing.
- HTTP Spec
- http://www.w3.org/pub/WWW/Protocols/HTTP/
+The CGI specification defines two kinds of script: the "Parsed Header"
+script, and the "Non Parsed Header" (NPH) script. Check your server
+documentation to see what it supports. "Parsed Header" scripts are
+simpler in various respects. The CGI specification allows any of the
+usual newline representations in the CGI response (it's the server's
+job to create an accurate HTTP response based on it). So "\n" written in
+text mode is technically correct, and recommended. NPH scripts are more
+tricky: they must put out a complete and accurate set of HTTP
+transaction response headers; the HTTP specification calls for records
+to be terminated with carriage-return and line-feed, i.e ASCII \015\012
+written in binary mode.
- HTML Spec
- http://www.w3.org/TR/REC-html40/
- http://www.w3.org/pub/WWW/MarkUp/
+Using CGI.pm gives excellent platform independence, including EBCDIC
+systems. CGI.pm selects an appropriate newline representation
+($CGI::CRLF) and sets binmode as appropriate.
- CGI Spec
- http://www.w3.org/CGI/
+=head2 My CGI script runs from the command line but not the browser. (500 Server Error)
+
+Several things could be wrong. You can go through the "Troubleshooting
+Perl CGI scripts" guide at
+
+ http://www.perl.org/troubleshooting_CGI.html
+
+If, after that, you can demonstrate that you've read the FAQs and that
+your problem isn't something simple that can be easily answered, you'll
+probably receive a courteous and useful reply to your question if you
+post it on comp.infosystems.www.authoring.cgi (if it's something to do
+with HTTP or the CGI protocols). Questions that appear to be Perl
+questions but are really CGI ones that are posted to comp.lang.perl.misc
+are not so well received.
+
+The useful FAQs, related documents, and troubleshooting guides are
+listed in the CGI Meta FAQ:
+
+ http://www.perl.org/CGI_MetaFAQ.html
- CGI Security FAQ
- http://www.go2net.com/people/paulp/cgi-security/safe-cgi.txt
=head2 How can I get better error messages from a CGI program?
Many folks attempt a simple-minded regular expression approach, like
C<< s/<.*?>//g >>, but that fails in many cases because the tags
may continue over line breaks, they may contain quoted angle-brackets,
-or HTML comment may be present. Plus folks forget to convert
-entities, like C<<> for example.
+or HTML comment may be present. Plus, folks forget to convert
+entities--like C<<> for example.
Here's one "simple-minded" approach, that works for most files:
If you want a more complete solution, see the 3-stage striphtml
program in
-http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/striphtml.gz
+http://www.cpan.org/authors/Tom_Christiansen/scripts/striphtml.gz
.
Here are some tricky cases that you should think about when picking
=head2 How do I extract URLs?
-A quick but imperfect approach is
+You can easily extract all sorts of URLs from HTML with
+C<HTML::SimpleLinkExtor> which handles anchors, images, objects,
+frames, and many other tags that can contain a URL. If you need
+anything more complex, you can create your own subclass of
+C<HTML::LinkExtor> or C<HTML::Parser>. You might even use
+C<HTML::SimpleLinkExtor> as an example for something specifically
+suited to your needs.
+
+You can use URI::Find to extract URLs from an arbitrary text document.
+
+Less complete solutions involving regular expressions can save
+you a lot of processing time if you know that the input is simple. One
+solution from Tom Christiansen runs 100 times faster than most
+module based approaches but only extracts URLs from anchors where the first
+attribute is HREF and there are no other attributes.
+
+ #!/usr/bin/perl -n00
+ # qxurl - tchrist@perl.com
+ print "$2\n" while m{
+ < \s*
+ A \s+ HREF \s* = \s* (["']) (.*?) \1
+ \s* >
+ }gsix;
- #!/usr/bin/perl -n00
- # qxurl - tchrist@perl.com
- print "$2\n" while m{
- < \s*
- A \s+ HREF \s* = \s* (["']) (.*?) \1
- \s* >
- }gsix;
-
-This version does not adjust relative URLs, understand alternate
-bases, deal with HTML comments, deal with HREF and NAME attributes
-in the same tag, understand extra qualifiers like TARGET, or accept
-URLs themselves as arguments. It also runs about 100x faster than a
-more "complete" solution using the LWP suite of modules, such as the
-http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/xurl.gz program.
=head2 How do I download a file from the user's machine? How do I open a file on another machine?
-In the context of an HTML form, you can use what's known as
-B<multipart/form-data> encoding. The CGI.pm module (available from
-CPAN) supports this in the start_multipart_form() method, which isn't
-the same as the startform() method.
+In this case, download means to use the file upload feature of HTML
+forms. You allow the web surfer to specify a file to send to your web
+server. To you it looks like a download, and to the user it looks
+like an upload. No matter what you call it, you do it with what's
+known as B<multipart/form-data> encoding. The CGI.pm module (which
+comes with Perl as part of the Standard Library) supports this in the
+start_multipart_form() method, which isn't the same as the startform()
+method.
+
+See the section in the CGI.pm documentation on file uploads for code
+examples and details.
=head2 How do I make a pop-up menu in HTML?
=head2 How do I decode or create those %-encodings on the web?
-Here's an example of decoding:
- $string = "http://altavista.digital.com/cgi-bin/query?pg=q&what=news&fmt=.&q=%2Bcgi-bin+%2Bperl.exe";
- $string =~ s/%([a-fA-F0-9]{2})/chr(hex($1))/ge;
+If you are writing a CGI script, you should be using the CGI.pm module
+that comes with perl, or some other equivalent module. The CGI module
+automatically decodes queries for you, and provides an escape()
+function to handle encoding.
+
-Encoding is a bit harder, because you can't just blindly change
-all the non-alphanumunder character (C<\W>) into their hex escapes.
-It's important that characters with special meaning like C</> and C<?>
-I<not> be translated. Probably the easiest way to get this right is
-to avoid reinventing the wheel and just use the URI::Escape module,
-available from CPAN.
+The best source of detailed information on URI encoding is RFC 2396.
+Basically, the following substitutions do it:
+
+ s/([^\w()'*~!.-])/sprintf '%%%02x', ord $1/eg; # encode
+
+ s/%([A-Fa-f\d]{2})/chr hex $1/eg; # decode
+
+However, you should only apply them to individual URI components, not
+the entire URI, otherwise you'll lose information and generally mess
+things up. If that didn't explain it, don't worry. Just go read
+section 2 of the RFC, it's probably the best explanation there is.
+
+RFC 2396 also contains a lot of other useful information, including a
+regexp for breaking any arbitrary URI into components (Appendix B).
=head2 How do I redirect to another page?
-Instead of sending back a C<Content-Type> as the headers of your
-reply, send back a C<Location:> header. Officially this should be a
-C<URI:> header, so the CGI.pm module (available from CPAN) sends back
-both:
+Specify the complete URL of the destination (even if it is on the same
+server). This is one of the two different kinds of CGI "Location:"
+responses which are defined in the CGI specification for a Parsed Headers
+script. The other kind (an absolute URLpath) is resolved internally to
+the server without any HTTP redirection. The CGI specifications do not
+allow relative URLs in either case.
- Location: http://www.domain.com/newpage
- URI: http://www.domain.com/newpage
+Use of CGI.pm is strongly recommended. This example shows redirection
+with a complete URL. This redirection is handled by the web browser.
-Note that relative URLs in these headers can cause strange effects
-because of "optimizations" that servers do.
+ use CGI qw/:standard/;
- $url = "http://www.perl.com/CPAN/";
- print "Location: $url\n\n";
- exit;
+ my $url = 'http://www.cpan.org/';
+ print redirect($url);
-To target a particular frame in a frameset, include the "Window-target:"
-in the header.
- print <<EOF;
- Location: http://www.domain.com/newpage
- Window-target: <FrameName>
+This example shows a redirection with an absolute URLpath. This
+redirection is handled by the local web server.
- EOF
+ my $url = '/CPAN/index.html';
+ print redirect($url);
+
+
+But if coded directly, it could be as follows (the final "\n" is
+shown separately, for clarity), using either a complete URL or
+an absolute URLpath.
+
+ print "Location: $url\n"; # CGI response header
+ print "\n"; # end of headers
-To be correct to the spec, each of those virtual newlines should really be
-physical C<"\015\012"> sequences by the time you hit the client browser.
-Except for NPH scripts, though, that local newline should get translated
-by your server into standard form, so you shouldn't have a problem
-here, even if you are stuck on MacOS. Everybody else probably won't
-even notice.
=head2 How do I put a password on my web pages?
-That depends. You'll need to read the documentation for your web
-server, or perhaps check some of the other FAQs referenced above.
+To enable authentication for your web server, you need to configure
+your web server. The configuration is different for different sorts
+of web servers---apache does it differently from iPlanet which does
+it differently from IIS. Check your web server documentation for
+the details for your particular server.
=head2 How do I edit my .htpasswd and .htgroup files with Perl?
The HTTPD::UserAdmin and HTTPD::GroupAdmin modules provide a
consistent OO interface to these files, regardless of how they're
-stored. Databases may be text, dbm, Berkley DB or any database with a
-DBI compatible driver. HTTPD::UserAdmin supports files used by the
+stored. Databases may be text, dbm, Berkeley DB or any database with
+a DBI compatible driver. HTTPD::UserAdmin supports files used by the
`Basic' and `Digest' authentication schemes. Here's an example:
use HTTPD::UserAdmin ();
=head2 How do I make sure users can't enter values into a form that cause my CGI script to do bad things?
-Read the CGI security FAQ, at
-http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html, and the
-Perl/CGI FAQ at
-http://www.perl.com/CPAN/doc/FAQs/cgi/perl-cgi-faq.html.
+See the security references listed in the CGI Meta FAQ
-In brief: use tainting (see L<perlsec>), which makes sure that data
-from outside your script (eg, CGI parameters) are never used in
-C<eval> or C<system> calls. In addition to tainting, never use the
-single-argument form of system() or exec(). Instead, supply the
-command and arguments as a list, which prevents shell globbing.
+ http://www.perl.org/CGI_MetaFAQ.html
=head2 How do I parse a mail header?
For a quick-and-dirty solution, try this solution derived
-from page 222 of the 2nd edition of "Programming Perl":
+from L<perlfunc/split>:
$/ = '';
$header = <MSG>;
RFC-822 (the mail header standard) compliant, and addresses that aren't
deliverable which are compliant.
+You can use the Email::Valid or RFC::RFC822::Address which check
+the format of the address, although they cannot actually tell you
+if it is a deliverable address (i.e. that mail to the address
+will not bounce). Modules like Mail::CheckUser and Mail::EXPN
+try to interact with the domain name system or particular
+mail servers to learn even more, but their methods do not
+work everywhere---especially for security conscious administrators.
+
Many are tempted to try to eliminate many frequently-invalid
mail addresses with a simple regex, such as
-C</^[\w.-]+\@([\w.-]\.)+\w+$/>. It's a very bad idea. However,
+C</^[\w.-]+\@(?:[\w-]+\.)+\w+$/>. It's a very bad idea. However,
this also throws out many valid ones, and says nothing about
-potential deliverability, so is not suggested. Instead, see
-http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/ckaddr.gz ,
+potential deliverability, so it is not suggested. Instead, see
+http://www.cpan.org/authors/Tom_Christiansen/scripts/ckaddr.gz ,
which actually checks against the full RFC spec (except for nested
comments), looks for addresses you may not wish to accept mail to
(say, Bill Clinton or your postmaster), and then makes sure that the
=head2 How do I decode a MIME/BASE64 string?
-The MIME-tools package (available from CPAN) handles this and a lot
-more. Decoding BASE64 becomes as simple as:
+The MIME-Base64 package (available from CPAN) handles this as well as
+the MIME/QP encoding. Decoding BASE64 becomes as simple as:
- use MIME::base64;
+ use MIME::Base64;
$decoded = decode_base64($encoded);
-A more direct approach is to use the unpack() function's "u"
+The MIME-Tools package (available from CPAN) supports extraction with
+decoding of BASE64 encoded attachments and content directly from email
+messages.
+
+If the string to decode is short (less than 84 bytes long)
+a more direct approach is to use the unpack() function's "u"
format after minor transliterations:
tr#A-Za-z0-9+/##cd; # remove non-base64 chars
=head2 How do I return the user's mail address?
-On systems that support getpwuid, the $< variable and the
+On systems that support getpwuid, the $< variable, and the
Sys::Hostname module (which is part of the standard perl distribution),
you can probably try using something like this:
The Mail::Internet module uses Net::SMTP which is less Unix-centric than
Mail::Mailer, but less reliable. Avoid raw SMTP commands. There
are many reasons to use a mail transport agent like sendmail. These
-include queueing, MX records, and security.
+include queuing, MX records, and security.
+
+=head2 How do I use MIME to make an attachment to a mail message?
+
+This answer is extracted directly from the MIME::Lite documentation.
+Create a multipart message (i.e., one with attachments).
+
+ use MIME::Lite;
+
+ ### Create a new multipart message:
+ $msg = MIME::Lite->new(
+ From =>'me@myhost.com',
+ To =>'you@yourhost.com',
+ Cc =>'some@other.com, some@more.com',
+ Subject =>'A message with 2 parts...',
+ Type =>'multipart/mixed'
+ );
+
+ ### Add parts (each "attach" has same arguments as "new"):
+ $msg->attach(Type =>'TEXT',
+ Data =>"Here's the GIF file you wanted"
+ );
+ $msg->attach(Type =>'image/gif',
+ Path =>'aaa000123.gif',
+ Filename =>'logo.gif'
+ );
+
+ $text = $msg->as_string;
+
+MIME::Lite also includes a method for sending these things.
+
+ $msg->send;
+
+This defaults to using L<sendmail> but can be customized to use
+SMTP via L<Net::SMTP>.
=head2 How do I read mail?
While you could use the Mail::Folder module from CPAN (part of the
-MailFolder package) or the Mail::Internet module from CPAN (also part
-of the MailTools package), often a module is overkill, though. Here's a
+MailFolder package) or the Mail::Internet module from CPAN (part
+of the MailTools package), often a module is overkill. Here's a
mail sorter.
#!/usr/bin/perl
- # bysub1 - simple sort by subject
+
my(@msgs, @sub);
my $msgno = -1;
$/ = ''; # paragraph reads
while (<>) {
- if (/^From/m) {
+ if (/^From /m) {
/^Subject:\s*(?:Re:\s*)*(.*)/mi;
$sub[++$msgno] = lc($1) || '';
}
=head2 How do I fetch a news article or the active newsgroups?
Use the Net::NNTP or News::NNTPClient modules, both available from CPAN.
-This can make tasks like fetching the newsgroup list as simple as:
+This can make tasks like fetching the newsgroup list as simple as
perl -MNews::NNTPClient
-e 'print News::NNTPClient->new->list("newsgroups")'
=head2 How can I do RPC in Perl?
-A DCE::RPC module is being developed (but is not yet available), and
+A DCE::RPC module is being developed (but is not yet available) and
will be released as part of the DCE-Perl package (available from
CPAN). The rpcgen suite, available from CPAN/authors/id/JAKE/, is
an RPC stub generator and includes an RPC::ONC module.
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
+Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
All rights reserved.
-When included as part of the Standard Version of Perl, or as part of
-its complete documentation whether printed or otherwise, this work
-may be distributed only under the terms of Perl's Artistic License.
-Any distribution of this file or derivatives thereof I<outside>
-of that package require that special arrangements be made with
-copyright holder.
+This documentation is free; you can redistribute it and/or modify it
+under the same terms as Perl itself.
Irrespective of its distribution, all code examples in this file
are hereby placed into the public domain. You are permitted and
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