3 perlfaq9 - Networking ($Revision: 1.13 $)
7 This section deals with questions related to networking, the internet,
10 =head2 My CGI script runs from the command line but not the browser. Can you help me fix it?
12 Sure, but you probably can't afford our contracting rates :-)
14 Seriously, if you can demonstrate that you've read the following FAQs
15 and that your problem isn't something simple that can be easily
16 answered, you'll probably receive a courteous and useful reply to your
17 question if you post it on comp.infosystems.www.authoring.cgi (if it's
18 something to do with HTTP, HTML, or the CGI protocols). Questions that
19 appear to be Perl questions but are really CGI ones that are posted to
20 comp.lang.perl.misc may not be so well received.
24 http://www.perl.com/perl/faq/idiots-guide.html
25 http://www3.pair.com/webthing/docs/cgi/faqs/cgifaq.shtml
26 http://www.perl.com/perl/faq/perl-cgi-faq.html
27 http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html
28 http://www.boutell.com/faq/
30 =head2 How do I remove HTML from a string?
32 The most correct way (albeit not the fastest) is to use HTML::Parse
33 from CPAN (part of the libwww-perl distribution, which is a must-have
34 module for all web hackers).
36 Many folks attempt a simple-minded regular expression approach, like
37 C<s/E<lt>.*?E<gt>//g>, but that fails in many cases because the tags
38 may continue over line breaks, they may contain quoted angle-brackets,
39 or HTML comment may be present. Plus folks forget to convert
40 entities, like C<<> for example.
42 Here's one "simple-minded" approach, that works for most files:
44 #!/usr/bin/perl -p0777
45 s/<(?:[^>'"]*|(['"]).*?\1)*>//gs
47 If you want a more complete solution, see the 3-stage striphtml
49 http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/striphtml.gz
52 =head2 How do I extract URLs?
54 A quick but imperfect approach is
57 # qxurl - tchrist@perl.com
60 A \s+ HREF \s* = \s* (["']) (.*?) \1
64 This version does not adjust relative URLs, understand alternate
65 bases, deal with HTML comments, or accept URLs themselves as
66 arguments. It also runs about 100x faster than a more "complete"
67 solution using the LWP suite of modules, such as the
68 http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/xurl.gz
71 =head2 How do I download a file from the user's machine? How do I open a file on another machine?
73 In the context of an HTML form, you can use what's known as
74 B<multipart/form-data> encoding. The CGI.pm module (available from
75 CPAN) supports this in the start_multipart_form() method, which isn't
76 the same as the startform() method.
78 =head2 How do I make a pop-up menu in HTML?
80 Use the B<E<lt>SELECTE<gt>> and B<E<lt>OPTIONE<gt>> tags. The CGI.pm
81 module (available from CPAN) supports this widget, as well as many
82 others, including some that it cleverly synthesizes on its own.
84 =head2 How do I fetch an HTML file?
86 Use the LWP::Simple module available from CPAN, part of the excellent
87 libwww-perl (LWP) package. On the other hand, and if you have the
88 lynx text-based HTML browser installed on your system, this isn't too
91 $html_code = `lynx -source $url`;
92 $text_data = `lynx -dump $url`;
94 =head2 how do I decode or create those %-encodings on the web?
96 Here's an example of decoding:
98 $string = "http://altavista.digital.com/cgi-bin/query?pg=q&what=news&fmt=.&q=%2Bcgi-bin+%2Bperl.exe";
99 $string =~ s/%([a-fA-F0-9]{2})/chr(hex($1))/ge;
101 Encoding is a bit harder, because you can't just blindly change
102 all the non-alphanumunder character (C<\W>) into their hex escapes.
103 It's important that characters with special meaning like C</> and C<?>
104 I<not> be translated. Probably the easiest way to get this right is
105 to avoid reinventing the wheel and just use the URI::Escape module,
106 which is part of the libwww-perl package (LWP) available from CPAN.
108 =head2 How do I redirect to another page?
110 Instead of sending back a C<Content-Type> as the headers of your
111 reply, send back a C<Location:> header. Officially this should be a
112 C<URI:> header, so the CGI.pm module (available from CPAN) sends back
115 Location: http://www.domain.com/newpage
116 URI: http://www.domain.com/newpage
118 Note that relative URLs in these headers can cause strange effects
119 because of "optimizations" that servers do.
121 =head2 How do I put a password on my web pages?
123 That depends. You'll need to read the documentation for your web
124 server, or perhaps check some of the other FAQs referenced above.
126 =head2 How do I edit my .htpasswd and .htgroup files with Perl?
128 The HTTPD::UserAdmin and HTTPD::GroupAdmin modules provide a
129 consistent OO interface to these files, regardless of how they're
130 stored. Databases may be text, dbm, Berkley DB or any database with a
131 DBI compatible driver. HTTPD::UserAdmin supports files used by the
132 `Basic' and `Digest' authentication schemes. Here's an example:
134 use HTTPD::UserAdmin ();
136 ->new(DB => "/foo/.htpasswd")
137 ->add($username => $password);
139 =head2 How do I parse an email header?
141 For a quick-and-dirty solution, try this solution derived
142 from page 222 of the 2nd edition of "Programming Perl":
146 $header =~ s/\n\s+/ /g; # merge continuation lines
147 %head = ( UNIX_FROM_LINE, split /^([-\w]+):\s*/m, $header );
149 That solution doesn't do well if, for example, you're trying to
150 maintain all the Received lines. A more complete approach is to use
151 the Mail::Header module from CPAN (part of the MailTools package).
153 =head2 How do I decode a CGI form?
155 A lot of people are tempted to code this up themselves, so you've
156 probably all seen a lot of code involving C<$ENV{CONTENT_LENGTH}> and
157 C<$ENV{QUERY_STRING}>. It's true that this can work, but there are
158 also a lot of versions of this floating around that are quite simply
161 Please do not be tempted to reinvent the wheel. Instead, use the
162 CGI.pm or CGI_Lite.pm (available from CPAN), or if you're trapped in
163 the module-free land of perl1 .. perl4, you might look into cgi-lib.pl
164 (available from http://www.bio.cam.ac.uk/web/form.html).
166 =head2 How do I check a valid email address?
170 Without sending mail to the address and seeing whether it bounces (and
171 even then you face the halting problem), you cannot determine whether
172 an email address is valid. Even if you apply the email header
173 standard, you can have problems, because there are deliverable
174 addresses that aren't RFC-822 (the mail header standard) compliant,
175 and addresses that aren't deliverable which are compliant.
177 Many are tempted to try to eliminate many frequently-invalid email
178 addresses with a simple regexp, such as
179 C</^[\w.-]+\@([\w.-]\.)+\w+$/>. However, this also throws out many
180 valid ones, and says nothing about potential deliverability, so is not
181 suggested. Instead, see
182 http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/ckaddr.gz ,
183 which actually checks against the full RFC spec (except for nested
184 comments), looks for addresses you may not wish to accept email to
185 (say, Bill Clinton or your postmaster), and then makes sure that the
186 hostname given can be looked up in DNS. It's not fast, but it works.
188 =head2 How do I decode a MIME/BASE64 string?
190 The MIME-tools package (available from CPAN) handles this and a lot
191 more. Decoding BASE64 becomes as simple as:
194 $decoded = decode_base64($encoded);
196 A more direct approach is to use the unpack() function's "u"
197 format after minor transliterations:
199 tr#A-Za-z0-9+/##cd; # remove non-base64 chars
200 tr#A-Za-z0-9+/# -_#; # convert to uuencoded format
201 $len = pack("c", 32 + 0.75*length); # compute length byte
202 print unpack("u", $len . $_); # uudecode and print
204 =head2 How do I return the user's email address?
206 On systems that support getpwuid, the $E<lt> variable and the
207 Sys::Hostname module (which is part of the standard perl distribution),
208 you can probably try using something like this:
211 $address = sprintf('%s@%s', getpwuid($<), hostname);
213 Company policies on email address can mean that this generates addresses
214 that the company's email system will not accept, so you should ask for
215 users' email addresses when this matters. Furthermore, not all systems
216 on which Perl runs are so forthcoming with this information as is Unix.
218 The Mail::Util module from CPAN (part of the MailTools package) provides a
219 mailaddress() function that tries to guess the mail address of the user.
220 It makes a more intelligent guess than the code above, using information
221 given when the module was installed, but it could still be incorrect.
222 Again, the best way is often just to ask the user.
224 =head2 How do I send/read mail?
226 Sending mail: the Mail::Mailer module from CPAN (part of the MailTools
227 package) is UNIX-centric, while Mail::Internet uses Net::SMTP which is
228 not UNIX-centric. Reading mail: use the Mail::Folder module from CPAN
229 (part of the MailFolder package) or the Mail::Internet module from
230 CPAN (also part of the MailTools package).
232 =head2 How do I find out my hostname/domainname/IP address?
234 A lot of code has historically cavalierly called the C<`hostname`>
235 program. While sometimes expedient, this isn't very portable. It's
236 one of those tradeoffs of convenience versus portability.
238 The Sys::Hostname module (part of the standard perl distribution) will
239 give you the hostname after which you can find out the IP address
240 (assuming you have working DNS) with a gethostbyname() call.
244 my $host = hostname();
245 my $addr = inet_ntoa(scalar(gethostbyname($name)) || 'localhost');
247 Probably the simplest way to learn your DNS domain name is to grok
248 it out of /etc/resolv.conf, at least under Unix. Of course, this
249 assumes several things about your resolv.conf configuration, including
252 (We still need a good DNS domain name-learning method for non-Unix
255 =head2 How do I fetch a news article or the active newsgroups?
257 Use the Net::NNTP or News::NNTPClient modules, both available from CPAN.
258 This can make tasks like fetching the newsgroup list as simple as:
260 perl -MNews::NNTPClient
261 -e 'print News::NNTPClient->new->list("newsgroups")'
263 =head2 How do I fetch/put an FTP file?
265 LWP::Simple (available from CPAN) can fetch but not put. Net::FTP (also
266 available from CPAN) is more complex but can put as well as fetch.
268 =head2 How can I do RPC in Perl?
270 A DCE::RPC module is being developed (but is not yet available), and
271 will be released as part of the DCE-Perl package (available from
272 CPAN). No ONC::RPC module is known.
274 =head1 AUTHOR AND COPYRIGHT
276 Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
277 All rights reserved. See L<perlfaq> for distribution information.