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diff --git a/pod/perlfaq9.pod b/pod/perlfaq9.pod new file mode 100644 index 0000000000..d7faca02e3 --- /dev/null +++ b/pod/perlfaq9.pod @@ -0,0 +1,331 @@ +=head1 NAME + +perlfaq9 - Networking ($Revision: 1.16 $, $Date: 1997/04/23 18:12:06 $) + +=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. Can you help me fix it? + +Sure, but you probably can't afford our contracting rates :-) + +Seriously, 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. + +The useful FAQs are: + + http://www.perl.com/perl/faq/idiots-guide.html + http://www3.pair.com/webthing/docs/cgi/faqs/cgifaq.shtml + http://www.perl.com/perl/faq/perl-cgi-faq.html + http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html + http://www.boutell.com/faq/ + +=head2 How do I remove HTML from a string? + +The most correct way (albeit not the fastest) is to use HTML::Parse +from CPAN (part of the libwww-perl distribution, which is a must-have +module for all web hackers). + +Many folks attempt a simple-minded regular expression approach, like +C<s/E<lt>.*?E<gt>//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. + +Here's one "simple-minded" approach, that works for most files: + + #!/usr/bin/perl -p0777 + s/<(?:[^>'"]*|(['"]).*?\1)*>//gs + +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 +. + +=head2 How do I extract URLs? + +A quick but imperfect approach is + + #!/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, 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. + +=head2 How do I make a pop-up menu in HTML? + +Use the B<E<lt>SELECTE<gt>> and B<E<lt>OPTIONE<gt>> tags. The CGI.pm +module (available from CPAN) supports this widget, as well as many +others, including some that it cleverly synthesizes on its own. + +=head2 How do I fetch an HTML file? + +One approach, if you have the lynx text-based HTML browser installed +on your system, is this: + + $html_code = `lynx -source $url`; + $text_data = `lynx -dump $url`; + +The libwww-perl (LWP) modules from CPAN provide a more powerful way to +do this. They work through proxies, and don't require lynx: + + # print HTML from a URL + use LWP::Simple; + getprint "http://www.sn.no/libwww-perl/"; + + # print ASCII from HTML from a URL + use LWP::Simple; + use HTML::Parse; + use HTML::FormatText; + my ($html, $ascii); + $html = get("http://www.perl.com/"); + defined $html + or die "Can't fetch HTML from http://www.perl.com/"; + $ascii = HTML::FormatText->new->format(parse_html($html)); + print $ascii; + +=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($1))/ge; + +Encoding is a bit harder, because you can't just blindly change +all the non-alphanumunder character (C<\W>) into their hex escapes. +It's important that characters with special meaning like C</> and C<?> +I<not> be translated. Probably the easiest way to get this right is +to avoid reinventing the wheel and just use the URI::Escape module, +which is part of the libwww-perl package (LWP) available from CPAN. + +=head2 How do I redirect to another page? + +Instead of sending back a C<Content-Type> as the headers of your +reply, send back a C<Location:> header. Officially this should be a +C<URI:> header, so the CGI.pm module (available from CPAN) sends back +both: + + Location: http://www.domain.com/newpage + URI: http://www.domain.com/newpage + +Note that relative URLs in these headers can cause strange effects +because of "optimizations" that servers do. + +=head2 How do I put a password on my web pages? + +That depends. You'll need to read the documentation for your web +server, or perhaps check some of the other FAQs referenced above. + +=head2 How do I edit my .htpasswd and .htgroup files with Perl? + +The HTTPD::UserAdmin and HTTPD::GroupAdmin modules provide a +consistent OO interface to these files, regardless of how they're +stored. Databases may be text, dbm, Berkley DB or any database with a +DBI compatible driver. HTTPD::UserAdmin supports files used by the +`Basic' and `Digest' authentication schemes. Here's an example: + + use HTTPD::UserAdmin (); + HTTPD::UserAdmin + ->new(DB => "/foo/.htpasswd") + ->add($username => $password); + +=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. + +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. + +=head2 How do I parse an email header? + +For a quick-and-dirty solution, try this solution derived +from page 222 of the 2nd edition of "Programming Perl": + + $/ = ''; + $header = <MSG>; + $header =~ s/\n\s+/ /g; # merge continuation lines + %head = ( UNIX_FROM_LINE, split /^([-\w]+):\s*/m, $header ); + +That solution doesn't do well if, for example, you're trying to +maintain all the Received lines. A more complete approach is to use +the Mail::Header module from CPAN (part of the MailTools package). + +=head2 How do I decode a CGI form? + +A lot of people are tempted to code this up themselves, so you've +probably all seen a lot of code involving C<$ENV{CONTENT_LENGTH}> and +C<$ENV{QUERY_STRING}>. It's true that this can work, but there are +also a lot of versions of this floating around that are quite simply +broken! + +Please do not be tempted to reinvent the wheel. Instead, use the +CGI.pm or CGI_Lite.pm (available from CPAN), or if you're trapped in +the module-free land of perl1 .. perl4, you might look into cgi-lib.pl +(available from http://www.bio.cam.ac.uk/web/form.html). + +=head2 How do I check a valid email address? + +You can't. + +Without sending mail to the address and seeing whether it bounces (and +even then you face the halting problem), you cannot determine whether +an email address is valid. Even if you apply the email header +standard, you can have problems, because there are deliverable +addresses that aren't RFC-822 (the mail header standard) compliant, +and addresses that aren't deliverable which are compliant. + +Many are tempted to try to eliminate many frequently-invalid email +addresses with a simple regexp, such as +C</^[\w.-]+\@([\w.-]\.)+\w+$/>. 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 , +which actually checks against the full RFC spec (except for nested +comments), looks for addresses you may not wish to accept email to +(say, Bill Clinton or your postmaster), and then makes sure that the +hostname given can be looked up in DNS. It's not fast, but it works. + +Here's an alternative strategy used by many CGI script authors: Check +the email address with a simple regexp (such as the one above). If +the regexp matched the address, accept the address. If the regexp +didn't match the address, request confirmation from the user that the +email address they entered was correct. + +=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: + + use MIME::base64; + $decoded = decode_base64($encoded); + +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 + tr#A-Za-z0-9+/# -_#; # convert to uuencoded format + $len = pack("c", 32 + 0.75*length); # compute length byte + print unpack("u", $len . $_); # uudecode and print + +=head2 How do I return the user's email address? + +On systems that support getpwuid, the $E<lt> variable and the +Sys::Hostname module (which is part of the standard perl distribution), +you can probably try using something like this: + + use Sys::Hostname; + $address = sprintf('%s@%s', getpwuid($<), hostname); + +Company policies on email address can mean that this generates addresses +that the company's email system will not accept, so you should ask for +users' email addresses when this matters. Furthermore, not all systems +on which Perl runs are so forthcoming with this information as is Unix. + +The Mail::Util module from CPAN (part of the MailTools package) provides a +mailaddress() function that tries to guess the mail address of the user. +It makes a more intelligent guess than the code above, using information +given when the module was installed, but it could still be incorrect. +Again, the best way is often just to ask the user. + +=head2 How do I send/read mail? + +Sending mail: the Mail::Mailer module from CPAN (part of the MailTools +package) is UNIX-centric, while Mail::Internet uses Net::SMTP which is +not UNIX-centric. Reading mail: 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). + + # sending mail + use Mail::Internet; + use Mail::Header; + # say which mail host to use + $ENV{SMTPHOSTS} = 'mail.frii.com'; + # create headers + $header = new Mail::Header; + $header->add('From', 'gnat@frii.com'); + $header->add('Subject', 'Testing'); + $header->add('To', 'gnat@frii.com'); + # create body + $body = 'This is a test, ignore'; + # create mail object + $mail = new Mail::Internet(undef, Header => $header, Body => \[$body]); + # send it + $mail->smtpsend or die; + +=head2 How do I find out my hostname/domainname/IP address? + +A lot of code has historically cavalierly called the C<`hostname`> +program. While sometimes expedient, this isn't very portable. It's +one of those tradeoffs of convenience versus portability. + +The Sys::Hostname module (part of the standard perl distribution) will +give you the hostname after which you can find out the IP address +(assuming you have working DNS) with a gethostbyname() call. + + use Socket; + use Sys::Hostname; + my $host = hostname(); + my $addr = inet_ntoa(scalar(gethostbyname($host || 'localhost'))); + +Probably the simplest way to learn your DNS domain name is to grok +it out of /etc/resolv.conf, at least under Unix. Of course, this +assumes several things about your resolv.conf configuration, including +that it exists. + +(We still need a good DNS domain name-learning method for non-Unix +systems.) + +=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: + + perl -MNews::NNTPClient + -e 'print News::NNTPClient->new->list("newsgroups")' + +=head2 How do I fetch/put an FTP file? + +LWP::Simple (available from CPAN) can fetch but not put. Net::FTP (also +available from CPAN) is more complex but can put as well as fetch. + +=head2 How can I do RPC in Perl? + +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). No ONC::RPC module is known. + +=head1 AUTHOR AND COPYRIGHT + +Copyright (c) 1997 Tom Christiansen and Nathan Torkington. +All rights reserved. See L<perlfaq> for distribution information. + |