PERLFAQ9(1) PERLFAQ9(1)
perlfaq9 - Networking ($Revision: 1.17 $, $Date: 1997/04/24 22:44:29 $)
This section deals with questions related to networking, the internet,
and a few on the web.
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/
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
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 < 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
.
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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.
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
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.
How do I make a pop-up menu in HTML?
Use the <SELECT> and <OPTION> 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.
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 $url`;
The libwww-perl (LWP) modules from CPAN provide a more powerful way to do
this. They work through proxies, and don't require lynx:
# print HTML from a URL
use LWP::Simple;
getprint "http://www.sn.no/libwww-perl/";
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# print ASCII from HTML from a URL
use LWP::Simple;
use HTML::Parse;
use HTML::FormatText;
my ($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;
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($1))/ge;
Encoding is a bit harder, because you can't just blindly change all the
non-alphanumunder character (\W) into their hex escapes. It's important
that characters with special meaning like / and ? not be translated.
Probably the easiest way to get this right is to avoid reinventing the
wheel and just use the URI::Escape module, which is part of the libwwwperl
package (LWP) available from CPAN.
How do I redirect to another page?
Instead of sending back a Content-Type as the headers of your reply, send
back a Location: header. Officially this should be a URI: header, so the
CGI.pm module (available from CPAN) sends back both:
Location: http://www.domain.com/newpage
URI: http://www.domain.com/newpage
Note that relative URLs in these headers can cause strange effects
because of "optimizations" that servers do.
How do I put a password on my web pages?
That depends. You'll need to read the documentation for your web server,
or perhaps check some of the other FAQs referenced above.
How do I edit my .htpasswd and .htgroup files with Perl?
The HTTPD::UserAdmin and HTTPD::GroupAdmin modules provide a consistent
OO interface to these files, regardless of how they're stored. Databases
may be text, dbm, Berkley DB or any database with a DBI compatible
driver. HTTPD::UserAdmin supports files used by the `Basic' and `Digest'
authentication schemes. Here's an example:
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use HTTPD::UserAdmin ();
HTTPD::UserAdmin
->new(DB => "/foo/.htpasswd")
->add($username => $password);
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 the perlsec manpage), which makes sure that
data from outside your script (eg, CGI parameters) are never used in eval
or 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.
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).
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 $ENV{CONTENT_LENGTH} and
$ENV{QUERY_STRING}. It's true that this can work, but there are also a
lot of versions of this floating around that are quite simply broken!
Please do not be tempted to reinvent the wheel. Instead, use the CGI.pm
or CGI_Lite.pm (available from CPAN), or if you're trapped in the
module-free land of perl1 .. perl4, you might look into cgi-lib.pl
(available from http://www.bio.cam.ac.uk/web/form.html).
How do I check a valid email address?
You can't.
Without sending mail to the address and seeing whether it bounces (and
even then you face the halting problem), you cannot determine whether an
email address is valid. Even if you apply the email header standard, you
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can have problems, because there are deliverable addresses that aren't
RFC-822 (the mail header standard) compliant, and addresses that aren't
deliverable which are compliant.
Many are tempted to try to eliminate many frequently-invalid email
addresses with a simple regexp, such as /^[\w.-]+\@([\w.-]\.)+\w+$/.
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.
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
How do I return the user's email address?
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:
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.
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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.
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;
How do I find out my hostname/domainname/IP address?
A lot of code has historically cavalierly called the `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 $addr = inet_ntoa(scalar(gethostbyname($name)) || '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.
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(We still need a good DNS domain name-learning method for non-Unix
systems.)
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")'
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.
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.
Copyright (c) 1997 Tom Christiansen and Nathan Torkington. All rights
reserved. See the perlfaq manpage for distribution information.
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