PERLFAQ9(1)
NNAAMMEE
perlfaq9 - Networking ($Revision: 1.17 $, $Date:
1997/04/24 22:44:29 $)
DDEESSCCRRIIPPTTIIOONN
This section deals with questions related to networking,
the internet, and a few on the web.
MMyy CCGGII ssccrriipptt rruunnss ffrroomm tthhee ccoommmmaanndd lliinnee bbuutt nnoott tthhee
bbrroowwsseerr.. CCaann yyoouu hheellpp mmee ffiixx iitt??
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/
HHooww ddoo II rreemmoovvee HHTTMMLL ffrroomm aa ssttrriinngg??
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
.
HHooww ddoo II eexxttrraacctt UURRLLss??
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.
HHooww ddoo II ddoowwnnllooaadd aa ffiillee ffrroomm tthhee uusseerr''ss mmaacchhiinnee?? HHooww ddoo
II ooppeenn aa ffiillee oonn aannootthheerr mmaacchhiinnee??
In the context of an HTML form, you can use what's known
as mmuullttiippaarrtt//ffoorrmm--ddaattaa 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.
HHooww ddoo II mmaakkee aa ppoopp--uupp mmeennuu iinn HHTTMMLL??
Use the <<<<SSEELLEECCTT>>>> and <<<<OOPPTTIIOONN>>>> 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.
HHooww ddoo II ffeettcchh aann HHTTMMLL ffiillee??
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;
hhooww ddoo II ddeeccooddee oorr ccrreeaattee tthhoossee %%--eennccooddiinnggss oonn tthhee wweebb??
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($1))/ge;
Encoding is a bit harder, because you can't just blindly
change all the non-alphanumunder character (\W) into their
hex escapes. It's important that characters with special
meaning like / and ? not be translated. Probably the
easiest way to get this right is to avoid reinventing the
wheel and just use the URI::Escape module, which is part
of the libwww-perl package (LWP) available from CPAN.
HHooww ddoo II rreeddiirreecctt ttoo aannootthheerr ppaaggee??
Instead of sending back a Content-Type as the headers of
your reply, send back a Location: header. Officially this
should be a URI: header, so the CGI.pm module (available
from CPAN) sends back both:
Location: http://www.domain.com/newpage
URI: http://www.domain.com/newpage
Note that relative URLs in these headers can cause strange
effects because of "optimizations" that servers do.
HHooww ddoo II ppuutt aa ppaasssswwoorrdd oonn mmyy wweebb ppaaggeess??
That depends. You'll need to read the documentation for
your web server, or perhaps check some of the other FAQs
referenced above.
HHooww ddoo II eeddiitt mmyy ..hhttppaasssswwdd aanndd ..hhttggrroouupp ffiilleess wwiitthh PPeerrll??
The HTTPD::UserAdmin and HTTPD::GroupAdmin modules provide
a consistent OO interface to these files, regardless of
how they're stored. Databases may be text, dbm, Berkley
DB or any database with a DBI compatible driver.
HTTPD::UserAdmin supports files used by the `Basic' and
`Digest' authentication schemes. Here's an example:
use HTTPD::UserAdmin ();
HTTPD::UserAdmin
->new(DB => "/foo/.htpasswd")
->add($username => $password);
HHooww ddoo II mmaakkee ssuurree uusseerrss ccaann''tt eenntteerr vvaalluueess iinnttoo aa ffoorrmm
tthhaatt ccaauussee mmyy CCGGII ssccrriipptt ttoo ddoo bbaadd tthhiinnggss??
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.
HHooww ddoo II ppaarrssee aann eemmaaiill hheeaaddeerr??
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).
HHooww ddoo II ddeeccooddee aa CCGGII ffoorrmm??
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 $ENV{QUERY_STRING}. It's true
that this can work, but there are also a lot of versions
of this floating around that are quite simply broken!
Please do not be tempted to reinvent the wheel. Instead,
use the CGI.pm or CGI_Lite.pm (available from CPAN), or if
you're trapped in the module-free land of perl1 .. perl4,
you might look into cgi-lib.pl (available from
http://www.bio.cam.ac.uk/web/form.html).
HHooww ddoo II cchheecckk aa vvaalliidd eemmaaiill aaddddrreessss??
You can't.
Without sending mail to the address and seeing whether it
bounces (and even then you face the halting problem), you
cannot determine whether an email address is valid. Even
if you apply the email header standard, you can have
problems, because there are deliverable addresses that
aren't RFC-822 (the mail header standard) compliant, and
addresses that aren't deliverable which are compliant.
Many are tempted to try to eliminate many frequently-
invalid email addresses with a simple regexp, such as
/^[\w.-]+\@([\w.-]\.)+\w+$/. 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.
HHooww ddoo II ddeeccooddee aa MMIIMMEE//BBAASSEE6644 ssttrriinngg??
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
HHooww ddoo II rreettuurrnn tthhee uusseerr''ss eemmaaiill aaddddrreessss??
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.
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.
HHooww ddoo II sseenndd//rreeaadd mmaaiill??
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;
HHooww ddoo II ffiinndd oouutt mmyy hhoossttnnaammee//ddoommaaiinnnnaammee//IIPP aaddddrreessss??
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.
(We still need a good DNS domain name-learning method for
non-Unix systems.)
HHooww ddoo II ffeettcchh aa nneewwss aarrttiiccllee oorr tthhee aaccttiivvee nneewwssggrroouuppss??
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")'
HHooww ddoo II ffeettcchh//ppuutt aann FFTTPP ffiillee??
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.
HHooww ccaann II ddoo RRPPCC iinn PPeerrll??
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.
AAUUTTHHOORR AANNDD CCOOPPYYRRIIGGHHTT
Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
All rights reserved. See the perlfaq manpage for
distribution information.