perlfaq9 - Networking ($Revision: 1.20 $, $Date: 1998/06/22 18:31:09 $)
This section deals with questions related to networking, the internet, and a few on the web.
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 and related documents are:
CGI FAQ
http://www.webthing.com/page.cgi/cgifaq
Web FAQ
http://www.boutell.com/faq/
WWW Security FAQ
http://www.w3.org/Security/Faq/
HTTP Spec
http://www.w3.org/pub/WWW/Protocols/HTTP/
HTML Spec
http://www.w3.org/TR/REC-html40/
http://www.w3.org/pub/WWW/MarkUp/
CGI Spec
http://www.w3.org/CGI/
CGI Security FAQ
http://www.go2net.com/people/paulp/cgi-security/safe-cgi.txt
Use the CGI::Carp module. It replaces warn and die, plus the normal Carp modules carp, croak, and confess functions with more verbose and safer versions. It still sends them to the
normal server error log.
use CGI::Carp;
warn "This is a complaint";
die "But this one is serious";
The following use of CGI::Carp also redirects errors to a file of your choice, placed in a BEGIN block to catch compile-time warnings as well:
BEGIN {
use CGI::Carp qw(carpout);
open(LOG, ">>/var/local/cgi-logs/mycgi-log")
or die "Unable to append to mycgi-log: $!\n";
carpout(*LOG);
}
You can even arrange for fatal errors to go back to the client browser, which is nice for your own debugging, but might confuse the end user.
use CGI::Carp qw(fatalsToBrowser);
die "Bad error here";
Even if the error happens before you get the
HTTP header out, the module will try to take care of
this to avoid the dreaded server 500 errors. Normal warnings still go out
to the server error log (or wherever you've sent them with carpout) with the application name and date stamp prepended.
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 .
Here are some tricky cases that you should think about when picking a solution:
<IMG SRC = "foo.gif" ALT = "A > B">
<IMG SRC = "foo.gif"
ALT = "A > B">
<!-- <A comment> -->
<script>if (a<b && a>c)</script>
<# Just data #>
<![INCLUDE CDATA [ >>>>>>>>>>>> ]]>
If HTML comments include other tags, those solutions would also break on text like this:
<!-- This section commented out.
<B>You can't see me!</B>
-->
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.
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.
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.
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:
# simplest version
use LWP::Simple;
$content = get($URL);
# or print HTML from a URL
use LWP::Simple;
getprint "http://www.sn.no/libwww-perl/";;
# or 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;
If you're submitting values using the
GET method, create a
URL and encode the form using the
query_form method:
use LWP::Simple;
use URI::URL;
my $url = url('http://www.perl.com/cgi-bin/cpan_mod');
$url->query_form(module => 'DB_File', readme => 1);
$content = get($url);
If you're using the POST method, create your own user agent and encode the content appropriately.
use HTTP::Request::Common qw(POST);
use LWP::UserAgent;
$ua = LWP::UserAgent->new();
my $req = POST 'http://www.perl.com/cgi-bin/cpan_mod',
[ module => 'DB_File', readme => 1 ];
$content = $ua->request($req)->as_string;
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.
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.
$url = "http://www.perl.com/CPAN/";;
print "Location: $url\n\n";
exit;
To be correct to the spec, each of those "\n"
should really each be "\015\012", but unless you're stuck on MacOS, you probably won't notice.
That depends. You'll need to read the documentation for your web server, or perhaps check some of the other FAQs referenced above.
The HTTPD::UserAdmin and HTTPD::GroupAdmin modules provide a consistent OO interface to these files, regardless of how they're stored. Databases may be text, dbm, Berkley DB or any database with a DBI compatible driver. HTTPD::UserAdmin supports files used by the `Basic' and `Digest' authentication schemes. Here's an example:
use HTTPD::UserAdmin ();
HTTPD::UserAdmin
->new(DB => "/foo/.htpasswd")
->add($username => $password);
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.
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).
You use a standard module, probably CGI.pm. Under no circumstances should you attempt to do so by hand!
You'll see a lot of
CGI programs that blindly read from
STDIN the number of bytes equal to
CONTENT_LENGTH for POSTs, or grab
QUERY_STRING for decoding GETs. These programs are very poorly written. They only work sometimes. They typically forget to check the return value of the
read() system call, which is a cardinal sin. They don't handle
HEAD requests. They don't handle multipart forms used for file uploads. They don't deal with
GET/POST combinations where query fields are in more than one place. They don't deal with keywords in the query string.
In short, they're bad hacks. Resist them at all costs. 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).
Make sure you know whether to use a GET or a POST in your form. GETs should only be used for something that doesn't update the server. Otherwise you can get mangled databases and repeated feedback mail messages. The fancy word for this is ``idempotency''. This simply means that there should be no difference between making a GET request for a particular URL once or multiple times. This is because the HTTP protocol definition says that a GET request may be cached by the browser, or server, or an intervening proxy. POST requests cannot be cached, because each request is independent and matters. Typically, POST requests change or depend on state on the server (query or update a database, send mail, or purchase a computer).
You can't, at least, not in real time. Bummer, eh?
Without sending mail to the address and seeing whether there's a human on the other hand to answer you, you cannot determine whether a mail address is valid. Even if you apply the mail 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 mail addresses
with a simple regexp, such as
/^[\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 , 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 hostname given can be looked up in the
DNS
MX records. It's not fast, but it works for what it tries to do.
Our best advice for verifying a person's mail address is to have them enter their address twice, just as you normally do to change a password. This usually weeds out typos. If both versions match, send mail to that address with a personal message that looks somewhat like:
Dear someuser@host.com,
Please confirm the mail address you gave us Wed May 6 09:38:41
MDT 1998 by replying to this message. Include the string
"Rumpelstiltskin" in that reply, but spelled in reverse; that is,
start with "Nik...". Once this is done, your confirmed address will
be entered into our records.
If you get the message back and they've followed your directions, you can be reasonably assured that it's real.
A related strategy that's less open to forgery is to give them a PIN (personal ID number). Record the address and PIN (best that it be a random one) for later processing. In the mail you send, ask them to include the PIN in their reply. But if it bounces, or the message is included via a ``vacation'' script, it'll be there anyway. So it's best to ask them to mail back a slight alteration of the PIN, such as with the characters reversed, one added or subtracted to each digit, etc.
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
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 mail address can mean that this generates addresses that the company's mail system will not accept, so you should ask for users' mail 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.
Use the sendmail program directly:
open(SENDMAIL, "|/usr/lib/sendmail -oi -t -odq")
or die "Can't fork for sendmail: $!\n";
print SENDMAIL <<"EOF";
From: User Originating Mail <me\@host>
To: Final Destination <you\@otherhost>
Subject: A relevant subject line
Body of the message goes here, in as many lines as you like.
EOF
close(SENDMAIL) or warn "sendmail didn't close nicely";
The -oi option prevents sendmail from interpreting a line consisting of a single dot as ``end of message''. The -t option says to use the headers to decide who to send the message to, and -odq says to put the message into the queue. This last option means your message won't be immediately delivered, so leave it out if you want immediate delivery.
Or use the CPAN module Mail::Mailer:
use Mail::Mailer;
$mailer = Mail::Mailer->new();
$mailer->open({ From => $from_address,
To => $to_address,
Subject => $subject,
})
or die "Can't open: $!\n";
print $mailer $body;
$mailer->close();
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.
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;
Often a module is overkill, though. 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) {
/^Subject:\s*(?:Re:\s*)*(.*)/mi;
$sub[++$msgno] = lc($1) || '';
}
$msgs[$msgno] .= $_;
}
for my $i (sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msgs)) {
print $msgs[$i];
}
Or more succinctly,
#!/usr/bin/perl -n00
# bysub2 - awkish sort-by-subject
BEGIN { $msgno = -1 }
$sub[++$msgno] = (/^Subject:\s*(?:Re:\s*)*(.*)/mi)[0] if /^From/m;
$msg[$msgno] .= $_;
END { print @msg[ sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msg) ] }
The normal way to find your own hostname is to call the `hostname`
program. While sometimes expedient, this has some problems, such as not
knowing whether you've got the canonical name or not. 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.)
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")'
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.
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, 1998 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 outside of that package require that special arrangements be made with copyright holder.
Irrespective of its distribution, all code examples in this file are hereby placed into the public domain. You are permitted and encouraged to use this code in your own programs for fun or for profit as you see fit. A simple comment in the code giving credit would be courteous but is not required.
If rather than formatting bugs, you encounter substantive content errors in these documents, such as mistakes in the explanations or code, please use the perlbug utility included with the Perl distribution.