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perlfaq9 - Networking
This section deals with questions related to networking, the internet, and a few on the web.
(Alan Flavell <flavell+www@a5.ph.gla.ac.uk> answers...)
The Common Gateway Interface (CGI) specifies a software interface between a program
("CGI script") and a web server (HTTPD). It is not specific to Perl, and has its own
FAQs and tutorials, and usenet group, comp.infosystems.www.authoring.cgi
The original CGI specification is at: http://hoohoo.ncsa.uiuc.edu/cgi/
Current best-practice RFC draft at: http://CGI-Spec.Golux.Com/
Other relevant documentation listed in: http://www.perl.org/CGI_MetaFAQ.html
These Perl FAQs very selectively cover some CGI issues. However, Perl programmers are
strongly advised to use the CGI.pm module, to take care of the details for them.
The similarity between CGI response headers (defined in the CGI specification) and HTTP
response headers (defined in the HTTP specification, RFC2616) is intentional, but can sometimes
be confusing.
The CGI specification defines two kinds of script: the "Parsed Header" script, and
the "Non Parsed Header" (NPH) script. Check your server documentation to see what it
supports. "Parsed Header" scripts are simpler in various respects. The CGI
specification allows any of the usual newline representations in the CGI response (it's the
server's job to create an accurate HTTP response based on it). So "\n" written in text
mode is technically correct, and recommended. NPH scripts are more tricky: they must put out a
complete and accurate set of HTTP transaction response headers; the HTTP specification calls for
records to be terminated with carriage-return and line-feed, i.e ASCII \015\012 written in
binary mode.
Using CGI.pm gives excellent platform independence, including EBCDIC systems. CGI.pm selects
an appropriate newline representation ($CGI::CRLF) and sets binmode as appropriate.
Several things could be wrong. You can go through the "Troubleshooting Perl CGI
scripts" guide at
http://www.perl.org/troubleshooting_CGI.html
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If, after that, you can demonstrate that you've read the 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 or the CGI protocols). Questions that appear to be Perl questions but are really
CGI ones that are posted to comp.lang.perl.misc are not so well received.
The useful FAQs, related documents, and troubleshooting guides are listed in the CGI Meta
FAQ:
http://www.perl.org/CGI_MetaFAQ.html
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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";
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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);
}
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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";
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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::Parser from CPAN. Another
mostly correct way is to use HTML::FormatText which not only removes HTML but also attempts to
do a little simple formatting of the resulting plain text.
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
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If you want a more complete solution, see the 3-stage striphtml program in http://www.cpan.org/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 [ >>>>>>>>>>>> ]]>
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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>
-->
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You can easily extract all sorts of URLs from HTML with HTML::SimpleLinkExtor
which handles anchors, images, objects, frames, and many other tags that can contain a URL. If
you need anything more complex, you can create your own subclass of HTML::LinkExtor
or HTML::Parser. You might even use HTML::SimpleLinkExtor as an
example for something specifically suited to your needs.
Less complete solutions involving regular expressions can save you a lot of processing time
if you know that the input is simple. One solution from Tom Christiansen runs 100 times faster
than most module based approaches but only extracts URLs from anchors where the first attribute
is HREF and there are no other attributes.
#!/usr/bin/perl -n00
# qxurl - tchrist@perl.com
print "$2\n" while m{
< \s*
A \s+ HREF \s* = \s* (["']) (.*?) \1
\s* >
}gsix;
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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`;
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The libwww-perl (LWP) modules from CPAN provide a more powerful way to do this. They don't
require lynx, but like lynx, can still work through proxies:
# simplest version
use LWP::Simple;
$content = get($URL);
# or print HTML from a URL
use LWP::Simple;
getprint "http://www.linpro.no/lwp/";
# or print ASCII from HTML from a URL
# also need HTML-Tree package from CPAN
use LWP::Simple;
use HTML::Parser;
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;
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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);
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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;
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If you are writing a CGI script, you should be using the CGI.pm module that comes with perl,
or some other equivalent module. The CGI module automatically decodes queries for you, and
provides an escape() function to handle encoding.
The best source of detailed information on URI encoding is RFC 2396. Basically, the following
substitutions do it:
s/([^\w()'*~!.-])/sprintf '%%%02x', ord $1/eg; # encode
s/%([A-Fa-f\d]{2})/chr hex $1/eg; # decode
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However, you should only apply them to individual URI components, not the entire URI,
otherwise you'll lose information and generally mess things up. If that didn't explain it, don't
worry. Just go read section 2 of the RFC, it's probably the best explanation there is.
RFC 2396 also contains a lot of other useful information, including a regexp for breaking any
arbitrary URI into components (Appendix B).
Specify the complete URL of the destination (even if it is on the same server). This is one
of the two different kinds of CGI "Location:" responses which are defined in the CGI
specification for a Parsed Headers script. The other kind (an absolute URLpath) is resolved
internally to the server without any HTTP redirection. The CGI specifications do not allow
relative URLs in either case.
Use of CGI.pm is strongly recommended. This example shows redirection with a complete URL.
This redirection is handled by the web browser.
use CGI qw/:standard/;
my $url = 'http://www.cpan.org/';
print redirect($url);
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This example shows a redirection with an absolute URLpath. This redirection is handled by the
local web server.
my $url = '/CPAN/index.html';
print redirect($url);
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But if coded directly, it could be as follows (the final "\n" is shown separately,
for clarity), using either a complete URL or an absolute URLpath.
print "Location: $url\n"; # CGI response header
print "\n"; # end of headers
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That depends. You'll need to read the documentation for your web server, or perhaps check
some of the other FAQs referenced above.
The HTTPD::UserAdmin and HTTPD::GroupAdmin modules provide a consistent OO interface to these
files, regardless of how they're stored. Databases may be text, dbm, Berkeley DB or any database
with a DBI compatible driver. HTTPD::UserAdmin supports files used by the `Basic' and `Digest'
authentication schemes. Here's an example:
use HTTPD::UserAdmin ();
HTTPD::UserAdmin
->new(DB => "/foo/.htpasswd")
->add($username => $password);
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See the security references listed in the CGI Meta FAQ
http://www.perl.org/CGI_MetaFAQ.html
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For a quick-and-dirty solution, try this solution derived from perlfunc/split:
$/ = '';
$header = <MSG>;
$header =~ s/\n\s+/ /g; # merge continuation lines
%head = ( UNIX_FROM_LINE, split /^([-\w]+):\s*/m, $header );
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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://cgi-lib.stanford.edu/cgi-lib/
).
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
regex, 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 it is not
suggested. Instead, see http://www.cpan.org/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.
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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-Base64 package (available from CPAN) handles this as well as the MIME/QP encoding.
Decoding BASE64 becomes as simple as:
use MIME::Base64;
$decoded = decode_base64($encoded);
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The MIME-Tools package (available from CPAN) supports extraction with decoding of BASE64
encoded attachments and content directly from email messages.
If the string to decode is short (less than 84 bytes long) 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
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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;
$address = sprintf('%s@%s', scalar getpwuid($<), hostname);
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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 after the blank line
in as many lines as you like.
EOF
close(SENDMAIL) or warn "sendmail didn't close nicely";
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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.
Alternate, less convenient approaches include calling mail (sometimes called mailx) directly
or simply opening up port 25 have having an intimate conversation between just you and the
remote SMTP daemon, probably sendmail.
Or you might be able 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();
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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 queuing, MX records, and security.
This answer is extracted directly from the MIME::Lite documentation. Create a multipart
message (i.e., one with attachments).
use MIME::Lite;
### Create a new multipart message:
$msg = MIME::Lite->new(
From =>'me@myhost.com',
To =>'you@yourhost.com',
Cc =>'some@other.com, some@more.com',
Subject =>'A message with 2 parts...',
Type =>'multipart/mixed'
);
### Add parts (each "attach" has same arguments as "new"):
$msg->attach(Type =>'TEXT',
Data =>"Here's the GIF file you wanted"
);
$msg->attach(Type =>'image/gif',
Path =>'aaa000123.gif',
Filename =>'logo.gif'
);
$text = $msg->as_string;
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MIME::Lite also includes a method for sending these things.
This defaults to using sendmail(1)
but can be customized to use SMTP via Net::SMTP.
While you could 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), often a module is
overkill. 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];
}
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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) ] }
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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 $addr = inet_ntoa(scalar gethostbyname($host || 'localhost'));
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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")'
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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). The rpcgen suite, available from CPAN/authors/id/JAKE/,
is an RPC stub generator and includes an RPC::ONC module.
Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington. All rights reserved.
This documentation is free; you can redistribute it and/or modify it under the same terms as
Perl itself.
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.
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