perlfaq9
PERLFAQ9(1) Perl Programmers Reference Guide PERLFAQ9(1)
NAME
perlfaq9 - Networking ($Revision: 1.28 $, $Date: 2005/12/31 00:54:37
$)
DESCRIPTION
This section deals with questions related to networking, the internet,
and a few on the web.
What is the correct form of response from a CGI script?
(Alan Flavell <flavell+www@a5.ph.gla.ac.uk> answers...)
The Common Gateway Interface (CGI) specifies a software interface
between a program ("CGI script") and a web server (HTTPD). It is not
specific to Perl, and has its own FAQs and tutorials, and usenet
group, comp.infosystems.www.authoring.cgi
The CGI specification is outlined in an informational RFC:
http://www.ietf.org/rfc/rfc3875
Other relevant documentation listed in:
http://www.perl.org/CGI_MetaFAQ.html
These Perl FAQs very selectively cover some CGI issues. However, Perl
programmers are strongly advised to use the CGI.pm module, to take
care of the details for them.
The similarity between CGI response headers (defined in the CGI speci-
fication) and HTTP response headers (defined in the HTTP specifica-
tion, RFC2616) is intentional, but can sometimes be confusing.
The CGI specification defines two kinds of script: the "Parsed Header"
script, and the "Non Parsed Header" (NPH) script. Check your server
documentation to see what it supports. "Parsed Header" scripts are
simpler in various respects. The CGI specification allows any of the
usual newline representations in the CGI response (it’s the server’s
job to create an accurate HTTP response based on it). So "\n" written
in text mode is technically correct, and recommended. NPH scripts are
more tricky: they must put out a complete and accurate set of HTTP
transaction response headers; the HTTP specification calls for records
to be terminated with carriage-return and line-feed, i.e ASCII
\015\012 written in binary mode.
Using CGI.pm gives excellent platform independence, including EBCDIC
systems. CGI.pm selects an appropriate newline representation
($CGI::CRLF) and sets binmode as appropriate.
My CGI script runs from the command line but not the browser. (500
Server Error)
Several things could be wrong. You can go through the "Troubleshoot-
ing Perl CGI scripts" guide at
http://www.perl.org/troubleshooting_CGI.html
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 some-
thing 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
How can I get better error messages from a CGI program?
Use the CGI::Carp module. It replaces "warn" and "die", plus the nor-
mal 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 mod-
ule 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.
How do I remove HTML from a string?
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 con-
tinue over line breaks, they may contain quoted angle-brackets, or
HTML comment may be present. Plus, folks forget to convert enti-
ties--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 pro-
gram in http://www.cpan.org/authors/Tom_Chris-
tiansen/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>
-->
How do I extract URLs?
You can easily extract all sorts of URLs from HTML with "HTML::Sim-
pleLinkExtor" 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.
You can use URI::Find to extract URLs from an arbitrary text document.
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;
How do I download a file from the user’s machine? How do I open a
file on another machine?
In this case, download means to use the file upload feature of HTML
forms. You allow the web surfer to specify a file to send to your web
server. To you it looks like a download, and to the user it looks
like an upload. No matter what you call it, you do it with what’s
known as multipart/form-data encoding. The CGI.pm module (which comes
with Perl as part of the Standard Library) supports this in the
start_multipart_form() method, which isn’t the same as the startform()
method.
See the section in the CGI.pm documentation on file uploads for code
examples and details.
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 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;
How do I automate an HTML form submission?
If you are doing something complex, such as moving through many pages
and forms or a web site, you can use "WWW::Mechanize". See its docu-
mentation for all the details.
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;
How do I decode or create those %-encodings on the web?
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() func-
tion 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
s/%([[:xdigit:]]{2})/chr hex $1/eg; # same thing
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 sec-
tion 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).
How do I redirect to another page?
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 speci-
fications 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);
This example shows a redirection with an absolute URLpath. This redi-
rection is handled by the local web server.
my $url = ’/CPAN/index.html’;
print redirect($url);
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: $url\n"; # CGI response header
print "\n"; # end of headers
How do I put a password on my web pages?
To enable authentication for your web server, you need to configure
your web server. The configuration is different for different sorts
of web servers---apache does it differently from iPlanet which does it
differently from IIS. Check your web server documentation for the
details for your particular server.
How do I edit my .htpasswd and .htgroup files with Perl?
The HTTPD::UserAdmin and HTTPD::GroupAdmin modules provide a consis-
tent OO interface to these files, regardless of how they’re stored.
Databases may be text, dbm, Berkeley DB or any database with a DBI
compatible driver. HTTPD::UserAdmin supports files used by the
"Basic" and "Digest" authentication schemes. Here’s an example:
use HTTPD::UserAdmin ();
HTTPD::UserAdmin
->new(DB => "/foo/.htpasswd")
->add($username => $password);
How do I make sure users can’t enter values into a form that cause my
CGI script to do bad things?
See the security references listed in the CGI Meta FAQ
http://www.perl.org/CGI_MetaFAQ.html
How do I parse a mail header?
For a quick-and-dirty solution, try this solution derived from "split"
in perlfunc:
$/ = ’’;
$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 main-
tain 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?
(contributed by brian d foy)
Use the CGI.pm module that comes with Perl. It’s quick, it’s easy,
and it actually does quite a bit of work to ensure things happen cor-
rectly. It handles GET, POST, and HEAD requests, multipart forms,
multivalued fields, query string and message body combinations, and
many other things you probably don’t want to think about.
It doesn’t get much easier: the CGI module automatically parses the
input and makes each value available through the "param()" function.
use CGI qw(:standard);
my $total = param( ’price’ ) + param( ’shipping’ );
my @items = param( ’item’ ); # multiple values, same field name
If you want an object-oriented approach, CGI.pm can do that too.
use CGI;
my $cgi = CGI->new();
my $total = $cgi->param( ’price’ ) + $cgi->param( ’shipping’ );
my @items = $cgi->param( ’item’ );
You might also try CGI::Minimal which is a lightweight version of the
same thing. Other CGI::* modules on CPAN might work better for you,
too.
Many people try to write their own decoder (or copy one from another
program) and then run into one of the many "gotchas" of the task.
It’s much easier and less hassle to use CGI.pm.
How do I check a valid mail address?
You can’t, at least, not in real time. Bummer, eh?
Without sending mail to the address and seeing whether there’s a human
on the other end to answer you, you cannot determine whether a mail
address is valid. Even if you apply the mail header standard, you can
have problems, because there are deliverable addresses that aren’t
RFC-822 (the mail header standard) compliant, and addresses that
aren’t deliverable which are compliant.
You can use the Email::Valid or RFC::RFC822::Address which check the
format of the address, although they cannot actually tell you if it is
a deliverable address (i.e. that mail to the address will not bounce).
Modules like Mail::CheckUser and Mail::EXPN try to interact with the
domain name system or particular mail servers to learn even more, but
their methods do not work everywhere---especially for security con-
scious administrators.
Many are tempted to try to eliminate many frequently-invalid mail
addresses with a simple regex, such as
"/^[\w.-]+\@(?:[\w-]+\.)+\w+$/". 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 com-
ments), looks for addresses you may not wish to accept mail to (say,
Bill Clinton or your postmaster), and then makes sure that the host-
name 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 pass-
word. 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.
How do I decode a MIME/BASE64 string?
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);
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
How do I return the user’s mail address?
On systems that support getpwuid, the $< variable, and the Sys::Host-
name 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);
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 infor-
mation as is Unix.
The Mail::Util module from CPAN (part of the MailTools package) pro-
vides 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 mail?
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";
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();
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.
How do I use MIME to make an attachment to a mail message?
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;
MIME::Lite also includes a method for sending these things.
$msg->send;
This defaults to using sendmail but can be customized to use SMTP via
Net::SMTP.
How do I read mail?
While you could use the Mail::Folder module from CPAN (part of the
MailFolder package) or the Mail::Internet module from CPAN (part of
the MailTools package), often a module is overkill. Here’s a mail
sorter.
#!/usr/bin/perl
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) ] }
How do I find out my hostname, domainname, or IP address?
gethostbyname, Socket, Net::Domain, Sys::Hostname" (contributed by
brian d foy)
The Net::Domain module, which is part of the standard distribution
starting in perl5.7.3, can get you the fully qualified domain name
(FQDN), the host name, or the domain name.
use Net::Domain qw(hostname hostfqdn hostdomain);
my $host = hostfqdn();
The "Sys::Hostname" module, included in the standard distribution
since perl5.6, can also get the hostname.
use Sys::Hostname;
$host = hostname();
To get the IP address, you can use the "gethostbyname" built-in func-
tion to turn the name into a number. To turn that number into the dot-
ted octet form (a.b.c.d) that most people expect, use the "inet_ntoa"
function from the <Socket> module, which also comes with perl.
use Socket;
my $address = inet_ntoa(
scalar gethostbyname( $host ││ ’localhost’ )
);
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?
(Contributed by brian d foy)
Use one of the RPC modules you can find on CPAN (
http://search.cpan.org/search?query=RPC&mode=all ).
AUTHOR AND COPYRIGHT
Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and other
authors as noted. 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.
perl v5.8.8 2006-01-07 PERLFAQ9(1)
