perlfaq9 - Networking ($Revision: 1.6 $, $Date: 2003/12/03
03:02:45 $)
This section deals with questions related to networking,
the internet, and a few on the web.
What is the correct form of response from a CGI script?
(Alan Flavell <flavell+[email protected]> answers...)
The Common Gateway Interface (CGI) specifies a software
interface between a program ("CGI script") and a web
server (HTTPD). It is not specific to Perl, and has its
own FAQs and tutorials, and usenet group, comp.infosystems.www.authoring.cgi
The original CGI specification is at:
http://hoohoo.ncsa.uiuc.edu/cgi/
Current best-practice RFC draft at:
http://CGI-Spec.Golux.Com/
Other relevant documentation listed in:
http://www.perl.org/CGI_MetaFAQ.html
These Perl FAQs very selectively cover some CGI issues.
However, Perl programmers are strongly advised to use the
CGI.pm module, to take care of the details for them.
The similarity between CGI response headers (defined in
the CGI specification) and HTTP response headers (defined
in the HTTP specification, RFC2616) is intentional, but
can sometimes be confusing.
The CGI specification defines two kinds of script: the
"Parsed Header" script, and the "Non Parsed Header" (NPH)
script. Check your server documentation to see what it
supports. "Parsed Header" scripts are simpler in various
respects. The CGI specification allows any of the usual
newline representations in the CGI response (it's the
server's job to create an accurate HTTP response based on
it). So "0 written in text mode is technically correct,
and recommended. NPH scripts are more tricky: they must
put out a complete and accurate set of HTTP transaction
response headers; the HTTP specification calls for records
to be terminated with carriage-return and line-feed, i.e
ASCII 15 12 written in binary mode.
Using CGI.pm gives excellent platform independence,
including EBCDIC systems. CGI.pm selects an appropriate
newline representation ($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
"Troubleshooting 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 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
How can I get better error messages from a CGI program?
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: $!0;
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.
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 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.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::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.
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 - [email protected]
print "$20 while m{
<
A HREF = (["']) (.*?) 1
>
}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'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() function to handle
encoding.
The best source of detailed information on URI encoding is
RFC 2396. Basically, the following substitutions do it:
s/([^1176
s/%([A-Fa-f]{2})/chr hex $1/eg; # decode
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).
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 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);
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);
But if coded directly, it could be as follows (the final
"0 is shown separately, for clarity), using either a
complete URL or an absolute URLpath.
print "Location: $url0; # CGI response header
print "0; # end of headers
How do I put a password on my web pages?
To enable authentication for your web server, you need to
configure your web server. The configuration is different
for different sorts of web servers---apache does it differently
from iPlanet which does it differently from IIS.
Check your web server documentation for the details for
your particular server.
How do I edit my .htpasswd and .htgroup files with Perl?
The HTTPD::UserAdmin and HTTPD::GroupAdmin modules provide
a consistent OO interface to these files, regardless of
how they're stored. Databases may be text, dbm, Berkeley
DB or any database with a DBI compatible driver.
HTTPD::UserAdmin supports files used by the `Basic' and
`Digest' authentication schemes. Here's an example:
use HTTPD::UserAdmin ();
HTTPD::UserAdmin
->new(DB => "/foo/.htpasswd")
->add($username => $password);
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/0s+/ /g; # merge continuation lines
%head = ( UNIX_FROM_LINE, split /^([-408
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?
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).
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 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.
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 conscious administrators.
Many are tempted to try to eliminate many frequentlyinvalid
mail addresses with a simple regex, such as
"/^[264
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_Chris-
tiansen/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 [email protected],
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 nonbase64 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::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);
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.
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:
$!0;
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: $!0;
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 =>'[email protected]',
To =>'[email protected]',
Cc =>'[email protected],
[email protected]',
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:(?:Re:)*(.*)/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:(?:Re:)*(.*)/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/IP address?
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'));
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.)
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). 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.
perl v5.8.5 2002-11-06 13 [ Back ] |
