perlfaq9
(1)
Name
perlfaq9 - Networking
Synopsis
Please see following description for synopsis
Description
Perl Programmers Reference Guide PERLFAQ9(1)
NAME
perlfaq9 - Networking
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 <[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 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 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.
My CGI script runs from the command line but not the browser.
(500 Server Error)
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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: $!\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
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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_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>
-->
How do I extract URLs?
You can easily extract all sorts of URLs from HTML with
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"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 "$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 an HTML pop-up menu with Perl?
(contributed by brian d foy)
The "CGI.pm" module (which comes with Perl) has functions to
create the HTML form widgets. See the "CGI.pm" documentation
for more examples.
use CGI qw/:standard/;
print header,
start_html('Favorite Animals'),
start_form,
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"What's your favorite animal? ",
popup_menu(
-name => 'animal',
-values => [ qw( Llama Alpaca Camel Ram ) ]
),
submit,
end_form,
end_html;
How do I fetch an HTML file?
(contributed by brian d foy)
Use the libwww-perl distribution. The "LWP::Simple" module
can fetch web resources and give their content back to you
as a string:
use LWP::Simple qw(get);
my $html = get( "http://www.example.com/index.html" );
It can also store the resource directly in a file:
use LWP::Simple qw(getstore);
getstore( "http://www.example.com/index.html", "foo.html" );
If you need to do something more complicated, you can use
"LWP::UserAgent" module to create your own user-agent (e.g.
browser) to get the job done. If you want to simulate an
interactive web browser, you can use the "WWW::Mechanize"
module.
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 documentation 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.
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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?
(contributed by brian d foy)
Those "%" encodings handle reserved characters in URIs, as
described in RFC 2396, Section 2. This encoding replaces the
reserved character with the hexadecimal representation of
the character's number from the US-ASCII table. For
instance, a colon, ":", becomes %3A.
In CGI scripts, you don't have to worry about decoding URIs
if you are using "CGI.pm". You shouldn't have to process the
URI yourself, either on the way in or the way out.
If you have to encode a string yourself, remember that you
should never try to encode an already-composed URI. You need
to escape the components separately then put them together.
To encode a string, you can use the the "URI::Escape"
module. The "uri_escape" function returns the escaped
string:
my $original = "Colon : Hash # Percent %";
my $escaped = uri_escape( $original );
print "$escaped\n"; # 'Colon%20%3A%20Hash%20%23%20Percent%20%25'
To decode the string, use the "uri_unescape" function:
my $unescaped = uri_unescape( $escaped );
print $unescaped; # back to original
If you wanted to do it yourself, you simply need to replace
the reserved characters with their encodings. A global
substitution is one way to do it:
# encode
$string =~ s/([^^A-Za-z0-9\-_.!~*'()])/ sprintf "%%%0x", ord $1 /eg;
#decode
$string =~ s/%([A-Fa-f\d]{2})/chr hex $1/eg;
How do I redirect to another page?
Specify the complete URL of the destination (even if it is
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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
"\n" is shown separately, for clarity), using either a
complete URL or an absolute URLpath.
print "Location: $url\n"; # CGI response header
print "\n"; # end of headers
How do I put a password on my web pages?
To enable authentication for your web server, you need to
configure your web server. The configuration is different
for different sorts of web servers--apache does it
differently from iPlanet which does it differently from IIS.
Check your web server documentation for the details for your
particular server.
How do I edit my .htpasswd and .htgroup files with Perl?
The "HTTPD::UserAdmin" and "HTTPD::GroupAdmin" modules
provide a consistent OO interface to these files, regardless
of how they're stored. Databases may be text, dbm, Berkeley
DB or any database with a DBI compatible driver.
"HTTPD::UserAdmin" supports files used by the "Basic" and
"Digest" authentication schemes. Here's an example:
use HTTPD::UserAdmin ();
HTTPD::UserAdmin
->new(DB => "/foo/.htpasswd")
->add($username => $password);
How do I make sure users can't enter values into a form that
cause my CGI script to do bad things?
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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 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?
(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 correctly. 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.pm" 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.
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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?
(partly contributed by Aaron Sherman)
This isn't as simple a question as it sounds. There are two
parts:
a) How do I verify that an email address is correctly
formatted?
b) How do I verify that an email address targets a valid
recipient?
Without sending mail to the address and seeing whether
there's a human on the other end to answer you, you cannot
fully answer part b, but either the "Email::Valid" or the
"RFC::RFC822::Address" module will do both part a and part b
as far as you can in real-time.
If you want to just check part a to see that the address is
valid according to the mail header standard with a simple
regular expression, you can have problems, because there are
deliverable addresses that aren't RFC-2822 (the latest mail
header standard) compliant, and addresses that aren't
deliverable which, are compliant. However, the following
will match valid RFC-2822 addresses that do not have
comments, folding whitespace, or any other obsolete or non-
essential elements. This just matches the address itself:
my $atom = qr{[a-zA-Z0-9_!#\$\%&'*+/=?\^`{}~|\-]+};
my $dot_atom = qr{$atom(?:\.$atom)*};
my $quoted = qr{"(?:\\[^\r\n]|[^\\"])*"};
my $local = qr{(?:$dot_atom|$quoted)};
my $quotedpair = qr{\\[\x00-\x09\x0B-\x0c\x0e-\x7e]};
my $domain_lit = qr{\[(?:$quotedpair|[\x21-\x5a\x5e-\x7e])*\]};
my $domain = qr{(?:$dot_atom|$domain_lit)};
my $addr_spec = qr{$local\@$domain};
Just match an address against "/^${addr_spec}$/" to see if
it follows the RFC2822 specification. However, because it
is impossible to be sure that such a correctly formed
address is actually the correct way to reach a particular
person or even has a mailbox associated with it, you must be
very careful about how you use this.
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
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versions match, send mail to that address with a personal
message. 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::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.
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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: $!\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
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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:\s*(?:Re:\s*)*(.*)/mi;
$sub[++$msgno] = lc($1) || '';
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}
$msgs[$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 function to turn the name into a number. To turn
that number into the dotted 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
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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) 1997-2010 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.
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
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