PERLFAQ9(1) Perl Programmers Reference Guide PERLFAQ9(1)NAMEperlfaq9 - 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 <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
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)
(contributed by brian d foy)
There are many things that might be wrong with your CGI program, and
only some of them might be related to Perl. Try going through the
troubleshooting guide on Perlmonks:
http://www.perlmonks.org/?node_id=380424
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" module's "carp", "croak", and "confess" functions with
more verbose and safer versions. It still sends them to the normal
server error log.
use CGI::Carp;
warn "This is a complaint";
die "But this one is serious";
The following use of "CGI::Carp" also redirects errors to a file of
your choice, placed in a "BEGIN" block to catch compile-time warnings
as well:
BEGIN {
use CGI::Carp qw(carpout);
open(LOG, ">>/var/local/cgi-logs/mycgi-log")
or die "Unable to append to mycgi-log: $!\n";
carpout(*LOG);
}
You can even arrange for fatal errors to go back to the client browser,
which is nice for your own debugging, but might confuse the end user.
use CGI::Carp qw(fatalsToBrowser);
die "Bad error here";
Even if the error happens before you get the HTTP header out, the
module will try to take care of this to avoid the dreaded server 500
errors. Normal warnings still go out to the server error log (or
wherever you've sent them with "carpout") with the application name and
date stamp prepended.
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 comments 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/<(?:[^>'"]*|(['"]).*?\g1)*>//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
"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 - tchrist@perl.com
print "$2\n" while m{
< \s*
A \s+ HREF \s* = \s* (["']) (.*?) \g1
\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,
"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.
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 "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 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?
(contributed by brian d foy)
You can't prevent people from sending your script bad data. Even if you
add some client-side checks, people may disable them or bypass them
completely. For instance, someone might use a module such as "LWP" to
access your CGI program. If you want to prevent data that try to use
SQL injection or other sorts of attacks (and you should want to), you
have to not trust any data that enter your program.
The perlsec documentation has general advice about data security. If
you are using the "DBI" module, use placeholder to fill in data. If
you are running external programs with "system" or "exec", use the list
forms. There are many other precautions that you should take, too many
to list here, and most of them fall under the category of not using any
data that you don't intend to use. Trust no one.
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.
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 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.
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 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
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
perl -MNews::NNTPClient
-e 'print News::NNTPClient->new->list("newsgroups")'
How do I fetch/put an FTP file?
(contributed by brian d foy)
The "LWP" family of modules (available on CPAN as the libwww-perl
distribution) can work with FTP just like it can with many other
protocols. "LWP::Simple" makes it quite easy to fetch a file:
use LWP::Simple;
my $data = get( 'ftp://some.ftp.site/some/file.txt' );
If you want more direct or low-level control of the FTP process, you
can use the "Net::FTP" module (in the Standard Library since Perl 5.8).
It's documentation has examples showing you just how to do that.
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.
perl v5.14.2 2011-09-26 PERLFAQ9(1)
