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
Other relevant documentation listed in:
http://www.perl.org/CGI_MetaFAQ.html
These Perl FAQs very selectively cover some CGI issues. How-
ever, 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 some-
times 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 sup-
ports. "Parsed Header" scripts are simpler in various
respects. The CGI specification allows any of the usual new-
line 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 recom-
mended. 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 ter-
minated with carriage-return and line-feed, i.e ASCII
\015\012 written in binary mode.
Using CGI.pm gives excellent platform independence, includ-
ing EBCDIC systems. CGI.pm selects an appropriate newline
representation ($CGI::CRLF) and sets binmode as appropriate.
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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: $!\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";
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Even if the error happens before you get the HTTP header
out, the module will try to take care of this to avoid the
dreaded server 500 errors. Normal warnings still go out to
the server error log (or wherever you've sent them with
"carpout") with the application name and date stamp
prepended.
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:
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<!-- 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* (["']) (.*?) \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 down-
load, 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
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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 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);
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If you're using the POST method, create your own user agent
and encode the content appropriately.
use HTTP::Request::Common qw(POST);
use LWP::UserAgent;
$ua = LWP::UserAgent->new();
my $req = POST 'http://www.perl.com/cgi-bin/cpan_mod',
[ module => 'DB_File', readme => 1 ];
$content = $ua->request($req)->as_string;
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/([^\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 com-
ponents, not the entire URI, otherwise you'll lose informa-
tion 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 com-
ponents (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/;
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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 com-
plete URL or an absolute URLpath.
print "Location: $url\n"; # CGI response header
print "\n"; # end of headers
How do I put a password on my web pages?
To enable authentication for your web server, you need to
configure your web server. The configuration is different
for different sorts of web servers---apache does it dif-
ferently from iPlanet which does it differently from IIS.
Check your web server documentation for the details for your
particular server.
How do I edit my .htpasswd and .htgroup files with Perl?
The HTTPD::UserAdmin and HTTPD::GroupAdmin modules provide a
consistent OO interface to these files, regardless of how
they're stored. Databases may be text, dbm, Berkeley DB or
any database with a DBI compatible driver. HTTPD::UserAdmin
supports files used by the "Basic" and "Digest" authentica-
tion schemes. Here's an example:
use HTTPD::UserAdmin ();
HTTPD::UserAdmin
->new(DB => "/foo/.htpasswd")
->add($username => $password);
How do I make sure users can't enter values into a form that
cause my CGI script to do bad things?
See the security references listed in the CGI Meta FAQ
http://www.perl.org/CGI_MetaFAQ.html
How do I parse a mail header?
For a quick-and-dirty solution, try this solution derived
from "split" in perlfunc:
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$/ = '';
$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 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 = $cgi->param( 'item' );
You might also try CGI::Minimal which is a lightweight ver-
sion of the same thing. Other CGI::* modules on CPAN might
work better for you, too.
Many people try to write their own decoder (or copy one from
another program) and then run into one of the many "gotchas"
of the task. It's much easier and less hassle to use
CGI.pm.
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How do I check a valid mail address?
You can't, at least, not in real time. Bummer, eh?
Without sending mail to the address and seeing whether
there's a human on the other end to answer you, you cannot
determine whether a mail address is valid. Even if you
apply the mail header standard, you can have problems,
because there are deliverable addresses that aren't RFC-822
(the mail header standard) compliant, and addresses that
aren't deliverable which are compliant.
You can use the Email::Valid or RFC::RFC822::Address which
check the format of the address, although they cannot actu-
ally tell you if it is a deliverable address (i.e. that mail
to the address will not bounce). Modules like
Mail::CheckUser and Mail::EXPN try to interact with the
domain name system or particular mail servers to learn even
more, but their methods do not work everywhere---especially
for security conscious administrators.
Many are tempted to try to eliminate many frequently-invalid
mail addresses with a simple regex, such as
"/^[\w.-]+\@(?:[\w-]+\.)+\w+$/". It's a very bad idea.
However, this also throws out many valid ones, and says
nothing about potential deliverability, so it is not sug-
gested. Instead, see
http://www.cpan.org/authors/Tom_Christiansen/scripts/ckaddr.gz
, which actually checks against the full RFC spec (except
for nested comments), looks for addresses you may not wish
to accept mail to (say, Bill Clinton or your postmaster),
and then makes sure that the hostname given can be looked up
in the DNS MX records. It's not fast, but it works for what
it tries to do.
Our best advice for verifying a person's mail address is to
have them enter their address twice, just as you normally do
to change a password. This usually weeds out typos. If both
versions match, send mail to that address with a personal
message that looks somewhat like:
Dear someuser@host.com,
Please confirm the mail address you gave us Wed May 6 09:38:41
MDT 1998 by replying to this message. Include the string
"Rumpelstiltskin" in that reply, but spelled in reverse; that is,
start with "Nik...". Once this is done, your confirmed address will
be entered into our records.
If you get the message back and they've followed your direc-
tions, you can be reasonably assured that it's real.
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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 "vaca-
tion" 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 gen-
erates 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 pack-
age) 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
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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.
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How do I use MIME to make an attachment to a mail message?
This answer is extracted directly from the MIME::Lite docu-
mentation. 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
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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" (contri-
buted by brian d foy)
The Net::Domain module, which is part of the standard dis-
tribution starting in perl5.7.3, can get you the fully qual-
ified 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 distri-
bution 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;
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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 avail-
able from CPAN. This can make tasks like fetching the news-
group 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 per-
mitted 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 2006-06-30 14
