net::libnetfaq(3)

NAME

libnetFAQ - libnet Frequently Asked Questions

DESCRIPTION

Where to get this document

This document is distributed with the libnet distribution,
and is also available on the libnet web page at

http://www.pobox.com/~gbarr/libnet/

How to contribute to this document

You may mail corrections, additions, and suggestions to me
gbarr@pobox.com.

Author and Copyright Information

Copyright (c) 1997-1998 Graham Barr. All rights reserved.
This document is free; you can redistribute it and/or mod
ify it under the terms of the Artistic License.

Disclaimer

This information is offered in good faith and in the hope
that it may be of use, but is not guaranteed to be cor
rect, up to date, or suitable for any particular purpose
whatsoever. The authors accept no liability in respect of
this information or its use.

Obtaining and installing libnet

What is libnet ?

libnet is a collection of perl5 modules which all related
to network programming. The majority of the modules avail
able provided the client side of popular server-client
protocols that are used in the internet community.

Which version of perl do I need ?

libnet has been know to work with versions of perl from
5.002 onwards. However if your release of perl is prior to
perl5.004 then you will need to obtain and install the IO
distribution from CPAN. If you have perl5.004 or later
then you will have the IO modules in your installation
already, but CPAN may contain updates.

What other modules do I need ?

The only modules you will need installed are the modules
from the IO distribution. If you have perl5.004 or later
you will already have these modules.

What machines support libnet ?

libnet itself is an entirely perl-code distribution so it
should work on any machine that perl runs on. However IO
may not work with some machines and earlier releases of
perl. But this should not be the case with perl version
5.004 or later.

Where can I get the latest libnet release

The latest libnet release is always on CPAN, you will find
it in

http://www.cpan.org/modules/by-module/Net/

The latest release and information is also available on
the libnet web page at

http://www.pobox.com/~gbarr/libnet/

Using Net::FTP

How do I download files from an FTP server ?

An example taken from an article posted to
comp.lang.perl.misc

#!/your/path/to/perl

# a module making life easier

use Net::FTP;

# for debuging: $ftp = Net::FTP->new('site','De

bug',10);
# open a connection and log in!

$ftp = Net::FTP->new('target_site.somewhere.xxx');
$ftp->login('username','password');

# set transfer mode to binary

$ftp->binary();

# change the directory on the ftp site

$ftp->cwd('/some/path/to/somewhere/');

foreach $name ('file1', 'file2', 'file3') {

# get's arguments are in the following order:
# ftp server's filename
# filename to save the transfer to on the local ma

chine
# can be simply used as get($name) if you want the

same name

$ftp->get($name,$name);

}

# ftp done!

$ftp->quit;

How do I transfer files in binary mode ?

To transfer files without <LF><CR> translation Net::FTP
provides the "binary" method

$ftp->binary;

How can I get the size of a file on a remote FTP server ?

How can I get the modification time of a file on a remote FTP server ?

How can I change the permissions of a file on a remote server ?

The FTP protocol does not have a command for changing the
permissions of a file on the remote server. But some ftp
servers may allow a chmod command to be issued via a SITE
command, eg

$ftp->quot('site chmod 0777',$filename);

But this is not guaranteed to work.

Can I do a reget operation like the ftp command ?

How do I get a directory listing from an FTP server ?

Changing directory to "" does not fail ?

Passing an argument of "" to ->cwd() has the same affect
of calling ->cwd() without any arguments. Turn on Debug
(See below) and you will see what is happening

$ftp = Net::FTP->new($host, Debug => 1);
$ftp->login;
$ftp->cwd("");

gives

Net::FTP=GLOB(0x82196d8)>>> CWD /
Net::FTP=GLOB(0x82196d8)<<< 250 CWD command success

ful.

I am behind a SOCKS firewall, but the Firewall option does not work ?

The Firewall option is only for support of one type of
firewall. The type supported is an ftp proxy.

To use Net::FTP, or any other module in the libnet distri
bution, through a SOCKS firewall you must create a socksified perl executable by compiling perl with the socks
library.

I am behind an FTP proxy firewall, but cannot access machines outside ?

Net::FTP implements the most popular ftp proxy firewall
approach. The scheme implemented is that where you log in
to the firewall with "user@hostname"

I have heard of one other type of firewall which requires
a login to the firewall with an account, then a second
login with "user@hostname". You can still use Net::FTP to
traverse these firewalls, but a more manual approach must
be taken, eg

$ftp = Net::FTP->new($firewall) or die $@;
$ftp->login($firewall_user, $firewall_passwd) or die

$ftp->message;
$ftp->login($ext_user . '@' . $ext_host, $ext_passwd)

or die $ftp->message.

My ftp proxy firewall does not listen on port 21

FTP servers usually listen on the same port number, port
21, as any other FTP server. But there is no reason why
this has to be the case.

If you pass a port number to Net::FTP then it assumes this
is the port number of the final destination. By default
Net::FTP will always try to connect to the firewall on
port 21.

Net::FTP uses IO::Socket to open the connection and
IO::Socket allows the port number to be specified as part
of the hostname. So this problem can be resolved by either
passing a Firewall option like "hostname:1234" or by set
ting the "ftp_firewall" option in Net::Config to be a
string in in the same form.

Is it possible to change the file permissions of a file on an FTP server ?

The answer to this is "maybe". The FTP protocol does not
specify a command to change file permissions on a remote
host. However many servers do allow you to run the chmod
command via the "SITE" command. This can be done with

$ftp->site('chmod','0775',$file);

I have seen scripts call a method message, but cannot find it documented ?

Net::FTP, like several other packages in libnet, inherits
from Net::Cmd, so all the methods described in Net::Cmd
are also available on Net::FTP objects.

Why does Net::FTP not implement mput and mget methods

The quick answer is because they are easy to implement
yourself. The long answer is that to write these in such a
way that multiple platforms are supported correctly would
just require too much code. Below are some examples how
you can implement these yourself.

sub mput {

my($ftp,$pattern) = @_;
foreach my $file (glob($pattern)) {

$ftp->put($file) or warn $ftp->message;

} }

sub mget {

my($ftp,$pattern) = @_;
foreach my $file ($ftp->ls($pattern)) {

$ftp->get($file) or warn $ftp->message;

} }

Using Net::SMTP

Why can't the part of an Email address after the @ be used as the hostname ?

The part of an Email address which follows the @ is not
necessarily a hostname, it is a mail domain. To find the
name of a host to connect for a mail domain you need to do
a DNS MX lookup

Why does Net::SMTP not do DNS MX lookups ?

Net::SMTP implements the SMTP protocol. The DNS MX lookup
is not part of this protocol.

The verify method always returns true ?

Well it may seem that way, but it does not. The verify
method returns true if the command succeeded. If you pass
verify an address which the server would normally have to
forward to another machine, the command will succeed with
something like

252 Couldn't verify <someone@there> but will attempt

delivery anyway

This command will fail only if you pass it an address in a
domain the server directly delivers for, and that address
does not exist.

Debugging scripts

How can I debug my scripts that use Net::* modules ?

Most of the libnet client classes allow options to be
passed to the constructor, in most cases one option is
called "Debug". Passing this option with a non-zero value
will turn on a protocol trace, which will be sent to
STDERR. This trace can be useful to see what commands are
being sent to the remote server and what responses are
being received back.

#!/your/path/to/perl

use Net::FTP;

my $ftp = new Net::FTP($host, Debug => 1);
$ftp->login('gbarr','password');
$ftp->quit;

this script would output something like

Net::FTP: Net::FTP(2.22)
Net::FTP: Exporter
Net::FTP: Net::Cmd(2.0801)
Net::FTP: IO::Socket::INET
Net::FTP: IO::Socket(1.1603)
Net::FTP: IO::Handle(1.1504)

Net::FTP=GLOB(0x8152974)<<< 220 imagine FTP server (Ver

sion wu-2.4(5) Tue Jul 29 11:17:18 CDT 1997) ready.
Net::FTP=GLOB(0x8152974)>>> user gbarr
Net::FTP=GLOB(0x8152974)<<< 331 Password required for

gbarr.
Net::FTP=GLOB(0x8152974)>>> PASS ....
Net::FTP=GLOB(0x8152974)<<< 230 User gbarr logged in.

Access restrictions apply.
Net::FTP=GLOB(0x8152974)>>> QUIT
Net::FTP=GLOB(0x8152974)<<< 221 Goodbye.

The first few lines tell you the modules that Net::FTP
uses and their versions, this is useful data to me when a
user reports a bug. The last seven lines show the communi
cation with the server. Each line has three parts. The
first part is the object itself, this is useful for sepa
rating the output if you are using multiple objects. The
second part is either "<<<<" to show data coming from the
server or "&gt&gt&gt&gt" to show data going to the server.
The remainder of the line is the command being sent or
response being received.

AUTHOR AND COPYRIGHT

Copyright (c) 1997 Graham Barr. All rights reserved.

$Id: //depot/libnet/Net/libnetFAQ.pod#5 $

perl v5.8.0 2002-06-01 Net::libnetFAQ(3)