Net::libnetFAQ(3pPerl Programmers Reference GuiNet::libnetFAQ(3p)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://search.cpan.org/~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 modify
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 correct,
up to date, or suitable for any particular purpose whatso-
ever. 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 available
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.
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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://search.cpan.org/~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','Debug',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 $name ('file1', 'file2', 'file3') {
# get's arguments are in the following order:
# ftp server's filename
# filename to save the transfer to on the local machine
# can be simply used as get($name) if you want the same name
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$ftp->get($name,$name);
}
# ftp done!
$ftp->quit;
How do I transfer files in binary mode ?
To transfer files without <LF><CR> translation Net::FTP pro-
vides 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 successful.
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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 distribu-
tion, through a SOCKS firewall you must create a socks-ified
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 $ftp->message.
My ftp proxy firewall does not listen on port 21
FTP servers usually listen on the same port number, port 21,
as any other FTP server. But there is no reason why this has
to be the case.
If you pass a port number to Net::FTP then it assumes this
is the port number of the final destination. By default
Net::FTP will always try to connect to the firewall on port
21.
Net::FTP uses IO::Socket to open the connection and
IO::Socket allows the port number to be specified as part of
the hostname. So this problem can be resolved by either
passing a Firewall option like "hostname:1234" or by setting
the "ftp_firewall" option in Net::Config to be a string in
in the same form.
Is it possible to change the file permissions of a file on
an FTP server ?
The answer to this is "maybe". The FTP protocol does not
specify a command to change file permissions on a remote
host. However many servers do allow you to run the chmod
command via the "SITE" command. This can be done with
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$ftp->site('chmod','0775',$file);
I have seen scripts call a method message, but cannot find
it documented ?
Net::FTP, like several other packages in libnet, inherits
from Net::Cmd, so all the methods described in Net::Cmd are
also available on Net::FTP objects.
Why does Net::FTP not implement mput and mget methods
The quick answer is because they are easy to implement your-
self. The long answer is that to write these in such a way
that multiple platforms are supported correctly would just
require too much code. Below are some examples how you can
implement these yourself.
sub mput {
my($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
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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 (Version 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 communication
with the server. Each line has three parts. The first part
is the object itself, this is useful for separating the out-
put if you are using multiple objects. The second part is
either "<<<<" to show data coming from the server or
">>>>" to show data going to the server. The
remainder of the line is the command being sent or response
being received.
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Copyright (c) 1997 Graham Barr. All rights reserved.
$Id: //depot/libnet/Net/libnetFAQ.pod#6 $
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