Net::libnetFAQ
Net::libnetFAQ(3) User Contributed Perl Documentation 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://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 docu-
ment 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 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 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 installa-
tion 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://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 debugging: $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
$ftp->get($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 successful.
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 distribution,
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 man-
ual 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 prob-
lem can be resolved by either passing a Firewall option like "host-
name: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
$ftp->site('chmod','0775',$file);
I have seen scripts call a method message, but cannot find it docu-
mented ?
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 plat-
forms 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 host-
name ?
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 (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 separat-
ing the output 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.
AUTHOR AND COPYRIGHT
Copyright (c) 1997 Graham Barr. All rights reserved.
perl v5.8.6 2007-08-26 Net::libnetFAQ(3)
