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Net::libnetFAQ(3pPerl Programmers Reference GuiNet::libnetFAQ(3p)

       libnetFAQ - libnet Frequently Asked Questions

       Where to get this document

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


       How to contribute to this document

       You may mail corrections, additions, and suggestions to me

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.


       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.

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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


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


Using Net::FTP
       How do I download files from an FTP server ?

       An example taken from an article posted to


           # 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');

           # set transfer mode to binary


           # change the directory on the ftp site


           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 done!


       How do I transfer files in binary mode ?

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


       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);


           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 distri-
       bution, through a SOCKS firewall you must create a socks-
       ified perl executable by compiling perl with the socks

       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

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       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

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       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.


           use Net::FTP;

           my $ftp = new Net::FTP($host, Debug => 1);

       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 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.

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

       $Id: //depot/libnet/Net/libnetFAQ.pod#6 $

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