MirBSD manpage: Net::libnetFAQ(3p)

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
     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.


     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

     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 pro-
     vides 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 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,

         $ftp = Net::FTP->new($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

     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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     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


         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 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
     "&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.

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

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

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