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libnetFAQ - libnet Frequently Asked Questions
Contents
Debugging Scripts
HowcanIdebugmyscriptsthatuseNet::*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 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.
Description
Wheretogetthisdocument
This document is distributed with the libnet distribution, and is also available on the libnet web page
at
<https://metacpan.org/release/libnet>
Howtocontributetothisdocument
You may report corrections, additions, and suggestions on the CPAN Request Tracker at
<https://rt.cpan.org/Public/Bug/Report.html?Queue=libnet>
Name
libnetFAQ - libnet Frequently Asked Questions
Obtaining And Installing Libnet
Whatislibnet?
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.
WhichversionofperldoIneed?
This version of libnet requires Perl 5.8.1 or higher.
WhatothermodulesdoIneed?
No non-core modules are required for normal use, except on os390, which requires Convert::EBCDIC.
Authen::SASL is required for AUTH support.
IO::Socket::SSL version 2.007 or higher is required for SSL support.
IO::Socket::IP version 0.25 or IO::Socket::INET6 version 2.62 is required for IPv6 support.
Whatmachinessupportlibnet?
libnet itself is an entirely perl-code distribution so it should work on any machine that perl runs on.
WherecanIgetthelatestlibnetrelease
The latest libnet release is always on CPAN, you will find it in
<https://metacpan.org/release/libnet>
Using Net::Ftp
HowdoIdownloadfilesfromanFTPserver?
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;
HowdoItransferfilesinbinarymode?
To transfer files without <LF><CR> translation Net::FTP provides the "binary" method
$ftp->binary;
HowcanIgetthesizeofafileonaremoteFTPserver?HowcanIgetthemodificationtimeofafileonaremoteFTPserver?HowcanIchangethepermissionsofafileonaremoteserver?
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.
CanIdoaregetoperationliketheftpcommand?HowdoIgetadirectorylistingfromanFTPserver?Changingdirectoryto""doesnotfail?
Passing an argument of "" to ->cwd() has the same affect of calling ->cwd() without any arguments. Turn
on Debug (Seebelow) 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.
IambehindaSOCKSfirewall,buttheFirewalloptiondoesnotwork?
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.
IambehindanFTPproxyfirewall,butcannotaccessmachinesoutside?
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.
Myftpproxyfirewalldoesnotlistenonport21
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 the same form.
IsitpossibletochangethefilepermissionsofafileonanFTPserver?
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);
Ihaveseenscriptscallamethodmessage,butcannotfinditdocumented?
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.
WhydoesNet::FTPnotimplementmputandmgetmethods
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
Whycan'tthepartofanEmailaddressafterthe@beusedasthehostname?
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
WhydoesNet::SMTPnotdoDNSMXlookups?
Net::SMTP implements the SMTP protocol. The DNS MX lookup is not part of this protocol.
Theverifymethodalwaysreturnstrue?
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.
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