[p5sagit/p5-mst-13.2.git] / lib / Net / Ping.pm

package Net::Ping;

# $Id: Ping.pm,v 1.11 2001/12/04 02:41:51 rob Exp $

require 5.002;
require Exporter;

use strict;
use vars qw(@ISA @EXPORT $VERSION
            $def_timeout $def_proto $max_datasize $pingstring);
use FileHandle;
use Socket qw( SOCK_DGRAM SOCK_STREAM SOCK_RAW PF_INET
               inet_aton sockaddr_in );
use Carp;

@ISA = qw(Exporter);
@EXPORT = qw(pingecho);
$VERSION = 2.07;

# Constants

$def_timeout = 5;           # Default timeout to wait for a reply
$def_proto = "udp";         # Default protocol to use for pinging
$max_datasize = 1024;       # Maximum data bytes in a packet
# The data we exchange with the server for the stream protocol
$pingstring = "pingschwingping!\n";

# Description:  The pingecho() subroutine is provided for backward
# compatibility with the original Net::Ping.  It accepts a host
# name/IP and an optional timeout in seconds.  Create a tcp ping
# object and try pinging the host.  The result of the ping is returned.

sub pingecho
{
    my ($host,              # Name or IP number of host to ping
        $timeout            # Optional timeout in seconds
        ) = @_;
    my ($p);                # A ping object

    $p = Net::Ping->new("tcp", $timeout);
    $p->ping($host);        # Going out of scope closes the connection
}

# Description:  The new() method creates a new ping object.  Optional
# parameters may be specified for the protocol to use, the timeout in
# seconds and the size in bytes of additional data which should be
# included in the packet.
#   After the optional parameters are checked, the data is constructed
# and a socket is opened if appropriate.  The object is returned.

sub new
{
    my ($this,
        $proto,             # Optional protocol to use for pinging
        $timeout,           # Optional timeout in seconds
        $data_size          # Optional additional bytes of data
        ) = @_;
    my  $class = ref($this) || $this;
    my  $self = {};
    my ($cnt,               # Count through data bytes
        $min_datasize       # Minimum data bytes required
        );

    bless($self, $class);

    $proto = $def_proto unless $proto;          # Determine the protocol
    croak('Protocol for ping must be "icmp", "udp", "tcp", "stream", or "external"')
        unless $proto =~ m/^(icmp|udp|tcp|stream|external)$/;
    $self->{"proto"} = $proto;

    $timeout = $def_timeout unless $timeout;    # Determine the timeout
    croak("Default timeout for ping must be greater than 0 seconds")
        if $timeout <= 0;
    $self->{"timeout"} = $timeout;

    $min_datasize = ($proto eq "udp") ? 1 : 0;  # Determine data size
    $data_size = $min_datasize unless defined($data_size) && $proto ne "tcp";
    croak("Data for ping must be from $min_datasize to $max_datasize bytes")
        if ($data_size < $min_datasize) || ($data_size > $max_datasize);
    $data_size-- if $self->{"proto"} eq "udp";  # We provide the first byte
    $self->{"data_size"} = $data_size;

    $self->{"data"} = "";                       # Construct data bytes
    for ($cnt = 0; $cnt < $self->{"data_size"}; $cnt++)
    {
        $self->{"data"} .= chr($cnt % 256);
    }

    $self->{"seq"} = 0;                         # For counting packets
    if ($self->{"proto"} eq "udp")              # Open a socket
    {
        $self->{"proto_num"} = (getprotobyname('udp'))[2] ||
            croak("Can't udp protocol by name");
        $self->{"port_num"} = (getservbyname('echo', 'udp'))[2] ||
            croak("Can't get udp echo port by name");
        $self->{"fh"} = FileHandle->new();
        socket($self->{"fh"}, &PF_INET(), &SOCK_DGRAM(),
               $self->{"proto_num"}) ||
            croak("udp socket error - $!");
    }
    elsif ($self->{"proto"} eq "icmp")
    {
        croak("icmp ping requires root privilege") if ($> and $^O ne 'VMS');
        $self->{"proto_num"} = (getprotobyname('icmp'))[2] ||
                    croak("Can't get icmp protocol by name");
        $self->{"pid"} = $$ & 0xffff;           # Save lower 16 bits of pid
        $self->{"fh"} = FileHandle->new();
        socket($self->{"fh"}, &PF_INET(), &SOCK_RAW(), $self->{"proto_num"}) ||
            croak("icmp socket error - $!");
    }
    elsif ($self->{"proto"} eq "tcp" || $self->{"proto"} eq "stream")
    {
        $self->{"proto_num"} = (getprotobyname('tcp'))[2] ||
            croak("Can't get tcp protocol by name");
        $self->{"port_num"} = (getservbyname('echo', 'tcp'))[2] ||
            croak("Can't get tcp echo port by name");
        $self->{"fh"} = FileHandle->new();
    }


    return($self);
}

# Description: Ping a host name or IP number with an optional timeout.
# First lookup the host, and return undef if it is not found.  Otherwise
# perform the specific ping method based on the protocol.  Return the
# result of the ping.

sub ping
{
    my ($self,
        $host,              # Name or IP number of host to ping
        $timeout            # Seconds after which ping times out
        ) = @_;
    my ($ip,                # Packed IP number of $host
        $ret                # The return value
        );

    croak("Usage: \$p->ping(\$host [, \$timeout])") unless @_ == 2 || @_ == 3;
    $timeout = $self->{"timeout"} unless $timeout;
    croak("Timeout must be greater than 0 seconds") if $timeout <= 0;

    $ip = inet_aton($host);
    return(undef) unless defined($ip);      # Does host exist?

    # Dispatch to the appropriate routine.
    return $self->ping_external($ip, $timeout) if $self->{"proto"} eq "external";
    return $self->ping_udp($ip, $timeout)      if $self->{"proto"} eq "udp";
    return $self->ping_icmp($ip, $timeout)     if $self->{"proto"} eq "icmp";
    return $self->ping_tcp($ip, $timeout)      if $self->{"proto"} eq "tcp";
    return $self->ping_stream($ip, $timeout)   if $self->{"proto"} eq "stream";

    croak("Unknown protocol \"$self->{proto}\" in ping()");
}

# Uses Net::Ping::External to do an external ping.
sub ping_external {
  my ($self,
      $ip,                # Packed IP number of the host
      $timeout            # Seconds after which ping times out
     ) = @_;

  eval { require Net::Ping::External; }
    or croak('Protocol "external" not supported on your system: Net::Ping::External not found');
  return Net::Ping::External::ping(ip => $ip, timeout => $timeout);
}

sub ping_icmp
{
    my ($self,
        $ip,                # Packed IP number of the host
        $timeout            # Seconds after which ping times out
        ) = @_;

    my $ICMP_ECHOREPLY = 0; # ICMP packet types
    my $ICMP_ECHO = 8;
    my $icmp_struct = "C2 S3 A";  # Structure of a minimal ICMP packet
    my $subcode = 0;        # No ICMP subcode for ECHO and ECHOREPLY
    my $flags = 0;          # No special flags when opening a socket
    my $port = 0;           # No port with ICMP

    my ($saddr,             # sockaddr_in with port and ip
        $checksum,          # Checksum of ICMP packet
        $msg,               # ICMP packet to send
        $len_msg,           # Length of $msg
        $rbits,             # Read bits, filehandles for reading
        $nfound,            # Number of ready filehandles found
        $finish_time,       # Time ping should be finished
        $done,              # set to 1 when we are done
        $ret,               # Return value
        $recv_msg,          # Received message including IP header
        $from_saddr,        # sockaddr_in of sender
        $from_port,         # Port packet was sent from
        $from_ip,           # Packed IP of sender
        $from_type,         # ICMP type
        $from_subcode,      # ICMP subcode
        $from_chk,          # ICMP packet checksum
        $from_pid,          # ICMP packet id
        $from_seq,          # ICMP packet sequence
        $from_msg           # ICMP message
        );

    $self->{"seq"} = ($self->{"seq"} + 1) % 65536; # Increment sequence
    $checksum = 0;                          # No checksum for starters
    $msg = pack($icmp_struct . $self->{"data_size"}, $ICMP_ECHO, $subcode,
                $checksum, $self->{"pid"}, $self->{"seq"}, $self->{"data"});
    $checksum = Net::Ping->checksum($msg);
    $msg = pack($icmp_struct . $self->{"data_size"}, $ICMP_ECHO, $subcode,
                $checksum, $self->{"pid"}, $self->{"seq"}, $self->{"data"});
    $len_msg = length($msg);
    $saddr = sockaddr_in($port, $ip);
    send($self->{"fh"}, $msg, $flags, $saddr); # Send the message

    $rbits = "";
    vec($rbits, $self->{"fh"}->fileno(), 1) = 1;
    $ret = 0;
    $done = 0;
    $finish_time = time() + $timeout;       # Must be done by this time
    while (!$done && $timeout > 0)          # Keep trying if we have time
    {
        $nfound = select($rbits, undef, undef, $timeout); # Wait for packet
        $timeout = $finish_time - time();   # Get remaining time
        if (!defined($nfound))              # Hmm, a strange error
        {
            $ret = undef;
            $done = 1;
        }
        elsif ($nfound)                     # Got a packet from somewhere
        {
            $recv_msg = "";
            $from_saddr = recv($self->{"fh"}, $recv_msg, 1500, $flags);
            ($from_port, $from_ip) = sockaddr_in($from_saddr);
            ($from_type, $from_subcode, $from_chk,
             $from_pid, $from_seq, $from_msg) =
                unpack($icmp_struct . $self->{"data_size"},
                       substr($recv_msg, length($recv_msg) - $len_msg,
                              $len_msg));
            if (($from_type == $ICMP_ECHOREPLY) &&
                ($from_ip eq $ip) &&
                ($from_pid == $self->{"pid"}) && # Does the packet check out?
                ($from_seq == $self->{"seq"}))
            {
                $ret = 1;                   # It's a winner
                $done = 1;
            }
        }
        else                                # Oops, timed out
        {
            $done = 1;
        }
    }
    return($ret)
}

# Description:  Do a checksum on the message.  Basically sum all of
# the short words and fold the high order bits into the low order bits.

sub checksum
{
    my ($class,
        $msg            # The message to checksum
        ) = @_;
    my ($len_msg,       # Length of the message
        $num_short,     # The number of short words in the message
        $short,         # One short word
        $chk            # The checksum
        );

    $len_msg = length($msg);
    $num_short = int($len_msg / 2);
    $chk = 0;
    foreach $short (unpack("S$num_short", $msg))
    {
        $chk += $short;
    }                                           # Add the odd byte in
    $chk += (unpack("C", substr($msg, $len_msg - 1, 1)) << 8) if $len_msg % 2;
    $chk = ($chk >> 16) + ($chk & 0xffff);      # Fold high into low
    return(~(($chk >> 16) + $chk) & 0xffff);    # Again and complement
}


# Description:  Perform a tcp echo ping.  Since a tcp connection is
# host specific, we have to open and close each connection here.  We
# can't just leave a socket open.  Because of the robust nature of
# tcp, it will take a while before it gives up trying to establish a
# connection.  Therefore, we use select() on a non-blocking socket to
# check against our timeout.  No data bytes are actually
# sent since the successful establishment of a connection is proof
# enough of the reachability of the remote host.  Also, tcp is
# expensive and doesn't need our help to add to the overhead.

sub ping_tcp
{
    my ($self,
        $ip,                # Packed IP number of the host
        $timeout            # Seconds after which ping times out
        ) = @_;
    my ($ret                # The return value
        );

    $@ = "";
    $ret = $self -> tcp_connect( $ip, $timeout);
    $ret = 1 if $@ =~ /(Connection Refused|Unknown Error)/i;
    $self->{"fh"}->close();
    return($ret);
}

sub tcp_connect
{
    my ($self,
        $ip,                # Packed IP number of the host
        $timeout            # Seconds after which connect times out
        ) = @_;
    my ($saddr);            # Packed IP and Port

    $saddr = sockaddr_in($self->{"port_num"}, $ip);

    my $ret = 0;            # Default to unreachable

    my $do_socket = sub {
      socket($self->{"fh"}, &PF_INET(), &SOCK_STREAM(), $self->{"proto_num"}) ||
        croak("tcp socket error - $!");
    };
    my $do_connect = sub {
      eval {
        die $! unless connect($self->{"fh"}, $saddr);
        $self->{"ip"} = $ip;
        $ret = 1;
      };
      $ret;
    };

    if ($^O =~ /Win32/i) {

      # Buggy Winsock API doesn't allow us to use alarm() calls.
      # Hence, if our OS is Windows, we need to create a separate
      # process to do the blocking connect attempt.

      $| = 1; # Clear buffer prior to fork to prevent duplicate flushing.
      my $pid = fork;
      if (!$pid) {
        if (!defined $pid) {
          # Fork did not work
          warn "Win32 Fork error: $!";
          return 0;
        }
        &{ $do_socket }();

        # Try a slow blocking connect() call
        # and report the status to the pipe.
        if ( &{ $do_connect }() ) {
          $self->{"fh"}->close();
          # No error
          exit 0;
        } else {
          # Pass the error status to the parent
          exit $!;
        }
      }

      &{ $do_socket }();

      my $patience = time + $timeout;

      require POSIX;
      my ($child);
      $? = 0;
      # Wait up to the timeout
      # And clean off the zombie
      do {
        $child = waitpid($pid, &POSIX::WNOHANG);
        $! = $? >> 8;
        $@ = $!;
        sleep 1;
      } while time < $patience && $child != $pid;

      if ($child == $pid) {
        # Since she finished within the timeout,
        # it is probably safe for me to try it too
        &{ $do_connect }();
      } else {
        # Time must have run out.
        $@ = "Timed out!";
        # Put that choking client out of its misery
        kill "KILL", $pid;
        # Clean off the zombie
        waitpid($pid, 0);
        $ret = 0;
      }
    } else { # Win32
      # Otherwise don't waste the resources to fork

      &{ $do_socket }();

      $SIG{'ALRM'} = sub { die "Timed out!"; };
      alarm($timeout);        # Interrupt connect() if we have to

      &{ $do_connect }();
      alarm(0);
    }

    return $ret;
}

# This writes the given string to the socket and then reads it
# back.  It returns 1 on success, 0 on failure.
sub tcp_echo
{
    my $self = shift;
    my $timeout = shift;
    my $pingstring = shift;

    my $ret = undef;
    my $time = time;
    my $wrstr = $pingstring;
    my $rdstr = "";

    eval <<'EOM';
        do {
                my $rin = "";
                vec($rin, $self->{"fh"}->fileno(), 1) = 1;

                my $rout = undef;
                if($wrstr) {
                        $rout = "";
                        vec($rout, $self->{"fh"}->fileno(), 1) = 1;
                }

                if(select($rin, $rout, undef, ($time + $timeout) - time())) {

                        if($rout && vec($rout,$self->{"fh"}->fileno(),1)) {
                                my $num = syswrite($self->{"fh"}, $wrstr);
                                if($num) {
                                        # If it was a partial write, update and try again.
                                        $wrstr = substr($wrstr,$num);
                                } else {
                                        # There was an error.
                                        $ret = 0;
                                }
                        }

                        if(vec($rin,$self->{"fh"}->fileno(),1)) {
                                my $reply;
                                if(sysread($self->{"fh"},$reply,length($pingstring)-length($rdstr))) {
                                        $rdstr .= $reply;
                                        $ret = 1 if $rdstr eq $pingstring;
                                } else {
                                        # There was an error.
                                        $ret = 0;
                                }
                        }

                }
        } until time() > ($time + $timeout) || defined($ret);
EOM

    return $ret;
}


# Description: Perform a stream ping.  If the tcp connection isn't
# already open, it opens it.  It then sends some data and waits for
# a reply.  It leaves the stream open on exit.

sub ping_stream
{
    my ($self,
        $ip,                # Packed IP number of the host
        $timeout            # Seconds after which ping times out
        ) = @_;

    # Open the stream if it's not already open
    if(!defined $self->{"fh"}->fileno()) {
        $self->tcp_connect($ip, $timeout) or return 0;
    }

    croak "tried to switch servers while stream pinging"
       if $self->{"ip"} ne $ip;

    return $self->tcp_echo($timeout, $pingstring);
}

# Description: opens the stream.  You would do this if you want to
# separate the overhead of opening the stream from the first ping.

sub open
{
    my ($self,
        $host,              # Host or IP address
        $timeout            # Seconds after which open times out
        ) = @_;

    my ($ip);               # Packed IP number of the host
    $ip = inet_aton($host);
    $timeout = $self->{"timeout"} unless $timeout;

    if($self->{"proto"} eq "stream") {
      if(defined($self->{"fh"}->fileno())) {
        croak("socket is already open");
      } else {
        $self->tcp_connect($ip, $timeout);
      }
    }
}


# Description:  Perform a udp echo ping.  Construct a message of
# at least the one-byte sequence number and any additional data bytes.
# Send the message out and wait for a message to come back.  If we
# get a message, make sure all of its parts match.  If they do, we are
# done.  Otherwise go back and wait for the message until we run out
# of time.  Return the result of our efforts.

sub ping_udp
{
    my ($self,
        $ip,                # Packed IP number of the host
        $timeout            # Seconds after which ping times out
        ) = @_;

    my $flags = 0;          # Nothing special on open

    my ($saddr,             # sockaddr_in with port and ip
        $ret,               # The return value
        $msg,               # Message to be echoed
        $finish_time,       # Time ping should be finished
        $done,              # Set to 1 when we are done pinging
        $rbits,             # Read bits, filehandles for reading
        $nfound,            # Number of ready filehandles found
        $from_saddr,        # sockaddr_in of sender
        $from_msg,          # Characters echoed by $host
        $from_port,         # Port message was echoed from
        $from_ip            # Packed IP number of sender
        );

    $saddr = sockaddr_in($self->{"port_num"}, $ip);
    $self->{"seq"} = ($self->{"seq"} + 1) % 256;    # Increment sequence
    $msg = chr($self->{"seq"}) . $self->{"data"};   # Add data if any
    send($self->{"fh"}, $msg, $flags, $saddr);      # Send it

    $rbits = "";
    vec($rbits, $self->{"fh"}->fileno(), 1) = 1;
    $ret = 0;                   # Default to unreachable
    $done = 0;
    $finish_time = time() + $timeout;       # Ping needs to be done by then
    while (!$done && $timeout > 0)
    {
        $nfound = select($rbits, undef, undef, $timeout); # Wait for response
        $timeout = $finish_time - time();   # Get remaining time

        if (!defined($nfound))  # Hmm, a strange error
        {
            $ret = undef;
            $done = 1;
        }
        elsif ($nfound)         # A packet is waiting
        {
            $from_msg = "";
            $from_saddr = recv($self->{"fh"}, $from_msg, 1500, $flags)
		or last; # For example an unreachable host will make recv() fail.
	    ($from_port, $from_ip) = sockaddr_in($from_saddr);
	    if (($from_ip eq $ip) &&        # Does the packet check out?
		($from_port == $self->{"port_num"}) &&
		($from_msg eq $msg))
	    {
		$ret = 1;       # It's a winner
		$done = 1;
	    }
	}
        else                    # Oops, timed out
        {
            $done = 1;
        }
    }
    return($ret);
}

# Description:  Close the connection unless we are using the tcp
# protocol, since it will already be closed.

sub close
{
    my ($self) = @_;

    $self->{"fh"}->close() unless $self->{"proto"} eq "tcp";
}


1;
__END__

=head1 NAME

Net::Ping - check a remote host for reachability

$Id: Ping.pm,v 1.11 2001/12/04 02:41:51 rob Exp $

=head1 SYNOPSIS

    use Net::Ping;

    $p = Net::Ping->new();
    print "$host is alive.\n" if $p->ping($host);
    $p->close();

    $p = Net::Ping->new("icmp");
    foreach $host (@host_array)
    {
        print "$host is ";
        print "NOT " unless $p->ping($host, 2);
        print "reachable.\n";
        sleep(1);
    }
    $p->close();

    $p = Net::Ping->new("tcp", 2);
    # Try connecting to the www port instead of the echo port
    $p->{port_num} = getservbyname("http", "tcp");
    while ($stop_time > time())
    {
        print "$host not reachable ", scalar(localtime()), "\n"
            unless $p->ping($host);
        sleep(300);
    }
    undef($p);

    # For backward compatibility
    print "$host is alive.\n" if pingecho($host);

=head1 DESCRIPTION

This module contains methods to test the reachability of remote
hosts on a network.  A ping object is first created with optional
parameters, a variable number of hosts may be pinged multiple
times and then the connection is closed.

You may choose one of four different protocols to use for the
ping. The "udp" protocol is the default. Note that a live remote host
may still fail to be pingable by one or more of these protocols. For
example, www.microsoft.com is generally alive but not pingable.

With the "tcp" protocol the ping() method attempts to establish a
connection to the remote host's echo port.  If the connection is
successfully established, the remote host is considered reachable.  No
data is actually echoed.  This protocol does not require any special
privileges but has higher overhead than the other two protocols.

Specifying the "udp" protocol causes the ping() method to send a udp
packet to the remote host's echo port.  If the echoed packet is
received from the remote host and the received packet contains the
same data as the packet that was sent, the remote host is considered
reachable.  This protocol does not require any special privileges.
It should be borne in mind that, for a udp ping, a host
will be reported as unreachable if it is not running the
appropriate echo service.  For Unix-like systems see L<inetd(8)>
for more information.

If the "icmp" protocol is specified, the ping() method sends an icmp
echo message to the remote host, which is what the UNIX ping program
does.  If the echoed message is received from the remote host and
the echoed information is correct, the remote host is considered
reachable.  Specifying the "icmp" protocol requires that the program
be run as root or that the program be setuid to root.

If the "external" protocol is specified, the ping() method attempts to
use the C<Net::Ping::External> module to ping the remote host.
C<Net::Ping::External> interfaces with your system's default C<ping>
utility to perform the ping, and generally produces relatively
accurate results. If C<Net::Ping::External> if not installed on your
system, specifying the "external" protocol will result in an error.

=head2 Functions

=over 4

=item Net::Ping->new([$proto [, $def_timeout [, $bytes]]]);

Create a new ping object.  All of the parameters are optional.  $proto
specifies the protocol to use when doing a ping.  The current choices
are "tcp", "udp" or "icmp".  The default is "udp".

If a default timeout ($def_timeout) in seconds is provided, it is used
when a timeout is not given to the ping() method (below).  The timeout
must be greater than 0 and the default, if not specified, is 5 seconds.

If the number of data bytes ($bytes) is given, that many data bytes
are included in the ping packet sent to the remote host. The number of
data bytes is ignored if the protocol is "tcp".  The minimum (and
default) number of data bytes is 1 if the protocol is "udp" and 0
otherwise.  The maximum number of data bytes that can be specified is
1024.

=item $p->ping($host [, $timeout]);

Ping the remote host and wait for a response.  $host can be either the
hostname or the IP number of the remote host.  The optional timeout
must be greater than 0 seconds and defaults to whatever was specified
when the ping object was created.  If the hostname cannot be found or
there is a problem with the IP number, undef is returned.  Otherwise,
1 is returned if the host is reachable and 0 if it is not.  For all
practical purposes, undef and 0 and can be treated as the same case.

=item $p->open($host);

When you are using the stream protocol, this call pre-opens the
tcp socket.  It's only necessary to do this if you want to
provide a different timeout when creating the connection, or
remove the overhead of establishing the connection from the
first ping.  If you don't call C<open()>, the connection is
automatically opened the first time C<ping()> is called.
This call simply does nothing if you are using any protocol other
than stream.

=item $p->open($host);

When you are using the stream protocol, this call pre-opens the
tcp socket.  It's only necessary to do this if you want to
provide a different timeout when creating the connection, or
remove the overhead of establishing the connection from the
first ping.  If you don't call C<open()>, the connection is
automatically opened the first time C<ping()> is called.
This call simply does nothing if you are using any protocol other
than stream.

=item $p->close();

Close the network connection for this ping object.  The network
connection is also closed by "undef $p".  The network connection is
automatically closed if the ping object goes out of scope (e.g. $p is
local to a subroutine and you leave the subroutine).

=item pingecho($host [, $timeout]);

To provide backward compatibility with the previous version of
Net::Ping, a pingecho() subroutine is available with the same
functionality as before.  pingecho() uses the tcp protocol.  The
return values and parameters are the same as described for the ping()
method.  This subroutine is obsolete and may be removed in a future
version of Net::Ping.

=back

=head1 WARNING

pingecho() or a ping object with the tcp protocol use alarm() to
implement the timeout.  So, don't use alarm() in your program while
you are using pingecho() or a ping object with the tcp protocol.  The
udp and icmp protocols do not use alarm() to implement the timeout.

=head1 NOTES

There will be less network overhead (and some efficiency in your
program) if you specify either the udp or the icmp protocol.  The tcp
protocol will generate 2.5 times or more traffic for each ping than
either udp or icmp.  If many hosts are pinged frequently, you may wish
to implement a small wait (e.g. 25ms or more) between each ping to
avoid flooding your network with packets.

The icmp protocol requires that the program be run as root or that it
be setuid to root.  The other protocols do not require special
privileges, but not all network devices implement tcp or udp echo.

Local hosts should normally respond to pings within milliseconds.
However, on a very congested network it may take up to 3 seconds or
longer to receive an echo packet from the remote host.  If the timeout
is set too low under these conditions, it will appear that the remote
host is not reachable (which is almost the truth).

Reachability doesn't necessarily mean that the remote host is actually
functioning beyond its ability to echo packets.  tcp is slightly better
at indicating the health of a system than icmp because it uses more
of the networking stack to respond.

Because of a lack of anything better, this module uses its own
routines to pack and unpack ICMP packets.  It would be better for a
separate module to be written which understands all of the different
kinds of ICMP packets.

=head1 AUTHOR(S)

  Current maintainer Net::Ping base code:
    colinm@cpan.org (Colin McMillen)

  Stream protocol:
    bronson@trestle.com (Scott Bronson)

  Original pingecho():
    karrer@bernina.ethz.ch (Andreas Karrer)
    pmarquess@bfsec.bt.co.uk (Paul Marquess)

  Original Net::Ping author:
    mose@ns.ccsn.edu (Russell Mosemann)

  Compatibility porting:
    bbb@cpan.org (Rob Brown)

=head1 COPYRIGHT

Copyright (c) 2001, Colin McMillen.  All rights reserved.
Copyright (c) 2001, Rob Brown.  All rights reserved.

This program is free software; you may redistribute it and/or
modify it under the same terms as Perl itself.

=cut