Perl Script cannot fork more than 10 times - perl

Question

My perl code does not allow more than 10 forks. For the following perl code, whenever I use more than 10 machines in the list of machines read in to the script, the perl script only forks 10 processes for 10 machines and for the rest it dies with error:
SSHProcessError The ssh process was terminated. at serverLogin.pl 44
It dies at the line where it says $ssh->waitfor('The authenticity of host*',15);.
PERL SCRIPT:
#!/usr/bin/perl -w
use Net::SSH::Expect;
use Term::ReadKey;
print "please enter filename:\n";
$filename = ReadLine;
chomp $filename;
print "please enter user ID:\n";
$userID = ReadLine;
chomp $userID;
print "please enter password:\n";
ReadMode 'noecho';
$passwordforuser = ReadLine 0;
chomp $passwordforuser;
ReadMode 'normal';
open READFILE,"<","$filename" or die "Could not open file listofmachines\n";
my #listofmachines = <READFILE>;
foreach $machine (#listofmachines)
{
my $pid=fork();
if ($pid){
push(#childprocs,$pid);
}
elsif ( $pid == 0 ) {
my $ssh = Net::SSH::Expect->new (
host => "$machine",
user => "$userID",
password=> "$passwordforuser",
timeout => 25,
raw_pty => 1,
);
my $login_output = $ssh->run_ssh or die "Could not launch SSH\n";
$ssh->waitfor('The authenticity of host*',15);
#print "This output for machine $machine\n";
$ssh->send("yes");
$ssh->waitfor('password: ', 15);
$ssh->send("$passwordforuser");
$ssh->waitfor('$ ', 10);
my #commresult=$ssh->exec("uptime");
print $login_output;
print #commresult;
exit 0;
}
else {
die "Could not Fork()\n";
}
}
foreach(#childprocs){
waitpid($_, 0)
}
Please help. Thanks, nblu.

Answer 1

Your script using Net::OpenSSH::Parallel instead of Net::SSH::Expect.
The number of simultaneous connections is limited to 10 to overcome any resource exhaustion problem as happening in your script (probably PTYs):
#!/usr/bin/perl -w
use Net::OpenSSH::Parallel;
use Term::ReadKey;
print "please enter filename:\n";
$filename = ReadLine;
chomp $filename;
print "please enter user ID:\n";
$userID = ReadLine;
chomp $userID;
print "please enter password:\n";
ReadMode 'noecho';
$passwordforuser = ReadLine 0;
chomp $passwordforuser;
ReadMode 'normal';
open READFILE,"<","$filename" or die "Could not open file listofmachines\n";
my #listofmachines = <READFILE>;
chomp #listofmachines;
my $pssh = Net::OpenSSH::Parallel->new(connections => 10);
$pssh->add_host($_,
user => $userID, password => $passwordforuser,
master_opts => [-o => 'StrictHostKeyChecking=no'])
for #listofmachines;
sub do_ssh_task {
my ($host, $ssh) = #_;
my $output = $ssh->capture('uptime');
print "$host: $output";
}
$pssh->all(parsub => \&do_ssh_task);
$pssh->run;
for my $host (#listofmachines) {
if (my $error = $pssh->get_error($host)) {
print STDERR "remote task failed for host $host: $error\n";
}
}

Answer 2

By default, the remote ssh daemon limits the number of concurrent ssh connections to something like 10 per userid. If that is a problem for you, you will need to change the server configuration...

Answer 3

Perhaps you have a limit to the number of processes you can create? Can you create 30 or more processes in a loop where the children just sleep(60)?
If in fact you have a limit of how many you can do at once, try using Parallel::ForkManager.
If this is from hitting a limit on pseudoterminals, how you set that depends on kernel version; what does uname -a say? also depends on whether the code is using BSD or SysV/UNIX98 ptys. If you see it opening files like /dev/ptyXY where X is one of a-e or p-z, it's the former, and you will have a hard limit of 256 systemwide.
You can change passwords without a pseudoterminal using usermod instead of passwd, but this momentarily exposes the crypted password in the process list; that may be acceptable in your case.