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Author image Salvador Fandiño García


Net::OpenSSH - Perl SSH client package implemented on top of OpenSSH


  use Net::OpenSSH;

  my $ssh = Net::OpenSSH->new($host);
  $ssh->error and
    die "Couldn't establish SSH connection: ". $ssh->error;

  $ssh->system("ls /tmp") or
    die "remote command failed: " . $ssh->error;

  my @ls = $ssh->capture("ls");
  $ssh->error and
    die "remote ls command failed: " . $ssh->error;

  my ($out, $err) = $ssh->capture2("find /root");
  $ssh->error and
    die "remote find command failed: " . $ssh->error;

  my ($rin, $pid) = $ssh->pipe_in("cat >/tmp/foo") or
    die "pipe_in method failed: " . $ssh->error;

  print $rin, "hello\n";
  close $rin;

  my ($rout, $pid) = $ssh->pipe_out("cat /tmp/foo") or
    die "pipe_out method failed: " . $ssh->error;

  while (<$rout>) { print }
  close $rout;

  my ($in, $out ,$pid) = $ssh->open2("foo");
  my ($pty, $pid) = $ssh->open2pty("foo");
  my ($in, $out, $err, $pid) = $ssh->open3("foo");
  my ($pty, $err, $pid) = $ssh->open3pty("login");

  my $sftp = $ssh->sftp();
  $sftp->error and die "SFTP failed: " . $sftp->error;


Net::OpenSSH is a secure shell client package implemented on top of OpenSSH binary client (ssh).

Under the hood

This package is implemented around the multiplexing feature found in later versions of OpenSSH. That feature allows reuse of a previous SSH connection for running new commands (I believe that OpenSSH 4.1 is the first one to provide all the required functionality).

When a new Net::OpenSSH object is created, the OpenSSH ssh client is run in master mode, establishing a permanent (actually, for the lifetime of the object) connection to the server.

Then, every time a new operation is requested a new ssh process is started in slave mode, effectively reusing the master SSH connection to send the request to the remote side.

Net::OpenSSH Vs Net::SSH::.* modules

Why should you use Net::OpenSSH instead of any of the other Perl SSH clients available?

Well, this is my (biased) opinion:

Net::SSH::Perl is not well maintained nowadays (update: a new maintainer has stepped in so this situation could change!!!), requires a bunch of modules (some of them very difficult to install) to be acceptably efficient and has an API that is limited in some ways.

Net::SSH2 is much better than Net::SSH::Perl, but not completely stable yet. It can be very difficult to install on some specific operative systems and its API is also limited, in the same way as Net::SSH::Perl.

Using Net::SSH::Expect, in general, is a bad idea. Handling interaction with a shell via Expect in a generic way just can not be reliably done.

Net::SSH is just a wrapper around any SSH binary commands available on the machine. It can be very slow as they establish a new SSH connection for every operation performed.

In comparison, Net::OpenSSH is a pure perl module that doesn't have any mandatory dependencies (obviously, besides requiring OpenSSH binaries).

Net::OpenSSH has a very perlish interface. Most operations are performed in a fashion very similar to that of the Perl builtins and common modules (i.e. IPC::Open2).

It is also very fast. The overhead introduced by launching a new ssh process for every operation is not appreciable (at least on my Linux box). The bottleneck is the latency intrinsic to the protocol, so Net::OpenSSH is probably as fast as an SSH client can be.

Being based on OpenSSH is also an advantage: a proved, stable, secure (to paranoic levels), interoperable and well maintained implementation of the SSH protocol is used.

On the other hand, Net::OpenSSH does not work on Windows, not even under Cygwin.

Net::OpenSSH specifically requires the OpenSSH SSH client (AFAIK, the multiplexing feature is not available from any other SSH client). However, note that it will interact with any server software, not just servers running OpenSSH sshd.

For password authentication, IO::Pty has to be installed. Other modules and binaries are also required to implement specific functionality (for instance Net::SFTP::Foreign, Expect or rsync(1)).

Net::OpenSSH and Net::SSH2 do not support version 1 of the SSH protocol.


Optional arguments

Almost all methods in this package accept as first argument an optional reference to a hash containing parameters (\%opts). For instance, these two method calls are equivalent:

  my $out1 = $ssh->capture(@cmd);
  my $out2 = $ssh->capture({}, @cmd);

Error handling

Most methods return undef (or an empty list) to indicate failure.

The "error" method can always be used to explicitly check for errors. For instance:

  my ($output, $errput) = $ssh->capture2({timeout => 1}, "find /");
  $ssh->error and die "ssh failed: " . $ssh->error;

Net::OpenSSH methods

These are the methods provided by the package:

Net::OpenSSH->new($host, %opts)

Creates a new SSH master connection

$host can be a hostname or an IP address. It may also contain the name of the user, her password and the TCP port number where the server is listening:

   my $ssh1 = Net::OpenSSH->new('jack@foo.bar.com');
   my $ssh2 = Net::OpenSSH->new('jack:secret@foo.bar.com:10022');
   my $ssh3 = Net::OpenSSH->new('jsmith@2001:db8::1428:57ab'); # IPv6

IPv6 addresses may optionally be enclosed in brackets:

   my $ssh4 = Net::OpenSSH->new('jsmith@[::1]:1022');

This method always succeeds in returning a new object. Error checking has to be performed explicitly afterwards:

  my $ssh = Net::OpenSSH->new($host, %opts);
  $ssh->error and die "Can't ssh to $host: " . $ssh->error;

If you have problems getting Net::OpenSSH to connect to the remote host read the troubleshooting chapter near the end of this document.

Accepted options:

user => $user_name

Login name

port => $port

TCP port number where the server is running

passwd => $passwd
password => $passwd

User given password for authentication.

Note that using password authentication in automated scripts is a very bad idea. When possible, you should use public key authentication instead.

passphrase => $passphrase

Use given passphrase to open private key.

key_path => $private_key_path

Use the key stored on the given file path for authentication.

ctl_dir => $path

Directory where the SSH master control socket will be created.

This directory and its parents must be writable only by the current effective user or root, otherwise the connection will be aborted to avoid insecure operation.

By default ~/.libnet-openssh-perl is used.

ssh_cmd => $cmd

Name or full path to OpenSSH ssh binary. For instance:

  my $ssh = Net::OpenSSH->new($host, ssh_cmd => '/opt/OpenSSH/bin/ssh');
scp_cmd => $cmd

Name or full path to OpenSSH scp binary.

By default it is inferred from the ssh one.

rsync_cmd => $cmd

Name or full path to rsync binary. Defaults to rsync.

timeout => $timeout

Maximum acceptable time that can elapse without network traffic or any other event happening on methods that are not immediate (for instance, when establishing the master SSH connection or inside methods capture, system, scp_get, etc.).

See also "Timeouts".

kill_ssh_on_timeout => 1

This option tells Net::OpenSSH to kill the local slave SSH process when some operation times out.

See also "Timeouts".

strict_mode => 0

By default, the connection will be aborted if the path to the socket used for multiplexing is found to be non-secure (for instance, when any of the parent directories is writable by other users).

This option can be used to disable that feature. Use with care!!!

async => 1

By default, the constructor waits until the multiplexing socket is available. That option can be used to defer the waiting until the socket is actually used.

For instance, the following code connects to several remote machines in parallel:

  my (%ssh, %ls);
  # multiple connections are stablished in parallel:
  for my $host (@hosts) {
      $ssh{$host} = Net::OpenSSH->new($host, async => 1);
  # then to run some command in all the hosts (sequentially):
  for my $host (@hosts) {
      $ssh{$host}->system('ls /');
master_opts => [...]

Additional options to pass to the ssh command when establishing the master connection. For instance:

  my $ssh = Net::OpenSSH->new($host,
      master_opts => [-o => "ProxyCommand corkscrew httpproxy 8080 $host"]);
default_stdin_fh => $fh
default_stdout_fh => $fh
default_stderr_fh => $fh

Default I/O streams for open_ex and derived methods (currently, that means any method but pipe_in and pipe_out and I plan to remove those exceptions soon!).

For instance:

  open my $stderr_fh, '>>', '/tmp/$host.err' or die ...;
  open my $stdout_fh, '>>', '/tmp/$host.log' or die ...;

  my $ssh = Net::OpenSSH->new($host, default_stderr_fh => $stderr_fh,
                                     default_stdout_fh => $stdout_fh);
  $ssh->error and die "SSH connection failed: " . $ssh->error;

  $ssh->scp_put("/foo/bar*", "/tmp")
    or die "scp failed: " . $ssh->error;
default_stdin_file = $fn
default_stdout_file = $fn
default_stderr_file = $fn

Opens the given filenames and use it as the defaults.

master_stdout_fh => $fh
master_stderr_fh => $fh

Redirect corresponding stdio streams of the master SSH process to given filehandles.

master_stdout_discard => $bool
master_stderr_discard => $bool

Discard corresponding stdio streams.

expand_vars => $bool

Activates variable expansion inside command arguments and file paths.

See "Variable expansion" below.

vars => \%vars

Initial set of variables.

external_master => 1

Instead of launching a new OpenSSH client in master mode, the module tries to reuse an already existent one. ctl_path must also be passed when this option is set. See also "get_ctl_path".


  $ssh = Net::OpenSSH->new('foo', external_master => 1, ctl_path = $path);
default_encoding => $encoding
default_stream_encoding => $encoding
default_argument_encoding => $encoding

Set default encodings. See "Data encoding".


Returns the error condition for the last performed operation.

The returned value is a dualvar as $! (see "$!" in perlvar) that renders an informative message when used in string context or an error number in numeric context (error codes appear in Net::OpenSSH::Constants).


Return the corresponding SSH login parameters.


Returns the path to the socket where the OpenSSH master process listens for new multiplexed connections.

($in, $out, $err, $pid) = $ssh->open_ex(\%opts, @cmd)

Note: this is a low level method that, probably, you don't need to use!

That method starts the command @cmd on the remote machine creating new pipes for the IO channels as specified on the %opts hash.

Returns four values, the first three ($in, $out and $err) correspond to the local side of the pipes created (they can be undef) and the fourth ($pid) to the PID of the new SSH slave process. An empty list is returned on failure.

Note that waitpid has to be used afterwards to reap the slave SSH process.

Accepted options:

stdin_pipe => 1

Creates a new pipe and connects the reading side to the stdin stream of the remote process. The writing side is returned as the first value ($in).

stdin_pty => 1

Similar to stdin_pipe, but instead of a regular pipe it uses a pseudo-tty (pty).

Note that on some OSs (i.e. HP-UX, AIX), ttys are not reliable. They can overflow when large chunks are written or when data is written faster than it is read.

stdin_fh => $fh

Duplicates $fh and uses it as the stdin stream of the remote process.

stdin_file => $filename
stdin_file => \@open_args

Opens the file of the given name for reading and uses it as the remote process stdin stream.

If an array reference is passed its contents are used as the arguments for the underlying open call. For instance:

  $ssh->system({stdin_file => ['-|', 'gzip -c -d file.gz']}, $rcmd);
stdin_discard => 1

Uses /dev/null as the remote process stdin stream.

stdout_pipe => 1

Creates a new pipe and connects the writting side to the stdout stream of the remote process. The reading side is returned as the second value ($out).

stdout_pty => 1

Connects the stdout stream of the remote process to the pseudo-pty. This option requires stdin_pty to be also set.

stdout_fh => $fh

Duplicates $fh and uses it as the stdout stream of the remote process.

stdout_file => $filename
stdout_file => \@open_args

Opens the file of the given filename and redirect stdout there.

stdout_discard => 1

Uses /dev/null as the remote process stdout stream.

stdinout_socket => 1

Creates a new socketpair, attachs the stdin an stdout streams of the slave SSH process to one end and returns the other as the first value ($in) and undef for the second ($out).


  my ($socket, undef, undef, $pid) = $ssh->open_ex({stdinout_socket => 1},
                                                   '/bin/netcat $dest');

See also "open2socket".

stdinout_dpipe => $cmd
stdinout_dpipe => \@cmd

Runs the given command locally attaching its stdio streams to those of the remote SSH command. Conceptually it is equivalent to the dpipe(1) shell command.

stderr_pipe => 1

Creates a new pipe and connects the writting side to the stderr stream of the remote process. The reading side is returned as the third value ($err).


  my $pid = $ssh->open_ex({stdinout_dpipe => 'vncviewer -stdio'},
                          x11vnc => '-inetd');
stderr_fh => $fh

Duplicates $fh and uses it as the stderr stream of the remote process.

stderr_file => $filename

Opens the file of the given name and redirects stderr there.

stderr_to_stdout => 1

Makes stderr point to stdout.

tty => $bool

Tells ssh to allocate a pseudo-tty for the remote process. By default, a tty is allocated if remote command stdin stream is attached to a tty.

When this flag is set and stdin is not attached to a tty, the ssh master and slave processes may generate spurious warnings about failed tty operations. This is caused by a bug present in older versions of OpenSSH.

close_slave_pty => 0

When a pseudo pty is used for the stdin stream, the slave side is automatically closed on the parent process after forking the ssh command.

This option dissables that feature, so that the slave pty can be accessed on the parent process as $pty->slave. It will have to be explicitly closed (see IO::Pty)

quote_args => $bool

See "Shell quoting" below.

ssh_opts => \@opts

List of extra options for the ssh command.

This feature should be used with care, as the given options are not checked in any way by the module, and they could interfere with it.

tunnel => $bool

Instead of executing a command in the remote host, this option instruct Net::OpenSSH to create a TCP tunnel. The arguments become the target IP and port.


  my ($in, $out, undef, $pid) = $ssh->open_ex({tunnel => 1}, $IP, $port);

See also "Tunnels".

encoding => $encoding
argument_encoding => $encoding

Set encodings. See "Data encoding".

Usage example:

  # similar to IPC::Open2 open2 function:
  my ($in_pipe, $out_pipe, undef, $pid) = 
      $ssh->open_ex( { stdin_pipe => 1,
                       stdout_pipe => 1 },
                     @cmd )
      or die "open_ex failed: " . $ssh->error;
  # do some IO through $in/$out
  # ...
$ssh->system(\%opts, @cmd)

Runs the command @cmd on the remote machine.

Returns true on sucess, undef otherwise.

The error status is set to OSSH_SLAVE_CMD_FAILED when the remote command exits with a non zero code (the code is available from $?, see "$?" in perlvar).


  $ssh->system('ls -R /')
    or die "ls failed: " . $ssh->error";

As for system builtin, SIGINT and SIGQUIT signals are blocked. (see "system" in perlfunc). Also, setting $SIG{CHLD} to IGNORE or to a custom signal handler will interfere with this method.

Accepted options:

stdin_data => $input
stdin_data => \@input

Sends the given data through the stdin stream to the remote process.

For example, the following code creates a file on the remote side:

  $ssh->system({stdin_data => \@data}, "cat >/tmp/foo")
    or die "unable to write file: " . $ssh->error;
timeout => $timeout

The operation is aborted after $timeout seconds elapsed without network activity.

See also "Timeouts".

async => 1

Does not wait for the child process to exit. The PID of the new process is returned.

Note that when this option is combined with stdin_data, the given data will be transferred to the remote side before returning control to the caller.

See also the "spawn" method documentation below.

stdin_fh => $fh
stdin_discard => $bool
stdout_fh => $fh
stdout_discard => $bool
stderr_fh => $fh
stderr_discard => $bool
stderr_to_stdout => $bool
stdinout_dpipe => $cmd
tty => $bool

See the "open_ex" method documentation for an explanation of these options.

$ok = $ssh->test(\%opts, @cmd);

Runs the given command and returns its success/failure exit status as 1 or 0 respectively. Returns undef when something goes wrong in the SSH layer.

Error status is not set to OSSH_SLAVE_CMD_FAILED when the remote command exits with a non-zero code.

By default this method discards the remote command stdout and sterr streams.

Usage example:

  if ($ssh->test(ps => -C => $executable)) {
    say "$executable is running on remote machine"
  else {
    die "something got wrong: ". $ssh->error if $ssh->error;

    say "$executable is not running on remote machine"

This method support the same set of options as system, except async and tunnel.

$output = $ssh->capture(\%opts, @cmd);
@output = $ssh->capture(\%opts, @cmd);

This method is conceptually equivalent to the perl backquote operator (i.e. `ls`): it runs the command on the remote machine and captures its output.

In scalar context returns the output as a scalar. In list context returns the output broken into lines (it honors $/, see "$/" in perlvar).

When an error happens while capturing (for instance, the operation times out), the partial captured output will be returned. Error conditions have to be explicitly checked using the "error" method. For instance:

  my $output = $ssh->capture({ timeout => 10 },
                             "echo hello; sleep 20; echo bye");
  $ssh->error and
      warn "operation didn't complete successfully: ". $ssh->error;
  print $output;

Setting $SIG{CHLD} to a custom signal handler or to IGNORE will interfere with this method.

Accepted options:

stdin_data => $input
stdin_data => \@input
timeout => $timeout

See "Timeouts".

stdin_fh => $fh
stdin_discard => $bool
stderr_fh => $fh
stderr_discard => $bool
stderr_to_stdout => $bool
tty => $bool

See the "open_ex" method documentation for an explanation of these options.

($output, $errput) = $ssh->capture2(\%opts, @cmd)

captures the output sent to both stdout and stderr by @cmd on the remote machine.

Setting $SIG{CHLD} to a custom signal handler or to IGNORE will also interfere with this method.

The accepted options are:

stdin_data => $input
stdin_data => \@input

See the "system" method documentation for an explanation of these options.

timeout => $timeout

See "Timeouts".

stdin_fh => $fh
stdin_discard => $bool
tty => $bool

See the "open_ex" method documentation for an explanation of these options.

($in, $pid) = $ssh->pipe_in(\%opts, @cmd)

This method is similar to the following Perl open call

  $pid = open $in, '|-', @cmd

but running @cmd on the remote machine (see "open" in perlfunc).

No options are currently accepted.

There is no need to perform a waitpid on the returned PID as it will be done automatically by perl when $in is closed.


  my ($in, $pid) = $ssh->pipe_in('cat >/tmp/fpp')
      or die "pipe_in failed: " . $ssh->error;
  print $in $_ for @data;
  close $in or die "close failed";
($out, $pid) = $ssh->pipe_out(\%opts, @cmd)

Reciprocal to previous method, it is equivalent to

  $pid = open $out, '-|', @cmd

running @cmd on the remote machine.

No options are currently accepted.

($in, $out, $pid) = $ssh->open2(\%opts, @cmd)
($pty, $pid) = $ssh->open2pty(\%opts, @cmd)
($socket, $pid) = $ssh->open2socket(\%opts, @cmd)
($in, $out, $err, $pid) = $ssh->open3(\%opts, @cmd)
($pty, $err, $pid) = $ssh->open3pty(\%opts, @cmd)

Shortcuts around "open_ex" method.

$pid = $ssh->spawn(\%opts, @_)

Another "open_ex" shortcut, it launches a new remote process in the background and returns the PID of the local slave SSH process.

At some later point in your script, waitpid should be called on the returned PID in order to reap the slave SSH process.

For instance, you can run some command on several hosts in parallel with the following code:

  my %conn = map { $_ => Net::OpenSSH->new($_, async => 1) } @hosts;
  my @pid;
  for my $host (@hosts) {
      open my($fh), '>', "/tmp/out-$host.txt"
        or die "unable to create file: $!";
      push @pid, $conn{$host}->spawn({stdout_fh => $fh}, $cmd);

  waitpid($_, 0) for @pid;

Note that spawn shouldn't be used to start detached remote processes that may survive the local program (see also the "FAQ" about running remote processes detached).

($socket, $pid) = $ssh->open_tunnel(\%opts, $dest_host, $port)

Similar to "open2socket", but instead of running a command, it opens a TCP tunnel to the given address. See also "Tunnels".

$out = $ssh->capture_tunnel(\%opts, $dest_host, $port)
@out = $ssh->capture_tunnel(\%opts, $dest_host, $port)

Similar to "capture", but instead of running a command, it opens a TCP tunnel.


  $out = $ssh->capture_tunnel({stdin_data => join("\r\n",
                                                  "GET / HTTP/1.0",
                                                  "Host: www.perl.org",
                                                  "", "") },
                              'www.perl.org', 80)

See also "Tunnels".

$ssh->scp_get(\%opts, $remote1, $remote2,..., $local_dir_or_file)
$ssh->scp_put(\%opts, $local, $local2,..., $remote_dir_or_file)

These two methods are wrappers around the scp command that allow transfers of files to/from the remote host using the existing SSH master connection.

When transferring several files, the target argument must point to an existing directory. If only one file is to be transferred, the target argument can be a directory or a file name or can be ommited. For instance:

  $ssh->scp_get({glob => 1}, '/var/tmp/foo*', '/var/tmp/bar*', '/tmp');

Both "scp_get" and "scp_put" methods return a true value when all the files are transferred correctly, otherwise they return undef.

Accepted options:

quiet => 0

By default, scp is called with the quiet flag -q enabled in order to suppress progress information. This option allows reenabling the progress indication bar.

recursive => 1

Copy files and directories recursively.

glob => 1

Allow expansion of shell metacharacters in the sources list so that wildcards can be used to select files.

glob_flags => $flags

Second argument passed to File::Glob::bsd_glob function. Only available for "scp_put" method.

copy_attrs => 1

Copies modification and access times and modes from the original files.

bwlimit => $Kbits

Limits the used bandwith, specified in Kbit/s.

timeout => $secs

The transfer is aborted if the connection does not finish before the given timeout elapses. See also "Timeouts".

async => 1

Doesn't wait for the scp command to finish. When this option is used, the method returns the PID of the child scp process.

For instance, it is possible to transfer files to several hosts in parallel as follows:

  use Errno;
  my (%pid, %ssh);
  for my $host (@hosts) {
    $ssh{$host} = Net::OpenSSH->new($host, async => 1);
  for my $host (@hosts) {
    $pid{$host} = $ssh{$host}->scp_put({async => 1}, $local_fn, $remote_fn)
      or warn "scp_put to $host failed: " . $ssh{$host}->error . "\n";
  for my $host (@hosts) {
    if (my $pid = $pid{$host}) {
      if (waitpit($pid, 0) > 0) {
        my $exit = ($? >> 8);
        $exit and warn "transfer of file to $host failed ($exit)\n";
      else {
        redo if ($! == EINTR);
        warn "waitpid($pid) failed: $!\n";
stdout_fh => $fh
stderr_fh => $fh
stderr_to_stdout => 1

These options are passed unchanged to method "open_ex", allowing capture of the output of the scp program.

Note that scp will not generate progress reports unless its stdout stream is attached to a tty.

$ssh->rsync_get(\%opts, $remote1, $remote2,..., $local_dir_or_file)
$ssh->rsync_put(\%opts, $local1, $local2,..., $remote_dir_or_file)

These methods use rsync over SSH to transfer files from/to the remote machine.

They accept the same set of options as the SCP ones.

Any unrecognized option will be passed as an argument to the rsync command (see rsync(1)). Underscores can be used instead of dashes in rsync option names.

For instance:

  $ssh->rsync_get({exclude => '*~',
                   verbose => 1,
                   safe_links => 1},
                  '/remote/dir', '/local/dir');
$sftp = $ssh->sftp(%sftp_opts)

Creates a new Net::SFTP::Foreign object for SFTP interaction that runs through the ssh master connection.

@call = $ssh->make_remote_command(%opts, @cmd)
$call = $ssh->make_remote_command(\%opts, @cmd)

This method returns the arguments required to execute a command on the remote machine via SSH. For instance:

  my @call = $ssh->make_remote_command(ls => "/var/log");
  system @call;

In scalar context, returns the arguments quoted and joined into one string:

  my $remote = $ssh->make_remote_comand("cd /tmp/ && tar xf -");
  system "tar cf - . | $remote";

When the connection has been established by calling the constructor with the async option, this call allows to advance the process.

If $async is true, it will perform any work that can be done inmediately without waiting (for instance, entering the password or checking for the existence of the multiplexing socket) and then return. If a false value is given, it will finalize the connection process and wait until the multiplexing socket is available.

It returns a true value after the connection has been succesfully established. False is returned if the connection process fails or if it has not yet completed (then, the "error" method can be used to distinguish between both cases).


This method runs several checks to ensure that the master connection is still alive.


Returns the list of arguments quoted so that they will be restored to their original form when parsed by the remote shell.

In scalar context returns the list of arguments quoted and joined.

Usually this task is done automatically by the module. See "Shell quoting" below.

This method can also be used as a class method.


  my $quoted_args = Net::OpenSSH->shell_quote(@args);
  system('ssh', '--', $host, $quoted_args);

This method is like the previous shell_quote but leaves wildcard characters unquoted.

It can be used as a class method also.


Enables/disables variable expansion feature (see "Variable expansion").


Returns current state of variable expansion feature.

$ssh->set_var($name, $value)
$ssh->get_var($name, $value)

These methods allow to change and to retrieve the value of the logical value of the given name.


Returns the PID of the master SSH process


This methods allows to tell the module that the master process has exited when we get its PID from some external wait or waitpid call. For instance:

  my $ssh = Net::OpenSSH->new('foo', async => 1);

  # create new processes
  # ...

  # rip them...
  my $master_pid = $ssh->master_pid;
  while ((my $pid = wait) > 0) {
    if ($pid == $master_pid) {

If your program rips the master process and this method is not called, the OS could reassign the PID to a new unrelated process and the module would try to kill it at object destruction time.

Shell quoting

By default, when invoking remote commands, this module tries to mimic perl system builtin in regard to argument processing. Quoting "system" in perlfunc:

  Argument processing varies depending on the number of arguments.  If
  there is more than one argument in LIST, or if LIST is an array with
  more than one value, starts the program given by the first element
  of the list with arguments given by the rest of the list.  If there
  is only one scalar argument, the argument is checked for shell
  metacharacters, and if there are any, the entire argument is passed
  to the system's command shell for parsing (this is "/bin/sh -c" on
  Unix platforms, but varies on other platforms).

Take for example Net::OpenSSH "system" method:

  $ssh->system("ls -l *");
  $ssh->system('ls', '-l', '/');

The first call passes the argument unchanged to ssh and it is executed in the remote side through the shell which interprets metacharacters.

The second call escapes any shell metacharacters so that, effectively, it is equivalent to calling the command directly and not through the shell.

Under the hood, as the Secure Shell protocol does not provide for this mode of operation and always spawns a new shell where it runs the given command, Net::OpenSSH quotes any shell metacharacters in the comand list.

All the methods that invoke a remote command (system, open_ex, etc.) accept the option quote_args that allows to force/disable shell quoting.

For instance:

  $ssh->system({quote_args => 1}, "/path with spaces/bin/foo");

will correctly handle the spaces in the program path.

The shell quoting mechanism implements some extensions (for instance, performing redirections to /dev/null on the remote side) that can be dissabled with the option quote_args_extended:

  $ssh->system({ stderr_discard => 1,
                 quote_args => 1, quote_args_extended => 0 },

The option quote_args can also be used to disable quoting when more than one argument is passed. For instance, to get some pattern expanded by the remote shell:

  $ssh->system({quote_args => 0}, 'ls', '-l', "/tmp/files_*.dat");

The method shell_quote can be used to selectively quote some arguments and leave others untouched:

  $ssh->system({quote_args => 0},
               $ssh->shell_quote('ls', '-l'),

When the glob option is set in scp and rsync file transfer methods, an alternative quoting method that knows about file wildcards and passes them unquoted is used. The set of wildcards recognized currently is the one supported by bash(1).

Another way to selectively use quote globing or fully disable quoting for some specific arguments is to pass them as scalar references or double scalar references respectively. In practice, that means prepending them with one or two backslashes. For instance:

  # quote the last argument for globing:
  $ssh->system('ls', '-l', \'/tmp/my files/filed_*dat');

  # append a redirection to the remote command
  $ssh->system('ls', '-lR', \\'>/tmp/ls-lR.txt');

  # expand remote shell variables and glob in the same command:
  $ssh->system('tar', 'czf', \\'$HOME/out.tgz', \'/var/log/server.*.log');

As shell quoting is a tricky matter, I expect bugs to appear in this area. You can see how ssh is called, and the quoting used setting the following debug flag:

  $Net::OpenSSH::debug |= 16;

Also, the current shell quoting implementation expects a shell compatible with Unix sh in the remote side. It will not work as expected if for instance, the remote machine runs Windows, VMS or it is a router.

As a workaround, do any required quoting yourself and pass the quoted command as a string so that no further quoting is performed. For instance:

  # for VMS
  $ssh->system('DIR/SIZE NFOO::USERS:[JSMITH.DOCS]*.TXT;0');

I plan to add support for different quoting mechanisms in the future... if you need it now, just ask for it!!!

The current quoting mechanism does not handle possible aliases defined by the remote shell. In that case, to force execution of the command instead of the alias, the full path to the command must be used.


In order to stop remote processes when they timeout, the ideal aproach would be to send them signals through the SSH connection as specified by the protocol standard.

Unfortunatelly OpenSSH does not implement that feature so Net::OpenSSH has to use other imperfect approaches:

  • close slave I/O streams

    Closing the STDIN and STDOUT streams of the unresponsive remote process will effectively deliver a SIGPIPE when it tries to access any of them.

    Remote processes may not access STDIN or STDOUT and even them, Net::OpenSSH can only close these channels when it is capturing them, so this approach does not always work.

  • killing the local SSH slave process

    This action may leave the remote process running, creating a remote orphan so Net::OpenSSH does not use it unless the construction option kill_ssh_on_timeout is set.

Luckily, future versions of OpenSSH will support signaling remote processes via the mux channel.

Variable expansion

The variable expansion feature allows to define variables that are expanded automatically inside command arguments and file paths.

This feature is disabled by default. It is intended to be used with Net::OpenSSH::Parallel and other similar modules.

Variables are delimited by a pair of percent signs (%), for instance %HOST%. Also, two consecutive percent signs are replaced by a single one.

The special variables HOST, USER and PORT are maintained internally by the module and take the obvious values.

Variable expansion is performed before shell quoting (see "Shell quoting").

Some usage example:

  my $ssh = Net::OpenSSH->new('server.foo.com', expand_vars => 1);
  $ssh->set_var(ID => 42);
  $ssh->system("ls >/tmp/ls.out-%HOST%-%ID%");

will redirect the output of the ls command to /tmp/ls.out-server.foo.com-42 on the remote host.


Besides running commands on the remote host, Net::OpenSSH also allows to tunnel TCP connections to remote machines reachable from the SSH server.

That feature is made available through the tunnel option of the "open_ex" method, and also through wrapper methods "open_tunnel" and "capture_tunnel" and most others where it makes sense.


  $ssh->system({tunnel => 1,
                stdin_data => "GET / HTTP/1.0\r\n\r\n",
                stdout_file => "/tmp/$server.res"},
               $server, 80)
      or die "unable to retrieve page: " . $ssh->error;

or capturing the output of several requests in parallel:

  my @pids;
  for (@servers) {
    my $pid = $ssh->spawn({tunnel => 1,
                           stdin_file => "/tmp/request.req",
                           stdout_file => "/tmp/$_.res"},
                          $_, 80);
    if ($pid) {
      push @pids, $pid;
    else {
      warn "unable to spawn tunnel process to $_: " . $ssh->error;
  waitpid ($_, 0) for (@pids);

Under the hood, in order to create a tunnel, a new ssh process is spawned with the option -W${address}:${port} (available from OpenSSH 5.4 and upwards) making it redirect its stdio streams to the remote given address. Unlike when ssh -L options is used to create tunnels, no TCP port is opened on the local machine at any time so this is a perfectly secure operation.

The PID of the new process is returned by the named methods. It must be reaped once the pipe or socket handlers for the local side of the tunnel have been closed.

OpenSSH 5.4 or later is required for the tunnels functionality to work. Also, note that tunnel forwarding may be administratively forbidden at the server side (see sshd(8) and sshd_config(5) or the documentation provided by your SSH server vendor).

Data encoding

Net::OpenSSH has some support for transparently converting the data send or received from the remote server to Perl internal unicode representation.

The methods supporting that feature are those that move data from/to Perl data structures (i.e. capture, capture2, capture_tunnel and methods supporting the stdin_data option). Data accessed through pipes, sockets or redirections is not affected by the encoding options.

It is also possible to set the encoding of the command and arguments passed to the remote server on the command line.

By default, if no encoding option is given on the constructor or on the method calls, Net::OpenSSH will not perform any encoding transformation, effectively processing the data as latin1.

When data can not be converted between the Perl internal representation and the selected encoding inside some Net::OpenSSH method, it will fail with an OSSH_ENCODING_ERROR error.

The supported encoding options are as follows:

stream_encoding => $encoding

sets the encoding of the data send and received on capture methods.

argument_encoding => $encoding

sets the encoding of the command line arguments

encoding => $encoding

sets both argument_encoding and stream_encoding.

The constructor also accepts default_encoding, default_stream_encoding and default_argument_encoding that set the defaults.



Sometimes you would like to use Expect to control some program running in the remote host. You can do it as follows:

  my ($pty, $pid) = $ssh->open2pty(@cmd)
      or die "unable to run remote command @cmd";
  my $expect = Expect->init($pty);

Then, you will be able to use the new Expect object in $expect as usual.

mod_perl and mod_perl2

mod_perl and mod_perl2 tie STDIN and STDOUT to objects that are not backed up by real file descriptors at the operative system level. Net::OpenSSH will fail if any of these handles is used explicetly or implicitly when calling some remote command.

The workaround is to redirect them to /dev/null or to some file:

  open my $def_in, '<', '/dev/null' or die "unable to open /dev/null";
  my $ssh = Net::OpenSSH->new($host,
                              default_stdin_fh => $def_in);

  my $out = $ssh->capture($cmd1);
  $ssh->system({stdout_discard => 1}, $cmd2);
  $ssh->system({stdout_to_file => '/tmp/output'}, $cmd3);

Also, note that from a security stand point, running ssh from inside the webserver process is not a great idea. An attacker exploiting some Apache bug would be able to access the ssh keys and passwords and gain unlimited access to the remote systems.

If you can, use a queue (as TheSchwartz) or any other mechanism to execute the ssh commands from another process running under a different user account.

At a minimum, ensure that ~www-data/.ssh (or similar) is not accessible through the web server!

Other modules

CPAN contains several modules that rely on SSH to perform their duties as for example IPC::PerlSSH or GRID::Machine.

Often, it is possible to instruct them to go through a Net::OpenSSH multiplexed connection employing some available constructor option. For instance:

  use Net::OpenSSH;
  use IPC::PerlIPC;
  my $ssh = Net::OpenSSH->new(...);
  $ssh->error and die "unable to connect to remote host: " . $ssh->error;
  my @cmd = $ssh->make_remote_command('/usr/bin/perl');
  my $ipc = IPC::PerlSSH->new(Command => \@cmd);
  my @r = $ipc->eval('...');


  use GRID::Machine;
  my @cmd = $ssh->make_remote_command('/usr/bin/perl');
  my $grid = GRID::Machine->new(command => \@cmd);
  my $r = $grid->eval('print "hello world!\n"');

In other cases, some kind of plugin mechanism is provided by the 3rd party modules to allow for different transports. The method open2 may be used to create a pair of pipes for transport in these cases.


Usually, Net::OpenSSH works out of the box, but when it fails, some users have a hard time finding the cause of the problem. This mini troubleshooting guide should help you to find and solve it.

1 - check the error message

Add in your script, after the Net::OpenSSH constructor call, an error check:

  $ssh = Net::OpenSSH->new(...);
  $ssh->error and die "SSH connection failed: " . $ssh->error;

The error message will tell what has gone wrong.

2 - OpenSSH version

Ensure that you have a version of ssh recent enough:

  $ ssh -V
  OpenSSH_5.1p1 Debian-5, OpenSSL 0.9.8g 19 Oct 2007

OpenSSH version 4.1 was the first to support the multiplexing feature and is the minimal required by the module to work. I advise you to use the latest OpenSSH (currently 5.8) or at least a more recent version.

The ssh_cmd constructor option lets you select the ssh binary to use. For instance:

  $ssh = Net::OpenSSH->new($host,
                           ssh_cmd => "/opt/OpenSSH/5.8/bin/ssh")

Some hardware vendors (i.e. Sun) include custom versions of OpenSSH bundled with the operative system. In priciple, Net::OpenSSH should work with these SSH clients as long as they are derived from some version of OpenSSH recent enough. Anyway, I advise you to use the real OpenSSH software if you can!

3 - run ssh from the command line

Check you can connect to the remote host using the same parameters you are passing to Net::OpenSSH. In particular, ensure that you are running ssh as the same local user.

If you are running your script from a webserver, the user would probably be www, apache or something alike.

Common problems are:

  • Remote host public key not present in known_hosts file.

    The SSH protocol uses public keys to identify the remote hosts so that they can not be supplanted by some malicious third parties.

    For OpenSSH, usually the server public key is stored in /etc/ssh/ssh_host_dsa_key.pub or in /etc/ssh/ssh_host_rsa_key.pub and that key should be copied into the ~/.ssh/known_hosts file in the local machine (other SSH implementations may use other file locations).

    Maintaining the server keys when several hosts and clients are involved may be somewhat inconvenient, so most SSH clients, by default, when a new connection is stablished to a host whose key is not in the known_hosts file, show the key and ask the user if he wants the key copied there.

  • Wrong remote host public key in known_hosts file.

    This is another common problem that happens when some server is replaced or reinstalled from scratch and its public key changes becomming different to that installed on the known_hosts file.

    The easiest way to solve that problem is to remove the old key from the known_hosts file by hand using any editor and then to connect to the server replying yes when asked to save the new key.

  • Wrong permissions for the ~/.ssh directory or its contents.

    OpenSSH client performs several checks on the access permissions of the ~/.ssh directory and its contents and refuses to use them when misconfigured. See the FILES section from the ssh(1) man page.

  • Incorrect settings for password or public key authentication.

    Check that you are using the right password or that the user public key is correctly installed on the server.

4 - security checks on the multiplexing socket

Net::OpenSSH performs some security checks on the directory where the multiplexing socket is going to be placed to ensure that it can not be accessed by other users.

The default location for the multiplexing socket is under ~/.libnet-openssh-perl. It can be changed using the ctl_dir and ctl_path constructor arguments.

The requirements for that directory and all its parents are:

  • They have to be owned by the user executing the script or by root

  • Their permission masks must be 0755 or more restrictive, so nobody else has permissions to perform write operations on them.

The constructor option strict_mode disables these security checks, but you should not use it unless you understand its implications.


Frequent questions about the module:

Connecting to switches, routers, etc.

Q: I can't get the method system, capture, etc., to work when connecting to some router, switch, etc. What I am doing wrong?

A: Roughly, the SSH protocol allows for two modes of operation: command mode and interactive mode.

Command mode is designed to run single commands on the remote host. It opens an SSH channel between both hosts, ask the remote computer to run some given command and when it finnish the channel is closed. It is what you get, for instance, when you run something as...

  $ ssh my.unix.box cat foo.txt

... and it is also the way Net::OpenSSH runs commands on the remote host.

Interactive mode launches a shell on the remote hosts with its stdio streams redirected to the local ones so that the user can transparently interact with it.

Some devices (as probably the one you are using) do not run an standard, general purpose shell (i.e. bash, csh or ksh) but some custom program specially targeted and limited to the task of configuring the device.

Usually, the SSH server running on these devices does not support command mode. It unconditionally attachs the restricted shell to any incomming SSH connection and waits for the user to enter commands through the redirected stdin stream.

The only way to workaround this limitation is to make your script talk to the restricted shell (1-open a new SSH session, 2-wait for the shell prompt, 3-send a command, 4-read the output until you get to the shell prompt again, repeat from 3). The best tool for this task is probably Expect, used alone, as wrapped by Net::SSH::Expect or combined with Net::OpenSSH (see "Expect").

There are some devices that support command mode but that only accept one command per connection. In that cases, using Expect is also probably the best option.

Connection fails

Q: I am unable to make the module connect to the remote host...

A: Have you read the trubleshooting section? (see "TROUBLESHOOTING").

Disable StrictHostKeyChecking

Q: Why don't you run ssh with StrictHostKeyChecking=no?

A: Using StrictHostKeyChecking=no relaxes the default security level of SSH and it will be relatively easy to end with a misconfigured SSH (for instance, when known_hosts is unwriteable) that could be forged to connect to a bad host in order to perform man-in-the-middle attacks, etc.

I advice you to do not use that option unless you fully understand its implications from a security point of view.

If you want to use it anyway, past it to the constructor:

  $ssh = Net::OpenSSH->new($host,
           master_opts => [-o => "StrictHostKeyChecking=no"],
child process 14947 does not exist: No child processes

Q: Calls to system, capture or capture2 fail with the previous error, what I am doing wrong?

A: That usually happens when $SIG{CHLD} is set to IGNORE or to some custom handler reaping child processes by itself. In order to solve the problem just disable the handler during the method call:

  local $SIG{CHLD};
child process STDIN/STDOUT/STDERR is not a real system file handle

Q: Calls to system, capture, etc. fail with the previous error, what's happening?

A: The reported stdio stream is closed or is not attached to a real file handle (i.e. it is a tied handle). Redirect it to /dev/null or to a real file:

  my $out = $ssh->capture({stdin_discard => 1, stderr_to_stdout => 1},

See also the mod_perl entry above.

Solaris (and AIX and probably others)

Q: I was trying Net::OpenSSH on Solaris and seem to be running into an issue...

A: The SSH client bundled with Solaris is an early fork of OpenSSH that does not provide the multiplexing functionality required by Net::OpenSSH. You will have to install the OpenSSH client.

Precompiled packages are available from Sun Freeware (http://www.sunfreeware.com). There, select your OS version an CPU architecture, download the OpenSSH package and its dependencies and install them. Note that you do not need to configure Solaris to use the OpenSSH server sshd.

Ensure that OpenSSH client is in your path before the system ssh or alternatively, you can hardcode the full path into your scripts as follows:

  $ssh = Net::OpenSSH->new($host,
                           ssh_cmd => '/usr/local/bin/ssh');

AIX and probably some other unixen, also bundle SSH clients lacking the multiplexing functionality and require installation of the real OpenSSH.

Can't change working directory

Q: I want to run some command inside a given remote directory but I am unable to change the working directory. For instance:

  $ssh->system('cd /home/foo/bin');

does not list the contents of /home/foo/bin.

What am I doing wrong?

A: Net::OpenSSH (and, for that matter, all the SSH modules available from CPAN but Net::SSH::Expect) runs every command in a new session so most shell builtins that are run for its side effects become useless (i.e. cd, export, ulimit, umask, etc., usually, you can list them running help from the shell).

A work around is to combine several commands in one, for instance:

  $ssh->system('cd /home/foo/bin && ls');

Note the use of the shell && operator instead of ; in order to abort the command as soon as any of the subcommands fail.

Also, several commands can be combined into one while still using the multi-argument quoting feature as follows:

  $ssh->system(@cmd1, \\'&&', @cmd2, \\'&&', @cmd3, ...);
Running detached remote processes

Q: I need to be able to ssh into several machines from my script, launch a process to run in the background there, and then return immediately while the remote programs keep running...

A: If the remote systems run some Unix/Linux variant, the right approach is to use nohup(1) that will disconnect the remote process from the stdio streams and to ask the shell to run the command on the background. For instance:

  $ssh->system("nohup $long_running_command &");

Also, it may be possible to demonize the remote program. If it is written in Perl you can use App::Daemon for that (actually, there are several CPAN modules that provided that kind of functionality).

In any case, note that you shouldn't use "spawn" for that.


OpenSSH client documentation ssh(1), ssh_config(5), the project web http://www.openssh.org and its FAQ http://www.openbsd.org/openssh/faq.html. scp(1) and rsync(1). The OpenSSH Wikibook http://en.wikibooks.org/wiki/OpenSSH.

Core perl documentation perlipc, "open" in perlfunc, "waitpid" in perlfunc.

IO::Pty to known how to use the pseudo tty objects returned by several methods on this package.

Net::SFTP::Foreign provides a compatible SFTP implementation.

Expect can be used to interact with commands run through this module on the remote machine (see also the expect.pl and <autosudo.pl> scripts in the sample directory).

SSH::OpenSSH::Parallel is an advanced scheduler that allows to run commands in remote hosts in parallel. It is obviously based on Net::OpenSSH.

SSH::Batch allows to run remote commands in parallel in a cluster. It is build on top on Net::OpenSSH also.

Other Perl SSH clients: Net::SSH::Perl, Net::SSH2, Net::SSH, Net::SSH::Expect, Net::SCP.

Net::OpenSSH::Compat is a package offering a set of compatibility layers for other SSH modules on top of Net::OpenSSH.

IPC::PerlSSH, GRID::Machine allow execution of Perl code in remote machines through SSH.

SSH::RPC implements an RPC mechanism on top of SSH using Net::OpenSSH to handle the connections.


Support for data encoding is highly experimental.

Support for passphrase handling is experimental.

Support for taint mode is experimental.

Tested on Linux, OpenBSD, NetBSD and Solaris with OpenSSH 5.1 to 5.8.

Net::OpenSSH does not work on Windows. OpenSSH multiplexing feature requires passing file handles through sockets something that is not supported by any version of Windows.

It doesn't work on VMS either... well, probably, it doesn't work on anything not resembling a modern Linux/Unix OS.

To report bugs send an email to the address that appear below or use the CPAN bug tracking system at http://rt.cpan.org.

Post questions related to how to use the module in Perlmonks http://perlmoks.org/, you will probably get faster responses than if you address me directly and I visit Perlmonks quite often, so I will see your question anyway.

The source code of this module is hosted at GitHub: http://github.com/salva/p5-Net-OpenSSH.

Commercial support

Commercial support, professional services and custom software development around this module are available through my current company. Drop me an email with a rough description of your requirements and we will get back to you ASAP.

My wishlist

If you like this module and you're feeling generous, take a look at my Amazon Wish List: http://amzn.com/w/1WU1P6IR5QZ42.

Also consider contributing to the OpenSSH project this module builds upon: http://www.openssh.org/donations.html.


- *** add tests for scp, rsync and sftp methods

- *** add support for more target OSs (quoting, OpenVMS, Windows & others)

- better timeout handling in system and capture methods

- make "pipe_in" and "pipe_out" methods "open_ex" based

- add scp_cat and similar methods

- async disconnect

- currently wait_for_master does not honor timeout

Send your feature requests, ideas or any feedback, please!


Copyright (C) 2008-2011 by Salvador Fandiño (sfandino@yahoo.com)

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.10.0 or, at your option, any later version of Perl 5 you may have available.