Provided by: libnet-openssh-perl_0.74-1_all 
NAME
Net::OpenSSH - Perl SSH client package implemented on top of OpenSSH
SYNOPSIS
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;
DESCRIPTION
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 one to run several sessions over a single SSH connection
(OpenSSH 4.1 was 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 persistent (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 operating 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 does not 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 (e.g. 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 paranoid
levels), inseparably 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.
API
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
password => $password
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
Uses given passphrase to open private key.
key_path => $private_key_path
Uses the key stored on the given file path for authentication.
gateway => $gateway
If the given argument is a gateway object as returned by "find_gateway" in
Net::OpenSSH::Gateway method, use it to connect to the remote host.
If it is a hash reference, call the "find_gateway" method first.
For instance, the following code fragments are equivalent:
my $gateway = Net::OpenSSH::Gateway->find_gateway(
proxy => 'http://proxy.corporate.com');
$ssh = Net::OpenSSH->new($host, gateway => $gateway);
and
$ssh = Net::OpenSSH->new($host,
gateway => { proxy => 'http://proxy.corporate.com'});
proxy_command => $proxy_command
Use the given command to establish the connection to the remote host (see
"ProxyCommand" on ssh_config(5)).
batch_mode => 1
Disables querying the user for password and passphrases.
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".
remote_shell => $name
Name of the remote shell. Used to select the argument quoter backend.
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 established 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 /');
}
connect => 0
Do not launch the master SSH process yet.
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_ssh_opts => [...]
Default slave SSH command line options for "open_ex" and derived methods.
For instance:
my $ssh = Net::OpenSSH->new($host,
default_ssh_opts => [-o => "ConnectionAttempts=0"]);
forward_agent => 1
Enables forwarding of the authentication agent.
This option can not be used when passing a passphrase (via "passphrase") to unlock
the login private key.
Note that Net::OpenSSH will not run "ssh-agent" for you. This has to be done ahead
of time and the environment variable "SSH_AUTH_SOCK" set pointing to the proper
place.
forward_X11 => 1
Enables forwarding of the X11 protocol
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 file names and use them 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".
Example:
$ssh = Net::OpenSSH->new('foo', external_master => 1, ctl_path = $path);
When "external_master" is set, the hostname argument becomes optional (0.0.0.0 is
passed to OpenSSH which does not use it at all).
default_encoding => $encoding
default_stream_encoding => $encoding
default_argument_encoding => $encoding
Set default encodings. See "Data encoding".
password_prompt => $string
password_prompt => $re
By default, when using password authentication, the module expects the remote side
to send a password prompt matching "/[?:]/".
This option can be used to override that default for the rare cases when a
different prompt is used.
Examples:
password_prompt => ']'; # no need to escape ']'
password_prompt => qr/[:?>]/;
login_handler => \&custom_login_handler
Some remote SSH server may require a custom login/authentication interaction not
natively supported by Net::OpenSSH. In that cases, you can use this option to
replace the default login logic.
The callback will be invoked repeatedly as "custom_login_handler($ssh, $pty,
$data)" where $ssh is the current Net::OpenSSH object, "pty" a IO::Pty object
attached to the slave "ssh" process tty and $data a reference to an scalar you can
use at will.
The login handler must return 1 after the login process has completed successfully
or 0 in case it still needs to do something else. If some error happens, it must
die.
Note, that blocking operations should not be performed inside the login handler
(at least if you want the "async" and "timeout" features to work).
See also the sample script "login_handler.pl" in the "samples" directory.
Usage of this option is incompatible with the "password" and "passphrase" options,
you will have to handle password or passphrases from the custom handler yourself.
master_setpgrp => 1
When this option is set, the master process is run as a different process group.
As a consequence it will not die when the user presses Ctrl-C at the terminal.
In order to allow the master SSH process to request any information from the user,
the module may set it as the terminal controlling process while the connection is
established (using "tcsetpgrp" in POSIX). Afterwards, the terminal controlling
process is reset.
This feature is highly experimental. Report any problems you may find, please.
$ssh->error
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).
$ssh->get_user
$ssh->get_host
$ssh->get_port
Return the corresponding SSH login parameters.
$ssh->get_ctl_path
Returns the path to the socket where the OpenSSH master process listens for new
multiplexed connections.
($in, $out, $err, $pid) = $ssh->open_ex(\%opts, @cmd)
Note: this is a low level method which, probably, you do not need to use!
That method starts the command @cmd on the remote machine creating new pipes for the
IO channels as specified on the %opts hash.
If @cmd is omitted, the remote user shell is run.
Returns four values, the first three ($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 operating systems (e.g. 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']}, $rcmd);
stdin_discard => 1
Uses /dev/null as the remote process stdin stream.
stdout_pipe => 1
Creates a new pipe and connects the writing side to the stdout stream of the
remote process. The reading side is returned as the second value ($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 => $filename
stdout_file => \@open_args
Opens the file of the given filename and redirect stdout there.
stdout_discard => 1
Uses /dev/null as the remote process stdout stream.
stdinout_socket => 1
Creates a new socketpair, attaches the stdin an stdout streams of the slave SSH
process to one end and returns the other as the first value ($in) and undef for
the second ($out).
Example:
my ($socket, undef, undef, $pid) = $ssh->open_ex({stdinout_socket => 1},
'/bin/netcat $dest');
See also "open2socket".
stdinout_dpipe => $cmd
stdinout_dpipe => \@cmd
Runs the given command locally attaching its stdio streams to those of the remote
SSH command. Conceptually it is equivalent to the dpipe(1) shell command.
stderr_pipe => 1
Creates a new pipe and connects the writing side to the stderr stream of the
remote process. The reading side is returned as the third value ($err).
Example:
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 => $bool
Tells "ssh" to allocate a pseudo-tty for the remote process. By default, a tty is
allocated if remote command stdin stream is attached to a tty.
When this flag is set and stdin is not attached to a tty, the ssh master and slave
processes may generate spurious warnings about failed tty operations. This is
caused by a bug present in older versions of OpenSSH.
close_slave_pty => 0
When a pseudo pty is used for the stdin stream, the slave side is automatically
closed on the parent process after forking the ssh command.
This option disables that feature, so that the slave pty can be accessed on the
parent process as "$pty->slave". It will have to be explicitly closed (see
IO::Pty)
quote_args => $bool
See "Shell quoting" below.
remote_shell => $shell
Sets the remote shell. Allows one to change the argument quoting mechanism in a
per-command fashion.
This may be useful when interacting with a Windows machine where argument parsing
may be done at the command level in custom ways.
Example:
$ssh->system({remote_shell => 'MSWin'}, echo => $line);
$ssh->system({remote_shell => 'MSCmd,MSWin'}, type => $file);
forward_agent => $bool
Enables/disables forwarding of the authentication agent.
This option can only be used when agent forwarding has been previously requested
on the constructor.
forward_X11 => $bool
Enables/disables forwarding of the X11 protocol.
This option can only be used when X11 forwarding has been previously requested on
the constructor.
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 => $bool
Instead of executing a command in the remote host, this option instruct
Net::OpenSSH to create a TCP tunnel. The arguments become the target IP and port
or the remote path for an Unix socket.
Example:
my ($in, $out, undef, $pid) = $ssh->open_ex({tunnel => 1}, $IP, $port);
my ($in, $out, undef, $pid) = $ssh->open_ex({tunnel => 1}, $socket_path);
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
# ...
waitpid($pid);
setpgrp => 1
Calls "setpgrp" after forking the child process. As a result it will not die when the
user presses Ctrl+C at the console. See also "setpgrp" in perlfunc.
Using this option without also setting "master_setpgrp" on the constructor call is
mostly useless as the signal will be delivered to the master process and all the
remote commands aborted.
This feature is experimental.
$ssh->system(\%opts, @cmd)
Runs the command @cmd on the remote machine.
Returns true on success, 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).
Example:
$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 (e.g. "`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).
The exit status of the remote command is returned in $?.
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 $in is closed.
Example:
my ($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" should not 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, $port)
($socket, $pid) = $ssh->open_tunnel(\%opts, $socket_path)
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.
Example:
$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 omitted. For instance:
$ssh->scp_get({glob => 1}, '/var/tmp/foo*', '/var/tmp/bar*', '/tmp');
$ssh->scp_put('/etc/passwd');
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 one to re-enable the progress indication
bar.
verbose => 1
Calls "scp" with the "-v" flag.
recursive => 1
Copies files and directories recursively.
glob => 1
Enables 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 bandwidth, 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
Does not 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 (waitpid($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_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.
$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";
The options accepted are as follows:
tty => $bool
Enables/disables allocation of a tty on the remote side.
forward_agent => $bool
Enables/disables forwarding of authentication agent.
This option can only be used when agent forwarding has been previously requested
on the constructor.
tunnel => 1
Return a command to create a connection to some TCP server reachable from the
remote host. In that case the arguments are the destination address and port. For
instance:
$cmd = $ssh->make_remote_command({tunnel => 1}, $host, $port);
$ssh->wait_for_master($async)
When the connection has been established by calling the constructor with the "async"
option, this call allows one to advance the process.
If $async is true, it will perform any work that can be done immediately 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 successfully 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).
From version 0.64 upwards, undef is returned when the master is still in an unstable
state (login, killing, etc.) and 0 when it is in a stable state (running, stopped or
gone).
$ssh->check_master
This method runs several checks to ensure that the master connection is still alive.
$ssh->shell_quote(@args)
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.
Example:
my $quoted_args = Net::OpenSSH->shell_quote(@args);
system('ssh', '--', $host, $quoted_args);
$ssh->shell_quote_glob(@args)
This method is like the previous "shell_quote" but leaves wildcard characters
unquoted.
It can be used as a class method also.
$ssh->set_expand_vars($bool)
Enables/disables variable expansion feature (see "Variable expansion").
$ssh->get_expand_vars
Returns current state of variable expansion feature.
$ssh->set_var($name, $value)
$ssh->get_var($name, $value)
These methods allow one to change and to retrieve the value of the given name.
$ssh->get_master_pid
Returns the PID of the master SSH process
$ssh->master_exited
This methods allows one 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) {
$ssh->master_exited;
}
}
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.
$ssh->disconnect($async)
Shuts down the SSH connection.
Usually, you don't need to call this method explicitly, but just let the Net::OpenSSH
object go out of scope.
If "async" is true, it doesn't wait for the SSH connection to terminate. In that case,
"wait_for_master" must be called repeatedly until the shutdown sequence terminates
(See the "AnyEvent" integration section below).
$pid = $ssh->sshfs_import(\%opts, $remote_fs, $local_mnt_point)
$pid = $ssh->sshfs_export(\%opts, $local_fs, $remote_mnt_point)
These methods use sshfs(1) to import or export a file system through the SSH
connection.
They return the $pid of the "sshfs" process or of the slave "ssh" process used to
proxy it. Killing that process unmounts the file system, though, it may be probably
better to use fusermount(1).
The options accepted are as follows:
ssh_opts => \@ssh_opts
Options passed to the slave "ssh" process.
sshfs_opts => \@sshfs_opts
Options passed to the "sshfs" command. For instance, to mount the file system in
read-only mode:
my $pid = $ssh->sshfs_export({sshfs_opts => [-o => 'ro']},
"/", "/mnt/foo");
Note that this command requires a recent version of "sshfs" to work (at the time of
writing, it requires the yet unreleased version available from the FUSE git
repository!).
See also the sshfs(1) man page and the "sshfs" and FUSE web sites at
<https://github.com/libfuse/sshfs> and <https://github.com/libfuse/libfuse>
respectively.
$or = $ssh->object_remote(@args)
Returns an Object::Remote::Connection instance running on top of the Net::OpenSSH
connection.
Example:
my $or = $ssh->object_remote;
my $hostname = Sys::Hostname->can::on($or, 'hostname');
say $hostname->();
See also Object::Remote.
$any = $ssh->any(%opts)
Wraps the current object inside a Net::SSH::Any one.
Example:
my $any = $ssh->any;
my $content = $any->scp_get_content("my-file.txt");
$pid = $ssh->disown_master
Under normal operation Net::OpenSSH controls the life-time of the master "ssh" process
and when the object is destroyed the master process and any connection running over it
are terminated.
In some (rare) cases, it is desirable to let the master process and all the running
connections survive. Calling this method does just that, it tells Net::OpenSSH object
that the master process is not its own anymore.
The return value is the PID of the master process.
Note also that disowning the master process does not affect the operation of the
module in any other regard.
For instance:
# See sample/sshfs_mount.pl for a working program
my $ssh = Net::OpenSSH->new($host);
my $sshfs_pid = $ssh->sshfs_import("/home/foo", "my-remote-home");
$ssh->disown_master;
$ssh->stop; # tells the master to stop accepting requests
exit(0);
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 command list.
All the methods that invoke a remote command (system, open_ex, etc.) accept the option
"quote_args" that allows one 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 disabled with the option
"quote_args_extended":
$ssh->system({ stderr_discard => 1,
quote_args => 1, quote_args_extended => 0 },
@cmd);
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'),
"/tmp/files_*.dat");
When the glob option is set in "scp" and "rsync" file transfer methods, an alternative
quoting method which 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;
By default, the module assumes the remote shell is some variant of a POSIX or Bourne shell
("bash", "dash", "ksh", etc.). If this is not the case, the construction option
"remote_shell" can be used to select an alternative quoting mechanism.
For instance:
$ssh = Net::OpenSSH->new($host, remote_shell => 'csh');
$ssh->system(echo => "hard\n to\n quote\n argument!");
Currently there are quoters available for POSIX (Bourne) compatible shells, "csh" and the
two Windows variants "MSWin" (for servers using Win32::CreateProcess, see
Net::OpenSSH::ShellQuoter::MSWin) and "MSCmd" (for servers using "cmd.exe", see
Net::OpenSSH::ShellQuoter::MSCmd).
In any case, you can always do the quoting yourself and pass the quoted remote command as
a single string:
# for VMS
$ssh->system('DIR/SIZE NFOO::USERS:[JSMITH.DOCS]*.TXT;0');
Note that 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.
Timeouts
In order to stop remote processes when they timeout, the ideal approach would be to send
them signals through the SSH connection as specified by the protocol standard.
Unfortunately 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 then, 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 one 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.
Tunnels
Besides running commands on the remote host, Net::OpenSSH also allows one 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.
Example:
$ssh->system({tunnel => 1,
stdin_data => "GET / HTTP/1.0\r\n\r\n",
stdout_file => "/tmp/$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).
When connecting to hosts running a recent version of OpenSSH sshd, it is also possible to
open connections targeting Unix sockets.
For instance:
my $response = $ssh->capture({tunnel => 1, stdin_data => $request },
"/tmp/socket-foo");
Currently, this feature requires a patched OpenSSH ssh client. The patch is available as
"patches/openssh-fwd-stdio-to-streamlocal-1.patch".
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
(e.g. "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.
Diverting "new"
When a code ref is installed at $Net::OpenSSH::FACTORY, calls to new will be diverted
through it.
That feature can be used to transparently implement connection caching, for instance:
my $old_factory = $Net::OpenSSH::FACTORY;
my %cache;
sub factory {
my ($class, %opts) = @_;
my $signature = join("\0", $class, map { $_ => $opts{$_} }, sort keys %opts);
my $old = $cache{signature};
return $old if ($old and $old->error != OSSH_MASTER_FAILED);
local $Net::OpenSSH::FACTORY = $old_factory;
$cache{$signature} = $class->new(%opts);
}
$Net::OpenSSH::FACTORY = \&factory;
... and I am sure it can be abused in several other ways!
3rd PARTY MODULE INTEGRATION
Expect
Sometimes you would like to use Expect to control some program running in the remote host.
You can do it as follows:
my ($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 $expect as usual.
Net::Telnet
This example is adapted from Net::Telnet documentation:
my ($pty, $pid) = $ssh->open2pty({stderr_to_stdout => 1})
or die "unable to start remote shell: " . $ssh->error;
my $telnet = Net::Telnet->new(-fhopen => $pty,
-prompt => '/.*\$ $/',
-telnetmode => 0,
-cmd_remove_mode => 1,
-output_record_separator => "\r");
$telnet->waitfor(-match => $telnet->prompt,
-errmode => "return")
or die "login failed: " . $telnet->lastline;
my @lines = $telnet->cmd("who");
...
$telnet->close;
waitpid($pid, 0);
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 operating system level. Net::OpenSSH will fail if any of these handles
is used explicitly or implicitly when calling some remote command.
The work-around 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 web server
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!
Net::SFTP::Foreign
See method "sftp".
Net::SSH::Any
See method "any".
Object::Remote
See method "object_remote".
AnyEvent (and similar frameworks)
Net::OpenSSH provides all the functionality required to be integrated inside event
oriented programming framework such as AnyEvent or IO::Async in the following way:
1. Create a disconnected Net::OpenSSH object:
my $ssh = Net::OpenSSH->new($host, async => 1, ...);
2. Let the object connect to the remote host:
Use a timer to call the "wait_for_master" method in async mode repeatedly until it
returns a true value indicating success.
Also, the object error state needs to be checked after every call in order to detect
failed connections. For instance:
my $ssh = Net::OpenSSH->new(..., async => 1);
my $w;
$w = AE::timer 0.1, 0.1, sub {
if ($ssh->wait_for_master(1)) {
# the connection has been established!
# remote commands can be run now
undef $w;
on_ssh_success(...);
}
elsif ($ssh->error) {
# connection can not be established
undef $w;
on_ssh_failure(...);
}
}
3. Use the event framework to launch the remote processes:
Call Net::OpenSSH "make_remote_command" to construct commands which can be run using
the framework regular facilities for launching external commands.
Error checking should also be performed at this point because the SSH connection could
be broken.
For instance:
if (defined(my $cmd = $ssh->make_remote_command(echo => 'hello!')) {
AnyEvent::Util::run_cmd($cmd, %run_cmd_opts);
}
else {
# something went wrong!
}
Alternatively, any of the "open*" methods provided by Net::OpenSSH could also be used
to launch remote commands.
4. When finished, disconnect asynchronously
After initiating an asynchronous disconnect with disconnect(1), repeatedly call
"wait_for_master" until you get a defined but false value:
$ssh->disconnect(1);
my $w; $w = AE::timer 0.1, 0.1, sub {
my $res = $ssh->wait_for_master(1);
if (defined $res && !$res) {
undef $w;
undef $ssh;
}
};
Be careful not to let the $ssh object go out of scope until the disconnection has
finished, otherwise its destructor will wait and block your program until the
disconnection has completed.
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('...');
or...
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.
TROUBLESHOOTING
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 (e.g. Sun, err... Oracle) include custom versions of OpenSSH
bundled with the operating system. In principle, Net::OpenSSH should work with these
SSH clients as long as they are derived from some version of OpenSSH recent enough.
Anyway, my advise is 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 web server, 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
established 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 becoming 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.
5 - file system must support sockets
Some file systems (as for instance FAT or AFS) do not support placing sockets inside
them.
Ensure that the "ctl_dir" path does not lay into one of those file systems.
DEBUGGING
Debugging of Net::OpenSSH internals is controlled through the variable
$Net::OpenSSH::debug. Every bit of this variable activates debugging of some subsystem as
follows:
bit 1 - errors
Dumps changes on the internal object attribute where errors are stored.
bit 2 - ctl_path
Dumps information about ctl_path calculation and the tests performed on that directory
in order to decide if it is secure to place the multiplexing socket inside.
bit 4 - connecting
Dumps information about the establishment of new master connections.
bit 8 - commands and arguments
Dumps the command and arguments for every system/exec call.
bit 16 - command execution
Dumps information about the progress of command execution.
bit 32 - destruction
Dumps information about the destruction of Net::OpenSSH objects and the termination of
the SSH master processes.
bit 64 - IO loop
Dumps information about the progress of the IO loop on capture operations.
bit 128 - IO hexdumps
Generates hexdumps of the information that travels through the SSH streams inside
capture operations.
bit 512 - OS tracing of the master process
Use the module Net::OpenSSH::OSTracer to trace the SSH master process at the OS level.
For instance, in order to activate all the debugging flags, you can use:
$Net::OpenSSH::debug = ~0;
Note that the meaning of the flags and the information generated is only intended for
debugging of the module and may change without notice between releases.
If you are using password authentication, enabling debugging for IO::Tty may also show
interesting information:
IO::Tty::DEBUG = 1;
Finally, by default debugging output is sent to "STDERR". You can override it pointing
$Net::OpenSSH::debug_fh to a different file handle. For instance:
BEGIN {
open my $out, '>', '/tmp/debug.txt' or warn $!;
$Net::OpenSSH::debug_fh = $out;
$Net::OpenSSH::debug = -1;
}
SECURITY
Q: Is this module secure?
A: Well, it tries to be!
From a security standpoint the aim of this module is to be as secure as OpenSSH, your
operating system, your shell and in general your environment allow it to be.
It does not take any shortcut just to make your life easier if that means lowering the
security level (for instance, disabling "StrictHostKeyChecking" by default).
In code supporting features that are not just proxied to OpenSSH, the module tries to keep
the same standards of security as OpenSSH (for instance, checking directory and file
permissions when placing the multiplexing socket).
On the other hand, and keeping with OpenSSH philosophy, the module lets you disable most
(all?) of those security measures. But just because it lets you do it it doesn't mean it
is a good idea to do so!!!
If you are a novice programmer or SSH user, and googling you have just found some flag
that you don't understand but that seems to magically solve your connection problems...
well, believe me, it is probably a bad idea to use it. Ask somebody how really knows
first!
Just to make thinks clear, if your code contains any of the keywords from the (non-
exclusive) list below and you don't know why, you are probably wrecking the security of
the SSH protocol:
strict_mode
StrictHostKeyChecking
UserKnownHostsFile
Other considerations related to security you may like to know are as follows:
Taint mode
The module supports working in taint mode.
If you are in an exposed environment, you should probably enable it for your script in
order to catch any unchecked command for being executed in the remote side.
Web environments
It is a bad idea to establish SSH connections from your webserver because if it
becomes compromised in any way, the attacker would be able to use the credentials from
your script to connect to the remote host and do anything he wishes there.
Command quoting
The module can quote commands and arguments for you in a flexible and powerful way.
This is a feature you should use as it reduces the possibility of some attacker being
able to inject and run arbitrary commands on the remote machine (and even for scripts
that are not exposed it is always advisable to enable argument quoting).
Having said that, take into consideration that argument-quoting is just a hack to
emulate the invoke-without-a-shell feature of Perl builtins such as "system" and
alike. There may be bugs(*) on the quoting code, your particular shell may have
different quoting rules with unhandled corner cases or whatever. If your script is
exposed to the outside, you should check your inputs and restrict what you accept as
valid.
[* even if this is one of the parts of the module more intensively tested!]
Shellshock
(see Shellshock <http://en.wikipedia.org/wiki/Shellshock_%28software_bug%29>)
When executing local commands, the module always avoids calling the shell so in this
way it is not affected by Shellshock.
Unfortunately, some commands ("scp", "rsync" and "ssh" when the "ProxyCommand" option
is used) invoke other commands under the hood using the user shell. That opens the
door to local Shellshock exploitation.
On the remote side invocation of the shell is unavoidable due to the protocol design.
By default, SSH does not forward environment variables but some Linux distributions
explicitly change the default OpenSSH configuration to enable forwarding and
acceptance of some specific ones (for instance "LANG" and "LC_*" on Debian and
derivatives, Fedora does alike) and this also opens the door to Shellshock
exploitation.
Note that the shell used to invoke commands is not "/bin/sh" but the user shell as
configured in "/etc/passwd", PAM or whatever authentication subsystem is used by the
local or remote operating system. Debian users, don't think you are not affected
because your "/bin/sh" points to "dash"!
FAQ
Frequent questions about the module:
Connecting to switches, routers, etc.
Q: I can not 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 a SSH
channel between both hosts, asks the remote computer to run some given command and
when it finishes, 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 (e.g. "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 attaches the restricted shell to any incoming SSH connection and waits
for the user to enter commands through the redirected stdin stream.
The only way to work-around 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 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.
Nowadays, there is a new player, Net::CLI::Interaction that may be more suitable than
Expect.
Connection fails
Q: I am unable to make the module connect to the remote host...
A: Have you read the troubleshooting section? (see "TROUBLESHOOTING").
Disable StrictHostKeyChecking
Q: Why is "ssh" not run 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 unwritable) 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 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 (e.g.
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},
$cmd);
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 not 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');
$ssh->systen('ls');
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) run every command in a new session so most shell builtins that are
run for its side effects become useless (e.g. "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 should not use "spawn" for that.
MaxSessions server limit reached
Q: I created an $ssh object and then fork a lot children processes which use this
object. When the children number is bigger than "MaxSessions" as defined in sshd
configuration (defaults to 10), trying to fork new remote commands will prompt the
user for the password.
A: When the slave SSH client gets a response from the remote servers saying that the
maximum number of sessions for the current connection has been reached, it fall backs
to open a new direct connection without going through the multiplexing socket.
To stop that for happening, the following hack can be used:
$ssh = Net::OpenSSH->new(host,
default_ssh_opts => ['-oConnectionAttempts=0'],
...);
Running remote commands with sudo
Q: How can I run remote commands using "sudo" to become root first?
A: The simplest way is to tell "sudo" to read the password from stdin with the "-S"
flag and to do not use cached credentials with the "-k" flag. You may also like to use
the "-p" flag to tell "sudo" to print an empty prompt. For instance:
my @out = $ssh->capture({ stdin_data => "$sudo_passwd\n" },
'sudo', '-Sk',
'-p', '',
'--',
@cmd);
If the version of sudo installed on the remote host does not support the "-S" flag (it
tells sudo to read the password from its STDIN stream), you can do it as follows:
my @out = $ssh->capture({ tty => 1,
stdin_data => "$sudo_passwd\n" },
'sudo', '-k',
'-p', '',
'--',
@cmd);
This may generate an spurious and harmless warning from the SSH master connection
(because we are requesting allocation of a tty on the remote side and locally we are
attaching it to a regular pair of pipes).
If for whatever reason the methods described above fail, you can always revert to
using Expect to talk to the remote "sudo". See the "sample/expect.pl" script from this
module distribution.
SEE ALSO
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>.
Net::OpenSSH::Gateway for detailed instruction about how to get this module to connect to
hosts through proxies and other SSH gateway servers.
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 one to run commands in remote
hosts in parallel. It is obviously based on Net::OpenSSH.
SSH::Batch allows one 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::SSH::Mechanize.
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.
Net::CLI::Interact allows one to interact with remote shells and other services. It is
specially suited for interaction with network equipment. The passphrase approach it uses
is very clever. You may also like to check the other modules
<https://metacpan.org/author/OLIVER> from its author, Oliver Gorwits.
BUGS AND SUPPORT
Experimental features
Object::Remote integration is highly experimental.
Support for tunnels targeting Unix sockets is highly experimental.
Support for the setpgrp feature is highly experimental.
Support for the gateway feature is highly experimental and mostly stalled.
Support for taint mode is experimental.
Known issues
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 does not work on VMS either... well, probably, it does not work on anything not
resembling a modern Linux/Unix OS.
Old versions of OpenSSH "ssh" may leave stdio streams in non-blocking mode. That can
result on failures when writing to "STDOUT" or "STDERR" after using the module. In order
to work-around this issue, Perl "fcntl" in perlfunc can be used to unset the non-blocking
flag:
use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
my $flags = fcntl(STDOUT, F_GETFL, 0);
fcntl(STDOUT, F_SETFL, $flags & ~O_NONBLOCK);
Reporting bugs and asking for help
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://perlmonks.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.
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 are 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>.
TODO
- Tests for "scp_*", "rsync_*" and "sftp" methods
- Make "pipe_in" and "pipe_out" methods "open_ex" based
- "auto_discard_streams" feature for mod_perl2 and similar environments
- Refactor open_ex support for multiple commands, maybe just keeping
tunnel, ssh and raw
Send your feature requests, ideas or any feedback, please!
CONTRIBUTING CODE
The source code of this module is hosted at GitHub:
<http://github.com/salva/p5-Net-OpenSSH>.
Code contributions to the module are welcome but you should obey the following rules:
Only Perl 5.8.4 required
Yes, that's pretty old, but Net::OpenSSH is intended to be also used by system
administrators that sometimes have to struggle with old systems. The reason to pick
5.8.4 is that it has been the default perl on Solaris for a long time.
Avoid the "All the world's a Linux PC" syndrome
The module should work on any (barely) sane Unix or Linux operating system. Specially,
it should not be assumed that the over-featured GNU utilities and toolchain are
available.
Dependencies are optional
In order to make the module very easy to install, no mandatory dependencies on other
CPAN modules are allowed.
Optional modules, that are loaded only on demand, are acceptable when they are used
for adding new functionality (as it is done, for instance, with IO::Pty).
Glue code for integration with 3rd party modules is also allowed (as it is done with
Expect).
Usage of language extension modules and alike is not acceptable.
Tests should be lax
We don't want false negatives when testing. In case of doubt tests should succeed.
Also, in case of tests invoking some external program, it should be checked that the
external program is available and that it works as expected or otherwise skip those
tests.
Backward compatibility
Nowadays Net::OpenSSH is quite stable and there are lots of scripts out there using it
that we don't want to break, so, keeping the API backward compatible is a top
priority.
Probably only security issues could now justify a backward incompatible change.
Follow my coding style
Look at the rest of the code.
I let Emacs do the formatting for me using cperl-mode PerlStyle.
Talk to me
Before making a large change or implementing a new feature get in touch with me.
I may have my own ideas about how things should be done. It is better if you know them
before hand, otherwise, you risk getting your patch rejected.
Well, actually you should know that I am quite good at rejecting patches but it is not my
fault!
Most of the patches I get are broken in some way: they don't follow the main module
principles, sometimes the author didn't get the full picture and solved the issue in a
short-sighted way, etc.
In any case, you should not be discouraged to contribute. Even if your patch is not
applied directly, seeing how it solves your requirements or, in the case of bugs, the
underlying problem analysis may be very useful and help me to do it... my way.
I always welcome documentation corrections and improvements.
COPYRIGHT AND LICENSE
Copyright (C) 2008-2017 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.