Remctl(3pm)

NAME

Net::Remctl - Perl bindings for remctl (Kerberos remote command
execution)

SYNOPSIS

# Simplified form.
use Net::Remctl;
my $result = remctl("hostname", undef, undef, "test", "echo", "Hi");
if ($result->error) {
    die "test echo failed with error ", $result->error, "\n";
} else {
    warn $result->stderr;
    print $result->stdout;
    exit $result->status;
}

# Full interface.
use Net::Remctl ();
my $remctl = Net::Remctl->new;
$remctl->open("hostname")
    or die "Cannot connect to hostname: ", $remctl->error, "\n";
$remctl->command("test", "echo", "Hi there")
    or die "Cannot send command: ", $remctl->error, "\n";
my $output;
do {
    $output = $remctl->output;
    if ($output->type eq 'output') {
        if ($output->stream == 1) {
            print $output->data;
        } elsif ($output->stream == 2) {
            warn $output->data;
        }
    } elsif ($output->type eq 'error') {
        warn $output->error, "\n";
    } elsif ($output->type eq 'status') {
        exit $output->status;
    } else {
        die "Unknown output token from library: ", $output->type, "\n";
    }
} while ($output->type eq 'output');

DESCRIPTION

Net::Remctl provides Perl bindings to the libremctl client library.
remctl is a protocol for remote command execution using GSS-API
authentication. The specific allowable commands must be listed in a
configuration file on the remote system and the remote system can map
the remctl command names to any local command without exposing that
mapping to the client. This module implements a remctl client.
Simplified Interface: If you want to run a single command on a remote system and get back the output and exit status, you can use the exported remctl() function:; remctl(HOSTNAME, PORT, PRINCIPAL, COMMAND, [ARGS, ...])
Runs a command on the remote system and returns a
Net::Remctl::Result object (see below). HOSTNAME is the remote
host to contact. PORT is the port of the remote remctld server and may be 0 to tell the library to use the default (first try 4373,
the registered remctl port, and fall back to the legacy 4444 port
if that fails). PRINCIPAL is the principal of the server to use
for authentication; pass in the empty string to use the default of host/HOSTNAME, with the realm determined by domain-realm mapping.
The remaining arguments are the remctl command and arguments passed to the remote server.

As far as the module is concerned, undef may be passed as PORT and PRINCIPAL and is the same as 0 and the empty string respectively.
However, Perl will warn about passing undef explicitly as a
function argument.

The return value is a Net::Remctl::Result object which supports the following methods:

error()
Returns the error message from either the remote host or from
the local client library (if, for instance, contacting the
remote host failed). Returns undef if there was no error.
Checking whether error() returns undef is the supported way of determining whether the remctl() call succeeded.

stdout()

Returns the command's standard output or undef if there was
none.

stderr()

Returns the command's standard error or undef if there was
none.

status()

Returns the command's exit status.

Each call to remctl() will open a new connection to the remote host and close it after retrieving the results of the command. To
maintain a persistent connection, use the full interface described below.
Full Interface: The full remctl library interface requires that the user do more
bookkeeping, but it provides more flexibility and allows one to issue
multiple commands on the same persistent connection (provided that the remote server supports protocol version two; if not, the library will
transparently fall back to opening a new connection for each command).; To use the full interface, first create a Net::Remctl object with new() and then connect() to a remote server. Then, issue a command() and call output() to retrieve output tokens (as Net::Remctl::Output objects) until a status token is received. Destroying the Net::Remctl object will close the connection.; The supported object methods are:; new()
Create a new Net::Remctl object. This doesn't attempt to connect
to a host and hence will only fail (by throwing an exception) if
the library cannot allocate memory.; error()
Retrieves the error message from the last failing operation and
returns it as a string.; open(HOSTNAME[, PORT[, PRINCIPAL]])
Connect to HOSTNAME on port PORT using PRINCIPAL as the remote
server's principal for authentication. If PORT is omitted or 0,
use the default (first try 4373, the registered remctl port, and
fall back to the legacy 4444 port if that fails). If PRINCIPAL is omitted or the empty string, use the default of host/HOSTNAME, with the realm determined by domain-realm mapping. Returns true on
success, false on failure. On failure, call error() to get the failure message.

As far as the module is concerned, undef may be passed as PORT and PRINCIPAL and is the same as 0 and the empty string respectively.
However, Perl will warn about passing undef explicitly as a
function argument.; command(COMMAND[, ARGS, ...])
Send the command and arguments to the remote host. The command and the arguments may, under the remctl protocol, contain any
character, but be aware that most remctl servers will reject
commands or arguments containing ASCII 0 (NUL), so currently this
cannot be used for upload of arbitrary unencoded binary data.
Returns true on success (meaning success in sending the command,
and implying nothing about the result of the command), false on
failure. On failure, call error() to get the failure message.; output()
Returns the next output token from the remote host. The token is
returned as a Net::Remctl::Output object, which supports the
following methods:

type()
Returns the type of the output token, which will be one of
"output", "error", "status", or "done". A command will result in either one "error" token or zero or more "output" tokens
followed by a "status" token. After either a "error" or
"status" token is seen, another command can be issued. If the caller tries to retrieve another output token when it has
already consumed all of them for that command, the library will return a "done" token.

data()

Returns the contents of the token. This method only makes
sense for "output" and "error" tokens; otherwise, it will
return undef. Note that the returned value may contain any
character, including ASCII 0 (NUL).

length()

Returns the length of the data in the token. As with data(), this method only makes sense for the "output" and "error"
tokens. It will return 0 if there is no data or if the data is zero-length.

stream()

For an "output" token, returns the stream with which the data
is associated. Currently, only two stream values will be used: 1, meaning standard output; and 2, meaning standard error. The value is undefined for all other output token types.

status()

For a "status" token, returns the exit status of the remote
command. The value is undefined for all other token types.

error()

For an "error" token, returns the remctl error code for the
protocol error. The text message will be returned by data(). The value is undefined for all other token types.
Note that, due to internal implementation details in the library, the Net::Remctl::Output object returned by output() will be invalidated by the next call to command() or output() or by destroying the producing Net::Remctl object. Therefore, any data in the output token should be processed and stored if needed before making any further Net::Remctl method calls on the same object.

CAVEATS

If the principal argument to remctl() or remctl_open() is NULL, most GSS-API libraries will canonicalize the host using DNS before deriving the principal name from it. This means that when connecting to a
remctl server via a CNAME, remctl() and remctl_open() will normally authenticate using a principal based on the canonical name of the host instead of the specified host parameter. This behavior may cause problems if two consecutive DNS lookups of host may return two different results, such as with some DNS-based load-balancing systems.

The canonicalization behavior is controlled by the GSS-API library;
with the MIT Kerberos GSS-API library, canonicalization can be disabled by setting "rdns" to false in the [libdefaults] section of krb5.conf. It can also be disabled by passing an explicit Kerberos principal name via the principal argument, which will then be used without changes. If canonicalization is desired, the caller may wish to canonicalizae
host before calling remctl() or remctl_open() to avoid problems with multiple DNS calls returning different results.

The default behavior, when the port is not specified, of trying 4373
and falling back to 4444 will be removed in a future version of this
module in favor of using the "remctl" service in /etc/services if set and then falling back on only 4373. 4444 was the poorly-chosen
original remctl port and should be phased out.

NOTES

The remctl port number, 4373, was derived by tracing the diagonals of a QWERTY keyboard up from the letters "remc" to the number row.

AUTHOR

Russ Allbery <rra@stanford.edu>

COPYRIGHT AND LICENSE

Permission to use, copy, modify, and distribute this software and its
documentation for any purpose and without fee is hereby granted,
provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in
supporting documentation, and that the name of Stanford University not be used in advertising or publicity pertaining to distribution of the
software without specific, written prior permission. Stanford
University makes no representations about the suitability of this
software for any purpose. It is provided "as is" without express or
implied warranty.

THIS SOFTWARE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED
WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.

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