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REMCTL(3)             remctl Library Reference             REMCTL(3)

       remctl, remctl_result_free - Simple remctl call to a remote

       #include <remctl.h>

       struct remctl_result *
        remctl(const char *host, unsigned short port,
               const char *principal, const char **command);

       void remctl_result_free(struct remctl_result *result);

       remctl() provides a simplified client API for the remctl
       protocol.  Given the host, port, service principal for
       authentication, and command to run, it opens a connection to
       the remote system, sends the command via the remctl protocol,
       reads the results, closes the connection, and returns the
       result as a remctl_result struct.

       host is a hostname or IP address and must be non-NULL.  port
       is the port to connect to; if 0, the library first attempts
       to connect to the registered port of 4373 and then tries the
       legacy port of 4444 if that fails.  Future versions of the
       library will drop this fallback to 4444.  principal is the
       service principal to use for authentication; if NULL,
       "host/host" is used, with the realm determined by domain-
       realm mapping.  command is the command to run as a NULL-
       terminated array of NUL-terminated strings.

       If no principal is specified and the default is used, the
       underlying GSS-API library may canonicalize host via DNS
       before determining the service principal, depending on your
       library configuration.  Specifying a principal disables this

       The remctl protocol uses Kerberos v5 via GSS-API for
       authentication.  The underlying GSS-API library will use the
       default ticket cache for authentication, so to successfully
       use remctl(), the caller should already have Kerberos tickets
       for an appropriate realm stored in its default ticket cache.
       The environment variable KRB5CCNAME can be used to control
       which ticket cache is used.

       remctl() returns a newly allocated remctl_result struct,
       which has the following members:

           struct remctl_result {
               char *error;                /* remctl error if non-NULL. */
               char *stdout_buf;           /* Standard output. */
               size_t stdout_len;          /* Length of standard output. */
               char *stderr_buf;           /* Standard error. */
               size_t stderr_len;          /* Length of standard error. */
               int status;                 /* Exit status of remote command. */

       If error is non-NULL, a protocol error occurred and the
       command was not successfully completed.  Otherwise, standard
       output from the command will be stored in stdout_buf with the
       length in stdout_len, standard error from the command will be
       stored in stderr_buf with the length in stderr_len, and
       status will hold the exit status of the command.  Following
       the standard Unix convention, a 0 status should normally be
       considered success and any non-zero status should normally be
       considered failure, although a given command may have its own
       exit status conventions.

       remctl_result_free() frees the remctl_result struct when the
       calling program is through with it.

       If you want more control over the steps of the protocol, if
       you want to issue multiple commands on the same connection,
       or if you need to send data as part of the command that
       contains NULs, use the full API described in remctl_new(3),
       remctl_open(3), remctl_commandv(3), and remctl_output(3).

       remctl() returns NULL on failure to allocate a new
       remctl_result struct or on failure to allocate space to store
       an error message.  Otherwise, it returns a newly allocated
       remctl_result struct with either an error message in the
       error field or the results of the command filled out as
       described above.  If remctl() returns NULL, errno will be set
       to an appropriate error code (generally ENOMEM).

       If the principal argument to remctl() 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() 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 canonicalize host before calling remctl() to avoid
       problems with multiple DNS calls returning different results.

       The default behavior, when a port of 0 is given, of trying
       4373 and falling back to 4444 will be removed in a future
       version of this library 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.

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

       remctl_new(3), remctl_open(3), remctl_command(3),
       remctl_commandv(3), remctl_output(3), remctl_close(3)

       The current version of the remctl library and complete
       details of the remctl protocol are available from its web
       page at <>.

       Russ Allbery <>

2.16                         2010-05-02                    REMCTL(3)
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