NAME
remctl, remctl_result_free - Simple remctl call to a remote server
SYNOPSIS
#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);
DESCRIPTION
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 behavior.
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).
RETURN VALUE
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).
CAVEATS
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.
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.
SEE ALSO
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
<http://www.eyrie.org/~eagle/software/remctl/>.
AUTHOR
Russ Allbery <rra@stanford.edu>