Provided by: remctl-server_3.18-1build1_amd64 
NAME
remctld - Server for remctl, a remote command execution utility
SYNOPSIS
remctld [-dFhmSvZ] [-b bind-address [-b bind-address ...]]
[-f config] [-k keytab] [-P file] [-p port]
[-s service]
DESCRIPTION
remctld is the server for remctl. It accepts a connection from remctl, receives the
command to execute and the arguments, verifies authorization of the user and executes the
command, returning the result back to the client. All connections are authenticated using
Kerberos GSS-API Kerberos, and all transmissions are also encrypted using the GSS-API
privacy layer.
remctld is normally started using tcpserver or from inetd, but it may be run in stand-
alone mode as a daemon using -m. Either -s must be given to use an alternate identity
(which will require the same flag be used for remctl client invocations), or it must be
run as root to read the host keytab file. remctld logs its activity using syslog (the
daemon facility).
The location of the configuration file may be specified with the -f option. The default
location is /etc/remctl/remctl.conf. For information on the format of the configuration
file, see "CONFIGURATION FILE" below.
When the command is run, several environment variables will be set providing information
about the remote connection. See ENVIRONMENT below for more information.
Command-line and configuration options and ACL methods are annotated below with the
version at which they were added. For version information for more general features, see
COMPATIBILITY below.
OPTIONS
The start of each option description is annotated with the version of remctld in which
that option was added with its current meaning.
-b bind-address
[2.17] When running as a standalone server, bind to the specified local address rather
than listening on all interfaces. This option may be given multiple times to bind to
multiple addresses. bind-address must be an IP address (either IPv4 or IPv6), not a
hostname. Only makes sense in combination with -m.
This option is ignored if remctld is passed already open sockets via the systemd
socket activation protocol. In that case, the bind addresses of the sockets should be
controlled via the systemd configuration.
-d [1.10] Enable verbose debug logging to syslog (or to standard output if -S is also
given).
-F [2.8] Normally when running in stand-alone mode (-m), remctld backgrounds itself to
run as a daemon, changes directory to /, and drops any controlling terminal. This
flag suppresses this behavior, usually for debugging or so that remctld can be
monitored by other processes.
-f config
[1.0] The configuration file for remctld, overriding the default path.
-h [1.10] Show a brief usage message and then exit. This usage method will include a
list of supported ACL types and can be used to determine if optional ACL methods were
compiled into a given remctld build.
-k keytab
[2.8] Use keytab as the keytab for server credentials rather than the system default
or the value of the KRB5_KTNAME environment variable. Using -k just sets the
KRB5_KTNAME environment variable internally in the process.
-m [2.8] Enable stand-alone mode. remctld will listen to its configured port and fork a
new child for each incoming connection. By default, when this option is used, remctld
also changes directory to /, backgrounds itself, and closes standard input, output,
and error. To not background, pass -F as well. To not close standard output and
error and continue using them for logging, pass -S as well.
To determine the port, remctld attempts to look up the "remctl" service in the local
/etc/services file and uses the port defined there. If the "remctl" service could not
be found, it uses 4373, the registered remctl port.
When running in stand-alone mode, send the SIGHUP signal to remctld to ask it to re-
read its configuration file and SIGTERM to ask it to exit.
-P file
[2.0] When running in stand-alone mode (-m), write the PID of remctld to file. This
option is ignored unless -m is also given.
-p port
[1.0] When running in stand-alone mode, listen on port port rather than the default.
This option does nothing unless used with -m.
This option is ignored if remctld is passed already open sockets via the systemd
socket activation protocol. In that case, the listening port should be controlled via
the systemd configuration.
-S [2.3] Rather than logging to syslog, log debug and routine connection messages to
standard output and error messages to standard error. This option is mostly useful
for testing and debugging.
-s service
[1.0] Specifies which principal is used as the server identity for client
authentication. The client must also use the same identity as the server identity for
authentication to succeed. By default, remctld accepts any principal with a key in
the default keytab file (which can be changed with the -k option). This is normally
the most desirable behavior.
-v [1.10] Print the version of remctld and exit.
-Z [3.7] When remctld is running in stand-alone mode, after it has set up its network
socket and is ready to answer requests, raise SIGSTOP. This signals to upstart, when
using "expect stop", that the daemon is ready to accept connections, and upstart will
raise SIGCONT to allow remctld to continue. This option is probably only useful when
using upstart as the init system. Only makes sense in combination with -m.
CONFIGURATION FILE
The configuration file defines the allowed commands and specifies access control
information. The configuration file format is lines of space- or tab-separated strings,
where each line is:
command subcommand executable [option=value ...] acl [acl ...]
Each command consists of a command, a subcommand, and zero or more arguments. Each
configuration line defines an acceptable command and subcommand (or, if "ALL" is used as
mentioned below under command and subcommand, a set of commands). The first configuration
line matching the received command is used, so list more specific entries before more
general entries.
Blank lines and lines beginning with "#" are ignored. Lines can be continued on the next
line by ending them with a backslash ("\"). Be aware that comments can be continued with
a backslash as well.
As a special case, a line like:
include file
will include file as if its contents were pasted verbatim into the configuration file at
that point. file may be a directory, in which case all files whose names do not contain a
period found in that directory will be included (in no particular order). file should be
a fully qualified path.
The configuration file is loaded when remctld starts and is normally not re-read. To
trigger a re-read of the configuration file when remctld is running in stand-alone mode,
send the SIGHUP signal to the remctld process.
The meaning of the fields on each configuration line are:
command
The command being issued or the special keyword "ALL". Normally, related commands
(such as all commands for managing a particular service) are grouped together as
subcommands under one command.
If the keyword "ALL" is used instead of a specific subcommand, this line matches all
commands with the given subcommand (so "ALL ALL" matches any command) and can be used
to dispatch all commands to the same executable with the same ACLs. Since the first
matching entry is used, list entries for specific commands first (if any) and then the
"ALL" catch-all.
Note that while the subcommand is passed to the executable as a command-line option,
the command is not. The command is available to the executable in the environment
variable REMCTL_COMMAND (see ENVIRONMENT below).
The command "help" is handled specially if no such command is defined in the
configuration file. See below under the "help" and "summary" options.
subcommand
The subcommand within the command being requested, such as "release" for the release
function of the AFS volume backend, or one of the special keywords "ALL" or "EMPTY".
If the keyword "ALL" is used instead of a specific subcommand, this line matches all
subcommands with the given command and can be used to dispatch all subcommands under
that command to the same executable with the same ACLs. Since the first matching
entry is used, list entries for specific services first (if any) and then the "ALL"
catch-all.
If the keyword "EMPTY" is used instead of a specific subcommand, this line matches
only commands where no subcommand was given.
The subcommand is always passed as the first argument to the executable program that
is listed for that service unless no subcommand was given.
executable
The full path to the command executable to run for this command and subcommand
combination. (See examples below.)
option=value
An option setting that applies to this command. Supported option settings, annotated
with the version at which that option was added in its current form, are:
help=arg
[3.2] Specifies the argument for this command that will print help for a
particular subcommand to standard output.
If remctld receives the command "help" with one or two arguments, and no "help"
command is defined in the configuration file, the server will take the command
arguments as a command and subcommand. It will then look through the
configuration for a configuration line matching that command and subcommand with a
"help" option set. If one is found and the user is authorized to run that
command, the server will run the specified executable with the argument arg and
second and optional third arguments taken from the arguments to the "help"
command, sending the output back to the user.
This permits a standard interface to get additional help for a particular remctl
command. Also see the "summary" option.
logmask=n[,...]
[1.4] Limit logging of command arguments. Any argument listed in the logmask list
will have its value logged as "**MASKED**". This is to avoid logging the
arguments of commands that take private information such as passwords. The
logmask list should contain argument numbers separated by commas, with the
subcommand considered argument 1. The command argument cannot be masked.
For example, if the command is "admin passwd username password", then you'd want
to set logmask to 3, so the password argument gets logged as "**MASKED**". If the
command is "user passwd username old-password new-password", you'd want to set
logmask to "3,4".
stdin=(n | "last")
[2.14] Specifies that the nth or last argument to the command be passed on
standard input instead of on the command line. The value of this option must
either be the number of argument to pass on standard input (with the subcommand
considered argument 1) or the special value "last", which indicates that the final
argument (no matter how many there are) be passed on standard input.
The command cannot be passed on standard input, so n must be at least 1. If this
option is set to "last" and no arguments are given except the command and possibly
the subcommand, nothing will be passed on standard input.
This option is used primarily for passing large amounts of data that may not fit
on the command line or data that contains NUL characters. It can also be used for
arguments like passwords that shouldn't be exposed on the command line. Only at
most one argument may be passed on standard input to the command. Be aware that
even if the subcommand is the designated argument to pass on standard input
("stdin=1"), the subcommand may not contain NUL characters.
sudo=(username | #uid)
[3.12] Run this command as the specified user using sudo. This is exactly
equivalent to prepending "sudo -u username --" to the command before running it.
The path to sudo is determined when remctld is built.
The user option is simpler and easier if remctld is running as root. However, it
may be desirable in some configurations to run remctld as a non-root user, and
remctl-shell (which shares the same configuration files) usually runs as a non-
root user. In those cases, this option can be used to use sudo to switch users
before running the command.
Since the argument is passed verbatim to sudo's -u option, you can specify a
numeric UID by prepending it with "#".
summary=arg
[3.13] Specifies the argument for this command that will print a usage summary to
standard output.
If remctld receives the command "help" with no arguments, and no "help" command is
defined in the configuration file, the server will look through the configuration
for any command with a "summary" option set. If this option is set, and the user
is authorized to run the command, the server will run the specified executable
with the argument arg, sending the output back to the user. It will do this for
every command in the configuration that meets the above criteria.
This allows display of a summary of available commands to the user based on which
commands that user is authorized to run. It's a lightweight form of service
discovery. Also see the "help" option.
user=(username | uid)
[3.1] Run this command as the specified user, which can be given as either a
username or as a UID. Even if given as a UID, the user must be found in the user
database (searched via getpwuid(3)). remctld will run the command as the
specified user, including that user's primary and supplemental groups.
acl One or more entries of the form [method:]data, where method specifies an access
control method to be used, and data contains parameters whose meaning depends on the
method. If the method is omitted, the data is processed as described for the "file"
method.
If method is omitted, acl must either begin with "/" or must not contain "=".
Otherwise, it will be parsed as an option instead. If there is any ambiguity, prepend
the method.
As a special exception for backward compatibility, the ACL "ANYUSER" (case-sensitive)
is treated as equivalent to "anyuser:auth".
Each entry is checked in order, and access is granted as soon as an entry matches. If
no entry matches, access is denied. The following methods may supported; however, be
aware that the availability of several ACL types depends on whether remctld was built
with that support. Each ACL type is annotated with the version in which it was added.
anyuser
[3.10] Permit access to any user. This comes in two forms:
anyuser:auth
Permit any authenticated user. This means not only the local Kerberos realm
but also any realm with which there is a cross-realm trust relationship.
anyuser:anonymous
Permit entirely anonymous users. This means no authentication whatsoever is
required to run the command. Any client with network access to the server can
run the command (using anonymous PKINIT), assuming that anonymous service
tickets are enabled for the local Kerberos realm.
For backwards compatibility, the ACL "ANYUSER" is treated as identical to
"anyuser:auth". This was the only supported any-user ACL syntax prior to remctl
3.10.
file
[2.13] The data is the full path of an ACL file or to a directory containing ACL
files. Directories are handled as described for the include directive in
configuration files. An ACL file contains one entry per line, in the
[method:]data form described above. Entries are handled exactly as if they had
appeared in the configuration file except that the default method is "princ"
instead of "file". Blank lines and lines beginning with "#" are ignored in the
ACL files.
For backward compatibility, a line like:
include [<method>:]<data>
in an ACL file behaves exactly as if the "include" directive had been omitted,
except that the default method is "file". Thus, writing:
include <path>
in an ACL file is the same as writing:
file:<path>
and is handled identically to the include directive in configuration files.
princ
[2.13] The data is the name of a Kerberos v5 principal which is to be granted
access, such as "username@EXAMPLE.ORG".
deny
[2.13] This method is used to selectively deny access. The data is parsed as a
[method:]data and evaluated as described above, with the default scheme being
"princ". If it matches, access is denied immediately without examining any
further entries. Otherwise, processing continues.
Remember that access is granted as soon as an entry matches. For "deny" rules to
be effective, they therefore must come before any ACLs they are intended to
override. Be careful when using "deny" when including a directory of ACL files,
since the files in that directory are read in an undefined order (not in
alphabetical order by filename). It's best to explicitly include the file
containing "deny" ACL rules first.
Note that "deny" only denies access; it never grants it. Thus, deny alone does
not grant access to anyone, and using deny on itself as in "deny:deny:foo" neither
denies nor grants access to anyone.
gput
[2.13] This method is used to grant access based on the CMU GPUT (Global
Privileged User Table -- see gput(5)). The data is either a GPUT role name or a
string of the form group[xform], where group is a GPUT role name and xform is a
GPUT transform string. Access is granted if the user is a member of the specified
GPUT group, after applying either the optional xform or the default transform.
This method is supported only if remctld was compiled with GPUT support by using
the "--with-gput" configure option.
localgroup
[3.9] This method is used to grant or deny access based on membership in local
UNIX groups. The data is taken to be a name of a local system group. The user
principal is converted to a local user name with krb5_aname_to_localname(3) and
then compared to the members of the given group.
For example, to allow access to the members of group "goodguys", use an ACL of
"localgroup:goodguys" syntax. To deny access to the members of group "badguys",
use "deny:localgroup:badguys".
krb5_aname_to_localname() follows local configuration rules to determine how to
convert Kerberos principal to local users. If the realm of the principal is not
in a local realm and is not otherwise covered by one of those rules, the principal
will be unchanged, which will almost certainly mean that it will not be a member
of any local group and access will be denied.
This method is supported only if remctld was built with Kerberos support and the
getgrnam_r(3) library function was supported by the C library when it was built.
pcre
[2.16] This method is used to grant or deny access based on Perl-compatible
regular expressions. The data is taken to be a Perl-compatible regular expression
and matched against the user identity. To deny access, use the "deny:pcre:regex"
syntax.
The regular expression is not automatically anchored, so be careful to anchor it
at the start and end (with "\A" and "\z") to ensure that the entire principal name
is matched, unless you intend to allow partial matches.
This method is supported only if remctld was compiled with PCRE support (either
PCRE2 or PCRE1).
regex
[2.16] This method is used to grant or deny access based on POSIX extended regular
expressions. The data is taken to be a POSIX extended regular expression (like
those used by egrep) and matched against the user identity. To deny access, use
the "deny:regex:regex" syntax.
The regular expression is not automatically anchored, so be careful to anchor it
at the start and end (with "^" and "$") to ensure that the entire principal name
is matched, unless you intend to allow partial matches.
This method is supported only if a library for POSIX-compatible regular
expressions was found when remctld was built.
To see the list of ACL types supported by a particular build of remctld, run "remctld
-h".
The keyword ANYUSER may be used instead of the ACLs to allow access to all users. The
user still needs to authenticate to remctld; this only affects authorization. This
can be used for backend programs that want to check ACLs themselves and will retrieve
the authenticated principal from the REMOTE_USER environment variable. Note that
ANYUSER accepts any authenticated user, including cross-realm users from foreign
Kerberos realms.
ENVIRONMENT
remctld itself uses the following environment variables when run in stand-alone mode (-m):
LISTEN_FDS
LISTEN_PID
If these environment variables are set, remctld will expect to be provided its
listening sockets via the systemd socket activation protocol and will not attempt to
bind its own sockets. For more details on the protocol, see daemon(7) and
sd_listen_fds(3).
NOTIFY_SOCKET
If this environment variable is set, remctld will notify the socket named in this
variable when it is ready to accept incoming packets using the systemd status
notification protocol. For more details, see daemon(7) and sd_notify(3).
Note that using socket activation is recommended when running under systemd in stand-
alone mode, and status notification is not necessary or useful when using socket
activation.
When running in stand-alone mode, these environment variables will be cleared by remctld
before running any commands.
The following environment variables will be set for any commands run via remctld
(annotated with the version at which they were added):
REMCTL_COMMAND
[2.16] The command string that caused this command to be run. This variable will
contain only the command, not the subcommand or any additional arguments (which are
passed as command arguments).
REMOTE_ADDR
[2.1] The IP address of the remote host. This may be IPv4 or IPv6.
REMOTE_EXPIRES
[3.10] The time (in seconds since UNIX epoch) when the authenticated remote session
will expire. This will normally be the expiration time of the Kerberos ticket used to
authenticate to the server.
REMOTE_HOST
[2.1] The hostname of the remote host, if it was available. If reverse name
resolution failed, this environment variable will not be set.
This is determined via a simple reverse DNS lookup and should be considered under the
control of the client. remctl commands should treat it with skepticism and not use it
for anything other than logging purposes.
REMOTE_USER
REMUSER
[1.0 for REMUSER, 2.1 for REMOTE_USER] Set to the Kerberos principal of the
authenticated client.
If the -k flag is used, remctld will also set KRB5_KTNAME to the provided keytab path.
This is primarily for communication with the GSS-API library, but this setting will also
be inherited by any commands run by remctld.
EXAMPLES
remctld is normally started in one of two ways: either as a stand-alone daemon, or via
some network management service (for example, systemd or inetd) that handles listening for
incoming connections and forking remctld as needed.
To start remctld in stand-alone mode instead, run:
remctld -m
To start remctld in stand-alone mode in the foreground, use:
remctld -F -m
This is a typical invocation with systemd using socket activation. For upstart (with
"expect stop"), use:
remctld -F -m -Z
To start remctld via inetd, add this line to /etc/inetd.conf:
4373 stream tcp nowait root /usr/sbin/tcpd /usr/sbin/remctld
or:
remctl stream tcp nowait root /usr/sbin/tcpd /usr/sbin/remctld
if the "remctl" service is listed in your /etc/services file.
Example configuration file:
# Comments can be used like this.
accounts create /usr/local/bin/doaccount /etc/acl/group1 \
/etc/acl/group2
accounts delete /usr/local/bin/doaccount /etc/acl/group3
accounts view /usr/local/bin/doaccount ANYUSER
accounts passwd /usr/local/bin/dopasswd logmask=3 /etc/acl/group1
printing ALL /usr/local/bin/printthing /etc/acl/group2
The commands "accounts create", "accounts delete", and so forth will all be passed to
/usr/local/bin/doaccount with the first argument being the specific subcommand, with the
exception of "accounts passwd". That command will be passed to /usr/local/bin/dopasswd
instead, but it will still get "passwd" as its first argument. The third argument to
"accounts passwd" (presumably the password) will not be logged to syslog. All commands
starting with "printing" will be passed to /usr/local/bin/printthing.
Example ACL file:
# This is a comment.
deny:baduser@EXAMPLE.ORG
file:/etc/remctl/acl/admins
principal:service/admin@EXAMPLE.ORG
service/other@EXAMPLE.ORG
This ACL file will reject "baduser@EXAMPLE.ORG" even if that user would have been allowed
by one of the other ACL rules. It will then grant access according to the ACL entries in
/etc/remctl/acl/admins and the specific principals "service/admin@EXAMPLE.ORG" and
"service/other@EXAMPLE.ORG". The last line takes advantage of the default ACL method of
"principal" when processing an ACL file.
COMPATIBILITY
The version at which various command-line and configuration options and ACL methods were
added to remctld are noted in their descriptions. Below is the version information for
more general features, in reverse order of when the feature was added.
Support for the systemd readiness protocol and socket activation, including honoring the
environment variables LISTEN_FDS, LISTEN_PID, and NOTIFY_SOCKET, was added in version 3.7.
Special handling of the "help" and "summary" commands was added in version 3.2.
Support for the "ALL" keyword in the command field of the configuration file was added in
version 2.15. (It has always been supported in the subcommand field.)
Support for the "EMPTY" keyword in the subcommand field of the configuration file was
added in version 2.15.
Support for ACL schemes and the method:data syntax was added in remctl 2.13. Prior
versions of remctld expected only files in the main remctld configuration file, and only
principals or lines starting with "include" in those files, without any method: prefixes.
The default listening port with the -m option was changed to the IANA-registered port of
4373 in version 2.11.
Support for IPv6 addresses in the REMOTE_ADDR environment variable was added in version
2.4.
remctld used to set the environment variable SCPRINCIPAL when running commands, for
(partial) backward compatibility with sysctld, but stopped doing so in version 2.1.
"include" directives in ACL files were added in version 1.11. "include" directives in
configuration files were added in version 1.8.
CAVEATS
When using Heimdal with triple-DES keys and talking to old clients that only speak version
one of the remctl protocol, remctld may have problems with MIC verification. This doesn't
affect new clients and servers since the version two protocol doesn't use MICs. If you
are using Heimdal and run into MIC verification problems, see the COMPATIBILITY section of
gssapi(3).
remctld does not itself impose any limits on the number of child processes or other system
resources. You may want to set resource limits in systemd, your inetd server, or the
equivalent, or with ulimit when running it as a standalone daemon or under tcpserver.
Command arguments may not contain NUL characters and must be shorter than the operating
system limit on the length of a command line since they're passed to the command as
command-line arguments. The exception is an argument passed via standard input using the
"stdin=" option in the configuration file. At most one argument may be passed that way.
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
remctld was originally written by Anton Ushakov. Updates and current maintenance are done
by Russ Allbery <eagle@eyrie.org>.
COPYRIGHT AND LICENSE
Copyright 2015-2016, 2018, 2022 Russ Allbery <eagle@eyrie.org>
Copyright 2002-2012, 2014 The Board of Trustees of the Leland Stanford Junior University
Copying and distribution of this file, with or without modification, are permitted in any
medium without royalty provided the copyright notice and this notice are preserved. This
file is offered as-is, without any warranty.
SPDX-License-Identifier: FSFAP
SEE ALSO
remctl(1), syslog(3), tcpserver(1)
The current version of this program is available from its web page at
<https://www.eyrie.org/~eagle/software/remctl/>.