REMCTLD(8) remctl REMCTLD(8)
NAME
remctld - Server for remctl, a remote command execution utility
SYNOPSIS
remctld [-dFhmSv] [-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 GSS-
API Kerberos v5, 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.
OPTIONS
-b bind-address
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.
-d Enable verbose debug logging to syslog (or to standard output if -S is also given).
-F 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
The configuration file for remctld, overriding the default path.
-h 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
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 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.
-P file
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
When running in stand-alone mode, Listen on port port rather than the default. This option does nothing unless used with -m.
-S 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
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 Print the version of remctld and exit.
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 meaning of these fields is:
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, the command is not. Prior to remctl 2.16, the program run will not be able to distinguish between different
commands. From remctl 2.16 on, the environment variable REMCTL_COMMAND will contain the command. (See the ENVIRONMENT section 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 are:
help=arg
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[,...]
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")
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.
summary=arg
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, the subcommand is "ALL",
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.
As mentioned above, this option is only meaningful on configuration lines with a subcommand of "ALL".
user=(username | uid)
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.
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 are supported:
file
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
The data is the name of a Kerberos v5 principal which is to be granted access, such as "username@EXAMPLE.ORG".
deny
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
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.
pcre
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.
This method is supported only if remctld was compiled with PCRE support by using the "--with-pcre" configure option.
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.
Support for ACL schemes is new 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.
ENVIRONMENT
The following environment variables will be set for any commands run via remctld:
REMOTE_USER
REMUSER
Set to the Kerberos principal of the authenticated client. REMUSER has always been set by remctld; REMOTE_USER is also set (to the
same value) starting with remctl 2.1.
REMOTE_ADDR
The IP address of the remote host. Currently, this is always an IPv4 address, but in the future it may be set to an IPv6 address.
This environment variable was added in remctl 2.1.
REMOTE_HOST
The hostname of the remote host, if it was available. If reverse name resolution failed, this environment variable will not be set.
This variable was added in remctl 2.1.
REMCTL_COMMAND
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). This variable was added in remctl 2.16.
remctld also used to set SCPRINCIPAL for (partial) backward compatibility with sysctld, but stopped doing so as of remctl 2.1.
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
Typically remctld is to be started as follows, where "hostname" is the machine where remctld will run, and 4373 is the port:
tcpserver hostname 4373 remctld
The equivalent line for /etc/inetd.conf is:
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.
To start remctld in stand-alone mode instead, run:
remctld -m
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 using the scheme support new in remctl 2.13:
# 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.
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 your inetd server 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.
SEE ALSO
remctl(1), syslog(3), tcpserver(1)
The current version of this program is available from its web page at <http://www.eyrie.org/~eagle/software/remctl/>.
AUTHOR
Anton Ushakov <antonu@stanford.edu> is the original author. Updates and current maintenance are done by Russ Allbery <rra@stanford.edu>.
COPYRIGHT AND LICENSE
Copyright 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 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.
3.2 2012-06-19 REMCTLD(8)