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REMCTLD

REMCTLD

Section: remctl (8) Updated: 2010-05-02
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NAME

remctld - Server for remctl, a remote command execution utility  

SYNOPSIS

remctld [-dFhmSv] [-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

-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, so the program run will not be able to distinguish between different commands.

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:
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 subcommand cannot be passed on standard input, so n must be at least 2. 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.

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 Board of Trustees, Leland Stanford Jr. University. All rights reserved.

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.


 

Index

NAME
SYNOPSIS
DESCRIPTION
OPTIONS
CONFIGURATION FILE
ENVIRONMENT
EXAMPLES
CAVEATS
NOTES
SEE ALSO
AUTHOR
COPYRIGHT AND LICENSE

This document was created by man2html, using the manual pages.
Time: 22:02:14 GMT, April 16, 2011