remctl-shell - Man Page

Restricted shell that mimics a remctl server

Synopsis

remctl-shell [-dhqSv] [-f config] -c command

remctl-shell [-dqS] [-f config] user

Description

remctl-shell is a restricted shell that mimics the behavior of the remctld server without using the remctl protocol, GSS-API, or Kerberos. It's intended to be run via ssh as either the shell or the forced command for a special user (by convention, remctl, although remctl-shell doesn't care), with an authorized_keys file that specifies the user identity corresponding to each key that is allowed to run remctl commands. All access control then works as normal.

The output of the command ran is returned on standard output and standard error, like a normal command run via ssh, and the exit status of remctl-shell will be the exit status of the command.  Only one command can be run per ssh connection, so this will be noticeably slower for each command execution than a well-designed remctl client and server design that holds connections open for multiple commands.

remctl-shell is designed to mimic the behavior of remctld and uses the same configuration syntax and environment variables.  See “CONFIGURATION FILE” in remctld(8) for configuration information and Environment below for more specific details about environment variable handling.  The location of the configuration file may be specified with the -f option.  The default location is /etc/remctl.conf.

Since remctl-shell is designed to be run by a potentially untrusted user as a shell, normally all error messages and logging is done via syslog and not sent to standard error.  See the -S, -d, and -q options when running it manually to debug problems.  (When running manually, you will also normally need to set the SSH_CONNECTION environment variable and either REMCTL_USER or SSH_ORIGINAL_COMMAND depending on how you invoke it.)

Quoting and Command Limitations

The ssh protocol is much less sophisticated than remctl at passing command arguments from the client to the server, so remctl-shell requires careful attention to command arguments and quoting.  ssh does no quoting of arguments, just adds a single space between each argument and passes them verbatim to the shell on the server side.  This means the client has to add quoting to any arguments containing whitespace.  remctl-shell supports single and double quotes, and supports using backslash to escape any character inside or outside either quotes.  However, be aware, when running ssh from the command line, that your shell will remove another level of quoting.  You will therefore usually have to double-quote arguments.

For example, to run the command log message with argument this is a message via ssh from the command line, use:

    ssh remctl@example.com log message "'this is a message'"

The first level of "" quoting will be removed by your local shell, and remctl-shell will interpret the second level of '' quotes.  Note that, because of how ssh does command argument passing, this is exactly equivalent to:

    ssh remctl@example.com "log message 'this is a message'"

since ssh doesn't preserve the distinction between separate arguments when creating the command to send to the remote server.  It may be less confusing to get in the habit of quoting the entire command.

Also be aware that the full command is passed via command line arguments, which means, when invoking remctl-shell as a shell, there is a tight limit on the length of the whole command plus arguments.  Expect to have problems if the total command length exceeds 1000 characters.  For the same reason, binary data including null characters cannot be passed via remctl-shell.  Invoking it as a forced command may work around these limitations by putting the command into the environment instead, but there may still be restrictions on that.  (The regular remctl protocol supports arbitrary-length arguments, limited only by server-side configuration and available server memory, and supports arbitrary binary data in arguments.)

authorized_keys Configuration

remctl-shell is intended for use via ssh using authorized_keys to manage authentication.  (If you have Kerberos available, it's generally better to use the normal remctld server and native remctl protocol.)

There are two ways to set up remctl-shell: either by specifying forced commands, or by configuring remctl-shell as the shell of the account. The forced command approach is recommended, since it doesn't require setting a non-default sshd_config option.

Using forced commands

For the role account that you want to use to run remctl commands (remctl by convention), create an authorized_keys file listing everyone who should be able to run commands.  Before each key, set the command option like the below:

    command="/usr/sbin/remctl-shell example@EXAMPLE.ORG"

where the argument to remctl-shell is the identity matching the ssh key on that line.  A more complete example of a line in authorized_keys:

    command="/usr/sbin/remctl-shell example@EXAMPLE.ORG",\
    no-agent-forwarding,no-port-forwarding,no-pty,no-user-rc,\
    no-X11-forwarding ssh-rsa AAAAB3NzaC1yc2EA... \
    example@some-host.example.org

Backslashes and line breaks were added for clarity.  The actual entry should be a single long line.  For more information on the other settings here, see Examples below.

Using a shell

When running remctl-shell as the shell of the account, instead of using forced commands, the authorized_keys configuration must be set up to associate each key with an identity by setting the REMCTL_USER environment variable.  Using user identities that look like Kerberos principal names is strongly recommended, since it may make it easier to use some of the ACL methods intended for the normal remctl server.

Since this relies on setting environment variables via authorized_keys, you unfortunately have to enable PermitUserEnvironment in sshd_config (this is not the default) by adding:

    PermitUserEnvironment yes

Other options

remctl-shell will not make use of forwarded connections or agents, and will not pass them along to the processes they run, so all such ssh options should normally be disabled for defense in depth security.

Examples

Here is a recommended line in authorized_keys for the account managed by remctl-shell, with appropriate restrictions and an example of how to set the REMCTL_USER variable.  Backslashes and line breaks were added for clarity.  The actual entry should be a single long line.

    environment="REMCTL_USER=example@EXAMPLE.ORG",no-agent-forwarding,\
    no-port-forwarding,no-pty,no-user-rc,no-X11-forwarding ssh-rsa \
    AAAAB3NzaC1yc2EA... example@some-host.example.org

Setting no-user-rc is particularly important for remctl-shell.  If you have OpenSSH 7.2 or later, which added the restrict keyword, you can instead use the much simpler:

    environment="REMCTL_USER=example@EXAMPLE.ORG",restrict ssh-rsa \
    AAAAB3NzaC1yc2EA... example@some-host.example.org

REMCTL_USER should be set to the identity string for the owner of that key pair, as used in the ACLs in your remctl configuration.

Options

remctl-shell is normally only run with either the -c option or with a user, since it's intended for use as a shell or forced command. However, it does support some other options for testing, which may be useful in authorized_keys.  If using it as a shell, one can use a small wrapper program as the configured shell that passes additional options into remctl-shell if needed.

The start of each option description is annotated with the version of remctl-shell in which that option was added with its current meaning.

-c command

[3.12] The command to run.  This is how ssh passes the command string into remctl-shell.  remctl-shell will then parse it into separate arguments using an algorithm similar to that used by a shell.  See the above discussion of quoting for more information.

This is mandatory when using remctl-shell as a shell.  If using it as a forced command, pass the user on the command line instead and do not use this option.

-d

[3.12] Enable verbose debug logging to syslog (or to standard output if -S is also given).

-f config

[3.12] The configuration file for remctld, overriding the default path.

-h

[3.12] 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 remctl-shell build.

-q

[3.12] Suppress the normal informational logging of what commands are being executed and by whom.  This is intended primarily to avoid spamming syslog during testing.

-S

[3.12] Rather than logging to syslog, log debug and routine connection messages to standard output and error messages to standard error.  In normal usage, this would send all the logging back to the client, intermixed with program output, so it's normally useful only for testing and debugging.

-v

[3.12] Print the version of remctl-shell and exit.

Environment

remctl-shell itself requires the following environment variables be set when it is invoked, or it exits with an error and doesn't do anything.

REMCTL_USER

The user used for logging and to make authorization decisions, unless that was passed on the command line.  The security of all remctl-shell authorization checks is based on the accuracy of this environment variable, so be sure that it is set correctly.  The best way to do this is via environment stanzas in authorized_keys as described above.  If the user is passed on the command line, this is ignored.

SSH_CONNECTION

sshd uses this environment variable to communication information about the local and remote IP addresses and ports of the ssh connection. remctl-shell expects the first space-separated token in this environment variable to be the IP address of the client.  It then uses that to set REMOTE_ADDR in the environment of any commands it runs.

SSH_ORIGINAL_COMMAND

When run as a forced command, the command run by the user is taken from this environment variable, which is set by sshd.

The following environment variables will be set for any commands run via remctl-shell (annotated with the version at which they were added). These are mostly the same as those set by remctld.  Differences are noted in each description.

REMCTL_COMMAND

[3.12] 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

[3.12] The IP address of the remote host.  This may be IPv4 or IPv6.  This is taken from the SSH_CONNECTION environment variable.

REMOTE_EXPIRES

[3.12] Normally, this communicates the time (in seconds since UNIX epoch) when the authenticated remote session will expire.  However, this is not a meaningful concept for ssh authentication via public key, and regardless is not communicated by sshd to the shell.  It is therefore always set to 0 by remctl-shell.

REMOTE_HOST

[3.12] 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

[3.12] Set to the value of REMCTL_CLIENT as set in the environment of remctl-shell. This should be set securely via authorized_keys as discussed above.

Note that REMOTE_HOST is not set by remctl-shell, at least currently.

Examples

Typically, remctl-shell will either be run as a forced command or set as the shell for a dedicated user, normally remctl, via the normal mechanism for local account creation.  That account should be configured with an ssh authorized_keys file as discussed above.  remctl-shell will then be invoked with:

    remctl-shell -c 'command subcommand argument'

(if used as a shell) or with:

    remctl-shell user@EXAMPLE.ORG

(if used as a forced command) by sshd for each incoming connection from a user that has a key in the authorized_keys file.

If you need to run a command manually for debugging, you can run the same command as above, but it's often more useful to send errors to standard error instead of to syslog.  You can do that with:

    remctl-shell -S -c 'command subcommand argument'

If you don't want to see the normal command logging, add the -q option as well.  You can test an alternate configuration file by specifying it with the -f option.  You will need to set SSH_CONNECTION and either REMCTL_USER (if using -c) or SSH_ORIGINAL_COMMAND (if passing the user on the command line).

Compatibility

remctl-shell was added in the remctl 3.12 release.

The forced command mode where the user can be passed on the command line and the command retrieved from SSH_ORIGINAL_COMMAND was added in the remctl 3.13 release.

Caveats

Most of the caveats and differences between remctl-shell and the normal remctld server are from quoting and the limitations of passing arguments via the command line.  Review the section on quoting above for more information.

Normally, remctl-shell runs as a dedicated non-root user (as opposed to often running as root like remctld), which means that all commands will normally run as that user and the user configuration option will not work.  The easiest way to run commands as other users is to have the underlying command use sudo or some other user switching mechanism, which will normally require additional local configuration.

User environment setting has to be enabled in sshd by setting the non-default PermitUserEnvironment configuration option.  A future version of remctl-shell may use forced commands with an argument instead of a shell to avoid this.

Author

remctl-shell was written by Russ Allbery <eagle@eyrie.org>.  Many thanks to Dropbox, Inc. for providing the time to write the initial implementation during Dropbox's annual Hack Week.

See Also

remctld(8), sshd(8)

The current version of this program is available from its web page at <https://www.eyrie.org/~eagle/software/remctl/>.

Info

2020-12-13 3.17 remctl