openvpn/doc/management-notes.txt

OpenVPN Management Interface Notes
----------------------------------

The OpenVPN Management interface allows OpenVPN to
be administratively controlled from an external program via
a TCP or unix domain socket.

The interface has been specifically designed for developers
who would like to programmatically or remotely control
an OpenVPN daemon, and can be used when OpenVPN is running
as a client or server.

The management interface is implemented using a client/server TCP
connection or unix domain socket where OpenVPN will listen on a
provided IP address and port for incoming management client connections.

The management protocol is currently cleartext without an explicit
security layer.  For this reason, it is recommended that the
management interface either listen on a unix domain socket,
localhost (127.0.0.1), or on the local VPN address.  It's possible
to remotely connect to the management interface over the VPN itself,
though some capabilities will be limited in this mode, such as the
ability to provide private key passwords.

The management interface is enabled in the OpenVPN
configuration file using the following directive:

--management

See the man page for documentation on this and related
directives.

Once OpenVPN has started with the management layer enabled,
you can telnet to the management port (make sure to use
a telnet client which understands "raw" mode).

Once connected to the management port, you can use
the "help" command to list all commands.

COMMAND -- bytecount
--------------------

The bytecount command is used to request real-time notification
of OpenVPN bandwidth usage.

Command syntax:

  bytecount n (where n > 0) -- set up automatic notification of
                               bandwidth usage once every n seconds
  bytecount 0 -- turn off bytecount notifications

If OpenVPN is running as a client, the bytecount notification
will look like this:

  >BYTECOUNT:{BYTES_IN},{BYTES_OUT}

BYTES_IN is the number of bytes that have been received from
the server and BYTES_OUT is the number of bytes that have been
sent to the server.

If OpenVPN is running as a server, the bytecount notification
will look like this:

  >BYTECOUNT_CLI:{CID},{BYTES_IN},{BYTES_OUT}
 
CID is the Client ID, BYTES_IN is the number of bytes that have
been received from the client and BYTES_OUT is the number of
bytes that have been sent to the client.

Note that when the bytecount command is used on the server, every
connected client will report its bandwidth numbers once every n
seconds.

When the client disconnects, the final bandwidth numbers will be
placed in the 'bytes_received' and 'bytes_sent' environmental variables
as included in the >CLIENT:DISCONNECT notification.

COMMAND -- echo
---------------

The echo capability is used to allow GUI-specific
parameters to be either embedded in the OpenVPN config file
or pushed to an OpenVPN client from a server.

Command examples:

  echo on      -- turn on real-time notification of echo messages
  echo all     -- print the current echo history list
  echo off     -- turn off real-time notification of echo messages
  echo on all  -- atomically enable real-time notification,
                  plus show any messages in history buffer

For example, suppose you are developing a OpenVPN GUI and
you want to give the OpenVPN server the ability to ask
the GUI to forget any saved passwords.

In the OpenVPN server config file, add:

  push "echo forget-passwords"

When the OpenVPN client receives its pulled list of directives
from the server, the "echo forget-passwords" directive will
be in the list, and it will cause the management interface
to save the "forget-passwords" string in its list of echo
parameters.

The management client can use "echo all" to output the full
list of echoed parameters, "echo on" to turn on real-time
notification of echoed parameters via the ">ECHO:" prefix,
or "echo off" to turn off real-time notification.

When the GUI connects to the OpenVPN management socket, it
can issue an "echo all" command, which would produce output
like this:

  1101519562,forget-passwords
  END

Essentially the echo command allowed us to pass parameters from
the OpenVPN server to the OpenVPN client, and then to the
management client (such as a GUI).  The large integer is the
unix date/time when the echo parameter was received.

If the management client had issued the command "echo on",
it would have enabled real-time notifications of echo
parameters.  In this case, our "forget-passwords" message
would be output like this:

  >ECHO:1101519562,forget-passwords

Like the log command, the echo command can atomically show
history while simultaneously activating real-time updates:

  echo on all

The size of the echo buffer is currently hardcoded to 100
messages.

COMMAND -- exit, quit
---------------------

Close the managment session, and resume listening on the
management port for connections from other clients. Currently,
the OpenVPN daemon can at most support a single management client
any one time.

COMMAND -- help
---------------

Print a summary of commands.

COMMAND -- hold
---------------

The hold command can be used to manipulate the hold flag,
or release OpenVPN from a hold state.

If the hold flag is set on initial startup or
restart, OpenVPN will hibernate prior to initializing
the tunnel until the management interface receives
a "hold release" command.

The --management-hold directive of OpenVPN can be used
to start OpenVPN with the hold flag set.

The hold flag setting is persistent and will not
be reset by restarts.

OpenVPN will indicate that it is in a hold state by
sending a real-time notification to the management
client, the parameter indicates how long OpenVPN would
wait without UI (as influenced by connect-retry exponential
backoff). The UI needs to wait for releasing the hold if it
wants similar behavior:

  >HOLD:Waiting for hold release:10

Command examples:

  hold         -- show current hold flag, 0=off, 1=on.
  hold on      -- turn on hold flag so that future restarts
                  will hold.
  hold off     -- turn off hold flag so that future restarts will
                  not hold.
  hold release -- leave hold state and start OpenVPN, but
                  do not alter the current hold flag setting.

COMMAND -- kill
---------------

In server mode, kill a particlar client instance.

Command examples:

  kill Test-Client -- kill the client instance having a
                      common name of "Test-Client".
  kill 1.2.3.4:4000 -- kill the client instance having a
                       source address and port of 1.2.3.4:4000

Use the "status" command to see which clients are connected.

COMMAND -- log
--------------

Show the OpenVPN log file.  Only the most recent n lines
of the log file are cached by the management interface, where
n is controlled by the OpenVPN --management-log-cache directive.

Command examples:

  log on     -- Enable real-time output of log messages.
  log all    -- Show currently cached log file history.
  log on all -- Atomically show all currently cached log file
                history then enable real-time notification of
                new log file messages.
  log off    -- Turn off real-time notification of log messages.
  log 20     -- Show the most recent 20 lines of log file history.

Real-time notification format:

Real-time log messages begin with the ">LOG:" prefix followed
by the following comma-separated fields:

  (a) unix integer date/time,
  (b) zero or more message flags in a single string:
      I -- informational
      F -- fatal error
      N -- non-fatal error
      W -- warning
      D -- debug, and
  (c) message text.

COMMAND -- mute
---------------

Change the OpenVPN --mute parameter.  The mute parameter is
used to silence repeating messages of the same message
category.

Command examples:

  mute 40 -- change the mute parameter to 40
  mute    -- show the current mute setting

COMMAND -- net
--------------

(Windows Only) Produce output equivalent to the OpenVPN
--show-net directive.  The output includes OpenVPN's view
of the system network adapter list and routing table based
on information returned by the Windows IP helper API.

COMMAND -- pid
--------------

Shows the process ID of the current OpenVPN process.

COMMAND -- password and username
--------------------------------

  The password command is used to pass passwords to OpenVPN.

  If OpenVPN is run with the --management-query-passwords
  directive, it will query the management interface for RSA
  private key passwords and the --auth-user-pass
  username/password.

  When OpenVPN needs a password from the management interface,
  it will produce a real-time ">PASSWORD:" message.

  Example 1:

    >PASSWORD:Need 'Private Key' password

  OpenVPN is indicating that it needs a password of type
  "Private Key".

  The management client should respond to this query as follows:

    password "Private Key" foo

  Example 2:

    >PASSWORD:Need 'Auth' username/password

  OpenVPN needs a --auth-user-pass password.  The management
  client should respond:

    username "Auth" foo
    password "Auth" bar

  The username/password itself can be in quotes, and special
  characters such as double quote or backslash must be escaped,
  for example,

    password "Private Key" "foo\"bar"

  The escaping rules are the same as for the config file.
  See the "Command Parsing" section below for more info.

  The PASSWORD real-time message type can also be used to
  indicate password or other types of authentication failure:

  Example 3: The private key password is incorrect and OpenVPN
  is exiting:

    >PASSWORD:Verification Failed: 'Private Key'

  Example 4: The --auth-user-pass username/password failed,
  and OpenVPN is exiting:

    >PASSWORD:Verification Failed: 'Auth'

  Example 5: The --auth-user-pass username/password failed,
  and the server provided a custom client-reason-text string
  using the client-deny server-side management interface command.

    >PASSWORD:Verification Failed: 'custom server-generated string'

  Example 6: If server pushes --auth-token to the client, the OpenVPN
  will produce a real-time PASSWORD message:

    >PASSWORD:Auth-Token:foobar

COMMAND -- forget-passwords
---------------------------

The forget-passwords command will cause the daemon to forget passwords
entered during the session.

Command example:

  forget-passwords -- forget passwords entered so far.

COMMAND -- signal
-----------------

The signal command will send a signal to the OpenVPN daemon.
The signal can be one of SIGHUP, SIGTERM, SIGUSR1, or SIGUSR2.

Command example:

  signal SIGUSR1 -- send a SIGUSR1 signal to daemon

COMMAND -- state
----------------

Show the current OpenVPN state, show state history, or
enable real-time notification of state changes.

These are the OpenVPN states:

CONNECTING    -- OpenVPN's initial state.
WAIT          -- (Client only) Waiting for initial response
                 from server.
AUTH          -- (Client only) Authenticating with server.
GET_CONFIG    -- (Client only) Downloading configuration options
                 from server.
ASSIGN_IP     -- Assigning IP address to virtual network
                 interface.
ADD_ROUTES    -- Adding routes to system.
CONNECTED     -- Initialization Sequence Completed.
RECONNECTING  -- A restart has occurred.
EXITING       -- A graceful exit is in progress.

Command examples:

  state        -- Print current OpenVPN state.
  state on     -- Enable real-time notification of state changes.
  state off    -- Disable real-time notification of state changes.
  state all    -- Print current state history.
  state 3      -- Print the 3 most recent state transitions.
  state on all -- Atomically show state history while at the
                  same time enable real-time state notification
		  of future state transitions.

The output format consists of up to 9 comma-separated parameters:
  (a) the integer unix date/time,
  (b) the state name,
  (c) optional descriptive string (used mostly on RECONNECTING
      and EXITING to show the reason for the disconnect),
  (d) optional TUN/TAP local IPv4 address
  (e) optional address of remote server,
  (f) optional port of remote server,
  (g) optional local address,
  (h) optional local port, and
  (i) optional TUN/TAP local IPv6 address.

Fields (e)-(h) are shown for CONNECTED state,
(d) and (i) are shown for ASSIGN_IP and CONNECTED states.

(e) is available starting from OpenVPN 2.1
(f)-(i) are available starting from OpenVPN 2.4

Real-time state notifications will have a ">STATE:" prefix
prepended to them.

COMMAND -- status
-----------------

Show current daemon status information, in the same format as
that produced by the OpenVPN --status directive.

Command examples:

status   -- Show status information using the default status
            format version.

status 3 -- Show status information using the format of
            --status-version 3.

COMMAND -- username
-------------------

See the "password" section above.

COMMAND -- verb
---------------

Change the OpenVPN --verb parameter.  The verb parameter
controls the output verbosity, and may range from 0 (no output)
to 15 (maximum output).  See the OpenVPN man page for additional
info on verbosity levels.

Command examples:

  verb 4  -- change the verb parameter to 4
  verb    -- show the current verb setting

COMMAND -- version
------------------

Show the current OpenVPN and Management Interface versions.


COMMAND -- auth-retry
---------------------

Set the --auth-retry setting to control how OpenVPN responds to
username/password authentication errors.  See the manual page
for more info.

Command examples:

  auth-retry interact -- Don't exit when bad username/passwords are entered.
                         Query for new input and retry.

COMMAND -- needok  (OpenVPN 2.1 or higher)
------------------------------------------

Confirm a ">NEED-OK" real-time notification, normally used by
OpenVPN to block while waiting for a specific user action.

Example:

  OpenVPN needs the user to insert a cryptographic token,
  so it sends a real-time notification:

    >NEED-OK:Need 'token-insertion-request' confirmation MSG:Please insert your cryptographic token

  The management client, if it is a GUI, can flash a dialog
  box containing the text after the "MSG:" marker to the user.
  When the user acknowledges the dialog box,
  the management client can issue this command:

     needok token-insertion-request ok
  or
     needok token-insertion-request cancel

COMMAND -- needstr  (OpenVPN 2.1 or higher)
-------------------------------------------

Confirm a ">NEED-STR" real-time notification, normally used by
OpenVPN to block while waiting for a specific user input.

Example:

  OpenVPN needs the user to specify some input, so it sends a
  real-time notification:

    >NEED-STR:Need 'name' input MSG:Please specify your name

  The management client, if it is a GUI, can flash a dialog
  box containing the text after the "MSG:" marker to the user.
  When the user acknowledges the dialog box,
  the management client can issue this command:

     needstr name "John"

COMMAND -- pkcs11-id-count  (OpenVPN 2.1 or higher)
---------------------------------------------------

Retrieve available number of certificates.

Example:

     pkcs11-id-count
     >PKCS11ID-COUNT:5

COMMAND -- pkcs11-id-get  (OpenVPN 2.1 or higher)
-------------------------------------------------

Retrieve certificate by index, the ID string should be provided
as PKCS#11 identity, the blob is BASE64 encoded certificate.

Example:

     pkcs11-id-get 1
     PKCS11ID-ENTRY:'1', ID:'<snip>', BLOB:'<snip>'

COMMAND -- client-auth  (OpenVPN 2.1 or higher)
-----------------------------------------------

Authorize a ">CLIENT:CONNECT" or ">CLIENT:REAUTH" request and specify
"client-connect" configuration directives in a subsequent text block.

The OpenVPN server should have been started with the
--management-client-auth directive so that it will ask the management
interface to approve client connections.


  client-auth {CID} {KID}
  line_1
  line_2
  ...
  line_n
  END

CID,KID -- client ID and Key ID.  See documentation for ">CLIENT:"
notification for more info.

line_1 to line_n -- client-connect configuration text block, as would be
returned by a --client-connect script.  The text block may be null, with
"END" immediately following the "client-auth" line (using a null text
block is equivalent to using the client-auth-nt command).

A client-connect configuration text block contains OpenVPN directives
that will be applied to the client instance object representing a newly
connected client.

COMMAND -- client-auth-nt  (OpenVPN 2.1 or higher)
--------------------------------------------------

Authorize a ">CLIENT:CONNECT" or ">CLIENT:REAUTH" request without specifying
client-connect configuration text.

The OpenVPN server should have been started with the
--management-client-auth directive so that it will ask the management
interface to approve client connections.

  client-auth-nt {CID} {KID}

CID,KID -- client ID and Key ID.  See documentation for ">CLIENT:"
notification for more info.

COMMAND -- client-deny  (OpenVPN 2.1 or higher)
-----------------------------------------------

Deny a ">CLIENT:CONNECT" or ">CLIENT:REAUTH" request.

  client-deny {CID} {KID} "reason-text" ["client-reason-text"]

CID,KID -- client ID and Key ID.  See documentation for ">CLIENT:"
notification for more info.

reason-text: a human-readable message explaining why the authentication
request was denied.  This message will be output to the OpenVPN log
file or syslog.

client-reason-text: a message that will be sent to the client as
part of the AUTH_FAILED message.

Note that client-deny denies a specific Key ID (pertaining to a
TLS renegotiation).  A client-deny command issued in response to
an initial TLS key negotiation (notified by ">CLIENT:CONNECT") will
terminate the client session after returning "AUTH-FAILED" to the client.
On the other hand, a client-deny command issued in response to
a TLS renegotiation (">CLIENT:REAUTH") will invalidate the renegotiated
key, however the TLS session associated with the currently active
key will continue to live for up to --tran-window seconds before
expiration.

To immediately kill a client session, use "client-kill".

COMMAND -- client-kill  (OpenVPN 2.1 or higher)
-----------------------------------------------

Immediately kill a client instance by CID.

  client-kill {CID}

CID -- client ID.  See documentation for ">CLIENT:" notification for more
info.

COMMAND -- client-pf  (OpenVPN 2.1 or higher)
---------------------------------------------

Push a packet filter file to a specific client.

The OpenVPN server should have been started with the
--management-client-pf directive so that it will require that
VPN tunnel packets sent or received by client instances must
conform to that client's packet filter configuration.

  client-pf {CID}
  line_1
  line_2
  ...
  line_n
  END

CID -- client ID.  See documentation for ">CLIENT:" notification for
more info.

line_1 to line_n -- the packet filter configuration file for this
client.

Packet filter file grammar:

 [CLIENTS DROP|ACCEPT]
 {+|-}common_name1
 {+|-}common_name2
 . . .
 [SUBNETS DROP|ACCEPT]
 {+|-}subnet1
 {+|-}subnet2
 . . .
 [END]

 Subnet: IP-ADDRESS | IP-ADDRESS/NUM_NETWORK_BITS | "unknown"

 CLIENTS refers to the set of clients (by their common-name) which
 this instance is allowed ('+') to connect to, or is excluded ('-')
 from connecting to.  Note that in the case of client-to-client
 connections, such communication must be allowed by the packet filter
 configuration files of both clients AND the --client-to-client
 directive must have been specified in the OpenVPN server config.

 SUBNETS refers to IP addresses or IP address subnets which this
 client instance may connect to ('+') or is excluded ('-') from
 connecting to, and applies to IPv4 and ARP packets.  The special
 "unknown" tag refers to packets of unknown type, i.e. a packet that
 is not IPv4 or ARP.

 DROP or ACCEPT defines default policy when there is no explicit match
 for a common-name or subnet.  The [END] tag must exist.

 Notes:

 * The SUBNETS section currently only supports IPv4 addresses and
   subnets.

 * A given client or subnet rule applies to both incoming and
   outgoing packets.

 * The CLIENTS list is order-invariant.  Because the list is stored
   as a hash-table, the order of the list does not affect its function.

 * The SUBNETS table is scanned sequentially, and the first item to
   match is chosen.  Therefore the SUBNETS table is NOT order-invariant.

 * No client-to-client communication is allowed unless the
   --client-to-client configuration directive is enabled AND
   the CLIENTS list of BOTH clients allows the communication.

Example packet filter spec, as transmitted to the management interface:

 client-pf 42
 [CLIENTS ACCEPT]
 -accounting
 -enigma
 [SUBNETS DROP]
 -10.46.79.9
 +10.0.0.0/8
 [END]
 END

The above example sets the packet filter policy for the client
identified by CID=42.  This client may connect to all other clients
except those having a common name of "accounting" or "enigma".
The client may only interact with external IP addresses in the
10.0.0.0/8 subnet, however access to 10.46.79.9 is specifically
excluded.

Another example packet filter spec, as transmitted to the
management interface:

 client-pf 99
 [CLIENTS DENY]
 +public
 [SUBNETS ACCEPT]
 +10.10.0.1
 -10.0.0.0/8
 -unknown
 [END]
 END

The above example sets the packet filter policy for the client
identified by CID=99.  This client may not connect to any other
clients except those having a common name of "public".  It may
interact with any external IP address except those in the
10.0.0.0/8 netblock.  However interaction with one address in
the 10.0.0.0/8 netblock is allowed: 10.10.0.1.  Also, the client
may not interact with external IP addresses using an "unknown"
protocol (i.e. one that is not IPv4 or ARP).

COMMAND -- remote  (OpenVPN AS 2.1.5/OpenVPN 2.3 or higher)
--------------------------------------------

Provide remote host/port in response to a >REMOTE notification
(client only).  Requires that the --management-query-remote
directive is used.

  remote ACTION [HOST PORT]

The "remote" command should only be given in response to a >REMOTE
notification.  For example, the following >REMOTE notification
indicates that the client config file would ordinarily connect
to vpn.example.com port 1194 (UDP):

  >REMOTE:vpn.example.com,1194,udp

Now, suppose we want to override the host and port, connecting
instead to vpn.otherexample.com port 1234.  After receiving
the above notification, use this command:

  remote MOD vpn.otherexample.com 1234

To accept the same host and port as the client would ordinarily
have connected to, use this command:

  remote ACCEPT

To skip the current connection entry and advance to the next one,
use this command:

  remote SKIP

COMMAND -- proxy  (OpenVPN 2.3 or higher)
--------------------------------------------

Provide proxy server host/port and flags in response to a >PROXY
notification (client only).  Requires that the --management-query-proxy
directive is used.

  proxy TYPE HOST PORT ["nct"]

The "proxy" command must only be given in response to a >PROXY
notification.  Use the "nct" flag if you only want to allow
non-cleartext auth with the proxy server.  The following >PROXY
notification indicates that the client config file would ordinarily
connect to the first --remote configured, vpn.example.com using TCP:

  >PROXY:1,TCP,vpn.example.com

Now, suppose we want to connect to the remote host using the proxy server
proxy.intranet port 8080 with secure authentication only, if required.
After receiving the above notification, use this command:

  proxy HTTP proxy.intranet 8080 nct

You can also use the SOCKS keyword to pass a SOCKS server address, like:

  proxy SOCKS fe00::1 1080

To accept connecting to the host and port directly, use this command:

  proxy NONE

COMMAND -- rsa-sig (OpenVPN 2.3 or higher)
------------------------------------------
Provides support for external storage of the private key. Requires the
--management-external-key option. This option can be used instead of "key"
in client mode, and allows the client to run without the need to load the
actual private key. When the SSL protocol needs to perform an RSA sign
operation, the data to be signed will be sent to the management interface
via a notification as follows:

>RSA_SIGN:[BASE64_DATA]

The management interface client should then create a PKCS#1 v1.5 signature of
the (decoded) BASE64_DATA using the private key and return the SSL signature as
follows:

rsa-sig
[BASE64_SIG_LINE]
.
.
.
END

Base64 encoded output of RSA_private_encrypt() (OpenSSL) or mbedtls_pk_sign()
(mbed TLS) will provide a correct signature.

This capability is intended to allow the use of arbitrary cryptographic
service providers with OpenVPN via the management interface.

COMMAND -- certificate (OpenVPN 2.4 or higher)
----------------------------------------------
Provides support for external storage of the certificate. Requires the
--management-external-cert option. This option can be used instead of "cert"
in client mode. On SSL protocol initialization a notification will be sent
to the management interface with a hint as follows:

>NEED-CERTIFICATE:macosx-keychain:subject:o=OpenVPN-TEST

The management interface client should use the hint to obtain the specific
SSL certificate and then return base64 encoded certificate as follows:

certificate
[BASE64_CERT_LINE]
.
.
.
END

This capability is intended to allow the use of certificates
stored outside of the filesystem (e.g. in Mac OS X Keychain)
with OpenVPN via the management interface.

OUTPUT FORMAT
-------------

(1) Command success/failure indicated by "SUCCESS: [text]" or
    "ERROR: [text]".

(2) For commands which print multiple lines of output,
    the last line will be "END".

(3) Real-time messages will be in the form ">[source]:[text]",
    where source is "CLIENT", "ECHO", "FATAL", "HOLD", "INFO", "LOG",
    "NEED-OK", "PASSWORD", or "STATE".

REAL-TIME MESSAGE FORMAT
------------------------

The OpenVPN management interface produces two kinds of
output: (a) output from a command, or (b) asynchronous,
real-time output which can be generated at any time.

Real-time messages start with a '>' character in the first
column and are immediately followed by a type keyword
indicating the type of real-time message.  The following
types are currently defined:

BYTECOUNT -- Real-time bandwidth usage notification, as enabled
             by "bytecount" command when OpenVPN is running as
             a client.

BYTECOUNT_CLI -- Real-time bandwidth usage notification per-client,
	         as enabled by "bytecount" command when OpenVPN is
                 running as a server.

CLIENT   -- Notification of client connections and disconnections
            on an OpenVPN server.  Enabled when OpenVPN is started
            with the --management-client-auth option.  CLIENT
            notifications may be multi-line.  See "The CLIENT
            notification" section below for detailed info.

ECHO     -- Echo messages as controlled by the "echo" command.

FATAL    -- A fatal error which is output to the log file just
            prior to OpenVPN exiting.

HOLD     -- Used to indicate that OpenVPN is in a holding state
            and will not start until it receives a
            "hold release" command.

INFO     -- Informational messages such as the welcome message.

LOG      -- Log message output as controlled by the "log" command.

NEED-OK  -- OpenVPN needs the end user to do something, such as
            insert a cryptographic token.  The "needok" command can
            be used to tell OpenVPN to continue.

NEED-STR -- OpenVPN needs information from end, such as
            a certificate to use.  The "needstr" command can
            be used to tell OpenVPN to continue.

PASSWORD -- Used to tell the management client that OpenVPN
            needs a password, also to indicate password
            verification failure.

STATE    -- Shows the current OpenVPN state, as controlled
            by the "state" command.

The CLIENT notification
-----------------------

The ">CLIENT:" notification is enabled by the --management-client-auth
OpenVPN configuration directive that gives the management interface client
the responsibility to authenticate OpenVPN clients after their client
certificate has been verified.  CLIENT notifications may be multi-line, and
the sequentiality of a given CLIENT notification, its associated environmental
variables, and the terminating ">CLIENT:ENV,END" line are guaranteed to be
atomic.

CLIENT notification types:

(1) Notify new client connection ("CONNECT") or existing client TLS session
    renegotiation ("REAUTH").  Information about the client is provided
    by a list of environmental variables which are documented in the OpenVPN
    man page.  The environmental variables passed are equivalent to those
    that would be passed to an --auth-user-pass-verify script.

    >CLIENT:CONNECT|REAUTH,{CID},{KID}
    >CLIENT:ENV,name1=val1
    >CLIENT:ENV,name2=val2
    >CLIENT:ENV,...
    >CLIENT:ENV,END

(2) Notify successful client authentication and session initiation.
    Called after CONNECT.

    >CLIENT:ESTABLISHED,{CID}
    >CLIENT:ENV,name1=val1
    >CLIENT:ENV,name2=val2
    >CLIENT:ENV,...
    >CLIENT:ENV,END

(3) Notify existing client disconnection.  The environmental variables passed
    are equivalent to those that would be passed to a --client-disconnect
    script.

    >CLIENT:DISCONNECT,{CID}
    >CLIENT:ENV,name1=val1
    >CLIENT:ENV,name2=val2
    >CLIENT:ENV,...
    >CLIENT:ENV,END

(4) Notify that a particular virtual address or subnet
    is now associated with a specific client.

    >CLIENT:ADDRESS,{CID},{ADDR},{PRI}

Variables:

CID --  Client ID, numerical ID for each connecting client, sequence = 0,1,2,...
KID --  Key ID, numerical ID for the key associated with a given client TLS session,
        sequence = 0,1,2,...
PRI --  Primary (1) or Secondary (0) VPN address/subnet.  All clients have at least
        one primary IP address.  Secondary address/subnets are associated with
        client-specific "iroute" directives.
ADDR -- IPv4 address/subnet in the form 1.2.3.4 or 1.2.3.0/255.255.255.0

In the unlikely scenario of an extremely long-running OpenVPN server,
CID and KID should be assumed to recycle to 0 after (2^32)-1, however this
recycling behavior is guaranteed to be collision-free.

Command Parsing
---------------

The management interface uses the same command line lexical analyzer
as is used by the OpenVPN config file parser.

Whitespace is a parameter separator.

Double quotation or single quotation characters ("", '') can be used
to enclose parameters containing whitespace.

Backslash-based shell escaping is performed, using the following
mappings, when not in single quotations:

\\       Maps to a single backslash character (\).
\"       Pass a literal doublequote character ("), don't
         interpret it as enclosing a parameter.
\[SPACE] Pass a literal space or tab character, don't
         interpret it as a parameter delimiter.

Challenge/Response Protocol
---------------------------

The OpenVPN Challenge/Response Protocol allows an OpenVPN server to
generate challenge questions that are shown to the user, and to see
the user's responses to those challenges.  Based on the responses, the
server can allow or deny access.

In this way, the OpenVPN Challenge/Response Protocol can be used
to implement multi-factor authentication.  Two different
variations on the challenge/response protocol are supported: the
"Dynamic" and "Static" protocols.

The basic idea of Challenge/Response is that the user must enter an
additional piece of information, in addition to the username and
password, to successfully authenticate.  Normally, this information
is used to prove that the user posesses a certain key-like device
such as cryptographic token or a particular mobile phone.

Dynamic protocol:

The OpenVPN dynamic challenge/response protocol works by returning
a specially formatted error message after initial successful
authentication.  This error message contains the challenge question,
and is formatted as such:

  CRV1:<flags>:<state_id>:<username_base64>:<challenge_text>

flags: a series of optional, comma-separated flags:
 E : echo the response when the user types it
 R : a response is required

state_id: an opaque string that should be returned to the server
 along with the response.

username_base64 : the username formatted as base64

challenge_text : the challenge text to be shown to the user

Example challenge:

  CRV1:R,E:Om01u7Fh4LrGBS7uh0SWmzwabUiGiW6l:Y3Ix:Please enter token PIN

After showing the challenge_text and getting a response from the user
(if R flag is specified), the client should submit the following
auth creds back to the OpenVPN server:

Username: [username decoded from username_base64]
Password: CRV1::<state_id>::<response_text>

Where state_id is taken from the challenge request and response_text
is what the user entered in response to the challenge_text.
If the R flag is not present, response_text may be the empty
string.

Example response (suppose the user enters "8675309" for the token PIN):

  Username: cr1 ("Y3Ix" base64 decoded)
  Password: CRV1::Om01u7Fh4LrGBS7uh0SWmzwabUiGiW6l::8675309

Static protocol:

The static protocol differs from the dynamic protocol in that the
challenge question and response field is given to the user in the
initial username/password dialog, and the username, password, and
response are delivered back to the server in a single transaction.

The "static-challenge" directive is used to give the challenge text
to OpenVPN and indicate whether or not the response should be echoed.

When the "static-challenge" directive is used, the management
interface will respond as such when credentials are needed:

  >PASSWORD:Need 'Auth' username/password SC:<ECHO>,<TEXT>

  ECHO: "1" if response should be echoed, "0" to not echo
  TEXT: challenge text that should be shown to the user to
      facilitate their response

For example:

  >PASSWORD:Need 'Auth' username/password SC:1,Please enter token PIN

The above notification indicates that OpenVPN needs a --auth-user-pass
password plus a response to a static challenge ("Please enter token PIN").
The "1" after the "SC:" indicates that the response should be echoed.

The management interface client in this case should add the static
challenge text to the auth dialog followed by a field for the user to
enter a response.  Then the client should pack the password and response
together into an encoded password:

  username "Auth" foo
  password "Auth" "SCRV1:<BASE64_PASSWORD>:<BASE64_RESPONSE>"

For example, if the user entered "bar" as the password and 8675309
as the PIN, the following management interface commands should be
issued:

  username "Auth" foo
  password "Auth" "SCRV1:Zm9v:ODY3NTMwOQ=="

Client-side support for challenge/response protocol:

Currently, the Access Server client and standalone OpenVPN
client support both static and dynamic challenge/response
protocols.  However, any OpenVPN client UI that drives OpenVPN
via the management interface needs to add explicit support
for the challenge/response protocol.