@@ -195,6 +195,13 @@ request before sending the next one of the same type. The server MAY
send replies in the order that the requests were received, but is not
required to.
+There is no requirement for the client or server to complete a negotiation
+if it does not wish to do so. If ithe client does not find an export it
+is looking for (for instance) it may simply close the TCP connection.
+Under certain circumstances either the client or the server may be required
+by this document to close the TCP connection. In each case, this is
+referred to as to 'terminate the session'.
+
### Transmission
There are three message types in the transmission phase: the request,
@@ -286,6 +293,284 @@ S: (*length* bytes of data if the request is of type `NBD_CMD_READ`)
This reply type MUST NOT be used except as documented by the
experimental `STRUCTURED_REPLY` extension; see below.
+## TLS support
+
+The NBD protocol supports Transport Layer Security (TLS) (see
+[RFC5246](https://tools.ietf.org/html/rfc5246)
+as updated by
+[RFC6176](https://tools.ietf.org/html/rfc6176)
+).
+
+TLS is negotiated with the `NBD_OPT_STARTTLS`
+option. This is performed as an in-session upgrade. Below the term
+'negotiation' is used to refer to the sending and receiving of
+NBD commands, and the term 'initiation' of TLS is used to refer to
+the actual upgrade to TLS.
+
+### Certificates, authentication and authorisation
+
+This standard does not specify what encryption, certification
+and signature algorithms are used. This standard does not
+specify authentication and authorisation (for instance
+whether client and/or server certificates are required and
+what they should contain); this is implementation dependent.
+
+TLS requires fixed newstyle negotiation to have completed.
+
+### Server-side requirements
+
+There are four modes of operation for a server. The
+server MUST support one of these modes.
+
+* The server operates entirely without TLS ('NOTLS'); OR
+
+* The server insists upon TLS, and forces the client to
+ upgrade by erroring any NBD options other than `NBD_OPT_STARTTLS`
+ with `NBD_REP_ERR_TLS_REQD` ('FORCEDTLS'); this in practice means
+ that all option negotiation (apart from the `NBD_OPT_STARTTLS`
+ itself) is carried out with TLS; OR
+
+* The server provides TLS, and it is mandatory on zero or more
+ exports, and is available at the client's option on all
+ other exports ('SELECTIVETLS'). The server does not force
+ the client to upgrade to TLS during option haggling (as
+ if the client ultimately were to choose a non-TLS-only export,
+ stopping TLS is not possible). Instead it permits the client
+ to upgrade as and when it chooses, but unless an upgrade to
+ TLS has already taken place, the server errors attempts
+ to enter transmission mode on TLS-only exports, MAY
+ refuse to provide information about TLS-only exports
+ via `NBD_OPT_INFO`, MAY refuse to provide information
+ about non-existent exports via `NBD_OPT_INFO`, and MAY omit
+ exports that are TLS-only from `NBD_OPT_LIST`.
+
+The server MAY determine the mode in which it operates
+dependent upon the session (for instance it might be
+more liberal with TCP connections made over the loopback
+interface) but it MUST be consistent in its mode
+of operation across the lifespan of a single TCP connection
+to the server. A client MUST NOT assume indications from
+a prior TCP session to a given server will be relevant
+to a subsequent session.
+
+The server MUST operate in NOTLS mode unless the server
+set flag NBD_FLAG_FIXED_NEWSTYLE and the client replied
+with NBD_FLAG_C_FIXED_NEWSTYLE in the fixed newstyle
+negotiation.
+
+These modes of operations are described in detail below.
+
+#### NOTLS mode
+
+If the server receives `NBD_OPT_STARTTLS` it MUST respond with
+`NBD_REP_ERR_POLICY` (if it does not support TLS for
+policy reasons) or `NBD_REP_ERR_UNSUP` (if it does not
+support the `NBD_OPT_STARTTLS` option at all. The server MUST NOT
+respond to any option request with `NBD_REP_ERR_TLS_REQD`.
+
+#### FORCEDTLS mode
+
+If the server receives `NBD_OPT_STARTTLS` it MUST reply with
+`NBD_REP_ACK`. After this reply has been sent, the server MUST
+be prepared for a TLS handshake, and all further data MUST
+be sent and received over TLS. There is no downgrade to a
+non-TLS session.
+
+As per the TLS standard, the handshake MAY be initiated either
+by the server (having sent the `NBD_REP_ACK`) or by the client.
+If the handshake is unsuccessful (for instance the client's
+certificate does not match) the server MUST terminate the
+session as by this stage it is too late to continue without TLS
+as the acknowledgement has been sent.
+
+If the server receives any other option, including `NBD_OPT_INFO`
+and unsupported options, it MUST reply with `NBD_REP_ERR_TLS_REQD`
+if TLS has not been initiated; `NBD_OPT_INFO` is included as in this
+mode, all exports are TLS-only. If the server receives a request to
+enter transmission mode via `NBD_OPT_EXPORT_NAME` when TLS has not
+been initiated, then as this request cannot error, it MUST
+terminate the session. If the server receives a request to
+enter transmission mode via `NBD_OPT_GO` when TLS has not been
+initiated, it MUST error with `NBD_REP_ERR_TLS_REQD`.
+
+The server MUST NOT send `NBD_REP_ERR_TLS_REQD` in reply to
+any command if TLS has already been initiated.
+
+The FORCEDTLS mode of operation has an implementation problem in
+that the client MAY legally simply send a `NBD_OPT_EXPORT_NAME`
+to enter transmission mode without previously sending any options.
+Therefore, if a server uses FORCEDTLS, it SHOULD implement the
+INFO extension.
+
+#### SELECTIVETLS mode
+
+If the server receives `NBD_OPT_STARTTLS` it MUST reply with
+`NBD_REP_ACK` and initiate TLS as set out under 'FORCEDTLS'
+above.
+
+If the server receives any other option except `NBD_OPT_INFO` and
+`NBD_OPT_GO`, it MUST NOT reply with `NBD_REP_ERR_TLS_REQD`. If the
+server receives `NBD_OPT_INFO` or `NBD_OPT_GO` and TLS has not been
+initiated, it MAY reply with `NBD_REP_ERR_TLS_REQD` if that
+export is non-existent, and MUST reply with `NBD_REP_ERR_TLS_REQD`
+if that export is TLS-only.
+
+If the server receives a request to enter transmission mode
+via `NBD_OPT_EXPORT_NAME` on a TLS-only export when TLS has not
+been initiated, then as this request cannot error, it MUST
+terminate the session.
+
+The server MUST NOT send `NBD_REP_ERR_TLS_REQD` in reply to
+any command if TLS has already been negotiated. The server
+MUST NOT send `NBD_REP_ERR_TLS_REQD` in response to any
+option other than `NBD_OPT_INFO` or `NBD_OPT_GO`, and
+only in those cases in respect of a TLS-only or non-existent
+export.
+
+There is a degenerate case of SELECTIVETLS where all
+exports are TLS-only. This is permitted in part to make programming
+of servers easier. Operation is a little different from FORCEDTLS,
+as the client is not forced to upgrade to TLS prior to any options
+being processed, and the server MAY choose to give information on
+non-existent exports via NBD_OPT_INFO exports prior to an upgrade
+to TLS.
+
+The SELECTIVETLS mode of operation has an implementation problem
+in that unless the INFO extension is supported, the client that
+does not use TLS may have its access to exports denied without
+it being able to ascertain the reason. For instance it may
+go into transmission mode using `NBD_OPT_EXPORT_NAME` - which
+does not return an error as no options will be denied with
+`NBD_REP_ERR_TLS_REQD`. Further there is no way to remotely
+determine whether an export requires TLS, and therefore this
+must be initiated between client and server out of band.
+Therefore, if a server uses SELECTIVETLS, it MUST implement
+the INFO extension.
+
+## Client-side requirements
+
+If the client supports TLS at all, it MUST be prepared
+to deal with servers operating in any of the above modes.
+Notwithstanding, a client MAY always terminate the session or
+refuse to connect to a particular export if TLS is
+not available and the user requires TLS.
+
+The client MUST NOT issue `NBD_OPT_STARTTLS` unless the server
+set flag NBD_FLAG_FIXED_NEWSTYLE and the client replied
+with NBD_FLAG_C_FIXED_NEWSTYLE in the fixed newstyle
+negotiation.
+
+The client MUST NOT issue `NBD_OPT_STARTTLS` if TLS has already
+been initiated.
+
+Subject to the above two limitations, the client MAY send
+`NBD_OPT_STARTTLS` at any time to initiate a TLS session. If the
+client receives `NBD_REP_ACK` in response, it MUST immediately
+upgrade the session to TLS. If it receives `NBD_REP_ERR_UNSUP`,
+`NBD_REP_ERR_POLICY` in response or any other error, it indicates
+that the server cannot or will not upgrade the session to TLS
+and therefore MUST either continue the session without TLS,
+or terminate the session.
+
+A client that prefers to use TLS irrespective of whether
+the server makes TLS mandatory SHOULD send `NBD_OPT_STARTTLS`
+as the first option. This will ensure option haggling is subject
+to TLS, and will thus prevent the possibility of options being
+compromised by a Man-in-the-Middle attack. Note that the
+`NBD_OPT_STARTTLS` itself may be compromised - see 'downgrade
+attacks' for more details. For this reason, a client which only
+wishes to use TLS SHOULD terminate the session if the
+`NBD_OPT_STARTTLS` replies with an error.
+
+If the TLS handshake is unsuccessful (for instance the server's
+certificate does not validate) the client MUST terminate the
+session as by this stage it is too late to continue without TLS.
+
+If the client receives an `NBD_REP_ERR_TLS_REQD` in response
+to any option, it implies that this option cannot be executed
+unless a TLS upgrade is performed. If the option is any
+option other than `NBD_OPT_INFO` or `NBD_OPT_GO`, this
+indicates that no option will succeed unless a TLS upgrade
+is performed; the client MAY therefore choose to issue
+a `NBD_OPT_STARTTLS`, or MAY terminate the session (if
+for instance it does not support TLS or does not have
+appropriate credentials for this server). If the client
+receives `NBD_REP_ERR_TLS_REQD` in response to
+`NBD_OPT_INFO` or `NBD_OPT_GO` this indicates that the
+export referred to within the option is either non-existent
+or requires TLS; the client MAY therefore choose to issue
+a `NBD_OPT_STARTTLS`, MAY terminate the session (if
+for instance it does not support TLS or does not have
+appropriate credentials for this server), or MAY continue
+in another manner without TLS, for instance by querying
+or using other exports.
+
+If a client supports TLS, it SHOULD also support the INFO
+extension, and SHOULD use `NBD_OPT_GO` if available in place
+of `NBD_OPT_EXPORT_NAME`. The reason for this is set out in
+the final paragraphs of the sections under 'FORCEDTLS'
+and 'SELECTIVETLS': this gives an opportunity for the
+server to transmit that an error going into transmission
+mode is due to the client's failure to initiate TLS,
+and the fact that the client may obtain information about
+which exports are TLS-only through `NBD_OPT_INFO`.
+
+### Security considerations
+
+#### TLS versions
+
+NBD implementations supporting TLS MUST support TLS version 1.2,
+SHOULD support any later versions. NBD implementations
+MAY support older versions but SHOULD NOT do so by default
+(i.e. they SHOULD only be available by a configuration change).
+Older versions SHOULD NOT be used where there is a risk of security
+problems with those older versions or of a downgrade attack
+against TLS versions.
+
+#### Protocol downgrade attacks
+
+A danger inherent in any scheme relying on the negotiation
+of whether TLS should be employed is downgrade attacks within
+the NBD protocol.
+
+There are two main dangers:
+
+* A Man-in-the-Middle (MitM) hijacks a session and impersonates
+ the server (possibly by proxying it) claiming not to support
+ TLS. In this manner, the client is confused into operating
+ in a plain-text manner with the MitM (with the session possibly
+ being proxied in plain-text to the server using the method
+ below).
+
+* The MitM hijacks a session and impersonates the client
+ (possibly by proxying it) claiming not to support TLS. In
+ this manner the server is confused into operating in a plain-text
+ manner with the MitM (with the session being possibly
+ proxied to the client with the method above).
+
+With regard to the first, any client that does not wish
+to be subject to potential downgrade attack SHOULD ensure
+that if a TLS endpoint is specified by the client, it
+ensures that TLS is negotiated prior to sending or
+requesting sensitive data. To recap, the client MAY send
+`NBD_OPT_STARTTLS` at any point during option haggling,
+and MAY terminate the session if `NBD_REP_ACK` is not
+provided.
+
+With regard to the second, any server that does not wish
+to be subject to a potential downgrade attack SHOULD either
+used FORCEDTLS mode, or should force TLS on those exports
+it is concerned about using SELECTIVE mode and TLS-only
+exports. It is not possible to avoid downgrade attacks
+on exports which may be served either via TLS or in plain
+text unless the client insists on TLS.
+
+### Status
+
+This functionality has not yet been implemented by the reference
+implementation, but was implemented by qemu and subsequently
+by other users, so has been moved out of the "experimental" section.
+
## Values
This section describes the value and meaning of constants (other than
@@ -366,7 +651,7 @@ of the newstyle negotiation.
Data: String, name of the export, as free-form text.
The length of the name is determined from the option header. If the
chosen export does not exist or requirements for the chosen export
- are not met (e.g., the client did not negotiate TLS for an export
+ are not met (e.g., the client did not initiate TLS for an export
where the server requires it), the server should close the
connection.
@@ -391,7 +676,9 @@ of the newstyle negotiation.
- `NBD_OPT_LIST` (3)
Return a number of `NBD_REP_SERVER` replies, one for each export,
- followed by `NBD_REP_ACK`.
+ followed by `NBD_REP_ACK`. The server SHOULD omit entries from this
+ list if TLS has not been negotiated, the server is operating in
+ SELECTIVETLS mode, and the entry concerned is a TLS-only export.
- `NBD_OPT_PEEK_EXPORT` (4)
@@ -400,21 +687,14 @@ of the newstyle negotiation.
- `NBD_OPT_STARTTLS` (5)
- The client wishes to initiate TLS. If the server replies with
- `NBD_REP_ACK`, then the client should immediately initiate a TLS
- handshake and continue the negotiation in the encrypted channel. If
- the server is unwilling to perform TLS, it should reply with
- `NBD_REP_ERR_POLICY`. For backwards compatibility, a client should
- also be prepared to handle `NBD_REP_ERR_UNSUP`. If the client sent
- along any data with the request, the server should send back
- `NBD_REP_ERR_INVALID`. The client MUST NOT send this option if
- it has already negotiated TLS; if the server receives
- `NBD_OPT_STARTTLS` when TLS has already been negotiated, the server
- MUST send back `NBD_REP_ERR_INVALID`.
-
- This functionality has not yet been implemented by the reference
- implementation, but was implemented by qemu so has been moved out of
- the "experimental" section.
+ The client wishes to initiate TLS.
+
+ The server MUST either reply with `NBD_REP_ACK` after which
+ point the connection is upgraded to TLS, or reply with
+ `NBD_REP_ERR_POLICY` (or if it does not support the option
+ at all, `NBD_REP_ERR_UNSUP`).
+
+ See the section on TLS above for further details.
- `NBD_OPT_INFO` (6)
@@ -489,20 +769,10 @@ case that data is an error message string suitable for display to the user.
* `NBD_REP_ERR_TLS_REQD` (2^31 + 5)
The server is unwilling to continue negotiation unless TLS is
- negotiated first. A server MUST NOT send this error if it has one or
- more exports that do not require TLS; not even if the client indicated
- interest (by way of `NBD_OPT_PEEK_EXPORT`) in an export which requires
- TLS.
-
- If this reply is used, servers SHOULD send it in reply to each and every
- unencrypted `NBD_OPT_*` message (apart from `NBD_OPT_STARTTLS`).
-
- This functionality has not yet been implemented by the reference
- implementation, but was implemented by qemu so has been moved out of
- the "experimental" section.
-
- The experimental `INFO` extension makes small but compatible
- changes to the semantics of this error message; see below.
+ initiated first. In the case of `NBD_OPT_INFO` and `NBD_OPT_GO`
+ this unwillingness MAY (depending on the TLS mode) be limited
+ to the export in question. See the section on TLS above for
+ further details.
* `NBD_REP_ERR_UNKNOWN` (2^31 + 6)
@@ -735,13 +1005,13 @@ Therefore these commands share common documentation.
- `NBD_REP_ERR_UNKNOWN`: The chosen export does not exist on this
server.
- `NBD_REP_ERR_TLS_REQD`: The server does not wish to export this
- block device unless the client negotiates TLS first.
+ block device unless the client initiates TLS first.
- `NBD_REP_SERVER`: The server accepts the chosen export.
- Additionally, if TLS has not been negotiated, the server MAY reply
+ Additionally, if TLS has not been initiated, the server MAY reply
with `NBD_REP_ERR_TLS_REQD` (instead of `NBD_REP_ERR_UNKNOWN`)
to requests for exports that are unknown. This is so that clients
- that have not negotiated TLS cannot enumerate exports.
+ that have not initiated TLS cannot enumerate exports.
In the case of `NBD_REP_SERVER`, the message's data takes on a different
interpretation than the default (so as to provide additional