TLS Metadata for Load Balancers
draft-schwartz-tls-lb-01
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Expired".
|
|
|---|---|---|---|
| Author | Benjamin M. Schwartz | ||
| Last updated | 2019-07-02 (Latest revision 2019-06-28) | ||
| RFC stream | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-schwartz-tls-lb-01
tls B. Schwartz
Internet-Draft Google LLC
Intended status: Standards Track July 02, 2019
Expires: January 3, 2020
TLS Metadata for Load Balancers
draft-schwartz-tls-lb-01
Abstract
A load balancer that does not terminate TLS may wish to provide some
information to the backend server, in addition to forwarding TLS
data. This draft proposes a protocol between load balancers and
backends that enables secure, efficient delivery of TLS with
additional information. The need for such a protocol has recently
become apparent in the context of split mode ESNI.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 3, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
Schwartz Expires January 3, 2020 [Page 1]
Internet-Draft TLS-LB July 2019
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Conventions and Definitions . . . . . . . . . . . . . . . . . 2
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . 4
6. Defined ProxyExtensions . . . . . . . . . . . . . . . . . . . 5
7. Use with TLS over TCP . . . . . . . . . . . . . . . . . . . . 6
8. Use with QUIC . . . . . . . . . . . . . . . . . . . . . . . . 6
9. Configuration . . . . . . . . . . . . . . . . . . . . . . . . 8
10. Security considerations . . . . . . . . . . . . . . . . . . . 9
10.1. Integrity . . . . . . . . . . . . . . . . . . . . . . . 9
10.2. Confidentiality . . . . . . . . . . . . . . . . . . . . 9
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
12.1. Normative References . . . . . . . . . . . . . . . . . . 10
12.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 11
Appendix B. Open Questions . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Data encodings are expressed in the TLS 1.3 presentation language, as
defined in Section 3 of [TLS13].
2. Background
A load balancer is a server or bank of servers that acts as an
intermediary between the client and a range of backend servers. As
the name suggests, a load balancer's primary function is to ensure
that client traffic is spread evenly across the available backend
servers. However load balancers also serve many other functions,
such as identifying connections intended for different backends and
forwarding them appropriately, or dropping connections that are
deemed malicious.
Schwartz Expires January 3, 2020 [Page 2]
Internet-Draft TLS-LB July 2019
A load balancer operates at a specific point in the protocol stack,
forwarding e.g. IP packets, TCP streams, TLS contents, HTTP
requests, etc. Most relevant to this proposal are TCP and TLS load
balancers. TCP load balancers terminate the TCP connection with the
client and establish a new TCP connection to the selected backend,
bidirectionally copying the TCP contents between these two
connections. TLS load balancers additionally terminate the TLS
connection, forwarding the plaintext to the backend server (typically
inside a new TLS connection). TLS load balancers must therefore hold
the private keys for the domains they serve.
When a TCP load balancer forwards a TLS stream, the load balancer has
no way to incorporate additional information into the stream.
Insertion of any additional data would cause the connection to fail.
However, the load-balancer and backend can share additional
information if they agree to speak a new protocol. The most popular
protocol used for this purpose is currently the PROXY protocol
[PROXY], developed by HAPROXY. This protocol prepends a plaintext
collection of metadata (e.g. client IP address) onto the TCP socket.
The backend can parse this metadata, then pass the remainder of the
stream to its TLS library.
The PROXY protocol is widely used, but it offers no confidentiality
or integrity protection, and therefore might not be suitable when the
load balancer and backend communicate over the public internet.
3. Goals
o Enable TCP load balancers to forward metadata to the backend.
o Reduce the need for TLS-terminating load balancers.
o Ensure confidentiality and integrity for all forwarded metadata.
o Enable split ESNI architectures.
o Prove to the backend that the load balancer intended to associate
this metadata with this connection.
o Achieve good CPU and memory efficiency.
o Don't impose additional latency.
o Support backends that receive a mixture of direct and load-
balanced TLS.
o Support use in QUIC.
Schwartz Expires January 3, 2020 [Page 3]
Internet-Draft TLS-LB July 2019
o Enable simple and safe implementation.
4. Overview
The proposed protocol provides one-way communication from a load
balancer to a backend server. It works by prepending information to
the forwarded connection:
+-----------+ +-----------+ +-----------+
| Backend A | | Backend B | | Backend C |
+-----------+ +-----------+ +-----------+
\/ /\
4. ServerHello, \/ /\ 2. EncryptedProxyData[SNI: "secret.b",
etc. \/ /\ client: 2, etc.]
\/ /\ 3. ClientHello (verbatim)
\/ /\
+---------------+
| Load balancer |
+---------------+
\/ /\
5. ServerHello, \/ /\ 1. ClientHello[ESNI: enc("secret.b")]
etc. (verbatim) \/ /\
\/ /\
+-----------+ +-----------+ +-----------+
| Client 1 | | Client 2 | | Client 3 |
+-----------+ +-----------+ +-----------+
Figure 1: Data flow diagram
5. Encoding
A ProxyExtension is identical in form to a standard TLS Extension
(Section 4.2 of [TLS13]), with a new identifier space for the
extension types.
struct {
ProxyExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} ProxyExtension;
The ProxyData contains a set of ProxyExtensions.
struct {
ProxyExtension proxy_data<0..2^16-1>;
} ProxyData;
Schwartz Expires January 3, 2020 [Page 4]
Internet-Draft TLS-LB July 2019
The EncryptedProxyData structure contains metadata associated with
the original ClientHello (Section 4.1.2 of [TLS13]), encrypted with a
pre-shared key that is configured out of band.
struct {
opaque psk_identity<1..2^16-1>;
opaque nonce<8..2^16-1>
opaque encrypted_proxy_data<1..2^16-1>;
} EncryptedProxyData;
o psk_identity: The identity of a PSK previously agreed upon by the
load balancer and the backend. Including the PSK identity allows
for updating the PSK without disruption.
o nonce: Non-repeating initializer for the AEAD. This prevents an
attacker from observing whether the same ClientHello is marked
with different metadata over time.
o encrypted_proxy_data: AEAD-Encrypt(key, nonce,
additional_data=ClientHello, plaintext=ProxyData). The key and
AEAD function are agreed out of band and associated with
psk_identity.
When the load balancer receives a ClientHello, it serializes any
relevant metadata into a ProxyData, then encrypts it with the
ClientHello as additional data, to produce EncryptedProxyData.
6. Defined ProxyExtensions
Like a standard TLS Extension, a ProxyExtension is identified by a
2-byte type number. There are initially three type numbers
allocated:
enum {
padding(0),
network_address(1),
esni_inner(2),
(65535)
} ProxyExtensionType;
The "padding" extension functions as described in [RFC7685]. It is
used here to avoid leaking information about the other extensions.
The "network_address" extension functions as described in
[I-D.kinnear-tls-client-net-address]. It conveys the client IP
address observed by the load balancer.
Schwartz Expires January 3, 2020 [Page 5]
Internet-Draft TLS-LB July 2019
The "esni_inner" extension can only be used if the ClientHello
contains the encrypted_server_name extension [ESNI]. The
extension_data is the ClientESNIInner (Section 5.1.1 of [ESNI]),
which contains the true SNI and nonce. This is useful when the load
balancer knows the ESNI private key and the backend does not, i.e.
split mode ESNI.
Load balancers SHOULD only include extensions that are specified for
use in ProxyData, and backends MUST ignore any extensions that they
do not recognize.
7. Use with TLS over TCP
When forwarding a TLS stream over TCP, the load balancer SHOULD send
a ProxyHeader at the beginning of the stream:
struct {
uint8 opaque_type = 0;
ProtocolVersion version = 0;
uint16 length = length(ProxyHeader.contents);
EncryptedProxyData contents;
} ProxyHeader;
The opaque_type field ensures that this header is distinguishable
from an ordinary TLS connection, whose first byte is always 22
(ContentType = handshake in Section 5.1 of [TLS13]). This structure
matches the layout of TLSPlaintext with a ContentType of "invalid",
potentially simplifying parsing.
Following the ProxyHeader, the load balancer MUST send the full
contents of the TCP stream, exactly as received from the client. The
backend will observe the ProxyHeader, immediately followed by a
TLSPlaintext frame containing the ClientHello. The backend will
decrypt the ProxyHeader using the ClientHello as associated data, and
process the ClientHello and the remainder of the stream as standard
TLS.
When receiving a ProxyHeader with an unrecognized version, the
backend SHOULD ignore this ProxyHeader and proceed as if the
following byte were the first byte received.
8. Use with QUIC
A QUIC load balancer provides this service by extracting the
ClientHello from any client Initial packet [I-D.ietf-quic-tls]. A
multi-tenant load balancer needs to perform this extraction anyway in
order to determine where the connection should be forwarded, either
by SNI or ESNI.
Schwartz Expires January 3, 2020 [Page 6]
Internet-Draft TLS-LB July 2019
Extracting a TLS ClientHello from a QUIC handshake is a version-
dependent action, so a load balancer cannot support unrecognized
versions of QUIC. If the load balancer receives a packet with an
unrecognized QUIC version, it MUST reply with a VersionNegotiation
packet indicating the supported versions (currently only version 1).
If the backend applies downgrade protection, it SHOULD account for
the impact of the load balancer.
In QUIC version 1, each handshake begins with an Initial packet sent
by the client. This packet uses the QUIC "long header" packet form,
starting with a "fixed bit" of 1 and a "frame type" of 0x0.
+-+-+-+-+-+-+-+-+
|1|1| 0 |R R|P P|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|DCIL(4)|SCIL(4)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Connection ID (0/32..144) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Connection ID (0/32..144) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Token Length (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Token (*) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Number (8/16/24/32) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload (*) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: QUIC Initial Packet
A client Initial packet contains a complete ClientHello, in a CRYPTO
frame in the payload. The load balancer extracts this ClientHello in
order to compute EncryptedProxyData.
TODO: Confirm that HelloRetryRequest elicits an Initial containing a
complete ClientHello. The QUIC draft text is unclear.
To send EncryptedProxyData to the backend, the load balancer
constructs a new packet with a header copied from the Initial, but
with a frame type of 0x1 and a new version (0xTBD). Its payload
consists of the old Initial's version number (currently always 1) and
the EncryptedProxyData.
Schwartz Expires January 3, 2020 [Page 7]
Internet-Draft TLS-LB July 2019
+-+-+-+-+-+-+-+-+
|1|1| 1 |R R|P P|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| New Version, 0xTBD (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|DCIL(4)|SCIL(4)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Connection ID (0/32..144) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Connection ID (0/32..144) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Token Length (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Token (*) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| New Length (i) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Number (8/16/24/32) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initial Version (32) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EncryptedProxyData ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: EncryptedProxyData packet to the backend
The load balancer then forwards the client Initial unmodified, except
for replacing its Version number with 0xTBD. All other QUIC packets
are forwarded entirely unmodified.
The backend, upon receipt of a packet with QUIC version 0xTBD and
type "0" or "1", waits for a second packet with version 0xTBD, the
other type value, and matching connection IDs, token, and packet
number. When both packets have been received, the backend can
reconstruct the original Initial packet and decrypt the
EncryptedProxyData.
If the second packet is not received within a brief time period (e.g.
100 ms), the backend SHOULD discard the first packet.
9. Configuration
The method of configuring of the PSK on the load balancer and backend
is not specified here. However, the PSK MAY be represented as a
ProxyKey:
Schwartz Expires January 3, 2020 [Page 8]
Internet-Draft TLS-LB July 2019
struct {
ProtocolVersion version = 0;
opaque psk_identity<1..2^16-1>;
CipherSuite cipher_suite;
opaque key<16..2^16-1>
} ProxyKey;
10. Security considerations
10.1. Integrity
This protocol is intended to provide the backend with a strong
guarantee of integrity for the metadata written by the load balancer.
For example, an active attacker cannot take metadata intended for one
stream and attach it to another, because each stream will have a
unique ClientHello, and the metadata is bound to the ClientHello by
AEAD.
One exception to this protection is in the case of an attacker who
deliberately reissues identical ClientHello messages. An attacker
who reuses a ClientHello can also reuse the metadata associated with
it, if they can first observe the EncryptedProxyData transferred
between the load balancer and the backend. This could be used by an
attacker to reissue data originally generated by a true client (e.g.
as part of a 0-RTT replay attack), or it could be used by a group of
adversaries who are willing to share a single set of client secrets
while initiating different sessions, in order to reuse metadata that
they find helpful.
As such, the backend SHOULD treat this metadata as advisory.
10.2. Confidentiality
This protocol is intended to maintain confidentiality of the metadata
transferred between the load balancer and backend, currently
consisting of the ESNI plaintext and the client IP address. An
observer between the client and the load balancer does not observe
this protocol at all, and an observer between the load balancer and
backend observes only ciphertext.
However, an adversary who can monitor both of these links can easily
observe that a connection from the client to the load balancer is
shortly followed by a connection from the load balancer to a backend,
with the same ClientHello. This reveals which backend server the
client intended to visit. In many cases, the choice of backend
server could be the sensitive information that ESNI is intended to
protect.
Schwartz Expires January 3, 2020 [Page 9]
Internet-Draft TLS-LB July 2019
11. IANA Considerations
Need to create a new ProxyExtensionType registry.
Need to allocate TBD as a reserved QUIC version code.
12. References
12.1. Normative References
[ESNI] Rescorla, E., Oku, K., Sullivan, N., and C. Wood,
"Encrypted Server Name Indication for TLS 1.3", draft-
ietf-tls-esni-03 (work in progress), March 2019.
[I-D.ietf-quic-tls]
Thomson, M. and S. Turner, "Using TLS to Secure QUIC",
draft-ietf-quic-tls-20 (work in progress), April 2019.
[I-D.kinnear-tls-client-net-address]
Kinnear, E., Pauly, T., and C. Wood, "TLS Client Network
Address Extension", draft-kinnear-tls-client-net-
address-00 (work in progress), March 2019.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7685] Langley, A., "A Transport Layer Security (TLS) ClientHello
Padding Extension", RFC 7685, DOI 10.17487/RFC7685,
October 2015, <https://www.rfc-editor.org/info/rfc7685>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[TLS13] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
12.2. Informative References
[PROXY] Tarreau, W., "The PROXY protocol", March 2017,
<https://www.haproxy.org/download/1.8/doc/
proxy-protocol.txt>.
Schwartz Expires January 3, 2020 [Page 10]
Internet-Draft TLS-LB July 2019
Appendix A. Acknowledgements
This is an elaboration of an idea proposed by Eric Rescorla during
the development of ESNI. Thanks to David Schinazi and David Benjamin
for suggesting important improvements.
Appendix B. Open Questions
Should the ProxyExtensionType registry have a reserved range for
private extensions?
Author's Address
Benjamin M. Schwartz
Google LLC
Email: bemasc@google.com
Schwartz Expires January 3, 2020 [Page 11]