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Network Working Group Babu Neelam
Internet-Draft Independent
Intended status: Standards Track August 19, 2007
Expires: February 15, 2008
TLS extension for Proxies to transfer Server certificate
draft-babu-serv-cert-trans-from-proxy-00
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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The list of current Internet-Drafts can be accessed at
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This Internet-Draft will expire on January 10, 2008.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
Intercepting transparent proxies splice the client-Server connection
into two connections: Client-Proxy connection, Proxy-server
connection. On Client-Proxy connection, proxy sends it's certificate
to the client. As client is generally (in such a scenario)
pre-configured to accept proxy's certificate, client accepts and
proceeds further with the connection. On Proxy-Server connection,
server sends its certificate to the proxy. Proxy typically doesn't
possess the information (like MX domain name in case of SMTP)
required to validate the certificate. The certificate validation is
at times very complex & hence it is better to offload this
reponsibility to the original client itself.
This document addresses this issue by extending TLS to let proxy
send server's certificate to the client for validation and suggests
how client can indicate certificate validation result to the proxy.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Mechanism Overview . . . . . . . . . . . . . . . . . . . . . . 2
3. Need Server certificate extension. . . . . . . . . . . . . . . 2
4. Handshake message to transfer server certificate . . . . . . . 3
5. various scenarios. . . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Normative References . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . . . . 7
1. Introduction
Today, intercepting transparent proxies are very common in
applications (say SMTP, HTTP) using [TLS]. In SMTP, these
intercepting proxies may provide functionality like anti-virus
scanning, anti-spam scanning. HTTP intercepting proxies may provide
functionality like anti-virus scanning, URL filtering etc.
Client ---- Transparent Proxy -------- Server
This document defines a way for proxy to send original server's
certificate to the original client and suggests how client can
indicate certificate validation result to the proxy. The mechanism
makes use of TLS extension framework defined in [RFC4366] and defines
a new TLS handshake message type.
The clients supporting this extension receive certificates of
intercepting proxy (if interception happens) as well as the original
server. So clients should be capable of handling validations on both
the certificates.
2. Mechanism Overview
This extension defines
- A new extension type (need_server_certificate) for extended
client hellos defined in [RFC4366].
- A new handshake message (Orig_server_certificate)
3. Need Server certificate extension
Who should send this extension ?
- A client which is configured to request the original server
certificate for validation includes an extension of type
"need_server_certificate" in (extended) client hello.
- It is possible that there could be more than one proxy
between client and server:
Client --- P1 --- P2 ------ Server
In such a scenario, P1 also includes "need_server_certificate"
in (extended) client hello in its connection to P2, unless it
has the knowledge that it is the last proxy between client and
server. If a proxy is configured that it is the edge proxy in
client's trust domain, then it need not send this extension.
How should a receiver respond to this ?
- If a proxy intercepts the connection, it SHOULD respond back to
the client with "need_server_certificate" extension.
- When there are no intercepting proxies, a server receives this
extension. A server which understands this extension should
ignore this. It is not clear from [RFC4366] what a server does
when it receives an extension which it doesn't understand.
This item is TBD.
- If a proxy which doesn't have the capability to validate server
certificate or is configured to offload this responsibility to
the original client doesn't receive "need_server_certificate"
extension, it should return a fatal error like handshake failure
or insufficient security (TBD).
When a proxy responds with "need_server_certificate" extension to the
client, proxy MUST send the its certificate as well as the original
server certificate to the client (discussed in section 4).
How should client handle ServerHello?
- If the client receives "need server_certificate" extension in
ServerHello, it MUST expect the nexthop proxy certificate as
well as the original server certificate. Client MUST perform
validations on both proxy certificate as well as original server
certificate. If a client doesn't receive server certificate, it
MUST abort the connection.
- If the client doesn't receive "need_server_certificate"
extention in SerVerHello, client MUST assume that there is no
proxy in between and MUST perform server certificate validations
on the received certificate.
The "extension_data" field of this extension in both clientHello as
well as ServerHello SHALL be empty.
Note on backward compatibility: Suppose a client supports this
extension, but a intercepting proxy or the actual server doesn't
understand extended hello or "need_server_certificate", client
MUST proceed with the connection and MUST perform server certificate
validations on the received certificate. By validating this way,
clientcan deny connections from any proxies (because certificate
validation fails) which do not support this mechanism, but still
accept connections from server which do not support this.
4. Handshake message to transfer server certificate
This docuemnt suggests the use of a new handshake message,
"orig_server_certificate" to transfer the original server's
certificate to the client. The new handshake message structure
therefore becomes:
enum {
hello_request(0), client_hello(1), server_hello(2),
certificate(11), server_key_exchange (12),
certificate_request(13), server_hello_done(14),
certificate_verify(15), client_key_exchange(16),
finished(20), certificate_url(21), certificate_status(22),
orig_server_certificate(23),
(255)
} HandshakeType;
struct {
HandshakeType msg_type; /* handshake type */
uint24 length; /* bytes in message */
select (HandshakeType) {
case hello_request: HelloRequest;
case client_hello: ClientHello;
case server_hello: ServerHello;
case certificate: Certificate;
case server_key_exchange: ServerKeyExchange;
case certificate_request: CertificateRequest;
case server_hello_done: ServerHelloDone;
case certificate_verify: CertificateVerify;
case client_key_exchange: ClientKeyExchange;
case finished: Finished;
case certificate_url: CertificateURL;
case certificate_status: CertificateStatus;
case orig_server_certificate: Certificate; /*new*/
} body;
} Handshake;
The structure of Certificate is defined in [RFC4346].
If proxy responded to the client with "need_server_certificate"
extension, this message MUST be sent immediately after the
"Certificate" handshake message in Client-Proxy connection.
The client MUST perform validations on the received proxy
certificate as well as the server certificate. If either proxy or
server certificate is not valid, client should respond with
certificate related error messages defined in [RFC4346]. On
reception of such an error, proxy MUST close the Proxy-Server
connection.
It should be noted that proxy transparency is lost at TLS layer
due to the fact that client is sent both proxy as well as original
server certificate for validation. Though transparency is not
possible at TLS layer, application protocols can still remain
transparent to the proxy operation.
5. various scenarios
Client, Proxy understand this extension.
Client Proxy Server
ClientHelo
(with
"need_orig_server") -->
ClientHelo
(with
"need_orig_server")--->
ServerHelo
(without
"need_orig_server")
Certificate
ServerkeyExchange
<--- ServerhelloDone
ServerHelo
(with "need_orig_server")
Certificate
orig_server_certificate
ServerkeyExchange
<-- ServerHelloDone
ClientKeyExchange
CertificateVerify
[ChangeCipherSpec]
[Finished] -->
[ChangeCipherSpec]
<-- [Finished]
[ChangeCipherSpec]
[Finished] -->
[ChangeCipherSpec]
<-- [Finished]
Client understands this extension. Proxy doesn't. In this case,
certificate validation fails on the received proxy certificate.
Client Proxy Server
ClientHelo
(with
"need_orig_server") -->
ServerHelo
(without "need_orig_server")
Certificate
orig_server_certificate
ServerkeyExchange
<-- ServerHelloDone
Certificate error
Client doesn;t understand this extension, but proxy is configured
to offload original server certificate responsibility to the
original client:
Client Proxy Server
ClientHelo
(with
"need_orig_server") -->
Fatal Error
TBD: Two proxies between client and server. Client as well as two
proxies undertsand this extension.
6. IANA Considerations
This document (if approved) requests IANA to allocate
"need-server_certificate" TLS extension and
"Orig_server_certificate" handhsake message.
Note to RFC Editor: this section may be removed on publication as an
RFC.
7. Security Considerations
Though this extension equips clients with an ability to validate
original server certificate as well as it's nexthop, it doesn't
provide a mechanism to transmit certficates of any proxies between
the first proxy and the original server. It is assumed that client
trusts the first proxy to either not allow any other proxies in
between or to allow only a proxy which is in the trusted domain.
8. Normative References
[RFC4346] The Transport Layer Security (TLS) Protocol Version 1.1.
T.Dierks, E. Rescorla. April 2006.
[RFC4366] Transport Layer Security (TLS) Extensions.
S. Blake-Wilson, M.Nystrom, D. Hopwood, J. Mikkelsen,
T. Wright. April 2006.
[RFC2818] HTTP Over TLS. E. Rescorla. May 2000.
[RFC3207] SMTP Service Extension for Secure SMTP over Transport
Layer Security. P. Hoffman. February 2002.
Author's Address
Babu Neelam
Intoto Software India Private Ltd.
8-3-1111/2, kesava nagar colony,
sriniagar colony main road,
punjagutta,
Hyderabad,
India.
Email: babun@intoto.com
Comments are solicited and should be addressed to the working
group's mailing list at ietf-http-wg@w3.org and/or the author(s).
Full Copyright Statement
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