[Users] TLS requirements and some brainstorming (long email)
klaus.mailinglists at pernau.at
Mon Nov 14 18:46:29 CET 2005
There are several scenarios where TLS will be used to interconnect SIP
proxies. (open)ser's TLS implementation should be generic enough to
handle all the useful scenarios. Thus, to better understand the
requirements, first I present some examples where (open)ser+TLS will be
useful. (I do not propose which of the following interconnect models are
good or bad. However, openser should be capable to handle all of them,
best in a mixed mode).
A company uses TLS to interconnect their SIP proxies via public
Internet. The proxies import the companies selfsigned CA-cert as trusted
CAs. The proxies trust other proxies as soon as their cert is validated
using the root CA.
This is already possible using openser 1.0.0 (= or ser+experimental TLS)
Some ITSPs form a federation. The federation-CA signs the certs of the
ITSPs. Here, the validation is like in the enterprise scenario.
(open)ser validates against the federations CA-cert. This works with
openser 1.0.0 as long as the ITSP is only in one federation, or uses
different egress/ingress points for each federation. If the ITSP is
member of two federations and uses one egress/ingress proxy, it has to
decide which certificate it should present to the peer. The originating
proxy could choose the proper client certificate for example by using a
table like (or having the certificate as blob directly in the DB):
Presenting the proper server certificate, is more difficult. The server
does not know if the incoming TLS request belongs to a member of fedA,
fedB or someone else. Thus, presenting the wrong certificate will lead
to the clients rejecting the certificate due to failed validation. One
solution would be sending the "trusted_ca_keys" (TLS extension) in
Client Hello. Unfortunatelly this is not supported in openssl (and
gnutls). Any workaround for this?
Anyway, in this scenario it is important to have the certificate
parameters (Subject, Issuer) available in the routing logic to make
routing decisions based on the TLS authenticaten and adding them to the
CDRs (e.g. via AVPs and extra accounting)
An ITSP has bilateral trust relationships. Each ITSP has its own CA
which signs the certs of this ITSP. If another ITSP wants to trust this
ISTP it only has to import the others CA-cert. This works already with
openser 1.0.0, but exporting the cert parameters for extra accounting
will be useful.
Hosted SIP scenario:
An ITSP hosts multiple SIP domains for its customers. If the server has
to offer a certificate which includes the proper SIP domain, the
server_name extension is needed to indicate the requested domain in the
client_hello request. Then the server will present the proper
certificate and domain validation (Subject domain == SIP domain) in the
client will succeed. This will work fine with initial (out-of-dialog)
requests as they usually will include the SIP domain in the request URI.
There will be problems for responses and in-dialog requests as usually
the Record-Route and Via headers only includes IP addresses. Thus, the
SIP proxy either has to insert the SIP domain into Via and Record-Route,
or the domain validation should only be done for in-dialog requests.
This leads to the problem of domain validation. The TLS connection will
be set up after all the routing logic, somewhere inside t_relay. Thus,
if we want domain validation, it will be inside t_relay. Maybe we can
use a certain flag to indicate if domain-validation should be done (on a
per-transaction basis). This might cause problems if there is already a
TLS connection to the requested destination, but without domain
validation or validation against a different domain (virtual domain
hosting). How to solve this?
I can't propose a solution to all scenarios. But I think I showed that
the certificate selection and validation should be very flexible, e.g.
by choosing the proper client certificate for each transaction and
different routing in the server depending on the presented client
certificate and the cerfiticate signer (e.g. based on a whitelist).
Further we have to take care to add certifcates and CA-certs during
runtime, e.g. using a FIFO command "tls_reload". This should also drop
all existing TLS connections. Having a maximum connection time after
which we force re-validation will also be useful.
Also (open)ser should allow to import CRL (certificate revocation lists)
(shouldn't be a problem with openssl) or usage of OCSP (Online
Certificate Status Protocol).
Now I'm ready for some discussions :-)
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