[Users] Re: [Devel] TLS requirements and some brainstorming (long email)

Bogdan-Andrei Iancu bogdan at voice-system.ro
Fri Nov 18 11:48:11 CET 2005

Hi Klaus,

indeed this is a long email ;).
please see my inline comments.


Klaus Darilion wrote:

> Hi all!
> 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).
>   Enterprise scenario:
> 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)
>   Federation scenario:
> 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):
>      dst_domain               certificate
>    sip.atlanta.com   /etc/openser/federationAcert.pem
>    sip.biloxy.com    /etc/openser/federationBcert.pem
>    sip.chicago.com   /etc/openser/federationAcert.pem
> 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?

As I understood from Cesc, gnutls already support this extension, but to 
migrate to gnutls and restart all testing may not pay the effort as time 
as it's just a matter of time until the extension will be also available 
in openssl.
As temporary solution I will suggest to go by default without the 
extension patch, but to provide the patch into the TLS directory and 
people interested in these multi-domain scenarios will have to apply and 
recompile the openssl lib. And maybe we should do some lobby (read 
pressure) on the openssl mailing list in order to push this extension in 
the official tree.

Just an idea.

> 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)
interesting but there might be some problems - the information you want 
to log comes from transport layer and you try you log it by using 
mechanism from the SIP level. It will works, but the info will be 
actually available only for requests that initiated the TLS connection 
(send or received) and not also for the requests that reuse the connection.

>   Bilateral scenario:
> 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.
not sure what you mean by cert parameters.......

>   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. 

the solution will also the mighty extension, indeed.....

> 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.

I don't thing we should worry about replies - they will return via same 
connexion - the expiration time of a tcp connection must be higher than 
the expiration time of a transaction..

But about the within the dialog requests - you have a strong case here!! 
But is actually more complex : you need to know the inbound and outbound 
domains actually - if you received the request from another peer via TLS 
and fed it also via TLS to another peer (relaying) will need to remember 
both domains since the within the dialog request may flow in both 
directions ;).
Maybe storing the domain names as RR param is the simplest and uglier 
solution...in the mean while I think is the only one without involving 
any dialog persistence.

> 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?

one premiss we should based on is the fact that cannot exists (in my 
opinion) connections that should or not require domain validation in 
different case. Argumentation: AFAIK only two types of connections can 
be: user oriented and peering oriented; the first type will not require 
validation at all and the second one may or may nor, based on local 
policy. So, I think, we cannot have a case when connection to X will 
require validation and later no.

To control the validation (and maybe other parameter of the connection), 
prior setting from the script may be the solution - I was investigating 
with Cesc the idea of building a TLS module which will be used for 
provisioning the cert and to control the connection params. The TLS 
engine itself will stay in core as now.

So, I would say we never reach the case when we want to reuse an 
existing connection but with different settings.

> 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).

Some utilities like this will became definitely needed in short time.....
maybe all this will find the way into the TLS module - that will be 
actually it;s purpose - pure management and provisioning.

> Now I'm ready for some discussions :-)
> regards
> klaus
> _______________________________________________
> Devel mailing list
> Devel at openser.org
> http://openser.org/cgi-bin/mailman/listinfo/devel

More information about the Users mailing list