Hi Klaus,
indeed this is a long email ;). please see my inline comments.
regards. Bogdan
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 certificatesip.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
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