| TRAM | P. Patil |
| Internet-Draft | T. Reddy |
| Intended status: Standards Track | D. Wing |
| Expires: July 9, 2016 | Cisco |
| January 6, 2016 |
TURN Server Auto Discovery
draft-ietf-tram-turn-server-discovery-06
Current Traversal Using Relays around NAT (TURN) server discovery mechanisms are relatively static and limited to explicit configuration. These are usually under the administrative control of the application or TURN service provider, and not the enterprise, ISP, or the network in which the client is located. Enterprises and ISPs wishing to provide their own TURN servers need auto discovery mechanisms that a TURN client could use with no or minimal configuration. This document describes three such mechanisms for TURN server discovery.
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This Internet-Draft will expire on July 9, 2016.
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TURN [RFC5766] is a protocol that is often used to improve the connectivity of Peer-to-Peer (P2P) applications (as defined in section 2.7 of [RFC5128]). TURN allows a connection to be established when one or both sides are incapable of a direct P2P connection. It is an important building block for interactive, real-time communication using audio, video, collaboration etc.
While TURN services are extensively used today, the means to auto discover TURN servers do not exist. TURN clients are usually explicitly configured with a well known TURN server. To allow TURN applications to operate seamlessly across different types of networks and encourage the use of TURN without the need for manual configuration, it is important that there exists an auto discovery mechanism for TURN services. Web Real-Time Communication (WebRTC) [I-D.ietf-rtcweb-overview] usages and related extensions, which are mostly based on web applications, need this immediately.
This document describes three discovery mechanisms, so as to maximize opportunity for discovery, based on the network in which the TURN client finds itself. The three discovery mechanisms are:
In general, if a client wishes to communicate using one of its interfaces using a specific IP address family, it SHOULD query the TURN server(s) that has been discovered for that specific interface and address family. How to select an interface and IP address family is out of the scope of this document.
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 [RFC2119].
A TURN client that implements the auto discovery algorithm uses the following mechanisms for discovery:
Not all TURN servers may be discovered using NAPTR records or DNS SD; Similarly, not all TURN servers may support anycast. For best results, a client SHOULD implement all discovery mechanisms described above.
The document does not prescribe a strict order that a client must follow for discovery. An implementation may choose to perform all the above steps in parallel for discovery OR choose to follow any desired order and stop the discovery procedure if a mechanism succeeds.
On hosts with more than one interface or address family (IPv4/v6), the TURN server discovery procedure has to be performed for each combination of interface and address family. A client MAY optionaly choose to perform the discovery procedure only for a desired interface/address combination if the client does not wish to discover a TURN server for all combinations of interface and address family.
This mechanism is performed in two steps:
1. A DNS domain name is retrieved for each combination of interface and address family.
2. Retrieved DNS domain names are then used for S-NAPTR lookups as per [RFC5928]. Further DNS lookups may be necessary to determine TURN server IP address(es).
A client has to determine the domain in which it is located. The following sections provide two possible mechanisms to learn the domain name, but other means of retrieving domain names may be used, which are outside the scope of this document e.g. local configuration.
Implementations may allow the user to specify a default name that is used if no specific name has been configured.
DHCP can be used to determine the domain name related to an interface's point of network attachment. Network operators may provide the domain name to be used for service discovery within an access network using DHCP. Sections 3.2 and 3.3 of [RFC5986] define DHCP IPv4 and IPv6 access network domain name options to identify a domain name that is suitable for service discovery within the access network. [RFC2132] defines the DHCP IPv4 domain name option; While this option is less suitable, it may still be useful if the options defined in [RFC5986] are not available.
For IPv6, the TURN server discovery procedure MUST try to retrieve DHCP option 57 (OPTION_V6_ACCESS_DOMAIN). If no such option can be retrieved, the procedure fails for this interface. For IPv4, the TURN server discovery procedure MUST try to retrieve DHCP option 213 (OPTION_V4_ACCESS_DOMAIN). If no such option can be retrieved, the procedure SHOULD try to retrieve option 15 (Domain Name). If neither option can be retrieved the procedure fails for this interface. If a result can be retrieved it will be used as an input for S-NAPTR resolution.
For a TURN client with an understanding of the protocol mechanics of calling applications, the client may wish to extract the domain name from its own identity i.e canonical identifier used to reach the user.
Example SIP : 'sip:alice@example.com' JID : 'alice@example.com' email : 'alice@example.com' 'example.com' is retrieved from the above examples.
The means to extract the domain name may be different based on the type of identifier and is outside the scope of this document.
Once the TURN discovery procedure has retrieved domain names, the resolution mechanism described in [RFC5928] is followed. An S-NAPTR lookup with 'RELAY' application service and the desired protocol tag is made to obtain information necessary to connect to the authoritative TURN server within the given domain.
In the example below, for domain 'example.net', the resolution algorithm will result in IP address, port, and protocol tuples as follows:
example.net.
IN NAPTR 100 10 "" RELAY:turn.udp "" example.net.
example.net.
IN NAPTR 100 10 S RELAY:turn.udp "" _turn._udp.example.net.
_turn._udp.example.net.
IN SRV 0 0 3478 a.example.net.
a.example.net.
IN A 192.0.2.1
+-------+----------+------------+------+
| Order | Protocol | IP address | Port |
+-------+----------+------------+------+
| 1 | UDP | 192.0.2.1 | 3478 |
+-------+----------+------------+------+
DNS-based Service Discovery (DNS-SD) [RFC6763] and Multicast DNS (mDNS) [RFC6762] provide generic solutions for discovering services available in a local network. DNS-SD/ mDNS define a set of naming rules for certain DNS record types that they use for advertising and discovering services. PTR records are used to enumerate service instances of a given service type. A service instance name is mapped to a host name and a port number using a SRV record. If a service instance has more information to advertise than the host name and port number contained in its SRV record, the additional information is carried in a TXT record.
Section 4.1 of [RFC6763] specifies that a service instance name in DNS-SD has the following structure:
<Instance> . <Service> . <Domain>
The <Domain> portion specifies the DNS sub-domain where the service instance is registered. It may be "local.", indicating the mDNS local domain, or it may be a conventional domain name such as "example.com.". The <Service> portion of the TURN service instance name MUST be "_turnserver._udp", "_turnserver._tcp".
The <Instance> portion is a DNS label, containing UTF-8-encoded text [RFC5198], limited to 63 octets in length. It is meant to be a user-friendly description of the service instance, suitable for a menu-like user interface display. Thus it can contain any characters including spaces, punctuation, and non-Latin characters as long as they can be encoded in UTF-8.
For example, TURN server advertises the following DNS records :
_turnserver._udp.local. PTR example.com._turnserver._udp.local. example.com._turnserver._udp.local. SRV 0 0 5030 example-turn- server.local. example-turn-server.local. A 198.51.100.2
In addition to the service instance name, IP address and the port number, DNS-SD provides a way to publish other information pertinent to the service being advertised. The additional data can be stored as name/value attributes in a TXT record with the same name as the SRV record for the service. Each name/value pair within the TXT record is preceded by a single length byte, thereby limiting the length of the pair to 255 bytes (See Section 6 of [RFC6763] and Section 3.3.14 of [RFC1035] for details).
A TURN client tries to discover the TURN servers being advertised in the site by multicasting a PTR query "_turnserver._udp.local." or "_turnserver._tcp.local" or the TURN server can send out gratuitous multicast DNS answer packets whenever it starts up, wakes from sleep, or detects a chance in network configuration. TURN clients receive these gratuitous packet and cache the information contained in it.
+------+ +-------------+
| TURN | | TURN Server |
|Client| | |
+------+ +-------------+
| |
| PTR query "_turnserver._udp.local." |
|--------------------------------------------->|
| PTR reply |
|<---------------------------------------------|
| SRV query |
|--------------------------------------------->|
| SRV reply |
|<---------------------------------------------|
| A/AAAA query reply |
|--------------------------------------------->|
| TURN Request |
|--------------------------------------------->|
| TURN Response |
|<---------------------------------------------|
Figure 1: TURN Server Discovery using mDNS
IP anycast can also be used for TURN service discovery. A packet sent to an anycast address is delivered to the "topologically nearest" network interface with the anycast address. Using the TURN anycast address, the only two things that need to be deployed in the network are the two things that actually use TURN.
When a client requires TURN services, it sends a TURN allocate request to the assigned anycast address. The TURN anycast server responds with a 300 (Try Alternate) error as described in [RFC5766]; The response contains the TURN unicast address in the ALTERNATE-SERVER attribute. For subsequent communication with the TURN server, the client uses the responding server's unicast address. This has to be done because two packets addressed to an anycast address may reach two different anycast servers. The client, thus, also needs to ensure that the initial request fits in a single packet. An implementation may choose to send out every new request to the anycast address to learn the closest TURN server each time.
A change of IP address on an interface may invalidate the result of the TURN server discovery procedure. For instance, if the IP address assigned to a mobile host changes due to host mobility, it may be required to re-run the TURN server discovery procedure without relying on earlier gained information. New requests should be made to the newly learned TURN servers learned after TURN discovery re-run. However, if an earlier learned TURN server is still accessible using the new IP address, procedures described for mobility using TURN defined in [I-D.ietf-tram-turn-mobility] can be used for ongoing streams.
A TURN client could attempt to discover multiple TURN servers so as to use recursively encapsulated TURN, as described in [I-D.ietf-rtcweb-return], to route traffic through multiple TURN servers for privacy.
IANA should allocate an IPv4 and an IPv6 well-known TURN anycast address. 192.0.0.0/24 and 2001:0000::/48 are reserved for IETF Protocol Assignments, as listed at
<http://www.iana.org/assignments/iana-ipv4-special-registry/> and
<http://www.iana.org/assignments/iana-ipv6-special-registry/>
Clients can use TURN servers provided by the local network or by the access network without authenticating with the TURN server. It is recommended that clients use (D)TLS with network provided TURN servers to validate the TURN server and prevent man-in-middle attacks. A TURN client may use the following techniques to validate a TURN server:
The primary attack against the methods described in this document is one that would lead to impersonation of a TURN server. An attacker could attempt to compromise the S-NAPTR resolution. Security considerations described in [RFC5928] are applicable here as well.
In addition to considerations related to S-NAPTR, it is important to recognize that the output of this is entirely dependent on its input. An attacker who can control the domain name can also control the final result. Because more than one method can be used to determine the domain name, a host implementation needs to consider attacks against each of the methods that are used.
If DHCP is used, the integrity of DHCP options is limited by the security of the channel over which they are provided. Physical security and separation of DHCP messages from other packets are commonplace methods that can reduce the possibility of attack within an access network; alternatively, DHCP authentication [RFC3188] can provide a degree of protection against modification. When using DHCP discovery, clients are encouraged to use unicast DHCP INFORM queries instead of broadcast queries which are more easily spoofed in insecure networks.
Since DNS-SD is just a specification for how to name and use records in the existing DNS system, it has no specific additional security requirements over and above those that already apply to DNS queries and DNS updates. For DNS queries, DNS Security Extensions (DNSSEC) [RFC4033] should be used where the authenticity of information is important. For DNS updates, secure updates [RFC2136][RFC3007] should generally be used to control which clients have permission to update DNS records.
For mDNS, in addition to what has been described above, a principal security threat is a security threat inherent to IP multicast routing and any application that runs on it. A rogue system can advertise that it is a TURN server. Discovery of such rogue systems as TURN servers, in itself, is not a security threat if there is a means for the TURN client to authenticate and authorize the discovered TURN servers.
In a network without any TURN server that is aware of the TURN anycast address, outgoing TURN requests could leak out onto the external Internet, possibly revealing information.
Using an IANA-assigned well-known TURN anycast address enables border gateways to block such outgoing packets. In the default-free zone, routers should be configured to drop such packets. Such configuration can occur naturally via BGP messages advertising that no route exists to said address.
Sensitive clients that do not wish to leak information about their presence can set an IP TTL on their TURN requests that limits how far they can travel into the public Internet.
The authors would like to thank Simon Perrault, Paul Kyzivat, Troy Shields, Eduardo Gueiros, Ted Hardie, Bernard Aboba, Karl Stahl and Brandon Williams for their review and valuable comments. Thanks to Adam Roach for his detailed review and suggesting DNS Service Discovery as an additional discovery mechanism.
[Note to RFC Editor: Please remove this section prior to publication.]