TURN Server Auto DiscoveryCisco Systems, Inc.BangaloreIndiapraspati@cisco.comCisco Systems, Inc.Cessna Business Park, Varthur HobliSarjapur Marathalli Outer Ring RoadBangaloreKarnataka560103Indiatireddy@cisco.comCisco Systems, Inc.170 West Tasman DriveSan JoseCalifornia95134USAdwing@cisco.comTRAMCurrent 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.TURN is a protocol that is often used to
improve the connectivity of Peer-to-Peer (P2P) applications (as defined
in section 2.7 of ). 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) 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:A resolution mechanism based on straightforward Naming Authority
Pointer (S-NAPTR) resource records in the Domain Name System (DNS).
describes details on retrieving a list of
server transport addresses from DNS that can be used to create a
TURN allocation.DNS Service DiscoveryA mechanism based on anycast address for TURN.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", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in .TURN clients, by default, discover TURN server(s) by means of local
or manual TURN configuration i.e., TURN servers configured at the system
level. For example, in case of Web Real-Time Communication (WebRTC)
[I-D.ietf-rtcweb-overview] usages and related extensions, which are
based on web applications, a Java Script specified TURN server MUST be
considered as local configuration. An implementation MAY give the user
an opportunity (e.g., by means of configuration file options or menu
items) to specify a TURN server for each address family. A client can
choose auto-discovery in the absence of local configuration, if local
configuration doesn’t work or on top of local configuration. This
document does not offer a recommendation on server selection.A TURN client that implements the auto discovery algorithm, to
discover TURN servers in the attached network, uses the following
mechanisms for discovery:Service Resolution : The TURN client attempts to perform TURN
service resolution using the host's DNS domain.DNS SD: DNS Service Discovery.Anycast : Send TURN allocate request to the assigned TURN anycast
request for each combination of interface and address family.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 optionally
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 . 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 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.
defines the DHCP IPv4 domain name option; While this option is less
suitable, it may still be useful if the options defined in 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.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 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:If no TURN-specific S-NAPTR records can be retrieved, the
discovery procedure fails for this domain name (and the corresponding
interface and IP protocol version). If more domain names are known,
the discovery procedure may perform the corresponding S-NAPTR lookups
immediately. However, before retrying a lookup that has failed, a
client MUST wait a time period that is appropriate for the encountered
error (NXDOMAIN, timeout, etc.).DNS-based Service Discovery (DNS-SD) and
Multicast DNS (mDNS) 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 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 , 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 :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 and Section
3.3.14 of 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.|
| 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 ; 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 can be used
for ongoing streams.WebRTC endpoints SHOULD treat any TURN server discovered through
the mechanims described in this specification as an enterprise/gateway
server, in accordance with Recursively Encapsulated TURN .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/>Use of STUN authentication is OPTIONAL for TURN servers provided by
the local network or by the access network. A network provided TURN
server MAY be configured to accept Allocation requests without STUN
authentication, and a TURN client MAY be configured to accept Allocation
success responses without STUN authentication from a network provided
TURN server. In order to protect against man-in-the-middle attacks when
accepting a TURN allocation response without STUN authentication, it is
RECOMMENDED that the TURN client use one of the following techniques
with (D)TLS to validate the TURN server:For certificate-based authentication, a pre-populated trust
anchor store allows a TURN client to
perform path validation for the server certificate obtained during
the (D)TLS handshake. If the client used a domain name to discover
the TURN server, that domain name also provides a mechanism for
validation of the TURN server. The client MUST use the rules and
guidelines given in section 6 of to
validate the TURN server identity.For TURN servers that don't have a certificate trust chain (e.g.,
because they are on a home network or a corporate network), a
configured list of TURN servers can contain the Subject Public Key
Info (SPKI) fingerprint of the TURN servers. The public key is used
for the same reasons HTTP pinning uses the
public key.Raw public key-based authentication, as defined in , could also be used to authenticate a TURN
server.An auto-discovered TURN server is considered to be only as trusted as
the path between the client and the TURN server. In order to safely use
auto-discovered TURN servers for sessions with 'strict privacy'
requirements, the user needs to be able to define privacy criteria (e.g.
a trusted list of servers, networks, or domains) that are considered
acceptable for such traffic. Any discovered TURN server outside the
criteria is considered untrusted and is not used for privacy sensitive
communication.In some auto-discovery scenarios, it might not be possible for the
TURN client to use (D)TLS authentication to validate the TURN server.
However, fall-back to clear text in such cases could leave the TURN
client open to on-path injection of spoofed TURN messages. For this
reason, it is beneficial for the TURN client to make use of
'opportunistic privacy', analogous to SMTP opportunistic encryption
, where one does not require privacy but one
desires privacy when possible. In this scenario, a TURN client attempts
(D)TLS with authentication and encryption, falling back to
encryption-only if the TURN server cannot be authenticated via (D)TLS.
If the TURN server does not support unauthenticated (D)TLS, it could
fall back to clear text, but fallback to clear text is NOT RECOMMENDED
because it makes the client more susceptible to man-in-the-middle
attacks and on-path packet injection. A TURN client SHOULD fall-back to
encryption-only (D)TLS when (D)TLS authentication is not available in
order to protect against on-path attackers who might attempt to inject
fake TURN messages.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 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 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) should be used where the
authenticity of information is important. For DNS updates, secure
updates 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.]Added IP address (Section 4.1.2) and Own identity (4.1.3) as
new means to obtain domain namesNew Section 4.2.1 SOA (inspired by draft-kist-alto-3pdisc)300 (Try Alternate) response for AnycastRemoved sections that describe reverse IP lookupAdded DNS Service Discovery as an additional discovery
mechanism