Internet Engineering Task Force (IETF) P. Patil
Request for Comments: 8155 T. Reddy
Updates: 5766 Cisco
Category: Standards Track D. Wing
ISSN: 2070-1721 April 2017 Traversal Using Relays around NAT (TURN) Server Auto Discovery
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 minimal or no
configuration. This document describes three such mechanisms for
TURN server discovery.
This document updates RFC 5766 to relax the requirement for mutual
authentication in certain cases.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
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
automatically 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 exist an auto-discovery
mechanism for TURN services. Web Real-Time Communication (WebRTC)
[WebRTC-Overview] usages and related extensions, which are mostly
based on web applications, need TURN server discovery mechanisms.
This document describes three discovery mechanisms, so as to maximize
the opportunity for discovery, based on the network in which the TURN
client finds itself. The three discovery mechanisms are:
o A resolution mechanism based on Straightforward-Naming Authority
Pointer (S-NAPTR) resource records in the Domain Name System
(DNS). [RFC5928] describes details on retrieving a list of server
transport addresses from the DNS that can be used to create a TURN
o DNS Service Discovery.
o A mechanism based on an 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
3. Discovery Procedure
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). Configuration discovered from an application, e.g., a
(WebRTC) [WebRTC-Overview] usages and related extensions, is
considered a 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 in addition to
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:
o Service Resolution: The TURN client attempts to perform TURN
service resolution using the host's DNS domain.
o DNS SD: DNS Service Discovery.
o Anycast: Send TURN Allocation 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 the discovery mechanisms
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
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 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.
4. Discovery Using Service Resolution
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).
4.1. Retrieving Domain Name
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
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,
OPTION_V4_ACCESS_DOMAIN and OPTION_V6_ACCESS_DOMAIN respectively, to
identify a domain name that is suitable for service discovery within
the access network.
For IPv4, the discovery procedure MUST request the access network
domain name option in a Parameter Request List option, as described
in [RFC2131]. [RFC2132] defines the DHCP IPv4 domain name option;
while this option is less suitable, a client MAY request it if the
access network domain name defined in [RFC5986] is not available.
For IPv6, the discovery procedure MUST request the access network
domain name option in an Options Request Option (ORO) within an
Information-request message, as described in [RFC3315].
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.
4.1.2. From Own Identity
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, the canonical identifier used to reach
SIP : 'sip:firstname.lastname@example.org'
Bare JID : 'email@example.com'
email : 'firstname.lastname@example.org'
'example.com' is retrieved from the above examples.
A client may support multiple users, potentially with different
domains, or a single user utilizing different domains for different
services. The means to choose and extract the domain name may be
different based on the type of identifier, service being used, etc.,
which are 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 the 'RELAY' application service and the desired protocol
tag is made to obtain the information necessary to connect to the
authoritative TURN server within the given domain.
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.).
5. DNS Service Discovery
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
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 "_turn._udp" or "_turn._tcp" or "_turns._udp" or
"_turns._tcp", as introduced in [RFC5766].
A TURN client can proactively discover TURN servers being advertised
in the site by multicasting a PTR query to one or all of the
A TURN server can send out gratuitous multicast DNS answer packets
whenever it starts up, wakes from sleep, or detects a change in
network configuration. TURN clients receive these gratuitous packets
and cache information contained in it.
6. Discovery Using Anycast
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 for discovery are the two things that actually use TURN.
When a client requires TURN services, it sends a TURN Allocation
request to the assigned anycast address. A TURN anycast server
performs checks 1 through 7 discussed in Section 6.2 of [RFC5766].
If all checks pass, the TURN anycast server MUST respond with a 300
(Try Alternate) error as described in Section 2.9 of [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 TURN Allocation
request to the anycast address to discover the closest and the most
optimal unicast address for the TURN server.
7. Deployment Considerations
7.1. Mobility and Changing IP Addresses
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 that were learned after TURN the
discovery was 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 [RFC8016] can be used for ongoing
7.2. Recursively Encapsulated TURN
WebRTC endpoints SHOULD treat any TURN server discovered through the
mechanisms described in this specification as an enterprise/gateway
or access network server, in accordance with Recursively Encapsulated
9. Security Considerations
Use of Session Traversal Utilities for NAT (STUN) [RFC5389]
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.
Making STUN authentication optional is a downgrade of a MUST level
requirement defined in [RFC5766]. The downgrade allows TURN servers
provided by the local or access network to accept Allocation requests
from new and/or guest users in the network who do not necessarily
possess long term credentials for STUN authentication. The intention
in such deployments is to provide TURN services to all users in the
local or access network. However, this opens up a TURN server to a
variety of attacks described in Section 17 of [RFC5766]. A TURN
server in such cases must be configured to only process STUN requests
from the trusted local network or subscribers of the access network.
Operational measures must be taken in order to protect the TURN
server; some of these measures include, but are not limited to,
access control by means of access lists, firewalls, subscriber quota
limits, ingress filtering, etc.
A TURN client in the absence of the STUN long-term credential
mechanism [RFC5389] or the STUN Extension for Third-Party
Authorization [RFC7635] MUST use (D)TLS unless it trusts the network
infrastructure to defend against attacks discussed in [RFC5766]. It
is RECOMMENDED that the TURN client use one of the following
techniques with (D)TLS to validate the TURN server:
o For certificate-based authentication, a pre-populated trust anchor
store [RFC6024] 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 [RFC6125] to validate the TURN server identity.
o Certification authorities that issue TURN server certificates
SHOULD support the CN-ID, DNS-ID, SRV-ID, and URI-ID identifier
types. TURN service providers SHOULD prefer the use of DNS-ID,
SRV-ID, and URI-ID over CN-ID identifier types in certificate
requests (as described in Section 2.3 from [RFC6125]) and the
wildcard character '*' SHOULD NOT be included in the presented
o 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 [RFC7469] uses the public
o Raw public key-based authentication, as defined in [RFC7250],
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 therefore MUST NOT
be 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, fallback to clear text in such cases could leave the TURN
client open to on-path injection of spoofed TURN messages. A TURN
client could fall back to encryption-only (D)TLS when (D)TLS
authentication is not available but MUST NOT fall back without
explicit administrator choice. Another reason to fall back to
encryption-only is for privacy, which is analogous to SMTP
opportunistic encryption [RFC7435] where one does not require privacy
but one desires privacy when possible.
In order to allow the TURN client to fall back to (D)TLS as described
above, a TURN server that does not require either STUN long-term
authentication [RFC5389] or STUN Extension for Third Party
Authorization [RFC7635] MUST support (D)TLS and, if the network
infrastructure is capable of defending against attacks discussed in
[RFC5766], then the TURN server MAY allow fallback to clear text.
A TURN client could fall back to clear text if it does not support
unauthenticated (D)TLS but MUST NOT fall back without explicit
administrator choice. 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.
9.1. Service Resolution
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
9.2. DNS Service Discovery
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
The authors would like to thank Simon Perrault, Paul Kyzivat, Troy
Shields, Eduardo Gueiros, Ted Hardie, Bernard Aboba, Karl Stahl,
Brian Weis, Ralph Dromes, Ben Campbell, Suresh Krishnan, 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.
Cisco Systems, Inc.
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
United States America