Internet DRAFT - draft-schwartz-tram-turnbyname
draft-schwartz-tram-turnbyname
Network Working Group B. Schwartz
Internet-Draft J. Uberti
Intended status: Standards Track Google
Expires: September 6, 2015 March 5, 2015
TURN by name: an extension to TURN for contacting an endpoint by its DNS
name.
draft-schwartz-tram-turnbyname-00
Abstract
When tunneling traffic through TURN, a client may sometimes desire to
contact a remote endpoint that it knows by its DNS name, not its IP
address. This document describes an extension to TURN that allows
such a client to contact a named endpoint.
Status of This Memo
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This Internet-Draft will expire on September 6, 2015.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. New Address Family for DNS names . . . . . . . . . . . . . . 3
3. Extension to the XOR-PEER-ADDRESS attribute format . . . . . 3
4. Changes to TURN server behavior . . . . . . . . . . . . . . . 4
4.1. Servers that do not support the extension . . . . . . . . 4
4.2. Supported attributes . . . . . . . . . . . . . . . . . . 4
4.3. Supported messages . . . . . . . . . . . . . . . . . . . 4
4.4. Name mapping storage . . . . . . . . . . . . . . . . . . 4
4.5. Lookup behavior . . . . . . . . . . . . . . . . . . . . . 5
4.6. CreatePermission . . . . . . . . . . . . . . . . . . . . 6
4.6.1. Implications of this permission model . . . . . . . . 6
4.7. Send . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.8. Channel Binding . . . . . . . . . . . . . . . . . . . . . 7
4.8.1. Implications of this channel binding model . . . . . 8
4.9. Receiving Data . . . . . . . . . . . . . . . . . . . . . 9
4.9.1. Implications of this data receipt model . . . . . . . 9
5. Changes to TURN client behavior . . . . . . . . . . . . . . . 10
5.1. When to use this extension . . . . . . . . . . . . . . . 10
5.2. Issuing Send, CreatePermission, and ChannelBind requests
for DNS names . . . . . . . . . . . . . . . . . . . . . . 10
5.3. Receiving Data indications . . . . . . . . . . . . . . . 10
5.4. Handling dynamic addressing . . . . . . . . . . . . . . . 11
5.5. Dual-stack behavior . . . . . . . . . . . . . . . . . . . 11
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
10.1. Normative References . . . . . . . . . . . . . . . . . . 14
10.2. Informative References . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
The TURN standard [RFC5766] extends STUN to allow proxying
connections directly through the server. Clients send messages to
the server in order to request the allocation of ports on the server,
and identify the remote peers with whom they want to exchange
packets. These remote peers are identified by an XOR-PEER-ADDRESS
attribute, which includes the remote peer's IP address and port.
TURN is most commonly used as a component of an ICE [RFC5245]
implementation, to allow communication between endpoints that each
send their own transport address to the other in the form of an ICE
candidate. These candidates are typically constructed using STUN, or
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by direct interrogation of the network interfaces, so they normally
contain IP addresses, although domain names are also allowed.
However, TURN is now attracting a wider range of use cases,
especially on enterprise networks and in conjunction with WebRTC.
Some use cases employ TURN as an "escape hatch" in an otherwise
tightly restricted network, with the intention that users would
tunnel much or all of their UDP traffic through the TURN server. On
some restricted networks, DNS access is also restricted, which may
prevent users from determining the IP address of a domain (e.g. an
application-specified TURN server, for RETURN
[I-D.schwartz-rtcweb-return], or an ICE candidate that contains a
domain name) that they wish to contact through the escape-hatch TURN
server.
Extending TURN to support named peers allows TURN to work for clients
who are attempting to contact an endpoint by name, on networks where
resolving those names is not otherwise possible.
2. New Address Family for DNS names
The Address Family 0x03 is defined to indicate that the specified
address is a DNS name. This family is only permitted under certain
circumstances, detailed below.
3. Extension to the XOR-PEER-ADDRESS attribute format
STUN/TURN attributes are Type-Length-Value encoded ([RFC5389],
Section 15). For both of the existing Families, the attribute's
encoded length is a known constant, because the length of the address
is constant. For the newly defined Family 0x03, the length is
variable, and the indicated length from the TLV encoding is necessary
in order to parse the attribute.
The DNS name is transmitted in the X-Address field, and is encoded by
the following procedure:
1. Define the "legacy transaction ID" as a 128-bit value consisting
of the 32-bit magic cookie followed by the 96-bit transaction ID.
2. Define the DNS name as a standard dot-separated UTF-8 byte-string
(not null-terminated).
3. Compute the encoded address (X-Address) by XOR'ing each byte of
the DNS name with the corresponding byte of the legacy
transaction ID.
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* If the DNS name is longer than 128 bits, the corresponding
byte with which to XOR wraps around to the beginning of the
legacy transaction ID.
This procedure is an extension of the encoding for families 0x01 and
0x02, so all three XOR-PEER-ADDRESS families can be encoded and
parsed by a single procedure, without any special cases.
4. Changes to TURN server behavior
4.1. Servers that do not support the extension
Servers are NOT REQUIRED to support this extension. No change is
required to servers that do not support the extension. Upon
receiving a message containing an XOR-PEER-ADDRESS attribute with
Family 0x03, existing compliant servers MUST reply with Error 440
(Address Family not Supported).
Servers that do support this extension MUST comply with the
requirements that follow in this section.
4.2. Supported attributes
Address Family 0x03 is only permitted in the context of the XOR-PEER-
ADDRESS attribute. All other attributes that use address families
remain restricted to families 0x01 and 0x02. The server MUST respond
with Error 440 (Address Family not Supported) when encountering this
address family in an attribute where it is not supported.
4.3. Supported messages
The XOR-PEER-ADDRESS attribute may only have family 0x03 in the
context of a CreatePermission, Send, Data, or ChannelBind message.
If the server encounters this address family in the context of any
other message type, it MUST respond with Error 440 (Address Family
not Supported).
If a TURN server supports address family 0x03 in one of these
messages, it MUST support it in all of these messages.
4.4. Name mapping storage
Baseline TURN servers must store two kinds of state for each
Allocation: Permissions and Channel Bindings. This extension adds a
third kind of state: Name Mappings. Each DNS Name Mapping consists
of:
o a DNS name
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o an IP address
o a reference count, which is always either 1 or 2
Name Mappings do not have an expiration time, but the server MUST
delete them if their reference count falls to zero. Like Permissions
and Channel Bindings, Name Mappings are scoped to a single
Allocation.
Each IP address appears in only one Name Mapping for an Allocation.
The requirements for CreatePermission and ChannelBind are structured
to maintain this invariant.
Server implementations SHOULD implement Name Mappings in a way that
enables fast bidirectional lookup.
4.5. Lookup behavior
When a DNS name lookup is required, the server's behavior depends on
the current Allocation. Each supported message is associated with an
Allocation, whose address family is IPv4, IPv6 [RFC6156], or Both
(via ADDITIONAL-ADDRESS-FAMILY [I-D.ietf-tram-turnbis]).
If the address family is IPv4, then the server MUST search for an A
record for the name, and similarly if the address family is IPv6, the
server MUST search for a AAAA record. The server MUST handle errors
as follows:
o If resolution fails due to a server error (e.g. DNS SERVFAIL),
reply with error code 500 (Server Error).
o If the resolution fails because there is no record of the required
type (e.g. DNS NOERROR), respond with error code 443 (Peer
Address Family Mismatch).
o For all other DNS errors, return error code 447 (Connection
timeout or failure).
The TURN server implementation MAY use a high-level DNS resolution
API, such as gethostbyname or getaddrinfo, to perform the lookup.
If the Allocation has both address families, then it MUST look for an
IPv6 address, and fall back to IPv4 only if a AAAA record is not
found.
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4.6. CreatePermission
In baseline TURN, each CreatePermission message creates or renews a
Permission to send and receive messages to some specified IP address.
With this extension, a Permission may indicate either an IP address
or a DNS name. Both types of Permissions are subject to the same
expiration policy.
At any given time, there is at most one Permission that specifies any
IP address, or any DNS name, but there may be a Permission specifying
a DNS name that resolves to an IP address that is specified in
another Permission.
Upon receiving a CreatePermission message on an Allocation, the
server MUST perform these steps:
1. If the CreatePermission message contains a peer address of family
0x01 or 0x02, create or update a Permission for the given
address. (No change from baseline.)
2. If the CreatePermission message contains peer address of family
0x03:
A. Look for an existing Permission with the given DNS name. If
one exists, refresh its expiration time and return success.
B. Otherwise, check if there is a Name Mapping for the DNS name.
i. If one exists, increment its reference count.
ii. Otherwise, perform a DNS lookup for the name. If it
succeeds, add a DNS name mapping for the name and the
resolved address, with reference count 1.
C. Install a new permission for the DNS name.
When a Permission containing a DNS name expires, the server MUST
decrement the reference count on the Name Mapping for this DNS name,
and delete the Name Mapping if its reference count falls to zero.
4.6.1. Implications of this permission model
As long as a permission is regularly refreshed with the same DNS
name, the effective IP address will not change.
Permission refreshes for an IP address do not extend the lifetime of
DNS resolutions to that address.
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Permission requests for an IP address are not sufficient to allow
Send requests to a DNS name that resolves to that IP address, and
vice versa.
4.7. Send
Upon receiving a Send message on an Allocation, the server MUST
perform these steps:
1. If the Send message contains a peer address of family 0x01 or
0x02, check for a Permission that indicates that IP address.
(There will be at most one.) If a Permission matches, send the
packet; otherwise silently drop it. (No change from baseline.)
2. If the Send message contains a peer address of family 0x03, check
if there is a Permission for the given DNS name. (There will be
at most one.) If one exists, send the packet to the IP address
indicated for that DNS name in its Name Mapping; otherwise
silently drop it.
4.8. Channel Binding
In baseline TURN, each ChannelBind message creates or renews a
channel binding, which consists of a transport ID, a peer's IP
address, and a port on that address. It also creates or renews a
permission for the peer's IP address, exactly as if a
CreatePermission message had been received for that IP address.
In this extension, each channel binding includes either an IP address
or a DNS name.
Upon receiving a ChannelBind message on an Allocation, the server
MUST perform these steps:
1. If the ChannelBind indicates a peer address of family 0x01 or
0x02
A. If a binding already exists with the specified transport ID,
IP address, and port, refresh the binding.
B. If a binding already exists for the specified transport ID
with a different or unspecified IP address or port, report
Error 400 (Bad Request).
C. If a binding already exists with this port and this IP
address, or a DNS name that maps to this IP address, report
Error 400 (Bad Request) and include a CHANNEL-NUMBER
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attribute that indicates the number of the conflicting
channel.
D. Otherwise, create a binding.
E. Install or refresh a permission for the originally indicated
peer IP address.
2. If the ChannelBind indicates a peer address of family 0x03
A. If a binding already exists with the specified transport ID,
DNS name, and port, refresh the binding, including the IP
address.
B. Otherwise, resolve the DNS name to an IP address, using the
name mapping table if it exists, and performing a DNS lookup
only if no name mapping exists for this DNS name.
i. If a binding already exists for the specified transport
ID with a different IP address or port, report Error
400 (Bad Request).
ii. If a binding already exists with a different transport
ID, for this port, and this IP address or a DNS name
that is mapped to this IP address, report Error 400
(Bad Request) and include a CHANNEL-NUMBER attribute
that indicates the number of the conflicting channel.
C. Install a channel binding with the specified transport ID,
DNS name, and port.
D. Increment the name mapping's reference count, or Install a
new name mapping if one does not already exist for this DNS
name.
E. Perform the steps required when receiving a CreatePermission
message for this DNS name.
When a channel binding that indicates a DNS name expires, the server
MUST decrement the reference count on the matching name mapping, and
delete the mapping if the reference count falls to zero.
4.8.1. Implications of this channel binding model
As long as a channel is refreshed before it times out, it will
continue to resolve to a constant address.
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There can never be two channels bound to the same remote transport
address. If that were possible, it would result in traffic
amplification (sending each received packet to all matching channels)
or other strange behaviors (e.g. selecting one arbitrary channel to
receive the packet).
Each time a new channel is bound for a DNS name, it checks for a Name
Mapping before doing any external resolution, so the resolved IP
address is guaranteed to be consistent with the active Permission for
this DNS name, if one exists. As a result, DNS resolution results
can persist indefinitely within an Allocation, longer than the DNS
TTL or any individual connection, if they are maintained by
ChannelBind or CreatePermission calls to different ports on the same
remote peer that overlap in time.
If two ChannelBind requests are received for the same port on two
different DNS names that resolve to the same IP address, the second
request will fail with a generic error code (400), but will also let
the client know which existing channel to use instead. The same is
true of collisions between IP and DNS channel binding requests.
Installing a channel binding to a DNS name also enables Send messages
to the DNS name, but not to the resolved IP address.
4.9. Receiving Data
Upon receiving an incoming packet on an Allocation, the server MUST
perform these steps:
1. Check if there is a channel binding to this source port and IP
address, or a DNS name that is mapped to this IP address. (There
will be at most one such channel.) If there is, let the channel
handle the packet.
2. Otherwise, check if there is any DNS permission that is mapped to
the source IP address. If there is, produce a Data message with
that DNS name.
3. Otherwise, check if there is any IP permission that matches the
source IP address. If there is, produce a Data message with the
source IP address; otherwise discard the packet.
4.9.1. Implications of this data receipt model
If a name mapping exists for an IP address, all packets received from
that address will be labeled with the DNS name, not the IP address.
Clients never learn the IP address for a DNS name unless they provoke
a conflict, similar to the naming model used by SOCKS5 [RFC1928].
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If a channel is bound for a port on a peer, all packets from that
port will be routed to the channel exclusively.
5. Changes to TURN client behavior
Clients are NOT REQUIRED to support this extension. No change is
required to existing clients. The requirements in this section only
apply to clients that opt to support the extension.
5.1. When to use this extension
When the client receives a request to contact an endpoint that is
identified by its DNS name, the client SHOULD attempt to use this
extension to reach that endpoint, and SHOULD NOT attempt to perform a
local DNS lookup for the name, so that connections may succeed even
if the local DNS server fails to return a correct result.
If the TURN server responds with Error 440 (Address Family Not
Supported), then the TURN client application SHOULD attempt to
perform a local DNS lookup for the name, and retry the connection by
IP address. (This functionality is logically separable from the TURN
protocol itself, and might best be implemented by having a TURN
client library that indicates the error, leaving the DNS lookup to be
the responsibility of the application that uses the library.)
5.2. Issuing Send, CreatePermission, and ChannelBind requests for DNS
names
When attempting to contact an endpoint by its DNS name, the client
SHOULD transmit a CreatePermission or ChannelBind request whose XOR-
PEER-ADDRESS attribute contains family 0x03, conveying the DNS name
formatted as described above.
If the server responds with Error 440 (Address family not supported),
then the client SHOULD abandon all requests using DNS, because the
server does not support this extension.
If a ChannelBind request fails with Error 400, but includes a
CHANNEL-NUMBER attribute, then that channel is already bound to the
remote transport address.
5.3. Receiving Data indications
Clients MAY send CreatePermission requests for both an IP address and
a DNS name that maps to that IP address, and both requests will
succeed. However, all Data messages from the remote peer will be
marked as being received from the DNS name. Therefore, clients MUST
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NOT assume that replies from a Send to an IP address are labeled with
that IP address.
5.4. Handling dynamic addressing
The IP address to which a DNS name resolves is not a constant. It
may change occasionally due to address reassignment, or it may even
change on every lookup, in the case of round-robin DNS.
The TURN server ensures that the IP address associated with a
permission or channel binding does not change as long as the
permission or binding is refreshed before it expires. Therefore,
clients that need to send messages to a stable IP address MUST
refresh their DNS name permissions and channel bindings even while
they are not in use, to ensure that they do not expire and later
resolve to a different IP address.
If the client has previously connected to a DNS name on an
Allocation, and wishes to connect again to the same DNS name with an
up-to-date IP address resolution, it SHOULD request a new Allocation,
and connect to the DNS name on the new Allocation.
5.5. Dual-stack behavior
If a specific address family is not indicated for the remote
endpoint, and the server does not support dual allocation (e.g.
ADDITIONAL-ADDRESS-FAMILY [I-D.ietf-tram-turnbis]]), then the
client's behavior is implementation-defined. For example, when
processing a request to send the first packet to a DNS name, the
client MAY use an approach inspired by Happy Eyeballs [RFC6555]:
o Create an Allocation for the system's preferred address family
(e.g. IPv6).
o Attempt to connect to the DNS name on this Allocation using a
ChannelBind message.
* If the server replies with error code 443 (Peer Address Family
Mismatch), immediately discard the Allocation and try again
with an Allocation of the other family.
* If a response message is received before some timeout (e.g. 300
ms), use this Allocation
* If no response message is received before some timeout (e.g.
300 ms), attempt to connect using a new Allocation of the other
address family, and use whichever Allocation receives a
response first. Discard the other Allocation.
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6. Examples
TURN TURN Peer DNS
client server A Server
| | | |
|-- ChannelBind req ---------------->| | |
| (peer-a.example.com to 0x4001) | | |
| |======= DNS query ========>|
| | (peer-a.example.com) |
| |<=======DNS result=========|
| | (192.0.2.15) |
|<---------- ChannelBind succ resp --| | |
| | | |
|-- [0x4001] data ------------------>| | |
| |=== data ===>| |
| | | |
| |<== data ====| |
|<------------------ [0x4001] data --| | |
Figure 1: Using DNS names with ChannelBind
TURN TURN Peer DNS
client server A Server
| | | |
|----- CreatePermission req -------->| | |
| (peer-a.example.com) | | |
| |======= DNS query ========>|
| | (peer-a.example.com) |
| |<=======DNS result=========|
| | (192.0.2.15) |
|<-- CreatePermission success resp --| | |
| | | |
|-- Send ind (peer-a.example.com) -->| | |
| |=== data ===>| |
| | | |
| |<== data ====| |
|<-- Data ind (peer-a.example.com) --| | |
| | | |
Figure 2: Using DNS names with CreatePermission and Send
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TURN TURN Peer DNS
client server A Server
| | | |
|------- CreatePermission req ------>| | |
| (peer-a.example.com) | | |
| |======= DNS query ========>|
| | (peer-a.example.com) |
| |<=======DNS result=========|
| | (192.0.2.15) |
|<-- CreatePermission success resp --| | |
| | | |
|------- ChannelBind req ----------->| | |
| (peer-a.example.com to 0x4001) | | |
| | | |
|<---- ChannelBind succ resp --------| | |
| | | |
|-- Send ind (peer-a.example.com) -->| | |
| |=== data ===>| |
| | | |
| |<== data ====| |
|<---------- [0x4001] data ----------| | |
| | | |
Figure 3: Sharing DNS names between CreatePermission and ChannelBind
7. Security Considerations
TURN servers that implement this specification can be made to parse
arbitrary DNS records. They should make sure to use secure, well-
tested DNS client implementations.
Clients can cause the TURN server to perform an arbitrary number of
DNS lookups. Server implementations MAY limit the rate at which an
individual client can trigger lookups, and return Error 508
(Insufficient Capacity) when a client exceeds the limit.
A malicious server could forward messages to the wrong IP address for
a specified domain name, but this does not represent a change in
security relative to the basic TURN standard.
To provide this functionality, the server is required to store a
number of DNS Name Mappings that is at most the number of active
permissions or channels. Implementers should take care to avoid
resource leaks in the DNS mapping implementation, to maintain this
bound.
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8. IANA Considerations
This draft adds a new STUN address family, 0x03 (DNS name).
9. Acknowledgements
Thanks to Warren Kumari for his early review.
10. References
10.1. Normative References
[I-D.ietf-tram-turnbis]
Reddy, T., Johnston, A., Matthews, P., and J. Rosenberg,
"Traversal Using Relays around NAT (TURN): Relay
Extensions to Session Traversal Utilities for NAT (STUN)",
draft-ietf-tram-turnbis-02 (work in progress), February
2015.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, April
2010.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
[RFC6156] Camarillo, G., Novo, O., and S. Perreault, "Traversal
Using Relays around NAT (TURN) Extension for IPv6", RFC
6156, April 2011.
10.2. Informative References
[I-D.schwartz-rtcweb-return]
Schwartz, B. and J. Uberti, "Recursively Encapsulated TURN
(RETURN) for Connectivity and Privacy in WebRTC", draft-
schwartz-return-04 (work in progress), November 2014.
[RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
L. Jones, "SOCKS Protocol Version 5", RFC 5766, March
1996.
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[RFC6555] Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
Dual-Stack Hosts", RFC 6555, April 2012.
Authors' Addresses
Benjamin M. Schwartz
Google, Inc.
111 8th Ave
New York, NY 10011
USA
Email: bemasc@webrtc.org
Justin Uberti
Google, Inc.
747 6th Street South
Kirkland, WA 98033
USA
Email: justin@uberti.name
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