Internet Engineering Task Force (IETF) J. Dickinson
Request for Comments: 7766 S. Dickinson
Obsoletes: 5966 Sinodun
Updates: 1035, 1123 R. Bellis
Category: Standards Track ISC
ISSN: 2070-1721 A. MankinD. Wessels
March 2016 DNS Transport over TCP - Implementation Requirements
This document specifies the requirement for support of TCP as a
transport protocol for DNS implementations and provides guidelines
towards DNS-over-TCP performance on par with that of DNS-over-UDP.
This document obsoletes RFC 5966 and therefore updates RFC 1035 and
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 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Most DNS [RFC1034] transactions take place over UDP [RFC768]. TCP
[RFC793] is always used for full zone transfers (using AXFR) and is
often used for messages whose sizes exceed the DNS protocol's
original 512-byte limit. The growing deployment of DNS Security
(DNSSEC) and IPv6 has increased response sizes and therefore the use
of TCP. The need for increased TCP use has also been driven by the
protection it provides against address spoofing and therefore
exploitation of DNS in reflection/amplification attacks. It is now
widely used in Response Rate Limiting [RRL1] [RRL2]. Additionally,
recent work on DNS privacy solutions such as [DNS-over-TLS] is
another motivation to revisit DNS-over-TCP requirements.
Section 220.127.116.11 of [RFC1123] states:
DNS resolvers and recursive servers MUST support UDP, and SHOULD
support TCP, for sending (non-zone-transfer) queries.
However, some implementors have taken the text quoted above to mean
that TCP support is an optional feature of the DNS protocol.
The majority of DNS server operators already support TCP, and the
default configuration for most software implementations is to support
TCP. The primary audience for this document is those implementors
whose limited support for TCP restricts interoperability and hinders
deployment of new DNS features.
This document therefore updates the core DNS protocol specifications
such that support for TCP is henceforth a REQUIRED part of a full DNS
There are several advantages and disadvantages to the increased use
of TCP (see Appendix A) as well as implementation details that need
to be considered. This document addresses these issues and presents
TCP as a valid transport alternative for DNS. It extends the content
of [RFC5966], with additional considerations and lessons learned from
research, developments, and implementation of TCP in DNS and in other
Whilst this document makes no specific requirements for operators of
DNS servers to meet, it does offer some suggestions to operators to
help ensure that support for TCP on their servers and network is
optimal. It should be noted that failure to support TCP (or the
blocking of DNS over TCP at the network layer) will probably result
in resolution failure and/or application-level timeouts.
2. Requirements Terminology
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].
o Persistent connection: a TCP connection that is not closed either
by the server after sending the first response nor by the client
after receiving the first response.
o Connection Reuse: the sending of multiple queries and responses
over a single TCP connection.
o Idle DNS-over-TCP session: Clients and servers view application-
level idleness differently. A DNS client considers an established
DNS-over-TCP session to be idle when it has no pending queries to
send and there are no outstanding responses. A DNS server
considers an established DNS-over-TCP session to be idle when it
has sent responses to all the queries it has received on that
o Pipelining: the sending of multiple queries and responses over a
single TCP connection but not waiting for any outstanding replies
before sending another query.
o Out-of-Order Processing: The processing of queries concurrently
and the returning of individual responses as soon as they are
available, possibly out of order. This will most likely occur in
recursive servers; however, it is possible in authoritative
servers that, for example, have different backend data stores.
In the absence of EDNS0 (Extension Mechanisms for DNS 0 [RFC6891];
see below), the normal behaviour of any DNS server that needs to send
a UDP response that would exceed the 512-byte limit is for the server
to truncate the response so that it fits within that limit and then
set the TC flag in the response header. When the client receives
such a response, it takes the TC flag as an indication that it should
retry over TCP instead.
RFC 1123 also says:
... it is also clear that some new DNS record types defined in the
future will contain information exceeding the 512 byte limit that
applies to UDP, and hence will require TCP. Thus, resolvers and
name servers should implement TCP services as a backup to UDP
today, with the knowledge that they will require the TCP service
in the future.
Existing deployments of DNSSEC [RFC4033] have shown that truncation
at the 512-byte boundary is now commonplace. For example, a Non-
Existent Domain (NXDOMAIN) (RCODE == 3) response from a DNSSEC-signed
zone using NextSECure 3 (NSEC3) [RFC5155] is almost invariably larger
than 512 bytes.
Since the original core specifications for DNS were written, the
extension mechanisms for DNS have been introduced. These extensions
can be used to indicate that the client is prepared to receive UDP
responses larger than 512 bytes. An EDNS0-compatible server
receiving a request from an EDNS0-compatible client may send UDP
packets up to that client's announced buffer size without truncation.
However, transport of UDP packets that exceed the size of the path
MTU causes IP packet fragmentation, which has been found to be
unreliable in many circumstances. Many firewalls routinely block
fragmented IP packets, and some do not implement the algorithms
necessary to reassemble fragmented packets. Worse still, some
network devices deliberately refuse to handle DNS packets containing
EDNS0 options. Other issues relating to UDP transport and packet
size are discussed in [RFC5625].
The MTU most commonly found in the core of the Internet is around
1500 bytes, and even that limit is routinely exceeded by DNSSEC-
The future that was anticipated in RFC 1123 has arrived, and the only
standardised UDP-based mechanism that may have resolved the packet
size issue has been found inadequate.
5. Transport Protocol Selection
Section 18.104.22.168 of [RFC1123] is updated: All general-purpose DNS
implementations MUST support both UDP and TCP transport.
o Authoritative server implementations MUST support TCP so that they
do not limit the size of responses to what fits in a single UDP
o Recursive server (or forwarder) implementations MUST support TCP
so that they do not prevent large responses from a TCP-capable
server from reaching its TCP-capable clients.
o Stub resolver implementations (e.g., an operating system's DNS
resolution library) MUST support TCP since to do otherwise would
limit the interoperability between their own clients and upstream
Regarding the choice of when to use UDP or TCP, Section 22.214.171.124 of
RFC 1123 also says:
... a DNS resolver or server that is sending a non-zone-transfer
query MUST send a UDP query first.
This requirement is hereby relaxed. Stub resolvers and recursive
resolvers MAY elect to send either TCP or UDP queries depending on
local operational reasons. TCP MAY be used before sending any UDP
queries. If the resolver already has an open TCP connection to the
server, it SHOULD reuse this connection. In essence, TCP ought to be
considered a valid alternative transport to UDP, not purely a retry
In addition, it is noted that all recursive and authoritative servers
MUST send responses using the same transport as the query arrived on.
In the case of TCP, this MUST also be the same connection.
6. Connection Handling
6.1. Current Practices
Section 4.2.2 of [RFC1035] says:
- The server should assume that the client will initiate connection
closing, and should delay closing its end of the connection until
all outstanding client requests have been satisfied.
- If the server needs to close a dormant connection to reclaim
resources, it should wait until the connection has been idle for a
period on the order of two minutes. In particular, the server
should allow the SOA and AXFR request sequence (which begins a
refresh operation) to be made on a single connection. Since the
server would be unable to answer queries anyway, a unilateral
close or reset may be used instead of graceful close.
Other more modern protocols (e.g., HTTP/1.1 [RFC7230], HTTP/2
[RFC7540]) have support by default for persistent TCP connections for
all requests. Connections are then normally closed via a 'connection
close' signal from one party.
The description in [RFC1035] is clear that servers should view
connections as persistent (particularly after receiving an SOA), but
unfortunately does not provide enough detail for an unambiguous
interpretation of client behaviour for queries other than a SOA.
Additionally, DNS does not yet have a signalling mechanism for
connection timeout or close, although some have been proposed.
There is no clear guidance today in any RFC as to when a DNS client
should close a TCP connection, and there are no specific
recommendations with regard to DNS client idle timeouts. However, at
the time of writing, it is common practice for clients to close the
TCP connection after sending a single request (apart from the SOA/
Many DNS server implementations use a long fixed idle timeout and
default to a small number of TCP connections. They also offer little
in the way of TCP connection management options. The disadvantages
of this include:
o Operational experience has shown that long server timeouts can
easily cause resource exhaustion and poor response under heavy
o Intentionally opening many connections and leaving them idle can
trivially create a TCP denial of service (DoS) attack as many DNS
servers are poorly equipped to defend against this by modifying
their idle timeouts or other connection management policies.
o A modest number of clients that all concurrently attempt to use
persistent connections with non-zero idle timeouts to such a
server could unintentionally cause the same DoS problem.
Note that this DoS is only on the TCP service. However, in these
cases, it affects not only clients that wish to use TCP for their
queries for operational reasons, but all clients that choose to fall
back to TCP from UDP after receiving a TC=1 flag.
The following sections include recommendations that are intended to
result in more consistent and scalable implementations of DNS-over-
6.2.1. Connection Reuse
One perceived disadvantage to DNS over TCP is the added connection
setup latency, generally equal to one RTT. To amortise connection
setup costs, both clients and servers SHOULD support connection reuse
by sending multiple queries and responses over a single persistent
When sending multiple queries over a TCP connection, clients MUST NOT
reuse the DNS Message ID of an in-flight query on that connection in
order to avoid Message ID collisions. This is especially important
if the server could be performing out-of-order processing (see
126.96.36.199. Query Pipelining
Due to the historical use of TCP primarily for zone transfer and
truncated responses, no existing RFC discusses the idea of pipelining
DNS queries over a TCP connection.
In order to achieve performance on par with UDP, DNS clients SHOULD
pipeline their queries. When a DNS client sends multiple queries to
a server, it SHOULD NOT wait for an outstanding reply before sending
the next query. Clients SHOULD treat TCP and UDP equivalently when
considering the time at which to send a particular query.
It is likely that DNS servers need to process pipelined queries
concurrently and also send out-of-order responses over TCP in order
to provide the level of performance possible with UDP transport. If
TCP performance is of importance, clients might find it useful to use
server processing times as input to server and transport selection
DNS servers (especially recursive) MUST expect to receive pipelined
queries. The server SHOULD process TCP queries concurrently, just as
it would for UDP. The server SHOULD answer all pipelined queries,
even if they are received in quick succession. The handling of
responses to pipelined queries is covered in Section 7.
6.2.2. Concurrent Connections
To mitigate the risk of unintentional server overload, DNS clients
MUST take care to minimize the number of concurrent TCP connections
made to any individual server. It is RECOMMENDED that for any given
client/server interaction there SHOULD be no more than one connection
for regular queries, one for zone transfers, and one for each
protocol that is being used on top of TCP (for example, if the
resolver was using TLS). However, it is noted that certain primary/
secondary configurations with many busy zones might need to use more
than one TCP connection for zone transfers for operational reasons
(for example, to support concurrent transfers of multiple zones).
Similarly, servers MAY impose limits on the number of concurrent TCP
connections being handled for any particular client IP address or
subnet. These limits SHOULD be much looser than the client
guidelines above, because the server does not know, for example, if a
client IP address belongs to a single client, is multiple resolvers
on a single machine, or is multiple clients behind a device
performing Network Address Translation (NAT).
6.2.3. Idle Timeouts
To mitigate the risk of unintentional server overload, DNS clients
MUST take care to minimise the idle time of established DNS-over-TCP
sessions made to any individual server. DNS clients SHOULD close the
TCP connection of an idle session, unless an idle timeout has been
established using some other signalling mechanism, for example,
To mitigate the risk of unintentional server overload, it is
RECOMMENDED that the default server application-level idle period be
on the order of seconds, but no particular value is specified. In
practice, the idle period can vary dynamically, and servers MAY allow
idle connections to remain open for longer periods as resources
permit. A timeout of at least a few seconds is advisable for normal
operations to support those clients that expect the SOA and AXFR
request sequence to be made on a single connection as originally
specified in [RFC1035]. Servers MAY use zero timeouts when they are
experiencing heavy load or are under attack.
DNS messages delivered over TCP might arrive in multiple segments. A
DNS server that resets its idle timeout after receiving a single
segment might be vulnerable to a "slow-read attack". For this
reason, servers SHOULD reset the idle timeout on the receipt of a
full DNS message, rather than on receipt of any part of a DNS
Under normal operation DNS clients typically initiate connection
closing on idle connections; however, DNS servers can close the
connection if the idle timeout set by local policy is exceeded.
Also, connections can be closed by either end under unusual
conditions such as defending against an attack or system failure/
DNS clients SHOULD retry unanswered queries if the connection closes
before receiving all outstanding responses. No specific retry
algorithm is specified in this document.
If a DNS server finds that a DNS client has closed a TCP session (or
if the session has been otherwise interrupted) before all pending
responses have been sent, then the server MUST NOT attempt to send
those responses. Of course, the DNS server MAY cache those
7. Response Reordering
RFC 1035 is ambiguous on the question of whether TCP responses may be
reordered -- the only relevant text is in Section 4.2.1, which
relates to UDP:
Queries or their responses may be reordered by the network, or by
processing in name servers, so resolvers should not depend on them
being returned in order.
For the avoidance of future doubt, this requirement is clarified.
Authoritative servers and recursive resolvers are RECOMMENDED to
support the preparing of responses in parallel and sending them out
of order, regardless of the transport protocol in use. Stub and
recursive resolvers MUST be able to process responses that arrive in
a different order than that in which the requests were sent,
regardless of the transport protocol in use.
In order to achieve performance on par with UDP, recursive resolvers
SHOULD process TCP queries in parallel and return individual
responses as soon as they are available, possibly out of order.
Since pipelined responses can arrive out of order, clients MUST match
responses to outstanding queries on the same TCP connection using the
Message ID. If the response contains a question section, the client
MUST match the QNAME, QCLASS, and QTYPE fields. Failure by clients
to properly match responses to outstanding queries can have serious
consequences for interoperability.
8. TCP Message Length Field
DNS clients and servers SHOULD pass the two-octet length field, and
the message described by that length field, to the TCP layer at the
same time (e.g., in a single "write" system call) to make it more
likely that all the data will be transmitted in a single TCP segment.
This is for reasons of both efficiency and to avoid problems due to
some DNS server implementations behaving undesirably when reading
data from the TCP layer (due to a lack of clarity in previous
documents). For example, some DNS server implementations might abort
a TCP session if the first "read" from the TCP layer does not contain
both the length field and the entire message.
To clarify, DNS servers MUST NOT close a connection simply because
the first "read" from the TCP layer does not contain the entire DNS
message, and servers SHOULD apply the connection timeouts as
specified in Section 6.2.3.
9. TCP Fast Open
This section is non-normative.
TCP Fast Open (TFO) [RFC7413] allows data to be carried in the SYN
packet, reducing the cost of reopening TCP connections. It also
saves up to one RTT compared to standard TCP.
TFO mitigates the security vulnerabilities inherent in sending data
in the SYN, especially on a system like DNS where amplification
attacks are possible, by use of a server-supplied cookie. TFO
clients request a server cookie in the initial SYN packet at the
start of a new connection. The server returns a cookie in its SYN-
ACK. The client caches the cookie and reuses it when opening
subsequent connections to the same server.
The cookie is stored by the client's TCP stack (kernel) and persists
if either the client or server processes are restarted. TFO also
falls back to a regular TCP handshake gracefully.
DNS services taking advantage of IP anycast [RFC4786] might need to
take additional steps when enabling TFO. From [RFC7413]:
Servers behind load balancers that accept connection requests to
the same server IP address should use the same key such that they
generate identical Fast Open cookies for a particular client IP
address. Otherwise, a client may get different cookies across
connections; its Fast Open attempts would fall back to the regular
When DNS-over-TCP is a transport for DNS private exchange, as in
[DNS-over-TLS], the implementor needs to be aware of TFO and to
ensure that data requiring protection (e.g. data for a DNS query) is
not accidentally transported in the clear. See [DNS-over-TLS] for
10. Security Considerations
Some DNS server operators have expressed concern that wider promotion
and use of DNS over TCP will expose them to a higher risk of DoS
attacks on TCP (both accidental and deliberate).
Although there is a higher risk of some specific attacks against TCP-
enabled servers, techniques for the mitigation of DoS attacks at the
network level have improved substantially since DNS was first
Readers are advised to familiarise themselves with [CPNI-TCP], a
security assessment of TCP that details known TCP attacks and
countermeasures and that references most of the relevant RFCs on this
To mitigate the risk of DoS attacks, DNS servers are advised to
engage in TCP connection management. This could include maintaining
state on existing connections, reusing existing connections, and
controlling request queues to enable fair use. It is likely to be
advantageous to provide configurable connection management options,
o total number of TCP connections
o maximum TCP connections per source IP address or subnet
o TCP connection idle timeout
o maximum DNS transactions per TCP connection
o maximum TCP connection duration
No specific values are recommended for these parameters.
Operators are advised to familiarise themselves with the
configuration and tuning parameters available in the TCP stack of the
operating system. However, detailed advice on this is outside the
scope of this document.
Appendix A. Summary of Advantages and Disadvantages to Using TCP for
The TCP handshake generally prevents address spoofing and, therefore,
the reflection/amplification attacks that plague UDP.
IP fragmentation is less of a problem for TCP than it is for UDP.
TCP stacks generally implement Path MTU Discovery so they can avoid
IP fragmentation of TCP segments. UDP, on the other hand, does not
provide reassembly; this means datagrams that exceed the path MTU
size must experience fragmentation [RFC5405]. Middleboxes are known
to block IP fragments, leading to timeouts and forcing client
implementations to "hunt" for EDNS0 reply size values supported by
the network path. Additionally, fragmentation may lead to cache
TCP setup costs an additional RTT compared to UDP queries. Setup
costs can be amortised by reusing connections, pipelining queries,
and enabling TCP Fast Open.
TCP imposes additional state-keeping requirements on clients and
servers. The use of TCP Fast Open reduces the cost of closing and
reopening TCP connections.
Long-lived TCP connections to anycast servers might be disrupted due
to routing changes. Clients utilizing TCP for DNS need to always be
prepared to re-establish connections or otherwise retry outstanding
queries. It might also be possible for Multipath TCP [RFC6824] to
allow a server to hand a connection over from the anycast address to
a unicast address.
There are many "middleboxes" in use today that interfere with TCP
over port 53 [RFC5625]. This document does not propose any
solutions, other than to make it absolutely clear that TCP is a valid
transport for DNS and support for it is a requirement for all
A more in-depth discussion of connection-oriented DNS can be found
Appendix B. Changes to RFC 5966
This document obsoletes [RFC5966] and differs from it in several
respects. An overview of the most substantial changes/updates that
implementors should take note of is given below.
1. A Terminology section (Section 3) is added defining several new
2. Paragraph 3 of Section 5 puts TCP on a more equal footing with
UDP than RFC 5966 does. For example, it states:
1. TCP MAY be used before sending any UDP queries.
2. TCP ought to be considered a valid alternative transport to
UDP, not purely a fallback option.
3. Section 6.2.1 adds a new recommendation that TCP connection
reuse SHOULD be supported.
4. Section 188.8.131.52 adds a new recommendation that DNS clients
SHOULD pipeline their queries and DNS servers SHOULD process
pipelined queries concurrently.
5. Section 6.2.2 adds new recommendations on the number and usage
of TCP connections for client/server interactions.
6. Section 6.2.3 adds a new recommendation that DNS clients SHOULD
close idle sessions unless using a signalling mechanism.
7. Section 7 clarifies that servers are RECOMMENDED to prepare TCP
responses in parallel and send answers out of order. It also
clarifies how TCP queries and responses should be matched by
8. Section 8 adds a new recommendation about how DNS clients and
servers should handle the 2-byte message length field for TCP
9. Section 9 adds a non-normative discussion of the use of TCP Fast
10. Section 10 adds new advice regarding DoS mitigation techniques.
The authors would like to thank Francis Dupont and Paul Vixie for
their detailed reviews, as well as Andrew Sullivan, Tony Finch,
Stephane Bortzmeyer, Joe Abley, Tatuya Jinmei, and the many others
who contributed to the mailing list discussion. Also, the authors
thank Liang Zhu, Zi Hu, and John Heidemann for extensive DNS-over-TCP
discussions and code, and Lucie Guiraud and Danny McPherson for
reviewing early draft versions of this document. We would also like
to thank all those who contributed to RFC 5966.
Sinodun Internet Technologies
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Sinodun Internet Technologies
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