draft-ietf-dprive-dnsodtls-02.txt   draft-ietf-dprive-dnsodtls-03.txt 
DPRIVE T. Reddy DPRIVE T. Reddy
Internet-Draft D. Wing Internet-Draft D. Wing
Intended status: Standards Track P. Patil Intended status: Standards Track P. Patil
Expires: April 20, 2016 Cisco Expires: May 27, 2016 Cisco
October 18, 2015 November 24, 2015
DNS over DTLS (DNSoD) DNS over DTLS (DNSoD)
draft-ietf-dprive-dnsodtls-02 draft-ietf-dprive-dnsodtls-03
Abstract Abstract
DNS queries and responses are visible to network elements on the path DNS queries and responses are visible to network elements on the path
between the DNS client and its server. These queries and responses between the DNS client and its server. These queries and responses
can contain privacy-sensitive information which is valuable to can contain privacy-sensitive information which is valuable to
protect. An active attacker can send bogus responses causing protect. An active attacker can send bogus responses causing
misdirection of the subsequent connection. misdirection of the subsequent connection.
To counter passive listening and active attacks, this document To counter passive listening and active attacks, this document
skipping to change at page 1, line 42 skipping to change at page 1, line 42
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 20, 2016. This Internet-Draft will expire on May 27, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Relationship to TCP Queries and to DNSSEC . . . . . . . . . . 3 1.1. Relationship to TCP Queries and to DNSSEC . . . . . . . . 3
3. Common problems with DNS Privacy . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Firewall Blocking Ports or DNS Privacy Protocol . . . . . 3 3. DTLS session initiation, Polling and Discovery . . . . . . . 3
3.2. Authenticating the DNS Privacy Server . . . . . . . . . . 4 4. Performance Considerations . . . . . . . . . . . . . . . . . 4
3.3. Downgrade attacks . . . . . . . . . . . . . . . . . . . . 5 5. Established sessions . . . . . . . . . . . . . . . . . . . . 5
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 6. Anycast . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Incremental Deployment . . . . . . . . . . . . . . . . . . . 6 7. Downgrade attacks . . . . . . . . . . . . . . . . . . . . . . 7
6. DTLS session initiation, Polling and Discovery . . . . . . . 6 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Performance Considerations . . . . . . . . . . . . . . . . . 7 9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Established sessions . . . . . . . . . . . . . . . . . . . . 8 9.1. Authenticating a DNS Privacy Server . . . . . . . . . . . 8
9. Fragmentation and Reassembly . . . . . . . . . . . . . . . . 9 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Generating fragmented packets . . . . . . . . . . . . . . 10 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.2. Receiving fragmented packets . . . . . . . . . . . . . . 11 11.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.3. The DNS-fragment Extension . . . . . . . . . . . . . . . 12 11.2. Informative References . . . . . . . . . . . . . . . . . 12
10. Anycast . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
12. Security Considerations . . . . . . . . . . . . . . . . . . . 14
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
14.1. Normative References . . . . . . . . . . . . . . . . . . 14
14.2. Informative References . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction 1. Introduction
The Domain Name System is specified in [RFC1034] and [RFC1035]. DNS The Domain Name System is specified in [RFC1034] and [RFC1035]. DNS
queries and responses are normally exchanged unencrypted and are thus queries and responses are normally exchanged unencrypted and are thus
vulnerable to eavesdropping. Such eavesdropping can result in an vulnerable to eavesdropping. Such eavesdropping can result in an
undesired entity learning domains that a host wishes to access, thus undesired entity learning domains that a host wishes to access, thus
resulting in privacy leakage. DNS privacy problem is further resulting in privacy leakage. DNS privacy problem is further
discussed in [I-D.bortzmeyer-dnsop-dns-privacy]. discussed in [I-D.bortzmeyer-dnsop-dns-privacy].
skipping to change at page 3, line 25 skipping to change at page 3, line 19
application until the lost packet is re-transmitted. DNSoD, application until the lost packet is re-transmitted. DNSoD,
because it uses UDP, does not suffer from network head-of-line because it uses UDP, does not suffer from network head-of-line
blocking. blocking.
o DTLS session resumption consumes 1 round trip whereas TLS session o DTLS session resumption consumes 1 round trip whereas TLS session
resumption can start only after TCP handshake is complete. resumption can start only after TCP handshake is complete.
Although TCP Fast Open [RFC7413] can reduce that handshake, TCP Although TCP Fast Open [RFC7413] can reduce that handshake, TCP
Fast Open is not yet available in commercially-popular operating Fast Open is not yet available in commercially-popular operating
systems. systems.
2. Relationship to TCP Queries and to DNSSEC 1.1. Relationship to TCP Queries and to DNSSEC
DNS queries can be sent over UDP or TCP. The scope of this document, DNS queries can be sent over UDP or TCP. The scope of this document,
however, is only UDP. DNS over TCP could be protected with TLS, as however, is only UDP. DNS over TCP could be protected with TLS, as
described in [I-D.ietf-dprive-dns-over-tls]. Alternatively, a shim described in [I-D.ietf-dprive-dns-over-tls].
protocol could be defined between DTLS and DNS, allowing large
responses to be sent over DTLS itself, see Section 7.
DNS Security Extensions (DNSSEC [RFC4033]) provides object integrity DNS Security Extensions (DNSSEC [RFC4033]) provides object integrity
of DNS resource records, allowing end-users (or their resolver) to of DNS resource records, allowing end-users (or their resolver) to
verify legitimacy of responses. However, DNSSEC does not protect verify legitimacy of responses. However, DNSSEC does not protect
privacy of DNS requests or responses. DNSoD works in conjunction privacy of DNS requests or responses. DNSoD works in conjunction
with DNSSEC, but DNSoD does not replace the need or value of DNSSEC. with DNSSEC, but DNSoD does not replace the need or value of DNSSEC.
3. Common problems with DNS Privacy 2. Terminology
This section describes problems common to any DNS privacy solution.
To achieve DNS privacy an encrypted and integrity-protected channel
is needed between the client and server. This channel can be
blocked, and the client needs to react to such blockages.
3.1. Firewall Blocking Ports or DNS Privacy Protocol
When sending DNS over an encrypted channel, there are two choices:
send the encrypted traffic over the DNS ports (UDP 53, TCP 53) or
send the encrypted traffic over a different port. The encrypted
traffic is not normal DNS traffic, but rather is a cryptographic
handshake followed by encrypted payloads. There can be firewalls,
other security devices, or intercepting DNS proxies which block the
non-DNS traffic or otherwise react negatively (e.g., quarantining the
host for suspicious behavior). Alternatively, if a different port is
used for the encrypted traffic, a firewall or other security device
might block that port or otherwise react negatively.
There is no panacea, and only experiments on the Internet will
uncover which technique or combination of techniques will work best.
This document describes using DNSoD on a well-known port.
3.2. Authenticating the DNS Privacy Server
DNS privacy requires encrypting the query (and response) from passive
attacks. Such encryption typically provides integrity protection as
a side-effect, which means on-path attackers cannot simply inject
bogus DNS responses. However, to provide stronger protection from
active attackers pretending to be the server, the server itself needs
to be authenticated.
To authenticate the server providing DNS privacy, the DNS client
needs to be configured with the names or IP addresses of those DNS
privacy servers. The server certificate MUST contain DNS-ID
(subjectAltName) as described in Section 4.1 of [RFC6125]. DNS names
and IP addresses can be contained in the subjectAltName entries. The
client MUST use the rules and guidelines given in section 6 of
[RFC6125] to validate the DNS server identity.
We imagine this could be implemented by adding the certificate name
to the /etc/resolv.conf file, such as below:
nameserver 8.8.8.8
certificate google-public-dns.google.com
nameserver 208.67.220.220
certificate resolver.opendns.com
For DNS privacy servers that don't have a certificate trust chain
(e.g., because they are on a home network or a corporate network),
the configured list of DNS privacy servers can contain the Subject
Public Key Info (SPKI) fingerprint of the DNS privacy server (i.e., a
simple whitelist of name and SPKI fingerprint). The public key is
used for the same reasons HTTP pinning [RFC7469] uses the public key.
Raw public key-based authentication mechanism defined in [RFC7250]
can be also used to authenticate the DNS server.
We imagine this could be implemented by adding the SPKI fingerprint
to the /etc/resolv.conf file, such as below (line split for Internet
Draft formatting):
nameserver 192.168.1.1
sha256 : "d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="
The only algorithm considered at this time is "sha256", i.e., the
hash algorithm SHA256 [RFC6234]; additional algorithms may be allowed
for use in this context in the future. The quoted-string is a
sequence of base 64 digits: the base64-encoded SPKI Fingerprint
[RFC4648].
3.3. Downgrade attacks
Using DNS privacy with an authenticated server is most preferred, DNS
privacy with an unauthenticated server is next preferred, and plain
DNS is least preferred. This section gives a non-normative
discussion on common behaviors and choices.
An implementation MAY attempt to obtain DNS privacy by contacting DNS
servers on the local network (provided by DHCP) and on the Internet,
and make those attempts in parallel to reduce user impact. If DNS
privacy cannot be successfully negotiated for whatever reason, the
client can do three things:
1. refuse to send DNS queries on this network, which means the
client cannot make effective use of this network, as modern
networks require DNS; or,
2. use opportunistic security, as described in [RFC7435]. or,
3. send plain DNS queries on this network, which means no DNS
privacy is provided.
Heuristics can improve this situation, but only to a degree (e.g.,
previous success of DNS privacy on this network may be reason to
alert the user about failure to establish DNS privacy on this network
now). Still, the client (in cooperation with the end user) has to
decide to use the network without the protection of DNS privacy.
4. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in "OPTIONAL" in this document are to be interpreted as described in
[RFC2119]. [RFC2119].
5. Incremental Deployment 3. DTLS session initiation, Polling and Discovery
DNSoD can be deployed incrementally by the Internet Service Provider
or as an Internet service.
If the ISP's DNS resolver supports DNSoD, then DNS queries are
protected from passive listening and from many active attacks along
that path.
DNSoD can be offered as an Internet service, and a stub resolver or
DNS resolver can be configured to point to that DNSoD server (rather
than to the ISP-provided DNS server).
6. DTLS session initiation, Polling and Discovery
Many modern operating systems already detect if a web proxy is Many modern operating systems already detect if a web proxy is
interfering with Internet communications, using proprietary interfering with Internet communications, using proprietary
mechanisms that are out of scope of this document. After that mechanisms that are out of scope of this document. After that
mechanism has run (and detected Internet connectivity is working), mechanism has run (and detected Internet connectivity is working),
the DNSoD procedure described in this document should commence. This the DNSoD procedure described in this document should commence. This
timing avoids delays in joining the network (and displaying an icon timing avoids delays in joining the network (and displaying an icon
indicating successful Internet connection), at the risk that those indicating successful Internet connection), at the risk that those
initial DNS queries will be sent without protection afforded by initial DNS queries will be sent without protection afforded by
DNSoD. DNSoD.
DNSoD MUST run over standard UDP port 853 as defined in Section 11. DNSoD MUST run over standard UDP port 853 as defined in Section 8. A
A DNS server that supports DNSoD MUST listen for and accept DTLS DNS server that supports DNSoD MUST listen for and accept DTLS
packets on a designated port 853. packets on a designated port 853.
The host should determine if the DNS server supports DNSoD by sending The host should determine if the DNS server supports DNSoD by sending
a DTLS ClientHello message. A DNS server that does not support DNSoD a DTLS ClientHello message. A DNS server that does not support DNSoD
will not respond to ClientHello messages sent by the client. The will not respond to ClientHello messages sent by the client. The
client MUST use timer values defined in Section 4.2.4.1 of [RFC6347] client MUST use timer values defined in Section 4.2.4.1 of [RFC6347]
for retransmission of ClientHello message and if no response is for retransmission of ClientHello message and if no response is
received from the DNS server. After 15 seconds, it MUST cease received from the DNS server. After 15 seconds, it MUST cease
attempts to re-transmit its ClientHello. If the DNS client receives attempts to re-transmit its ClientHello. If the DNS client receives
a hard ICMP error [RFC1122], it MUST immediately cease attempts to a hard ICMP error [RFC1122], it MUST immediately cease attempts to
re-transmit its ClientHello. Thereafter, the client MAY repeat that re-transmit its ClientHello. Thereafter, the client MAY repeat that
procedure in the event the DNS server has been upgraded to support procedure in the event the DNS server has been upgraded to support
DNSoD, but such probing SHOULD NOT be done more frequently than every DNSoD, but such probing SHOULD NOT be done more frequently than every
24 hours and MUST NOT be done more frequently than every 15 minutes. 24 hours and MUST NOT be done more frequently than every 15 minutes.
This mechanism requires no additional signaling between the client This mechanism requires no additional signaling between the client
and server. and server.
7. Performance Considerations 4. Performance Considerations
To reduce number of octets of the DTLS handshake, especially the size To reduce number of octets of the DTLS handshake, especially the size
of the certificate in the ServerHello (which can be several of the certificate in the ServerHello (which can be several
kilobytes), DNS client and server can use raw public keys [RFC7250] kilobytes), DNS client and server can use raw public keys [RFC7250]
or Cached Information Extension [I-D.ietf-tls-cached-info]. Cached or Cached Information Extension [I-D.ietf-tls-cached-info]. Cached
Information Extension avoids transmitting the server's certificate Information Extension avoids transmitting the server's certificate
and certificate chain if the client has cached that information from and certificate chain if the client has cached that information from
a previous TLS handshake. a previous TLS handshake.
Multiple DNS queries can be sent over a single DTLS session and the Multiple DNS queries can be sent over a single DTLS session and the
DNSoD client need not wait for an outstanding reply before sending DNSoD client need not wait for an outstanding reply before sending
the next query. The existing Query ID allows multiple requests and the next query. The existing Query ID allows multiple requests and
responses to be interleaved in whatever order they can be fulfilled responses to be interleaved in whatever order they can be fulfilled
by the DNS server. This means DNSoD reduces the consumption of UDP by the DNS server. This means DNSoD reduces the consumption of UDP
port numbers, and because DTLS protects the communication between the port numbers, and because DTLS protects the communication between a
DNS client and its server, the resolver SHOULD NOT use random DNS client and its server, the resolver SHOULD NOT use random
ephemeral source ports (Section 9.2 of [RFC5452]) because such source ephemeral source ports (Section 9.2 of [RFC5452]) because such source
port use would incur additional, unnecessary DTLS load on the DNSoD port use would incur additional, unnecessary DTLS load on the DNSoD
server. When sending multiple queries over a single DTLS session, server. When sending multiple queries over a single DTLS session,
clients MUST take care to avoid Message ID collisions. In other clients MUST take care to avoid Message ID collisions. In other
words, they MUST not re-use the DNS Message ID of an in-flight query. words, they MUST not re-use the DNS Message ID of an in-flight query.
It is highly advantageous to avoid server-side DTLS state and reduce It is highly advantageous to avoid server-side DTLS state and reduce
the number of new DTLS sessions on the server which can be done with the number of new DTLS sessions on the server which can be done with
[RFC5077]. This also eliminates a round-trip for subsequent DNSoD [RFC5077]. This also eliminates a round-trip for subsequent DNSoD
queries, because with [RFC5077] the DTLS session does not need to be queries, because with [RFC5077] the DTLS session does not need to be
re-established. re-established.
Compared to normal DNS, DTLS adds at least 13 octets of header, plus Compared to normal DNS, DTLS adds at least 13 octets of header, plus
cipher and authentication overhead to every query and every response. cipher and authentication overhead to every query and every response.
This reduces the size of the DNS payload that can be carried. This reduces the size of the DNS payload that can be carried. DNS
Certain DNS responses are large (e.g., many AAAA records, TXT, SRV) client and server MUST support the EDNS0 option defined in [RFC6891]
and don't fit into a single UDP packet, causing a partial response so that the DNS client can indicate to the DNS server the maximum DNS
with the truncation (TC) bit set. The client is then expected to response size it can handle without IP fragmentation. If the DNS
repeat the query over TCP, which causes additional name resolution sever's response exceeds the EDNS0 value, the DNS server sets the TC
delay. We have considered two ideas, one that reduces the need to (truncated) bit. On receiving a response with the TC bit set, the
switch to TCP and another that eliminates the need to switch to TCP: client establishes a DNS-over-TLS connection to the same server, and
sends a new DNS request for the same resource record
o To avoid IP fragmentation, DTLS handshake messages incorporate
their own fragment offset and fragment length, but this is only
for the handshake. Payloads that cause the DTLS packet to exceed
the path maximum MTU need their own fragmentation support
Section 9.
o DNS client and server MUST support the EDNS0 option defined in
[RFC6891] so that the DNS client can indicate to the DNS server
the maximum DNS response size it can handle without IP
fragmentation.
DNSoD puts an additional computational load on servers. The largest DNSoD puts an additional computational load on servers. The largest
gain for privacy is to protect the communication between the DNS gain for privacy is to protect the communication between the DNS
client (the end user's machine) and its caching resolver. client (the end user's machine) and its caching resolver.
Implementing DNSoD on root servers is outside the scope of this Implementing DNSoD on root servers is outside the scope of this
document. document.
8. Established sessions 5. Established sessions
In DTLS, all data is protected using the same record encoding and In DTLS, all data is protected using the same record encoding and
mechanisms. When the mechanism described in this document is in mechanisms. When the mechanism described in this document is in
effect, DNS messages are encrypted using the standard DTLS record effect, DNS messages are encrypted using the standard DTLS record
encoding. When a user of DTLS wishes to send an DNS message, it encoding. When a user of DTLS wishes to send an DNS message, it
delivers it to the DTLS implementation as an ordinary application delivers it to the DTLS implementation as an ordinary application
data write (e.g., SSL_write()). A single DTLS session can be used to data write (e.g., SSL_write()). To reduce client and server
receive multiple DNS requests and generate DNS multiple responses. workload, clients SHOULD re-use the DTLS session. A single DTLS
session can be used to send multiple DNS requests and receive
multiple DNS responses.
DNSoD client and server can use DTLS heartbeat [RFC6520] to verify DNSoD client and server can use DTLS heartbeat [RFC6520] to verify
that the peer still has DTLS state. DTLS session is terminated by that the peer still has DTLS state. DTLS session is terminated by
the receipt of an authenticated message that closes the connection the receipt of an authenticated message that closes the connection
(e.g., a DTLS fatal alert). (e.g., a DTLS fatal alert).
Client Server Client Server
------ ------ ------ ------
ClientHello --------> ClientHello -------->
skipping to change at page 9, line 38 skipping to change at page 6, line 38
NewSessionTicket NewSessionTicket
[ChangeCipherSpec] [ChangeCipherSpec]
<-------- Finished <-------- Finished
DNS Request ---------> DNS Request --------->
<--------- DNS Response <--------- DNS Response
Message Flow for Full Handshake Issuing New Session Ticket Message Flow for Full Handshake Issuing New Session Ticket
9. Fragmentation and Reassembly 6. Anycast
This section describes an optional procedure the client and server
can negotiate to send large DNS responses without IP fragmentation or
reassembly.
Large DNS responses cannot exceed the DNS maximum payload size (512)
unless a larger size is negotiated with EDNS0. Even using EDNS0,
requesting responses larger the path MTU causes IP fragmentation. If
the response exceeds that size it is truncated and the TC bit set,
forcing a DNS client that wants the entire response to establish a
TCP connection and send the query again over TCP. This slows down
DNS lookups, and is even more troublesome if a TLS session also needs
to be established.
To avoid these problems with DNS over DTLS, the DNS client and the
DNS server can indicate support for a new application-layer
fragmentation and reassembly mechanism, by using the new DTLS
extension "DNS-fragment" in the DTLS ClientHello, and indicate how
many fragments the client is willing to receive. If the server
supports this extension, it includes "DNS-fragment" in its DTLS
ServerHello and indicates how many fragments it is willing to send in
a response. The EDNS0 value controls the size of the responses,
including the size of fragmented responses. If both the DTLS client
and DTLS server indicate support DNS-fragment, and the DNS server's
response exceeds the EDNS0-indicated size, the DNS server fragments
the response into packets that are no larger than the EDNS0-indicated
size, and sends them all to the DNS client. Logically, the layering
of the fragmentation is like this,
| DNS |
| fragmentation |
| DTLS |
| UDP |
| IP |
9.1. Generating fragmented packets
The response is formed, and separate packets are sent with their own
fragmentation header, as follows:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| DNS Query ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR| 1| M| M| Fragment-count | Fragment-ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| Master Fragment Sequence Number (MFSN) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
The above fragment header appears at the beginning of all fragments.
The fields are defined as follows:
DNS Query ID: Is the same as the ID value of the DNS response;
this means the first fragment has the DNS Query ID value appear
twice in the packet, and means subsequent fragments will contain
their associated DNS Query ID in the fragmentation header.
QR: This is the QR field in a normal DNS packet, and is always
set, because fragmentation/reassembly is only defined in this
document for responses.
OP: This bit is always set. This bit corresponds to first bit of
the Opcode field of a normal DNS packet; that Opcode field for a
normal DNS packet must be all 0. By setting this bit to 1, this
fragmentation header is distinguished from a normal DNS packet.
M: The next two "M" bits must be 0, for future use.
Fragment-count: For each query ID that generates a fragmented
response, this field is set to the number of fragments that will
be sent (that is, the highest Fragment-ID minus 1).
Fragment-ID: Starts at 0 for the first fragmented segment and is
incremented for each fragment.
Master Fragment Sequence Number: starts at 0 and is incremented
for the first packet of each fragmented response.
The length of each fragment is calculated from the UDP packet size
minus the DTLS overhead.
It is RECOMMENDED that a value not exceeding 10 is used by the DNS
client and DNS server during their DNS-fragment negotiation
[RFC6928]. If the server needs to generate more fragments than were
negotiated, MUST set the TC bit and SHOULD send the number of
fragments negotiated.
The fragments MUST be at least 64 bytes (minimum Ethernet MTU) minus
DTLS, UDP, and IP overhead. The fragments need not all be the same
size. The DNS client indicates the maximum DNS size using EDNS0,
which constrains the size of the response packet on the wire. When
generating fragmented packets, the DNS server MUST NOT generate
fragments that exceed the maximum DNS size.
9.2. Receiving fragmented packets
Upon receipt of a DTLS packet, DTLS processing is performed and the
Opcode field is examined to determine if reassembly is required
before processing as a DNS packet, as depicted below:
+--------+
| 00000 -+------------> DNS processing
Opcode = | | ^
| | |
| 1xxxx -+--->reassembly----+
+--------+
If reassembly needs to be performed, the packets are matched
according to their DNS query ID value (at the top of the fragment
header), their Master Fragment Sequence Number, and ordered by their
Fragment-ID. Once all the fragments have been received (that is, all
fragments from 0 through the Fragment-ID matching the frag-count
minus 1), the fragment headers are removed and the DNS payload is
handed to the DNS layer. Due to network loss or packet corruption,
some fragments might not be received, which will cause the DNS layer
to perform a normal re-transmission of the DNS query, with the same
query ID. The re-transmitted answer, which will be fragmented
identically to the original answer (assuming that resource record did
not change between the two answers), will have a different Master
Fragment Sequence Number.
Design Note: Tha MFSN protects against corruption caused by DNS
resource record changing between the initial query and its re-
transmitted query.
After a time out, incomplete fragments are discarded by the receiver.
If the Fragment-ID is 0 and the DNS Query ID value in the fragment
header does not match the ID value in the DNS header, a DTLS Alert is
generated and an error is logged.
9.3. The DNS-fragment Extension
A new extension type ("DNS_fragment(TBA)") is defined and MUST be
included by the client in its "ClientHello" message if it wants to
use fragmentation, and MUST be included in the ServerHello if the
server agrees to use fragmentation.
enum { DNS-fragment(TBA), (65535) } ExtensionType;
The "extension_data" field of the "DNS-fragment" extension MUST
contain a "MaxNumOfFragments" value, which is the maximum number of
fragments the client wants to receive (indicated in the ClientHello),
and the maximum number of fragments the server will send (indicated
in the ServerHello).
uint8 MaxNoOfFragments;
The value indicated in the ServerHello MUST be less than or equal to
the value indicated in the ClientHello, and if not the client MUST
terminate the DTLS association with an Alert, and MAY establish a new
DTLS association without the dns_fragment extension.
10. Anycast
DNS servers are often configured with anycast addresses. While the DNS servers are often configured with anycast addresses. While the
network is stable, packets transmitted from a particular source to an network is stable, packets transmitted from a particular source to an
anycast address will reach the same server that has the cryptographic anycast address will reach the same server that has the cryptographic
context from the DNS over DTLS handshake. But when the network context from the DNS over DTLS handshake. But when the network
configuration changes,a DNS over DTLS packet can be received by a configuration changes, a DNS over DTLS packet can be received by a
server that does not have the necessary cryptographic context. To server that does not have the necessary cryptographic context. To
encourage the client to initiate a new DTLS handshake, DNS servers encourage the client to initiate a new DTLS handshake, DNS servers
SHOULD generate a DTLS Alert message in response to receiving a DTLS SHOULD generate a DTLS Alert message in response to receiving a DTLS
packet for which the server does not have any cryptographic context. packet for which the server does not have any cryptographic context.
Upon receipt of an un-authenicated DTLS alert, the DTLS client
validates the Alert is within the replay window, as usual
(Section 4.1.2.6 of [RFC6347]). It is difficult for the DTLS client
to validate the DTLS alert was generated by the DTLS server in
response to a request or was generated by an on- or off-path
attacker. Thus, upon receipt of an in-window DTLS Alert, the client
SHOULD continue re-transmitting the DTLS packet (in the event the
Alert was spoofed), and at the same time it SHOULD initiate DTLS
session resumption.
11. IANA Considerations 7. Downgrade attacks
This document adds a new extension for DTLS, in accordance with Using DNS privacy with an authenticated server is most preferred, DNS
[RFC5246]: privacy with an unauthenticated server is next preferred, and plain
DNS is least preferred. This section gives a non-normative
discussion on common behaviors and choices.
enum { DNS-Fragment(TBA), (65535) } ExtensionType; An implementation MAY attempt to obtain DNS privacy by contacting DNS
servers on the local network (provided by DHCP) and on the Internet,
and make those attempts in parallel to reduce user impact. If DNS
privacy cannot be successfully negotiated for whatever reason, the
client can do three things:
[[ NOTE: This value needs to be assigned by IANA ]] 1. refuse to send DNS queries on this network, which means the
client cannot make effective use of this network, as modern
networks require DNS; or,
This extension MUST only be used with DTLS. 2. use opportunistic security, as described in [RFC7435]. or,
3. send plain DNS queries on this network, which means no DNS
privacy is provided.
Heuristics can improve this situation, but only to a degree (e.g.,
previous success of DNS privacy on this network may be reason to
alert the user about failure to establish DNS privacy on this network
now). Still, the client (in cooperation with the end user) has to
decide to use the network without the protection of DNS privacy.
8. IANA Considerations
IANA is requested to add the following value to the "Service Name and IANA is requested to add the following value to the "Service Name and
Transport Protocol Port Number Registry" registry in the System Transport Protocol Port Number Registry" registry in the System
Range. The registry for that range requires IETF Review or IESG Range. The registry for that range requires IETF Review or IESG
Approval [RFC6335] and such a review has been requested using the Approval [RFC6335] and such a review has been requested using the
Early Allocation process [RFC7120] for the well-known UDP port in Early Allocation process [RFC7120] for the well-known UDP port in
this document. this document.
Service Name domain-s Service Name domain-s
Transport Protocol(s) UDP/TCP Transport Protocol(s) UDP/TCP
Port 853 Port 853
Assignee IESG Assignee IESG
Contact dwing@cisco.com Contact dwing@cisco.com
Description DNS query-response protocol runs over Description DNS query-response protocol runs over
DTLS and TLS DTLS and TLS
Reference This document Reference This document
12. Security Considerations 9. Security Considerations
The interaction between a DNS client and DNS server requires Datagram The interaction between a DNS client and DNS server requires Datagram
Transport Layer Security (DTLS) with a ciphersuite offering Transport Layer Security (DTLS) with a ciphersuite offering
confidentiality protection and guidance given in [RFC7525] must be confidentiality protection and guidance given in [RFC7525] must be
followed to avoid attacks on DTLS. Once a DNSoD client has followed to avoid attacks on DTLS. Once a DNSoD client has
established a security association with a particular DNS server, and established a security association with a particular DNS server, and
outstanding normal DNS queries with that server (if any) have been outstanding normal DNS queries with that server (if any) have been
received, the DNSoD client MUST ignore any subsequent normal DNS received, the DNSoD client MUST ignore any subsequent normal DNS
responses from that server, as all subsequent responses should be responses from that server, as all subsequent responses should be
encrypted. This behavior mitigates all possible attacks described in encrypted. This behavior mitigates all possible attacks described in
Measures for Making DNS More Resilient against Forged Answers Measures for Making DNS More Resilient against Forged Answers
[RFC5452]. [RFC5452].
The DNS Fragment extension does not impact security of DTLS session The DNS Fragment extension does not impact security of DTLS session
establishment or application data exchange. DNS Fragment provides establishment or application data exchange. DNS Fragment provides
fragmentation and reassembly of the encrypted DNS payload. fragmentation and reassembly of the encrypted DNS payload.
13. Acknowledgements A malicious client might attempt to perform a high number of DTLS
handshakes with a server. As the clients are not uniquely identified
by the protocol and can be obfuscated with IPv4 address sharing and
with IPv6 temporary addresses, a server needs to mitigate the impact
of such an attack. Such mitigation might involve rate limiting
handshakes from a certain subnet or more advanced DoS/DDoS techniques
beyond the scope of this paper.
9.1. Authenticating a DNS Privacy Server
DNS privacy requires encrypting the query (and response) from passive
attacks. Such encryption typically provides integrity protection as
a side-effect, which means on-path attackers cannot simply inject
bogus DNS responses. However, to provide stronger protection from
active attackers pretending to be the server, the server itself needs
to be authenticated.
To authenticate the server providing DNS privacy, the DNS client
needs to be configured with the names or IP addresses of those DNS
privacy servers. The server certificate MUST contain DNS-ID
(subjectAltName) as described in Section 4.1 of [RFC6125]. DNS names
and IP addresses can be contained in the subjectAltName entries. The
client MUST use the rules and guidelines given in section 6 of
[RFC6125] to validate the DNS server identity.
This could be implemented by adding the certificate name to the /etc/
resolv.conf file, such as below:
nameserver 8.8.8.8
certificate google-public-dns.google.com
nameserver 208.67.220.220
certificate resolver.opendns.com
For DNS privacy servers that don't have a certificate trust chain
(e.g., because they are on a home network or a corporate network),
the configured list of DNS privacy servers can contain the Subject
Public Key Info (SPKI) fingerprint of the DNS privacy server (i.e., a
simple whitelist of name and SPKI fingerprint). The public key is
used for the same reasons HTTP pinning [RFC7469] uses the public key.
Raw public key-based authentication mechanism defined in [RFC7250]
can be also used to authenticate the DNS server.
This could be implemented by adding the SPKI fingerprint to the /etc/
resolv.conf file, such as below (line split for Internet Draft
formatting):
nameserver 192.168.1.1
sha256 : "d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="
The only algorithm considered at this time is "sha256", i.e., the
hash algorithm SHA256 [RFC6234]; additional algorithms may be allowed
for use in this context in the future. The quoted-string is a
sequence of base 64 digits: the base64-encoded SPKI Fingerprint
[RFC4648].
10. Acknowledgements
Thanks to Phil Hedrick for his review comments on TCP and to Josh Thanks to Phil Hedrick for his review comments on TCP and to Josh
Littlefield for pointing out DNSoD load on busy servers (most notably Littlefield for pointing out DNSoD load on busy servers (most notably
root servers). The authors would like to thank Simon Josefsson, root servers). The authors would like to thank Simon Josefsson,
Daniel Kahn Gillmor, Bob Harold, Ilari Liusvaara and Sara Dickinson Daniel Kahn Gillmor, Bob Harold, Ilari Liusvaara and Sara Dickinson
for discussions and comments on the design of DNSoD. for discussions and comments on the design of DNSoD.
14. References 11. References
11.1. Normative References
14.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<http://www.rfc-editor.org/info/rfc1034>. <http://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>. November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
skipping to change at page 16, line 38 skipping to change at page 12, line 15
[RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning [RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
2015, <http://www.rfc-editor.org/info/rfc7469>. 2015, <http://www.rfc-editor.org/info/rfc7469>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer "Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <http://www.rfc-editor.org/info/rfc7525>. 2015, <http://www.rfc-editor.org/info/rfc7525>.
14.2. Informative References 11.2. Informative References
[I-D.bortzmeyer-dnsop-dns-privacy] [I-D.bortzmeyer-dnsop-dns-privacy]
Bortzmeyer, S., "DNS privacy considerations", draft- Bortzmeyer, S., "DNS privacy considerations", draft-
bortzmeyer-dnsop-dns-privacy-02 (work in progress), April bortzmeyer-dnsop-dns-privacy-02 (work in progress), April
2014. 2014.
[I-D.ietf-dprive-dns-over-tls] [I-D.ietf-dprive-dns-over-tls]
Zi, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., Zi, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "DNS over TLS: Initiation and Performance and P. Hoffman, "DNS over TLS: Initiation and Performance
Considerations", draft-ietf-dprive-dns-over-tls-01 (work Considerations", draft-ietf-dprive-dns-over-tls-01 (work
in progress), October 2015. in progress), October 2015.
[I-D.ietf-tls-cached-info] [I-D.ietf-tls-cached-info]
Santesson, S. and H. Tschofenig, "Transport Layer Security Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension", draft-ietf-tls- (TLS) Cached Information Extension", draft-ietf-tls-
cached-info-19 (work in progress), March 2015. cached-info-20 (work in progress), October 2015.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989, DOI 10.17487/RFC1122, October 1989,
<http://www.rfc-editor.org/info/rfc1122>. <http://www.rfc-editor.org/info/rfc1122>.
[RFC3749] Hollenbeck, S., "Transport Layer Security Protocol [RFC3749] Hollenbeck, S., "Transport Layer Security Protocol
Compression Methods", RFC 3749, DOI 10.17487/RFC3749, May Compression Methods", RFC 3749, DOI 10.17487/RFC3749, May
2004, <http://www.rfc-editor.org/info/rfc3749>. 2004, <http://www.rfc-editor.org/info/rfc3749>.
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