DNSOP Working Group T. Reddy
Internet-Draft D. Wing
Intended status: Standards Track P. Patil
Expires: October 24, 2014 Cisco
April 22, 2014
DNS over DTLS (DNSoD)
draft-wing-dnsop-dnsodtls-00
Abstract
DNS queries and responses are visible to network elements on the path
between the DNS client and its server. These queries and responses
can contain privacy-sensitive information which is valuable to
protect. An active attacker can send bogus responses causing
misdirection of the subsequent connection.
To counter passive listening and active attacks, this document
proposes the use of Datagram Transport Layer Security (DTLS) for DNS,
to protect against passive listeners and certain active attacks. As
DNS needs to remain fast, this proposal also discusses mechanisms to
reduce DTLS round trips and reduce DTLS handshake size. The proposed
mechanism runs over the default DNS port and can also run over an
alternate port.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 24, 2014.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Relationship to TCP Queries and to DNSSEC . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Incremental Deployment . . . . . . . . . . . . . . . . . . . 3
5. Demultiplexing, Polling, Port Usage, and Discovery . . . . . 3
6. Performance Considerations . . . . . . . . . . . . . . . . . 4
7. In-Band Signaling . . . . . . . . . . . . . . . . . . . . . . 6
8. Authenticating a DNS Server . . . . . . . . . . . . . . . . . 6
9. Established sessions . . . . . . . . . . . . . . . . . . . . 7
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
11. Security Considerations . . . . . . . . . . . . . . . . . . . 8
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
13.1. Normative References . . . . . . . . . . . . . . . . . . 9
13.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
The Domain Name System is specified in [RFC1034] and [RFC1035]. DNS
queries and responses are normally exchanged unencrypted and are thus
vulnerable to eavesdropping. Such eavesdropping can result in an
undesired entity learning domains that a host wishes to access, thus
resulting in privacy leakage. DNS privacy problem is further
discussed in [I-D.bortzmeyer-dnsop-dns-privacy].
Active attackers have long been successful at injecting bogus
responses, causing cache poisoning and causing misdirection of the
subsequent connection (if attacking A or AAAA records). A popular
mitigation against that attack is to use ephemeral and random source
ports for DNS queries.
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This document defines DNS over DTLS (DNSoD, pronounced "dee-enn-sod")
which provides confidential DNS communication for stub resolvers,
recursive resolvers, iterative resolvers and authoritative servers.
2. Relationship to TCP Queries and to DNSSEC
DNS queries can be sent over UDP or TCP. This document scope is only
UDP. DNS over TCP could be protected with TLS, such as described by
[I-D.hzhwm-start-tls-for-dns]. Alternatively, a shim protocol could
be defined between DTLS and DNS, allowing large responses to be sent
over DTLS itself, see Section 6.
DNS Security Extensions (DNSSEC [RFC4033]) provides object integrity
of DNS resource records, allowing end-users (or their resolver) to
verify legitimacy of responses. However, DNSSEC does not protect
privacy of DNS requests or responses. DNSoD works in conjunction
with DNSSEC, but DNSoD does not replace the need or value of DNSSEC.
3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
4. Incremental Deployment
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).
5. Demultiplexing, Polling, Port Usage, and Discovery
[Note - This section requires further discussion]
Many modern operating systems already detect if a web proxy is
interfering with Internet communications, using proprietary
mechanisms that are out of scope of this document. After that
mechanism has run (and detected Internet connectivity is working),
the DNSoD procedure described in this document should commence. This
timing avoids delays in joining the network (and displaying an icon
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indicating successful Internet connection), at the risk that those
initial DNS queries will be sent without protection afforded by
DNSoD.
DNSoD can run over standard UDP port 53 as defined in [RFC1035]. A
DNS client or server that does not implement this specification will
not respond to the incoming DTLS packets because they don't parse as
DNS packets (the DNS Opcode would be 15, which is undefined). A DNS
client or server that does implement this specification can
demultiplex DNS and DTLS packets by examining the third octet. For
TLS 1.2, which is what is defined by this specification, a DTLS
packet will contain 253 in the third octet, whereas a DNS packet will
never contain 253 in the third octet.
There has been some concern with sending DNSoD traffic over the same
port as normal, un-encrypted DNS traffic. The intent of this section
is to show that DNSoD could successfully be sent over port 53.
Further analysis and testing on the Internet may be valuable to
determine if multiplexing on port 53, using a separate port, or some
fallback between a separate port and port 53 brings the most success.
After performing the above steps, the host should determine if the
DNS server supports DNSoD by sending a DTLS ClientHello message. A
DNS server that does not support DNSoD will not respond to
ClientHello messages sent by the client, because they are not valid
DNS requests (specifically, the DNS Opcode is invalid). The client
MUST use timer values defined in Section 4.2.4.1 of [RFC6347] for
retransmission of ClientHello message and if no response is received
from the DNS server. After 15 seconds, it MUST cease attempts to re-
transmit its ClientHello. Thereafter, the client MAY repeat that
procedure in the event the DNS server has been upgraded to support
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.
This mechanism requires no additional signaling between the client
and server.
6. Performance Considerations
To reduce number of octets of the DTLS handshake, especially the size
of the certificate in the ServerHello (which can be several
kilobytes), we should consider using plain public keys
[I-D.ietf-tls-oob-pubkey]. Considering that to authorize a certain
DNS server the client already needs explicit configuration of the DNS
servers it trusts, maybe the public key configuration problem is
really no worse than the configuration problem of those whitelisted
certificates?
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Multiple DNS queries can be sent over a single DNSoD security
association. The existing QueryID allows multiple requests and
responses to be interleaved in whatever order they can be fulfilled
by the DNS server. This means DNSoD reduces the consumption of UDP
port numbers, and because DTLS protects the communication between the
DNS client and its server, the resolver SHOULD NOT use random
ephemeral source ports (Section 9.2 of [RFC5452]) because such source
port use would incur additional, unnecessary DTLS load on the DNSoD
server.
It is highly advantageous to avoid server-side DTLS state and reduce
the number of new DTLS security associations on the server which can
be done with [RFC5077]. This also eliminates a round-trip for
subsequent DNSoD queries, because with [RFC5077] the DTLS security
association does not need to be re-established. Note: with the shim
(described below) perhaps we could send the query and the restore
server-side state in the ClientHello packet.
Compared to normal DNS, DTLS adds at least 13 octets of header, plus
cipher and authentication overhead to every query and every response.
This reduces the size of the DNS payload that can be carried.
Certain DNS responses are large (e.g., many AAAA records, TXT, SRV)
and don't fit into a single UDP packet, causing a partial response
with the truncation (TC) bit set. The client is then expected to
repeat the query over TCP, which causes additional name resolution
delay. We have considered two ideas, one that reduces the need to
switch to TCP and another that eliminates the need to switch to TCP:
o Path MTU can be determined using Packetization Layer Path MTU
Discovery [RFC4821] using DTLS heartbeat. [RFC4821] does not rely
on ICMP or ICMPv6, and would not affect DNS state or
responsiveness on the client or server. However, it would be
additional chattiness.
o To avoid IP fragmentation, DTLS handshake messages incorporate
their own fragment offset and fragment length. We might utilize a
similar mechanism in a shim layer between DTLS and DNS, so that
large DNS messages could be carried without causing IP
fragmentation.
DNSoD puts an additional computational load on servers. The largest
gain for privacy is to protect the communication between the DNS
client (the end user's machine) and its caching resolver. Because of
the load imposed, and because of the infrequency of queries to root
servers means the DTLS overhead is unlikely to be amoritized over the
DNS queries sent over that DTLS connection, implementing DNSoD on
root servers is NOT RECOMMENDED.
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7. In-Band Signaling
Probing with DTLS ClientHello packets to determine if a DNS server
supports DNSoD is somewhat inefficient, as it requires multiple probe
packets to each DNS server. It may well be preferable to have a
method of determining, within the DNS protocol itself, if a certain
DNS server supports DNSoD. Two approaches are discussed below, one
using DNS SRV query or Name Server Instance [RFC4892] query, and
another extending the EDNS0 OPT meta-RR [I-D.hzhwm-start-tls-for-dns]
to describe DNSoD support.
To use Name Server Instance [RFC4892] query, we would define a new
CHAOS class resource record. The name is not significant, so let's
pretend it would be called "DNSoD". Note: we might to indicate an
alternate port, querying a CHAOS SRV resource record is probably most
ideal.
To extend the EDNS0 OPT meta-RR defined in
[I-D.hzhwm-start-tls-for-dns], we can add another bit indicating
support for DNSoD, denoted as "SO". Clients and servers indicate
their support for, and desire to use, DNSoD by setting a bit in the
Flags field of the EDNS0 [RFC6891] OPT meta-RR. The "DTLS OK" (SO)
bit is defined as the third bit of the third and fourth bytes of the
"extended RCODE and flags" portion of the EDNS0 OPT meta-RR,
immediately adjacent to the "TLS OK" (TO) bit defined in
[I-D.hzhwm-start-tls-for-dns], as shown below:
+0 (MSB) +1 (LSB)
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
0: | EXTENDED-RCODE | VERSION |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
2: |DO|TO|SO| Must be zero |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
8. Authenticating a DNS Server
As DTLS provides integrity protection, some protection from on-path
attackers sending bogus responses is provided by simply using DTLS.
That is, a device on the path between a DNS client and its DNS server
cannot simply inject a bogus DNS response, as could be done with
normal DNS. However, that protection is not terribly strong, because
that attacker could intercept the DTLS handshake itself and pretend
to be the actual DNS server. To prevent that, we need to resolve two
problems: identifying the trusted servers and determining an
appropriate action when no trusted servers are available.
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The first problem, of identifying the legitimate server, requires
extending the host configuration. This is because DNS clients have
their DNS servers configured by IP address (from DHCP or from a local
configuration file such as /etc/resolv.conf), but certificates have a
DNS name in the SubjectAltName field. We can't change how
certificates are issued, but we already have to modify the DNS client
to support DNSoD. With that in mind, the DNS client would have a
list of SubjectAltNames it trusts, configured by the user. That
could be configured in the resolv.conf file itself, as shown below,
or could be maintained in a separate file.
nameserver 192.0.2.1 dns.example.net
nameserver 198.51.100.1 dns.example.com
The second problem is determining an appropriate action when no
trusted servers are available. Although the DNS client could still
use DNSoD with an un-trusted server which still provides protection
from on-path passive listeners and from on- and off-path active
attackers, this use does not protect from a malicious server
returning bogus responses. While DNSSEC can protect against bogus
responses (lies), DNSSEC cannot protect against a server that simply
does not return certain answers.
[[Editor's Note: Is there more we could do? Trust certain
responses (like for my enterprise VPN or other sites where I will
be doing a TLS handshake)? Keep in mind that with web portals at
hotels/airports, we won't have access to a trusted DNS server on
initial connect at all.]]
9. Established sessions
In DTLS, all data is protected using the same record encoding and
mechanisms. When the mechanism described in this document is in
effect, DNS messages are encrypted using the standard DTLS record
encoding. When a user of DTLS wishes to send an DNS message, it
delivers it to the DTLS implementation as an ordinary application
data write (e.g., SSL_write()). A single DTLS session can be used to
receive multiple DNS requests and generate DNS multiple responses.
Client Server
------ ------
ClientHello -------->
<------- HelloVerifyRequest
(contains cookie)
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ClientHello -------->
(contains cookie)
(empty SessionTicket extension)
ServerHello
(empty SessionTicket extension)
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished -------->
NewSessionTicket
[ChangeCipherSpec]
<-------- Finished
DNS Request --------->
<--------- DNS Response
Message Flow for Full Handshake Issuing New Session Ticket
10. IANA Considerations
This document defines a new bit ("SO") in the Flags field of the
EDNS0 OPT meta-RR. At the time of approval of this draft in the
standards track, as per the IANA Considerations of [RFC6891], IANA is
requested to reserve the third leftmost bit of the flags as the SO
bit, immediately adjacent to the DNSSEC SO bit, as shown in
Section 7.
If demultiplexing DTLS and DNS (using the third octet, Section 5) is
useful, we should reserve DNS Opcode 15 to ensure DNS always has a 0
bit where DTLS always has a 1 bit.
11. Security Considerations
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Once a DNSoD client has established a security association with a
particular DNS server, and outstanding normal DNS queries with that
server (if any) have been received, the DNSoD client MUST ignore any
subsequent normal DNS responses from that server, as all subsequent
responses should be inside DNSoD. This behavior mitigates all (?)
attacks described in Measures for Making DNS More Resilient against
Forged Answers [RFC5452].
Security considerations discussed in DTLS [RFC6347] also apply to
this document.
12. Acknowledgements
The EDNS0 OPT meta-RR described in Section 7 are an extension of the
technique described in Starting TLS over DNS
[I-D.hzhwm-start-tls-for-dns].
Thanks to Phil Hedrick for his review comments on TCP and to Josh
Littlefield for pointing out DNSoD load on busy servers (most notably
root servers).
13. References
13.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, January 2008.
[RFC5452] Hubert, A. and R. van Mook, "Measures for Making DNS More
Resilient against Forged Answers", RFC 5452, January 2009.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
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[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.
13.2. Informative References
[I-D.bortzmeyer-dnsop-dns-privacy]
Bortzmeyer, S., "DNS privacy problem statement", draft-
bortzmeyer-dnsop-dns-privacy-01 (work in progress),
December 2013.
[I-D.hzhwm-start-tls-for-dns]
Zi, Z., Zhu, L., Heidemann, J., Mankin, A., and D.
Wessels, "Starting TLS over DNS", draft-hzhwm-start-tls-
for-dns-00 (work in progress), February 2014.
[I-D.ietf-tls-oob-pubkey]
Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., and
T. Kivinen, "Using Raw Public Keys in Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", draft-ietf-tls-oob-pubkey-11 (work in progress),
January 2014.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[RFC4892] Woolf, S. and D. Conrad, "Requirements for a Mechanism
Identifying a Name Server Instance", RFC 4892, June 2007.
Authors' Addresses
Tirumaleswar Reddy
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India
Email: tireddy@cisco.com
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, California 95134
USA
Email: dwing@cisco.com
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Prashanth Patil
Cisco Systems, Inc.
Bangalore
India
Email: praspati@cisco.com
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