| DPRIVE | T. Reddy |
| Internet-Draft | D. Wing |
| Intended status: Standards Track | P. Patil |
| Expires: April 20, 2016 | Cisco |
| October 18, 2015 |
DNS over DTLS (DNSoD)
draft-ietf-dprive-dnsodtls-02
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 port 853.
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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 20, 2016.
Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved.
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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 [RFC5452].
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.
The motivations for proposing DNSoD are that
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 described in [I-D.ietf-dprive-dns-over-tls]. Alternatively, a shim 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 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.
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.
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.
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].
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:
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.
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].
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).
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 indicating successful Internet connection), at the risk that those initial DNS queries will be sent without protection afforded by DNSoD.
DNSoD MUST run over standard UDP port 853 as defined in Section 11. A DNS server that supports DNSoD MUST listen for and accept DTLS packets on a designated port 853.
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. 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. If the DNS client receives a hard ICMP error [RFC1122], it MUST immediately 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.
To reduce number of octets of the DTLS handshake, especially the size of the certificate in the ServerHello (which can be several kilobytes), DNS client and server can use raw public keys [RFC7250] or Cached Information Extension [I-D.ietf-tls-cached-info]. Cached Information Extension avoids transmitting the server's certificate and certificate chain if the client has cached that information from a previous TLS handshake.
Multiple DNS queries can be sent over a single DTLS session and the DNSoD client need not wait for an outstanding reply before sending the next query. The existing Query ID 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. When sending multiple queries over a single DTLS session, 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.
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 [RFC5077]. This also eliminates a round-trip for subsequent DNSoD queries, because with [RFC5077] the DTLS session does not need to be re-established.
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:
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. Implementing DNSoD on root servers is outside the scope of this document.
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.
DNSoD client and server can use DTLS heartbeat [RFC6520] to verify that the peer still has DTLS state. DTLS session is terminated by the receipt of an authenticated message that closes the connection (e.g., a DTLS fatal alert).
Client Server
------ ------
ClientHello -------->
<------- HelloVerifyRequest
(contains cookie)
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
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 |
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:
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.
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.
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.
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.
DNS servers are often configured with anycast addresses. While the network is stable, packets transmitted from a particular source to an anycast address will reach the same server that has the cryptographic context from the DNS over DTLS handshake. But when the network configuration changes,a DNS over DTLS packet can be received by a server that does not have the necessary cryptographic context. To encourage the client to initiate a new DTLS handshake, DNS servers SHOULD generate a DTLS Alert message in response to receiving a DTLS packet for which the server does not have any cryptographic context.
This document adds a new extension for DTLS, in accordance with [RFC5246]:
enum { DNS-Fragment(TBA), (65535) } ExtensionType;
[[ NOTE: This value needs to be assigned by IANA ]]
This extension MUST only be used with DTLS.
IANA is requested to add the following value to the "Service Name and Transport Protocol Port Number Registry" registry in the System Range. The registry for that range requires IETF Review or IESG Approval [RFC6335] and such a review has been requested using the Early Allocation process [RFC7120] for the well-known UDP port in this document.
Service Name domain-s
Transport Protocol(s) UDP/TCP
Port 853
Assignee IESG
Contact dwing@cisco.com
Description DNS query-response protocol runs over
DTLS and TLS
Reference This document
The interaction between a DNS client and DNS server requires Datagram Transport Layer Security (DTLS) with a ciphersuite offering confidentiality protection and guidance given in [RFC7525] must be followed to avoid attacks on DTLS. 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 encrypted. This behavior mitigates all possible attacks described in Measures for Making DNS More Resilient against Forged Answers [RFC5452].
The DNS Fragment extension does not impact security of DTLS session establishment or application data exchange. DNS Fragment provides fragmentation and reassembly of the encrypted DNS payload.
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). The authors would like to thank Simon Josefsson, Daniel Kahn Gillmor, Bob Harold, Ilari Liusvaara and Sara Dickinson for discussions and comments on the design of DNSoD.
| [I-D.bortzmeyer-dnsop-dns-privacy] | Bortzmeyer, S., "DNS privacy considerations", Internet-Draft draft-bortzmeyer-dnsop-dns-privacy-02, April 2014. |
| [I-D.ietf-dprive-dns-over-tls] | Zi, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D. and P. Hoffman, "DNS over TLS: Initiation and Performance Considerations", Internet-Draft draft-ietf-dprive-dns-over-tls-01, October 2015. |
| [I-D.ietf-tls-cached-info] | Santesson, S. and H. Tschofenig, "Transport Layer Security (TLS) Cached Information Extension", Internet-Draft draft-ietf-tls-cached-info-19, March 2015. |
| [RFC1122] | Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, DOI 10.17487/RFC1122, October 1989. |
| [RFC3749] | Hollenbeck, S., "Transport Layer Security Protocol Compression Methods", RFC 3749, DOI 10.17487/RFC3749, May 2004. |
| [RFC4821] | Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007. |
| [RFC6928] | Chu, J., Dukkipati, N., Cheng, Y. and M. Mathis, "Increasing TCP's Initial Window", RFC 6928, DOI 10.17487/RFC6928, April 2013. |
| [RFC7250] | 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)", RFC 7250, DOI 10.17487/RFC7250, June 2014. |
| [RFC7413] | Cheng, Y., Chu, J., Radhakrishnan, S. and A. Jain, "TCP Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014. |