DNSCurve: Link-Level Security for the Domain Name System
draft-dempsky-dnscurve-01

Abstract

This document describes DNSCurve, a protocol extension that adds link-level security to the Domain Name System (DNS).

Status of this Memo

This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79.

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Copyright Notice

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Table of Contents

1.  Introduction
    1.1.  Terminology
2.  Overview
3.  Base-32 Encoding
    3.1.  Examples
4.  Encoding Public Keys in Name Server Names
5.  Nonce Generation
6.  DNSCurve Expanded Formats
    6.1.  Streamlined Format
    6.2.  TXT Format
7.  UDP and TCP
8.  Security Considerations
9.  IANA Considerations
10.  Acknowledgements
11.  References
    11.1.  Normative References
    11.2.  Informative References
§  Author's Address

1.  Introduction

DNSCurve adds link-level security to the Domain Name System (DNS). It includes a key distribution mechanism compatible with today's name server software and registry services, and two packet formats: a simple streamlined format requiring minimal space and processing overhead and a mostly backwards-compatible format intended for use with strict firewalls and DNS proxies.

DNSCurve packets include a cryptographic MAC (aka authenticator) to provide integrity and availability. Clients can be confident that verified responses came from the appropriate server and were not forged by a blind or even sniffing attacker, while servers can be confident that responses will not be replayed against other unintended clients. Additionally, DNSCurve packets are encrypted to provide some confidentiality.

1.1.  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 RFC 2119 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

2.  Overview

DNSCurve uses Curve25519XSalsa20Poly1305, a particular combination of the Curve25519, Salsa20, and Poly1305 primitives as described in [naclcrypto] (Bernstein, D., “Cryptography in NaCl,” March 2009.). In particular, it is a cryptosystem featuring 256-bit public and secret keys, 192-bit nonces, and 128-bit authenticators.

Each DNSCurve client and server has a secret key and a corresponding public key. DNSCurve servers distribute their public keys by encoding them in name server names embedded in standard DNS NS records (as described in Section 4 (Encoding Public Keys in Name Server Names)), while DNSCurve clients distribute their public keys by including them in their query packets. (Additional mechanisms for key distribution like DNSSEC's trust anchors and DLV are possible, but not defined by this document.)

When a DNSCurve client is about to send a DNS query to a name server, if the client knows a DNSCurve public key for that name server, it MAY instead use that public key along with its own DNSCurve secret key and a nonce to protect its query in a "cryptographic box" as described in [naclcrypto] (Bernstein, D., “Cryptography in NaCl,” March 2009.). The client then encodes this cryptographic box along with the nonce and its own public key as an expanded DNSCurve query packet, which it sends to the DNSCurve server instead of the original DNS query.

Upon receiving a DNS query packet, a DNSCurve name server should first treat the packet as a DNSCurve query packet by extracting the client's DNSCurve public key, nonce, and boxed query and trying to open the box using the extracted public key and its own secret key. However, if this fails (i.e., the packet is not formatted as an expanded DNSCurve query packet or the box's authenticator is invalid), then the server responds to the packet as a normal DNS packet.

Assuming the unboxing succeeds, then the server discovers the client's original query packet. To send a response, the server chooses a nonce extension to append to the client-chosen nonce, and protects its response packet in a cryptographic box using the extend nonce and same keys used to unbox the client's DNS query. The server then encodes this cryptographic box as an expanded DNSCurve response packet, which it sends to the DNSCurve client instead of the original DNS response.

Meanwhile, the DNSCurve client waits for an expanded DNSCurve response packet. If it receives a non-DNSCurve response packet, an expanded DNSCurve response packet with an invalid nonce (i.e., not an extension of its original nonce) or an invalid cryptographic box (i.e., cannot be opened using the same keys and the extended nonce), then it discards the packet and continues waiting. Once it receives a valid expanded DNSCurve response packet, it opens the cryptographic box to discover the server's original DNS response.

3.  Base-32 Encoding

Sometimes DNSCurve communicates arbitrary byte strings inside domain names. While the DNS protocol is 8-bit safe for names and labels (except for case-insensitive handling of ASCII alphabetic characters), many tools have trouble with arbitrary characters in domain names, in particular domain registrar software. To cope with this limitation, DNSCurve encodes byte strings using a set of safe alphanumeric characters.

In DNSCurve's base-32 encoding, a byte string is interpreted as a number in little-endian form. Each 5-bit sequence of this number, from least significant to most significant, is encoded as one of the standard "digits" "0123456789bcdfghjklmnpqrstuvwxyz". A final sequence of fewer than 5 bits is zero-extended before encoding. Decoders MUST accept "BCDFGHJKLMNPQRSTUVWXYZ" as synonyms for "bcdfghjklmnpqrstuvwxyz".

For example, the two-byte string with bytes {0x64,0x88} (i.e., {100,136} decimal) is interpreted as the integer 0x8864 (i.e., 34916). The bits 1000100001100100 of this integer are divided into 5-bit parts 00100, 00011, 00010, 00001, which in turn are encoded as "4", "3", "2", "1". The original string is therefore encoded as the string "4321".

N.B., this is not the same encoding as defined in [RFC4648] (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” October 2006.). In particular, the byte string is chunked into 5-bit sequences differently, and a different alphabet is used. The first allows DNSCurve public keys to be encoded slightly more compactly (see Section 4 (Encoding Public Keys in Name Server Names)), and the second helps to further prevent false positives when searching for base-32 encoded strings in domain names.

3.1.  Examples

4.  Encoding Public Keys in Name Server Names

DNSCurve public keys are encoded in name server names as a 54-byte label consisting of the magic string "uz5" followed by the first 51 bytes of the base-32 encoding of the public key. (Curve25519 public keys are actually 255-bit integers in little-endian, so the 52nd byte of the base-32 encoding will always be "0".)

When a DNSCurve client is searching a name server name for a DNSCurve public key, it MUST check every label for an encoded public key. If multiple public keys are found, the left-most label MUST be chosen. String comparison with "uz5" MUST be performed case-insensitively.

5.  Nonce Generation

For every request, DNSCurve clients generate a 96-bit nonce, and for every response, DNSCurve servers generate a 96-bit nonce extension. Nonces MUST be unique for distinct packets for the same client-server key pair. A simple way to achieve this is to choose a unique nonce for each packet and for each retransmission. Additionally, servers MUST use a non-zero nonce extension (because nonces are zero extended in query packets). However, subject to these constraints, clients and servers may generate nonces however they choose.

Two recommended ways to generate a 96-bit nonce or nonce extension are

1. a 64-bit counter (starting at 1) followed by a 32-bit random number and
2. a 64-bit timestamp (e.g., nanoseconds since 1970) followed by a 32-bit random number.

In either case the 64-bit value MUST NOT decrease even if the software restarts or the system clock jumps backwards.

If multiple clients or multiple servers share a DNSCurve secret key, then they MUST make sure no two separate clients or servers generate the same nonce. A simple way to achieve this is to use nonce separation; e.g., if two servers share a DNSCurve key pair, one server could use only even nonces and the other could use only odd nonces.

6.  DNSCurve Expanded Formats

DNSCurve defines two expanded formats: "streamlined" and "TXT". Each includes a format for expanded queries and a format for expanded responses. DNSCurve clients may send DNSCurve expanded queries using whichever format it chooses, but they are encouraged to use the streamlined format when possible. A DNSCurve server MUST support DNSCurve expanded queries in either format and MUST send expanded responses using the corresponding format.

6.1.  Streamlined Format

An expanded query packet in streamlined format has the following bytes:

• 8 bytes: the magic string "Q6fnvWj8".
• 32 bytes: the client's DNSCurve public key.
• 12 bytes: a client-selected nonce for this packet.
• A cryptographic box containing the original DNS query packet.

An expanded response packet in streamlined format has the following bytes:

• 8 bytes: the magic string "R6fnvWJ8".
• 12 bytes: the client's nonce.
• 12 bytes: a server-selected nonce extension.
• A cryptographic box containing the original DNS response packet.

Note that this streamlined response format does not repeat the client's query name, and in particular does not repeat the client's public key. However, it does repeat the client's nonce.

6.2.  TXT Format

The "TXT" format receives its name from the fact that expanded query and response packets in this format appear to casual inspection to be standard DNS packets with two possible exceptions: 1) the query name may exceed 255 bytes and 2) the total packet may exceed 512 bytes.

When encoding an expanded query packet in TXT format, a DNSCurve client MUST create a DNS standard query packet with the AA, TC, RD, RA, Z, and RCODE bits cleared, a single entry in the question section, and no records in the answer, authority, or additional records sections. The one question MUST ask for Internet-class TXT records for the query name constructed from the concatenation of the following labels:

• One or more labels, each label except the last being exactly 50 bytes, with the last label being at most 50 bytes. The concatenation of these labels is the base-32 encoding of a 96-bit client-selected nonce for this packet followed by a cryptographic box containing the original DNS query packet.
• One 54-byte label: the client's DNSCurve public key, encoded as described in Section 4 (Encoding Public Keys in Name Server Names), except with the magic string "x1a" instead of "uz5".
• Zero or more additional labels specifying the name of the zone served by this server; i.e., the owner name of the relevant NS record.

A DNSCurve server SHOULD be lenient in decoding expanded query packets in TXT format. In particular, it MUST allow the RD bit to either be set or clear, MUST allow records in the answer, authority records, and additional records sections, and MUST allow any labels to follow the DNSCurve public key in the query name. However, it MUST discard packets with the QR bit set.

When encoding an expanded response packet in TXT format, a DNSCurve server MUST create a DNS standard response packet copying the ID, RD bit, and questions section from the expanded query packet, setting the AA bit, leaving the TC and RA bits cleared and Z and RCODE values set to 0, containing one record in the answer section, and no records in the authority records or additional records section. The record in the answer section MUST be an Internet-class TXT record for the query name from the questions section with a TTL of 0. The RDATA of this record is the 96-bit server-selected nonce extension followed by a cryptographic box containing the original DNS response packet, encoded as a sequence of one or more strings of at most 255 bytes in standard DNS TXT RDATA format.

Similarly, a DNSCurve client SHOULD be lenient in decoding expanded response packets in TXT format. In particular, it MUST allow the server to alter the case of the query name when repeating it in the questions section.

7.  UDP and TCP

If a normal DNS response packet is larger than 512 bytes then the server replaces it by an explicitly truncated packet. The client then tries again through TCP. Servers are not required to support TCP if no responses are above 512 bytes; clients are permitted to try TCP only if the server has explicitly indicated truncation.

DNSCurve does not require TCP support from servers that were not already supporting TCP. If the original DNS response packet is at most 512 bytes then the server is permitted to send the expanded response packet as a UDP packet. DNSCurve clients are required to set aside a 4096-byte buffer for receiving a UDP response packet.

If the original DNS response packet is larger than 512 bytes then it is replaced by an explicitly truncated packet and the truncated packet is protected by DNSCurve. In this case the client tries again by TCP, sending its DNSCurve query packet through TCP and receiving the DNSCurve response through TCP.

TCP is considerably more expensive for clients and servers than UDP is, and TCP has no protection against denial of service, so server administrators are advised to stay below 512 bytes if possible. DNSCurve adds some denial-of-service protection for UDP but cannot do anything to help TCP.

If a protected DNS query includes an EDNS0 OPT record, then the payload size field refers to how large the original DNS response packet can be before encoding as a DNSCurve response packet. Clients MUST reduce the payload size they advertise to account for overhead from encoding the response as an expanded response packet. If a server builds a response within the payload size limit, but then cannot fit the encoded response in 4096 bytes, it MAY silently discard the response.

Even when DNSCurve transactions take place over UDP, they may still be vulnerable to denial-of-service attacks due to spoofed IP fragments if response packets are large enough to require IP fragmentation. Therefore, servers SHOULD try to keep response packets within the path's MTU limits.

8.  Security Considerations

The security of the Curve25519XSalsa20Poly1305 cryptosystem and its underlying cryptographic primitives is discussed in [naclcrypto] (Bernstein, D., “Cryptography in NaCl,” March 2009.). In summary, it is designed to meet the standard notions of privacy and third-party unforgeability for a public-key authenticated-encryption scheme using nonces.

DNSCurve only provides link-level security between a client-server pair. It does not attempt to ensure end-to-end security for queries and responses relayed by untrusted DNS proxies and caches.

DNSCurve clients are free to choose whether or not to use DNSCurve on a per query basis; e.g., a client may decide to fallback to standard DNS after a few failed DNSCurve queries. Of course, DNSCurve cannot make any security guarantees for transactions that do not use DNSCurve, so clients are encouraged to use DNSCurve if possible.

DNSCurve adds some confidentiality by encrypting DNS packet contents but does not attempt to hide the length of the original DNS packet nor the source or destination of the packet. Additionally, the TXT format requires clients to reveal the zone they are querying.

9.  IANA Considerations

This document has no actions for IANA.

10.  Acknowledgements

The DNSCurve protocol was first introduced by Dan Berstein. Thanks also to Adam Langley and George Barwood for their contributions to early DNSCurve implementations.

Much thanks for feedback regarding this draft from George Barwood, Sjoerd Langkemper and Nikos Mavrogiannopoulos.

11.  References

11.1. Normative References

11.2. Informative References

Author's Address