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Frank Denis edited this page Jun 21, 2020 · 29 revisions

DNS Stamps

Server stamps encode all the parameters required to connect to a secure DNS server as a single string. Think about stamps as QR code, but for DNS.

Online DNS stamp calculator

Stamps can be quickly viewed/modified/created using this VueJS component.

An online demo is accessible here: https://stamps.dnscrypt.info

An example list of public secure DNS resolvers, with their DNS stamps.

Common definitions

  • a || b is the concatenation of a and b
  • a | b is the result of the logical OR operation between a and b.
  • len(x) is a single byte representation of the length of x, in bytes. Strings don't have to be zero-terminated and do not require invidual encoding.
  • vlen(x) is equal to len(x) if x is the last element of a set, and 0x80 | len(x) if there are more elements in the set.
  • LP(x) is len(x) || x, i.e x prefixed by its length.
  • VLP(x1, x2, ...xn) encodes a set, as vlen(x1) || x1 || vlen(x2) || x2 ... || vlen(xn) || xn. Since vlen(xn) == len(xn) (length of the last element doesn't have the high bit set), for a set with a single element, we have VLP(x) == LP(x).
  • [ Q ] means that Q is optional.
  • base64url(s) is the URL-safe base64 encoding of s. None of the parameters are individually base64-encoded. Base64-encoding is only applied once, on the entire concatenation of all length-prefixed parameters.

Encoding examples:

  • LP("10.0.0.1") is [ 0x08, 0x31, 0x30, 0x2e, 0x30, 0x2e, 0x30, 0x2e, 0x31 ]
  • VLP("10.0.0.1", "10.0.0.2") evaluates to [ 0x88, 0x31, 0x30, 0x2e, 0x30, 0x2e, 0x30, 0x2e, 0x31, 0x08, 0x31, 0x30, 0x2e, 0x30, 0x2e, 0x30, 0x2e, 0x32 ]
  • base64url(VLP("10.0.0.1", "10.0.0.2")) is iDEwLjAuMC4xCDEwLjAuMC4y

DNSCrypt stamps

Format:

"sdns://" || base64url(0x01 || props || LP(addr) || LP(pk) || LP(providerName))

0x01 is the protocol identifier for DNSCrypt.

props is a small-endian 64 bit value that represents informal properties about the resolver. It is a logical OR combination of the following values:

  • 1: the server supports DNSSEC
  • 2: the server doesn't keep logs
  • 4: the server doesn't intentionally block domains

For example, a server that supports DNSSEC, stores logs, but doesn't block anything on its own should set props as the following 8 bytes sequence: [ 0x05, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 ].

addr is the IP address, as a string, with a port number if the server is not accessible over the standard port for the protocol (443). IPv6 strings must be included in square brackets: [fe80::6d6d:f72c:3ad:60b8]. Scopes are permitted.

pk is the DNSCrypt provider's Ed25519 public key, as 32 raw bytes.

providerName is the DNSCrypt provider name.

DNS-over-HTTPS stamps

Format:

"sdns://" || base64url(0x02 || props || LP(addr) || VLP(hash1, hash2, ...hashn) ||
                       LP(hostname) || LP(path)
                       [ || VLP(bootstrap_ip1, bootstrap_ip2, ...bootstrap_ipn) ])

addr is the IP address of the server. It can be an empty string, or just a port number, represented with a preceding colon (:443). In that case, the host name will be resolved to an IP address using another resolver.

hashi is the SHA256 digest of one of the TBS certificate found in the validation chain, typically the certificate used to sign the resolver's certificate. Multiple hashes can be provided for seamless rotations.

hostname is the server host name which will also be used as a SNI name. If the host name contains characters outside the URL-permitted range, these characters should be sent as-is, without any extra encoding (neither URL-encoded nor punycode).

path is the absolute URI path, such as /dns-query.

bootstrap_ipi is the IP address of a recommended resolver accessible over standard DNS in order to resolve hostname. This is optional, and clients can ignore this information.

DNS-over-TLS stamps

Format:

"sdns://" || base64url(0x03 || props || LP(addr) || VLP(hash1, hash2, ...hashn) ||
                       LP(hostname) ||
                       [ || vlen(bootstrap_ip) || bootstrap_ip ])

addr is the IP address of the server. It can be an empty string, or just a port number. In that case, the host name will be resolved to an IP address using another resolver. IPv6 strings must be included in square brackets: [fe80::6d6d:f72c:3ad:60b8]. Scopes are permitted.

hashi is the SHA256 digest of one of the TBS certificate found in the validation chain, typically the certificate used to sign the resolver's certificate. Multiple hashes can be provided for seamless rotations.

hostname is the server host name which will also be used as a SNI name.

bootstrap_ipi is the IP address of a recommended resolver accessible over standard DNS in order to resolve hostname. This is optional, and clients can ignore this information.

Plain DNS stamps

Format:

"sdns://" || base64url(0x00 || props || LP(addr))

addr is the IP address of the server. IPv6 strings must be included in square brackets: [fe80::6d6d:f72c:3ad:60b8]. Scopes are permitted.

Anonymized DNSCrypt relay stamps

Format:

"sdns://" || base64url(0x81 || LP(addr))

0x81 is the protocol identifier for a DNSCrypt relay.

addr is the IP address and port, as a string. IPv6 strings must be included in square brackets: [fe80::6d6d:f72c:3ad:60b8]:443.

Implementations

Libraries

Applications

DNS stamps are known to be used in the following applications:

Servers lists as DNS stamps

In somecases if you want to run dnscrypt-proxy as a non-root user you'll get the error "[FATAL] listen udp 0.0.0.0:53: bind: permission denied"

to solve this problem you can run the following command and allow dnscrypt to have access to a low level port :

sudo setcap cap_net_bind_service=+ep $(which dnscrypt-proxy)

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