SVCB and HTTPS

Shared wire-format representation for the SVCB service binding record (RFC 9460 section 2) and its HTTPS-specific variant (RFC 9460 section 9).

                                1  1  1  1  1  1
  0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                  SvcPriority                  |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/                  TargetName                   /
/                                               /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/                   SvcParams                   /
/                                               /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

The type parameter n (either N_https or N_svcb) determines the RR type. Each has its own type synonym (T_https, T_svcb) and matching record pattern synonym (T_HTTPS, T_SVCB) with the corresponding field-name prefix (https, svcb). The wire format is shared, but the type role of n is nominal: a T_svcb value cannot be used where a T_https is expected. This is deliberate — the two RR types serve different transports, and future SvcParamKeys may apply to only one of them.

Note that T_https and T_svcb are just type aliases! The T_HTTPS and T_SVCB record pattern synonyms and their fields are bundled with the underlying X_svcb data type. In many cases it is sufficient to import just X_svcb(..), but if you also need the type aliases, they need to be imported separately:

import Net.DNSBase (X_svcb(..), T_https, T_svcb)

The target field is not subject to wire-form name compression (RFC 3597 section 4) and is not in the RFC 4034 section 6.2 list of types that lower-case their RDATA names — it is compared case-sensitively in canonical form. The Ord instance compares structurally on the parsed fields rather than on the wire form, so it is not canonical: callers that need RFC 4034 canonical ordering must serialise to wire form first.

The SvcParams field is a list of (key, length, value) triples — possibly empty — making up the rest of the RData:

                                1  1  1  1  1  1
  0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                  SvcParamKey                  |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                  SvcParamLen                  |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/                  SvcParamValue                /
/                                               /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

On the wire the list must be in strictly ascending key order; the presentation form may list keys in any order. Each value has the type associated with its key in the decoder state configured for the RR type — keys absent from the state decode as OpaqueSPV, preserving the raw bytes.

The record pattern synonyms T_SVCB and T_HTTPS build the corresponding T_svcb or T_https value directly, with their own field-name prefixes (svc and https):

let s = T_SVCB  { svcPriority      = 0
                , svcTarget        = RootDomain
                , svcParamValues   = [] }
    h = T_HTTPS { httpsPriority    = 0
                , httpsTarget      = RootDomain
                , httpsParamValues = [] }
 in RData s : RData h : []

Functions that work on either RR type can use the underscore-prefixed selectors on the shared X_svcb record:

aliasDomain :: forall n. X_svcb n -> Maybe Domain
aliasDomain r | x_svcPriority r == 0 = Just $ x_svcTarget r
              | otherwise           = Nothing

X_svcb specialised to HTTPS records.

Note that T_https is just a type alias! The T_HTTPS record pattern synonym and its fields are bundled with the underlying X_svcb data type. In many cases it is sufficient to import just X_svcb(..), but if you also need the type alias, it needs to be imported separately:

import Net.DNSBase (X_svcb(..), T_https)

X_svcb specialised to SVCB records.

Note that T_svcb is just a type alias! The T_SCVB record pattern synonym and its fields are bundled with the underlying X_svcb data type. In many cases it is sufficient to import just X_svcb(..), but if you also need the type alias, it needs to be imported separately:

import Net.DNSBase (X_svcb(..), T_svcb)

The class of types representing the value side of a service parameter inside an SVCB or HTTPS record. Each instance corresponds to a specific SVCParamKey; the encodeSPV and decodeSPV methods handle only the value bytes.

The Presentable instance builds the RFC 9460 zone-file presentation form: the key name followed (where the value is non-empty) by = and the value. The Show instance is typically derived and aims to produce syntactically valid Haskell.

The set of service parameters in an SVCB or HTTPS record, with at most one value per SVCParamKey. The Monoid instance provides the empty set; the Semigroup instance is left-biased on key collisions.

Look up the parameter at the key associated with type a, returning a Just only when the stored value really is of type a. A value of the same key code but held as OpaqueSPV (because the typed instance was not registered when the record was decoded) does not match — opaque values must be retrieved through SPVMap.