feat: specify the DNS data format

docs/spec/data-format/df-000-dns.md

@@ -0,0 +1,178 @@
+# DNS data format
+
+|              |                                                |
+|--------------|------------------------------------------------|
+| Author       | [@bassosimone](https://github.com/bassosimone) |
+| Last-Updated | 2024-11-18                                     |
+
+This document describes the format of DNS measurements emitted
+by [RBMK](https://github.com/rbmk-project/rbmk) and implemented by
+the [dnscore](https://github.com/rbmk-project/dnscore) library.
+
+
+## Table of contents
+
+- [Overview](#overview)
+- [Address and Protocol](#address-and-protocol)
+- [Query](#query)
+- [Response](#response)
+- [Example](#example)
+
+
+## Overview
+
+The [dnscore](https://github.com/rbmk-project/dnscore) library
+emits a "query" event before sending a DNS query and one or more
+"response" events when receiving responses. In general, we
+expect a single "response" per query. However, when using DNS
+over UDP, we may receive multiple responses for a single query,
+due to network misconfiguration or internet censorship.
+
+Specifically, the Great Firewall of China (GFW) may cause
+multiple responses for the same query, with the last response
+typically being the legitimate response from the server, and
+previous responses being fake and for arbitrary addresses.
+
+
+## Address and Protocol
+
+A remote server is identified by an address and a protocol.
+
+The protocol is an enumeration string with the following values:
+
+- `"udp"` for DNS over UDP;
+
+- `"tcp"` for DNS over TCP;
+
+- `"dot"` for DNS over TLS;
+
+- `"doh"` for DNS over HTTPS.
+
+For `"udp"`, `"tcp"`, and `"dot"`, the address is a string
+containing an IP address and a port separated by `":"`. When
+the address is and IPv6 address, it is enclosed in square
+brackets. The following are valid addresses:
+
+- `"8.8.8.8:53"`;
+
+- `"[2001:4860:4860::8888]:53"`.
+
+For `"doh"`, the address is a string containing the URL of the
+server. The following are valid addresses:
+
+- `"https://dns.google/dns-query"`;
+
+- `"https://cloudflare-dns.com/dns-query"`.
+
+
+## Query
+
+The JSON serialization of the query message contains
+*at least* the following fields:
+
+```JSON
+{
+  "msg":"dnsQuery",
+  "rawQuery":"",
+  "serverAddr":"",
+  "serverProtocol":"",
+  "t":""
+}
+```
+
+Where:
+
+- `"msg"` (string) is the message type and is
+always equal to `"dnsQuery"`;
+
+- `"rawQuery"` (base64 string) contains the
+raw DNS query in base64 encoding;
+
+- `"serverAddr"` (string) is the address of the
+server, whose format depends on the protocol;
+
+- `"serverProtocol"` (string) is the DNS protocol we
+are using for the query;
+
+- `"t"` (string) is the RFC3339 representation
+of the time right before sending the query.
+
+The current [dnscore](https://github.com/rbmk-project/dnscore)
+implementation uses [log/slog](https://pkg.go.dev/log/slog), which
+causes the generated message to contain additional fields that
+you can safely ignore when processing the message.
+
+
+## Response
+
+The JSON serialization of the response message contains
+*at least* the following fields:
+
+```JSON
+{
+  "msg":"dnsResponse",
+  "rawQuery":"",
+  "rawResponse":"",
+  "serverAddr":"",
+  "serverProtocol":"",
+  "t0":"",
+  "t":""
+}
+```
+
+Where:
+
+- `"msg"` (string) is the message type and is
+always equal to `"dnsQuery"`;
+
+- `"rawQuery"` (string) contains the
+raw DNS query in base64 encoding;
+
+- `"rawResponse"` (string) contains the
+raw DNS response in base64 encoding;
+
+- `"serverAddr"` (string) is the address of the
+server, whose format depends on the protocol;
+
+- `"serverProtocol"` (string) is the DNS protocol we
+are using for the query;
+
+- `"t0"` (string) is the RFC3339 representation of
+the time right before sending the query;
+
+- `"t"` (string) is the RFC3339 representation
+of the time when the response was received.
+
+The current [dnscore](https://github.com/rbmk-project/dnscore)
+implementation uses [log/slog](https://pkg.go.dev/log/slog), which
+causes the generated message to contain additional fields that
+you can safely ignore when processing the message.
+
+
+## Example
+
+Here is an example of a `"dnsQuery"` message:
+
+```JSON
+{
+  "msg":"dnsQuery",
+  "rawQuery":"yHUBAAABAAAAAAABA3d3dwdleGFtcGxlA2NvbQAAAQABAAApBNAAAAAAAAA=",
+  "serverAddr":"8.8.8.8:53",
+  "serverProtocol":"udp",
+  "t":"2024-11-18T15:31:53.05491+01:00",
+}
+```
+
+Here is an example of a `"dnsResponse"` message:
+
+```JSON
+{
+  "msg":"dnsResponse",
+  "rawQuery":"yHUBAAABAAAAAAABA3d3dwdleGFtcGxlA2NvbQAAAQABAAApBNAAAAAAAAA=",
+  "rawResponse":"yHWBgAABAAEAAAABA3d3dwdleGFtcGxlA2NvbQAAAQABwAwAAQABAAANVAAEXbjXDgAAKQIAAAAAAAAA",
+  "serverAddr":"8.8.8.8:53",
+  "serverProtocol":"udp",
+  "t0":"2024-11-18T15:31:53.05491+01:00",
+  "t":"2024-11-18T15:31:53.072107+01:00"
+}
+```