Architecture

nxdns is a self-hosted DNS sinkhole for a household LAN: one static Zig binary that answers DNS on UDP/TCP 53 (optionally DoH and DoT), filters against blocklists, and serves an embedded admin SPA over HTTP. This document is a map of the source tree and the few design rules that hold everywhere.

Module map

Top-level files:

File	Role
`src/main.zig`	Process shell: writers, argv, dispatch, exit code.
`src/cli.zig`	Every command body (`run`, `check`, `export`, `import`, `version`, `help`); takes its writers as parameters so tests capture output without a process.
`src/app.zig`	The composition root: everything `nxdns run` owns, built in order. Nothing else constructs a collaborator.
`src/version.zig`	Build-time version strings.
`src/tests.zig`	Test root; imports each file directly.

Directories:

Directory	Role
`src/dns/`	Pure DNS wire format: header, names, questions, records, whole packets, EDNS(0)/ECS (`edns.zig`), enums and limits (`types.zig`). No allocation, no `std.Io` beyond writing to a caller's writer.
`src/filter/`	Blocklist pipeline: line parsers (hosts, domains, ABP), the compiler that turns a downloaded list into `.list`/`.wild` bodies, `domain_set.zig` (exact-match set, no Bloom filter), `matcher.zig` (the immutable snapshot every query evaluates against), per-group `rules.zig`, `wildcard.zig`, `safesearch.zig`, blocked-response synthesis (`response.zig`). Two I/O edges live here too: `fetcher.zig` (HTTP download) and `manager.zig` (files + DB + snapshot swap).
`src/local/`	Local DNS records and conditional forward zones: immutable lookup tables built once from DB rows (`records.zig`, `forward_zones.zig`), plus the plain UDP/TCP client for LAN resolvers (`forward_client.zig`).
`src/cache/`	`dns_cache.zig`: bounded in-memory TTL cache of whole response messages, keyed by the question. The clock arrives as a parameter.
`src/upstream/`	Upstream resolution: shared vocabulary and the `Client` interface (`transport.zig`), DoH client (RFC 8484), DoT client (RFC 7858), per-endpoint health/backoff (`health.zig`), and `pool.zig` — priority-ordered sequential failover that is itself a `transport.Client`, so the handler sees one interface.
`src/server/`	The serving side: UDP/TCP/DoH/DoT listeners, `handler.zig` (the whole query pipeline), `cert_store.zig` (refcounted TLS cert holder), `rate_limiter.zig`, `pause.zig`, `clients.zig` (client auto-materialisation), `local_tables.zig` (published local-answer tables), `query_sink.zig` (log/SSE fanout), `shutdown.zig` (SIGINT/SIGTERM → one `std.Io.Event`).
`src/storage/`	SQLite ownership: `db.zig` is the only file that calls SQLite, `config_schema.zig` + `migrations.zig` for `config.db`, `querylog_schema.zig` (open-or-recreate), async query `logger.zig`, `retention.zig`, `disk_monitor.zig`, and one repository per table under `repositories/`.
`src/config/`	The one configuration model (`model.zig`), the pure validator (`validate.zig`), `import.zig`/`export.zig` (ZON ⇄ `config.db`, byte-stable round trip), `bootstrap.zig` (first-start seeding — a policy wrapper over import).
`src/web/`	The admin HTTP layer: `server.zig` (listener), `router.zig`/`routes.zig`, one file per resource under `handlers/`, `auth.zig` (sessions), `sse.zig` (live query fanout), `static.zig` (embedded SPA), `metrics.zig` (Prometheus), `openapi.zig` (served contract), `api_limiter.zig`, `http_util.zig`.
`src/platform/`	OS and TLS edges: IP address values, the `std.log` sink (`logging.zig`), `statfs.zig` (free-space query via libc), client TLS over `std.crypto.tls` (`tls_client.zig`), server TLS over vendored Mbed TLS (`tls_server.zig`).

The SPA source lives in web/ at the repo root; the build embeds its dist/ output as the web_assets module (-Dweb-dist).

main.zig ── cli.zig ── app.zig (composition root)
                          │ injects std.Io + collaborators
   ┌──────────────────────┴───────────────────────┐
   │  server/    web/    upstream/    storage/    │  I/O edge
   │  platform/  config/{import,export,bootstrap} │
   ├──────────────────────────────────────────────┤
   │  dns/   filter/*   local/*   cache/          │  pure core:
   │  config/{model,validate}                     │  bytes in, bytes out
   └──────────────────────────────────────────────┘
   * except filter/{fetcher,manager}.zig and local/forward_client.zig,
     which are those directories' named I/O edges

The purity rule

dns/, filter/, local/ and cache/ take bytes and return bytes: no std.Io, no sockets, no clocks hidden inside (AGENTS.md). Anything that needs a timestamp takes it as a parameter — the cache, the rate limiter and the pause flag all work this way, so every decision is testable without a backend. The exceptions are deliberate and few: filter/fetcher.zig downloads lists, filter/manager.zig owns the compiled files, the DB columns and the snapshot swap, and local/forward_client.zig speaks UDP/TCP to a LAN resolver. The decision path a query takes through these directories allocates nothing and opens nothing.

std.Io injection

There is one std.Io in the process. main receives it through std.process.Init — on the standard start path this is the Threaded backend (std.Io.Threaded, constructed in the stdlib's start code) — and hands it to cli.Runner, from which app.zig threads it into every collaborator as a parameter. No module constructs its own event loop or reads an ambient clock; tests build their own std.Io.Threaded instance and pass it the same way. The one deliberate exception is storage/db.zig: SQLite performs its own file I/O through its VFS, so that file takes no std.Io at all.

Life of one query

The pipeline in src/server/handler.zig (its order is PLAN §4; the stages below are the code's actual call chain — Handler.handle then Context.run):

UDP/53  TCP/53  DoH  DoT          (src/server/{udp,tcp,doh,dot}_server.zig)
   └──────┴──────┴────┘
          │ raw query bytes, listener-owned buffers
          ▼
  handler.handle
     ├─ header parse (too short / QR set → counted drop)
     ├─ rate limit (over budget → REFUSED)
     ├─ packet + EDNS validation (FORMERR / NOTIMP)
     ├─ client tracking, group lookup (snapshot.groupForClient)
     │
     ├─ local records ────────────────► authoritative answer
     ├─ forward zones ─► cache ─► LAN resolver ─► answer
     │
     └─ upstream path
          ├─ filter snapshot evaluate ─► blocked? synthesized block reply
          ├─ safe-search rewrite (per group)
          ├─ cache get ─► hit? answer
          ├─ upstream pool: priority failover across DoH/DoT endpoints
          ├─ CNAME uncloak: walk the answer's chain, re-evaluate each target
          └─ cache put
          ▼
  reply bytes ─► listener sends
          │
          └─► QuerySink ─► SSE hub (GET /api/queries/live)
                        └─► async logger ─► querylog.db

Local records win over forward zones, and both win over filtering: a name nxdns answers itself never reaches a blocklist. Pause suspends filtering only; local records, forward zones, cache, upstream and the query log keep running. handle returns no error union — every failure is either a DNS response the client can act on or a counted drop. The query path never waits on the database: QuerySink copies the entry, the SSE hub gets it first, and one writer task owns the querylog.db handle behind an std.Io.Queue.

Storage

Two databases with opposite contracts:

config.db is the truth. Schema DDL is carried verbatim by migrations.zig as step 1; a schema change is a new migration step, applied inside one transaction. nxdns import replaces its whole content atomically (BEGIN IMMEDIATE; a failed import changes nothing), nxdns export renders it back as canonical ZON, byte-identical across round trips. A config file seeds the DB exactly once at first start (config/bootstrap.zig); the DB is truth thereafter.
querylog.db is expendable. It is never migrated: its schema carries a fingerprint derived from the DDL text, and a mismatch at open replaces the file (storage/querylog_schema.zig). Retention deletes old rows daily and periodically rewrites the file; config.db is walled off from that churn.

Web stack

web/server.zig runs one std.http.Server per connection over its own accept loop, with fixed pre-allocated connection slots, optionally behind TLS. The SPA is embedded at build time: static.zig serves the web_assets module — bytes, content type, strong ETag, and a pre-compressed .gz sibling where it paid off — via a linear scan, no filesystem at runtime. GET /api/queries/live is server-sent events over chunked transfer, fed by the same QuerySink the logger reads. Routing is a flat table (routes.zig) matched linearly; the OpenAPI YAML is hand-written, embedded and served at GET /api/openapi.yaml, kept honest by tests that assert every served route appears in it.

Auth (web/auth.zig): the operator's password is verified against an argon2id PHC string (web.password_hash — the plaintext is hashed on import and never stored). A successful login mints a 256-bit token carried in a cookie; the in-memory session table holds only SHA-256 digests of tokens, compared in constant time, capped at 32 sessions with LRU eviction. Nothing is persisted — a restart logs everyone out. Monitoring endpoints (health, version, metrics), the served OpenAPI contract and login itself are unauthenticated; everything else requires the cookie, and the API has its own token-bucket rate limiter.

DoH, DoT and certificate hot-reload

server/doh_server.zig (RFC 8484 over HTTP/1.1 + TLS) and server/dot_server.zig (RFC 7858) mirror the plain listeners' shape. Server TLS terminates in Mbed TLS (platform/tls_server.zig), exposing plaintext as std.Io.Reader/std.Io.Writer.

Certificates hot-reload through server/cert_store.zig: one refcounted CertStore per endpoint owns the published TLS context generation; listeners acquire it per connection and release it when the connection ends, so a reload never frees a context mid-handshake. Reload publishes nothing on failure — both PEM files are read and a whole new context built before anything swaps, and any failure leaves the old generation serving. A watcher polls mtime+size of both files every 30 s; POST /api/certs/reload triggers the same path on demand and reports the per-endpoint outcome as its payload.

Failure visibility

Every failure mode must be visible, and the surface is counters, not log lines (AGENTS.md). The handler counts every outcome — drops, FORMERR, SERVFAIL, blocked, truncated, cache hits, paused and unfiltered queries — in atomics; listeners count dropped datagrams instead of queueing them unboundedly. GET /metrics renders all of it as Prometheus text 0.0.4, and GET /api/health rolls it up for a monitor (always 200: "degraded" is a fact about the box, not a failed request). The disk monitor classifies free space against thresholds and gates non-essential writes; the query logger holds its batches while writes are disallowed. std.log is reserved for failures nobody else records, with upstream-error deduplication so a flapping resolver cannot fill a disk.

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