Initial commit

.gitignore

@@ -0,0 +1,2 @@
+checker-tls
+checker-tls.so

Dockerfile

@@ -0,0 +1,14 @@
+FROM golang:1.25-alpine AS builder
+
+ARG CHECKER_VERSION=custom-build
+
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN CGO_ENABLED=0 go build -ldflags "-X main.Version=${CHECKER_VERSION}" -o /checker-tls .
+
+FROM scratch
+COPY --from=builder /checker-tls /checker-tls
+EXPOSE 8080
+ENTRYPOINT ["/checker-tls"]

LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2026 The happyDomain Authors
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the “Software”), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

Makefile

@@ -0,0 +1,25 @@
+CHECKER_NAME := checker-tls
+CHECKER_IMAGE := happydomain/$(CHECKER_NAME)
+CHECKER_VERSION ?= custom-build
+
+CHECKER_SOURCES := main.go $(wildcard checker/*.go)
+
+GO_LDFLAGS := -X main.Version=$(CHECKER_VERSION)
+
+.PHONY: all plugin docker clean
+
+all: $(CHECKER_NAME)
+
+$(CHECKER_NAME): $(CHECKER_SOURCES)
+	go build -ldflags "$(GO_LDFLAGS)" -o $@ .
+
+plugin: $(CHECKER_NAME).so
+
+$(CHECKER_NAME).so: $(CHECKER_SOURCES) $(wildcard plugin/*.go)
+	go build -buildmode=plugin -ldflags "$(GO_LDFLAGS)" -o $@ ./plugin/
+
+docker:
+	docker build --build-arg CHECKER_VERSION=$(CHECKER_VERSION) -t $(CHECKER_IMAGE) .
+
+clean:
+	rm -f $(CHECKER_NAME) $(CHECKER_NAME).so

NOTICE

@@ -0,0 +1,26 @@
+checker-dummy
+Copyright (c) 2026 The happyDomain Authors
+
+This product is licensed under the MIT License (see LICENSE).
+
+-------------------------------------------------------------------------------
+Third-party notices
+-------------------------------------------------------------------------------
+
+This product includes software developed as part of the checker-sdk-go
+project (https://git.happydns.org/happyDomain/checker-sdk-go), licensed
+under the Apache License, Version 2.0:
+
+    checker-sdk-go
+    Copyright 2020-2026 The happyDomain Authors
+
+    This product includes software developed as part of the happyDomain
+    project (https://happydomain.org).
+
+    Portions of this code were originally written for the happyDomain
+    server (licensed under AGPL-3.0 and a commercial license) and are
+    made available there under the Apache License, Version 2.0 to enable
+    a permissively licensed ecosystem of checker plugins.
+
+You may obtain a copy of the Apache License 2.0 at:
+    http://www.apache.org/licenses/LICENSE-2.0

README.md

@@ -0,0 +1,153 @@
+# checker-tls
+
+TLS posture checker for happyDomain.
+
+Consumes `DiscoveryEntry` records of type `tls.endpoint.v1` published by
+service checkers (xmpp, srv, caldav, carddav, …), performs a real TCP
+dial, optional protocol-specific STARTTLS upgrade, and TLS handshake on
+each, and exports per-endpoint posture under the observation key
+`tls_probes`.
+
+## For producers: the `contract` package
+
+Service checkers that want TLS probing on the endpoints they discover
+should depend on `git.happydns.org/checker-tls/contract` and emit
+`DiscoveryEntry` records through it. The contract is a tiny
+producer↔consumer package; it has no dependency on checker-tls itself
+beyond the SDK.
+
+```go
+import (
+    sdk "git.happydns.org/checker-sdk-go/checker"
+    tlsct "git.happydns.org/checker-tls/contract"
+)
+
+// DiscoverEntries is the sdk.DiscoveryPublisher hook on your provider.
+func (p *xmppProvider) DiscoverEntries(data any) ([]sdk.DiscoveryEntry, error) {
+    d := data.(*XMPPData)
+    var out []sdk.DiscoveryEntry
+    for _, srv := range d.Client {
+        e, err := tlsct.NewEntry(tlsct.TLSEndpoint{
+            Host:            srv.Target,
+            Port:            srv.Port,
+            SNI:             d.Domain, // only when it differs from Host
+            STARTTLS:        "xmpp-client",
+            RequireSTARTTLS: true,
+        })
+        if err != nil {
+            return nil, err
+        }
+        out = append(out, e)
+    }
+    return out, nil
+}
+```
+
+### `TLSEndpoint` fields
+
+| Field             | Meaning                                                                 |
+| ----------------- | ----------------------------------------------------------------------- |
+| `Host`            | Target hostname (trailing dot tolerated).                               |
+| `Port`            | TCP port.                                                               |
+| `SNI`             | TLS SNI; leave empty when equal to `Host`.                              |
+| `STARTTLS`        | SRV service name of the upgrade protocol; empty for direct TLS.         |
+| `RequireSTARTTLS` | `true` when absence of STARTTLS must be reported crit, not opportunistic. |
+
+### Helpers
+
+| Symbol                   | Role                                                                           |
+| ------------------------ | ------------------------------------------------------------------------------ |
+| `contract.Type`          | The `DiscoveryEntry.Type` string (`"tls.endpoint.v1"`).                        |
+| `contract.NewEntry(ep)`  | Builds a `DiscoveryEntry` with a deterministic `Ref` and marshaled payload.    |
+| `contract.Ref(ep)`       | Exposes the Ref derivation so producers can reference it ahead of time.        |
+| `contract.ParseEntry(e)` | Decodes a single entry; errors on wrong `Type` or malformed payload.           |
+| `contract.ParseEntries(s)` | Filters a slice to this contract, decodes each, returns warnings for the malformed. |
+
+The `Ref` is a deterministic 16-hex-digit hash of
+`(Host, Port, effective SNI, STARTTLS proto, RequireSTARTTLS)`. It
+appears as the key in `tls_probes.probes` and as the value consumers
+will see in any future `RelatedObservation.Ref` field.
+
+### Supported STARTTLS protocols
+
+| `TLSEndpoint.STARTTLS` | Upgrade performed                              |
+| ---------------------- | ---------------------------------------------- |
+| *(empty)*              | Direct TLS on connect                          |
+| `smtp`                 | ESMTP EHLO + STARTTLS (RFC 3207)               |
+| `submission`           | Same as `smtp`                                 |
+| `imap`                 | IMAP CAPABILITY + STARTTLS (RFC 3501)          |
+| `pop3`                 | POP3 CAPA + STLS (RFC 2595)                    |
+| `xmpp-client`          | XMPP c2s stream + `<starttls/>` (RFC 6120)     |
+| `xmpp-server`          | XMPP s2s stream + `<starttls/>` (RFC 6120)     |
+| any other value        | Probe reports a `handshake_failed` issue       |
+
+The protocol table lives in `checker/starttls.go`; new entries are added
+there, not in this repo's consumers.
+
+### Versioning
+
+`DiscoveryEntry.Type` ends in `.v1`. If a future schema adds a field the
+consumer can't ignore safely, this package will introduce
+`tls.endpoint.v2` alongside a new `TLSEndpoint` type, and old consumers
+will silently skip v2 entries (they do not match the expected `Type`).
+
+## Observation payload
+
+Observation data written under `tls_probes`:
+
+```json
+{
+  "probes": {
+    "<ref>": {
+      "host": "example.net",
+      "port": 5222,
+      "endpoint": "example.net:5222",
+      "type": "starttls-xmpp-client",
+      "sni": "example.net",
+      "tls_version": "TLS1.3",
+      "cipher_suite": "TLS_AES_128_GCM_SHA256",
+      "hostname_match": true,
+      "chain_valid": true,
+      "not_after": "2026-07-01T00:00:00Z",
+      "issuer": "Let's Encrypt",
+      "issues": []
+    }
+  },
+  "collected_at": "2026-04-21T12:34:56Z"
+}
+```
+
+The map is keyed by `contract.Ref(ep)` — the same value the host exposes
+on the lineage side so that a consumer knows which probe corresponds to
+which entry it originally published.
+
+The `type` field inside each probe preserves the human-readable
+`"tls"` / `"starttls-<proto>"` shape for backward compatibility with
+existing downstream parsers.
+
+## Issues reported
+
+- `tcp_unreachable` — dial failed.
+- `handshake_failed` — TLS handshake or STARTTLS upgrade failed.
+- `starttls_not_offered` — server didn't advertise STARTTLS. Severity is
+  `crit` when `TLSEndpoint.RequireSTARTTLS` is `true`, `warn` otherwise.
+- `chain_invalid` — leaf does not chain to a system-trusted root.
+- `hostname_mismatch` — cert SANs don't cover the SNI.
+- `expired` / `expiring_soon` — cert expiry posture.
+- `weak_tls_version` — negotiated TLS < 1.2.
+
+## Options
+
+| Id               | Type   | Default | Description                                  |
+| ---------------- | ------ | ------- | -------------------------------------------- |
+| `probeTimeoutMs` | number | 10000   | Per-endpoint dial + handshake timeout in ms. |
+
+## Running
+
+```bash
+# Plugin (loaded by happyDomain at startup)
+make plugin
+
+# Standalone HTTP server
+make && ./checker-tls -listen :8080
+```

checker/collect.go

@@ -0,0 +1,63 @@
+package checker
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"sync"
+	"time"
+
+	sdk "git.happydns.org/checker-sdk-go/checker"
+	"git.happydns.org/checker-tls/contract"
+)
+
+func (p *tlsProvider) Collect(ctx context.Context, opts sdk.CheckerOptions) (any, error) {
+	raw, ok := sdk.GetOption[[]sdk.DiscoveryEntry](opts, OptionEndpoints)
+	if !ok {
+		return nil, fmt.Errorf("no discovery entries in options: did the host wire AutoFillDiscoveryEntries?")
+	}
+
+	timeoutMs := sdk.GetIntOption(opts, OptionProbeTimeoutMs, DefaultProbeTimeoutMs)
+	if timeoutMs <= 0 {
+		timeoutMs = DefaultProbeTimeoutMs
+	}
+	timeout := time.Duration(timeoutMs) * time.Millisecond
+
+	entries, warnings := contract.ParseEntries(raw)
+	for _, w := range warnings {
+		log.Printf("checker-tls: discarding malformed entry: %v", w)
+	}
+	// An empty entry set is not an error: it is the steady state on any
+	// target where no producer has published yet, and the first run after
+	// a fresh publication when the producer hasn't finished its own cycle.
+	// The rule surfaces this as StatusUnknown rather than StatusError so a
+	// freshly-enrolled domain doesn't flap red.
+	if len(entries) == 0 {
+		return &TLSData{Probes: map[string]TLSProbe{}, CollectedAt: time.Now()}, nil
+	}
+
+	probes := make(map[string]TLSProbe, len(entries))
+	var mu sync.Mutex
+	var wg sync.WaitGroup
+	sem := make(chan struct{}, MaxConcurrentProbes)
+	for _, e := range entries {
+		wg.Add(1)
+		sem <- struct{}{}
+		go func() {
+			defer wg.Done()
+			defer func() { <-sem }()
+			pr := probe(ctx, e.Endpoint, timeout)
+			log.Printf("checker-tls: %s %s:%d → tls=%s issues=%d elapsed=%dms err=%q",
+				pr.Type, pr.Host, pr.Port, pr.TLSVersion, len(pr.Issues), pr.ElapsedMS, pr.Error)
+			mu.Lock()
+			probes[e.Ref] = pr
+			mu.Unlock()
+		}()
+	}
+	wg.Wait()
+
+	return &TLSData{
+		Probes:      probes,
+		CollectedAt: time.Now(),
+	}, nil
+}

checker/definition.go

@@ -0,0 +1,52 @@
+package checker
+
+import (
+	"time"
+
+	sdk "git.happydns.org/checker-sdk-go/checker"
+)
+
+// Version defaults to "built-in"; standalone and plugin builds override it via
+// -ldflags "-X .../checker.Version=...".
+var Version = "built-in"
+
+// Definition returns the CheckerDefinition for the TLS checker.
+func (p *tlsProvider) Definition() *sdk.CheckerDefinition {
+	return &sdk.CheckerDefinition{
+		ID:      "tls",
+		Name:    "TLS",
+		Version: Version,
+		Availability: sdk.CheckerAvailability{
+			ApplyToDomain: true,
+		},
+		ObservationKeys: []sdk.ObservationKey{ObservationKeyTLSProbes},
+		Options: sdk.CheckerOptionsDocumentation{
+			UserOpts: []sdk.CheckerOptionDocumentation{
+				{
+					Id:          OptionProbeTimeoutMs,
+					Type:        "number",
+					Label:       "Per-endpoint probe timeout (ms)",
+					Description: "Maximum time allowed for dial + STARTTLS + TLS handshake on a single endpoint.",
+					Default:     float64(DefaultProbeTimeoutMs),
+				},
+			},
+			RunOpts: []sdk.CheckerOptionDocumentation{
+				{
+					Id:          OptionEndpoints,
+					Label:       "Discovery entries",
+					Description: "Entries published by other checkers for this domain; this checker decodes the tls.endpoint.v1 contract and ignores the rest.",
+					AutoFill:    sdk.AutoFillDiscoveryEntries,
+					Hide:        true,
+				},
+			},
+		},
+		Rules: []sdk.CheckRule{
+			Rule(),
+		},
+		Interval: &sdk.CheckIntervalSpec{
+			Min:     6 * time.Hour,
+			Max:     7 * 24 * time.Hour,
+			Default: 24 * time.Hour,
+		},
+	}
+}

checker/prober.go

@@ -0,0 +1,231 @@
+package checker
+
+import (
+	"context"
+	"crypto/tls"
+	"crypto/x509"
+	"errors"
+	"fmt"
+	"net"
+	"strconv"
+	"strings"
+	"time"
+
+	"git.happydns.org/checker-tls/contract"
+)
+
+// probeTypeString renders the TLSProbe.Type string from a TLSEndpoint.
+// Observation consumers already parse this field in its "tls" /
+// "starttls-<proto>" shape; the contract-level split of direct vs.
+// STARTTLS is collapsed back here so the wire format of tls_probes
+// stays unchanged.
+func probeTypeString(ep contract.TLSEndpoint) string {
+	if ep.STARTTLS == "" {
+		return "tls"
+	}
+	return "starttls-" + ep.STARTTLS
+}
+
+// probe performs a TLS handshake (or STARTTLS upgrade + handshake) on the
+// given endpoint and returns a populated TLSProbe. It never returns an error:
+// transport/handshake failures are recorded on the probe so the caller can
+// still surface them in the report.
+func probe(ctx context.Context, ep contract.TLSEndpoint, timeout time.Duration) TLSProbe {
+	start := time.Now()
+	host := strings.TrimSuffix(ep.Host, ".")
+	addr := net.JoinHostPort(host, strconv.Itoa(int(ep.Port)))
+	sni := ep.SNI
+	if sni == "" {
+		sni = host
+	}
+
+	p := TLSProbe{
+		Host:     host,
+		Port:     ep.Port,
+		Endpoint: addr,
+		Type:     probeTypeString(ep),
+		SNI:      sni,
+	}
+
+	dialCtx, cancel := context.WithTimeout(ctx, timeout)
+	defer cancel()
+
+	d := &net.Dialer{}
+	conn, err := d.DialContext(dialCtx, "tcp", addr)
+	if err != nil {
+		p.Error = "dial: " + err.Error()
+		p.Issues = append(p.Issues, Issue{
+			Code:     "tcp_unreachable",
+			Severity: SeverityCrit,
+			Message:  fmt.Sprintf("Cannot open TCP connection to %s: %v", addr, err),
+			Fix:      "Check DNS, firewall, and that the service listens on this port.",
+		})
+		p.ElapsedMS = time.Since(start).Milliseconds()
+		return p
+	}
+	defer conn.Close()
+
+	if deadline, ok := dialCtx.Deadline(); ok {
+		_ = conn.SetDeadline(deadline)
+	}
+
+	tlsConn, err := handshake(conn, ep, sni)
+	if err != nil {
+		p.Error = err.Error()
+		p.Issues = append(p.Issues, classifyHandshakeError(ep, err))
+		p.ElapsedMS = time.Since(start).Milliseconds()
+		return p
+	}
+	defer tlsConn.Close()
+
+	state := tlsConn.ConnectionState()
+	p.TLSVersion = tls.VersionName(state.Version)
+	p.CipherSuite = tls.CipherSuiteName(state.CipherSuite)
+
+	if len(state.PeerCertificates) == 0 {
+		p.Issues = append(p.Issues, Issue{
+			Code:     "no_peer_cert",
+			Severity: SeverityCrit,
+			Message:  "Server presented no certificate.",
+		})
+		p.ElapsedMS = time.Since(start).Milliseconds()
+		return p
+	}
+
+	leaf := state.PeerCertificates[0]
+	p.NotAfter = leaf.NotAfter
+	p.Issuer = leaf.Issuer.CommonName
+	p.Subject = leaf.Subject.CommonName
+	p.DNSNames = append(p.DNSNames, leaf.DNSNames...)
+
+	hostnameMatch := leaf.VerifyHostname(sni) == nil
+	p.HostnameMatch = &hostnameMatch
+
+	// Chain verification against system roots, using intermediates presented
+	// by the server. We run this independently from Go's tls.Config
+	// verification so we can report a dedicated "chain invalid" issue rather
+	// than failing the whole handshake.
+	intermediates := x509.NewCertPool()
+	for _, c := range state.PeerCertificates[1:] {
+		intermediates.AddCert(c)
+	}
+	now := time.Now()
+	_, verifyErr := leaf.Verify(x509.VerifyOptions{
+		DNSName:       sni,
+		Intermediates: intermediates,
+		CurrentTime:   now,
+	})
+	chainValid := verifyErr == nil
+	p.ChainValid = &chainValid
+	if !chainValid {
+		msg := "Invalid certificate chain"
+		if verifyErr != nil {
+			msg = "Invalid certificate chain: " + verifyErr.Error()
+		}
+		p.Issues = append(p.Issues, Issue{
+			Code:     "chain_invalid",
+			Severity: SeverityCrit,
+			Message:  msg,
+			Fix:      "Serve the full intermediate chain and ensure the root is trusted.",
+		})
+	}
+	if !hostnameMatch {
+		p.Issues = append(p.Issues, Issue{
+			Code:     "hostname_mismatch",
+			Severity: SeverityCrit,
+			Message:  fmt.Sprintf("Certificate does not cover %q (SANs: %s)", sni, strings.Join(leaf.DNSNames, ", ")),
+			Fix:      "Re-issue the certificate with a matching SAN.",
+		})
+	}
+	if leaf.NotAfter.Before(now) {
+		p.Issues = append(p.Issues, Issue{
+			Code:     "expired",
+			Severity: SeverityCrit,
+			Message:  "Certificate expired on " + leaf.NotAfter.Format(time.RFC3339),
+			Fix:      "Renew the certificate.",
+		})
+	} else if leaf.NotAfter.Sub(now) < 14*24*time.Hour {
+		p.Issues = append(p.Issues, Issue{
+			Code:     "expiring_soon",
+			Severity: SeverityWarn,
+			Message:  "Certificate expires in less than 14 days (" + leaf.NotAfter.Format(time.RFC3339) + ")",
+			Fix:      "Renew before expiry.",
+		})
+	}
+	if state.Version < tls.VersionTLS12 {
+		p.Issues = append(p.Issues, Issue{
+			Code:     "weak_tls_version",
+			Severity: SeverityWarn,
+			Message:  "Negotiated TLS version " + p.TLSVersion + " is below the recommended TLS 1.2.",
+			Fix:      "Disable TLS 1.0/1.1 on the server.",
+		})
+	}
+
+	p.ElapsedMS = time.Since(start).Milliseconds()
+	return p
+}
+
+// handshake performs STARTTLS upgrade (when ep.STARTTLS is non-empty) and
+// then a TLS handshake. InsecureSkipVerify is true on purpose: we verify
+// the chain separately in probe so an invalid chain becomes a structured
+// Issue rather than aborting the handshake.
+func handshake(conn net.Conn, ep contract.TLSEndpoint, sni string) (*tls.Conn, error) {
+	cfg := &tls.Config{
+		ServerName:         sni,
+		InsecureSkipVerify: true,
+	}
+
+	if ep.STARTTLS == "" {
+		tlsConn := tls.Client(conn, cfg)
+		if err := tlsConn.Handshake(); err != nil {
+			return nil, fmt.Errorf("tls-handshake: %w", err)
+		}
+		return tlsConn, nil
+	}
+
+	up, ok := starttlsUpgraders[ep.STARTTLS]
+	if !ok {
+		return nil, fmt.Errorf("unsupported starttls protocol %q", ep.STARTTLS)
+	}
+	if err := up(conn, sni); err != nil {
+		return nil, fmt.Errorf("starttls-%s: %w", ep.STARTTLS, err)
+	}
+	tlsConn := tls.Client(conn, cfg)
+	if err := tlsConn.Handshake(); err != nil {
+		return nil, fmt.Errorf("tls-handshake-after-starttls: %w", err)
+	}
+	return tlsConn, nil
+}
+
+// classifyHandshakeError converts a dial/handshake error into a structured
+// Issue, distinguishing "server doesn't offer STARTTLS" (which is opportunistic
+// for some endpoints) from hard failures.
+func classifyHandshakeError(ep contract.TLSEndpoint, err error) Issue {
+	msg := err.Error()
+
+	if ep.STARTTLS != "" && isStartTLSUnsupported(err) {
+		sev := SeverityWarn
+		if ep.RequireSTARTTLS {
+			sev = SeverityCrit
+		}
+		return Issue{
+			Code:     "starttls_not_offered",
+			Severity: sev,
+			Message:  fmt.Sprintf("Server on %s:%d does not advertise STARTTLS: %s", ep.Host, ep.Port, msg),
+			Fix:      "Enable STARTTLS on the server or publish a direct-TLS endpoint.",
+		}
+	}
+
+	return Issue{
+		Code:     "handshake_failed",
+		Severity: SeverityCrit,
+		Message:  fmt.Sprintf("TLS handshake failed on %s:%d: %s", ep.Host, ep.Port, msg),
+		Fix:      "Inspect the server's TLS configuration and certificate.",
+	}
+}
+
+var errStartTLSNotOffered = errors.New("starttls not advertised by server")
+
+func isStartTLSUnsupported(err error) bool {
+	return errors.Is(err, errStartTLSNotOffered)
+}

checker/prober_test.go

@@ -0,0 +1,95 @@
+package checker
+
+import (
+	"context"
+	"net"
+	"net/http/httptest"
+	"net/url"
+	"strconv"
+	"testing"
+	"time"
+
+	"git.happydns.org/checker-tls/contract"
+)
+
+func TestProbe_DirectTLS_OK(t *testing.T) {
+	srv := httptest.NewTLSServer(nil)
+	defer srv.Close()
+
+	u, _ := url.Parse(srv.URL)
+	host, portStr, _ := net.SplitHostPort(u.Host)
+	port, _ := strconv.ParseUint(portStr, 10, 16)
+
+	probe := probe(context.Background(), contract.TLSEndpoint{
+		Host: host,
+		Port: uint16(port),
+		SNI:  host,
+	}, 5*time.Second)
+
+	if probe.Error != "" {
+		t.Fatalf("unexpected error: %s", probe.Error)
+	}
+	if probe.TLSVersion == "" {
+		t.Errorf("expected TLSVersion, got empty")
+	}
+	if probe.CipherSuite == "" {
+		t.Errorf("expected CipherSuite, got empty")
+	}
+	if probe.ChainValid == nil || *probe.ChainValid {
+		t.Errorf("httptest self-signed chain should NOT be valid (chain_valid=%v)", probe.ChainValid)
+	}
+	if probe.HostnameMatch == nil {
+		t.Errorf("expected HostnameMatch to be populated")
+	}
+	if probe.NotAfter.IsZero() {
+		t.Errorf("expected NotAfter populated")
+	}
+}
+
+func TestProbe_TCPUnreachable(t *testing.T) {
+	// Grab a free port then immediately close it so we know nothing listens.
+	l, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	addr := l.Addr().(*net.TCPAddr)
+	_ = l.Close()
+
+	probe := probe(context.Background(), contract.TLSEndpoint{
+		Host: "127.0.0.1",
+		Port: uint16(addr.Port),
+	}, 1*time.Second)
+
+	if probe.Error == "" {
+		t.Errorf("expected an error for unreachable port")
+	}
+	if len(probe.Issues) == 0 || probe.Issues[0].Code != "tcp_unreachable" {
+		t.Errorf("expected tcp_unreachable issue, got %+v", probe.Issues)
+	}
+}
+
+func TestProbe_UnsupportedStartTLSProto(t *testing.T) {
+	// Listen so the dial succeeds, but the type maps to an unknown proto.
+	l, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer l.Close()
+	go func() {
+		c, err := l.Accept()
+		if err == nil {
+			c.Close()
+		}
+	}()
+
+	addr := l.Addr().(*net.TCPAddr)
+	probe := probe(context.Background(), contract.TLSEndpoint{
+		Host:     "127.0.0.1",
+		Port:     uint16(addr.Port),
+		STARTTLS: "totallyfake",
+	}, 2*time.Second)
+
+	if probe.Error == "" {
+		t.Errorf("expected handshake error for unsupported starttls protocol")
+	}
+}

checker/provider.go

@@ -0,0 +1,14 @@
+package checker
+
+import sdk "git.happydns.org/checker-sdk-go/checker"
+
+// Provider returns a new TLS observation provider.
+func Provider() sdk.ObservationProvider {
+	return &tlsProvider{}
+}
+
+type tlsProvider struct{}
+
+func (p *tlsProvider) Key() sdk.ObservationKey {
+	return ObservationKeyTLSProbes
+}

checker/rule.go

@@ -0,0 +1,137 @@
+package checker
+
+import (
+	"context"
+	"fmt"
+
+	sdk "git.happydns.org/checker-sdk-go/checker"
+)
+
+// Rule returns the rule that aggregates per-endpoint TLS probe outcomes into
+// a single status for this checker run.
+func Rule() sdk.CheckRule {
+	return &tlsRule{}
+}
+
+type tlsRule struct{}
+
+func (r *tlsRule) Name() string { return "tls_posture" }
+
+func (r *tlsRule) Description() string {
+	return "Summarises TLS handshake, certificate validity, hostname match and expiry across all probed endpoints"
+}
+
+func (r *tlsRule) ValidateOptions(opts sdk.CheckerOptions) error {
+	return nil
+}
+
+func (r *tlsRule) Evaluate(ctx context.Context, obs sdk.ObservationGetter, opts sdk.CheckerOptions) sdk.CheckState {
+	var data TLSData
+	if err := obs.Get(ctx, ObservationKeyTLSProbes, &data); err != nil {
+		return sdk.CheckState{
+			Status:  sdk.StatusError,
+			Message: fmt.Sprintf("Failed to read tls_probes: %v", err),
+			Code:    "tls_observation_error",
+		}
+	}
+
+	var (
+		total     = len(data.Probes)
+		okCount   int
+		warnCount int
+		critCount int
+		firstCrit string
+		firstWarn string
+	)
+
+	// Steady state when no producer has published entries for this target
+	// yet (or when the last producer run cleared them). Report Unknown so
+	// we don't flap red during the eventual-consistency window between a
+	// fresh enrollment and the first producer cycle.
+	if total == 0 {
+		return sdk.CheckState{
+			Status:  sdk.StatusUnknown,
+			Message: "No TLS endpoints have been discovered for this target yet",
+			Code:    "tls_no_endpoints",
+		}
+	}
+	for _, p := range data.Probes {
+		worst, critMsg, warnMsg := summarize(p.Issues)
+		switch worst {
+		case SeverityCrit:
+			critCount++
+			if firstCrit == "" {
+				firstCrit = fmt.Sprintf("%s (%s)", p.Endpoint, critMsg)
+			}
+		case SeverityWarn:
+			warnCount++
+			if firstWarn == "" {
+				firstWarn = fmt.Sprintf("%s (%s)", p.Endpoint, warnMsg)
+			}
+		default:
+			okCount++
+		}
+	}
+
+	meta := map[string]any{
+		"probes": total,
+		"ok":     okCount,
+		"warn":   warnCount,
+		"crit":   critCount,
+	}
+
+	switch {
+	case critCount > 0:
+		return sdk.CheckState{
+			Status:  sdk.StatusCrit,
+			Message: fmt.Sprintf("%d/%d TLS endpoint(s) have critical issues: %s", critCount, total, firstCrit),
+			Code:    "tls_critical",
+			Meta:    meta,
+		}
+	case warnCount > 0:
+		return sdk.CheckState{
+			Status:  sdk.StatusWarn,
+			Message: fmt.Sprintf("%d/%d TLS endpoint(s) have warnings: %s", warnCount, total, firstWarn),
+			Code:    "tls_warning",
+			Meta:    meta,
+		}
+	default:
+		return sdk.CheckState{
+			Status:  sdk.StatusOK,
+			Message: fmt.Sprintf("%d TLS endpoint(s) OK", total),
+			Code:    "tls_ok",
+			Meta:    meta,
+		}
+	}
+}
+
+// summarize walks the issues once and returns (worst severity, first
+// critical message, first warning message). Picking the messages during the
+// same pass avoids a second iteration in the caller.
+func summarize(issues []Issue) (worst, firstCrit, firstWarn string) {
+	for _, is := range issues {
+		msg := is.Message
+		if msg == "" {
+			msg = is.Code
+		}
+		switch is.Severity {
+		case SeverityCrit:
+			worst = SeverityCrit
+			if firstCrit == "" {
+				firstCrit = msg
+			}
+		case SeverityWarn:
+			if worst == "" || worst == SeverityInfo {
+				worst = SeverityWarn
+			}
+			if firstWarn == "" {
+				firstWarn = msg
+			}
+		case SeverityInfo:
+			if worst == "" {
+				worst = SeverityInfo
+			}
+		}
+	}
+	return
+}

checker/starttls.go

@@ -0,0 +1,281 @@
+package checker
+
+import (
+	"bufio"
+	"encoding/xml"
+	"fmt"
+	"io"
+	"net"
+	"strings"
+)
+
+// starttlsUpgrader performs the plaintext portion of a STARTTLS upgrade on
+// conn, leaving conn ready for tls.Client(conn, …).Handshake(). On success
+// the returned function returns nil; on failure it returns a descriptive
+// error (wrap errStartTLSNotOffered when the server advertises no STARTTLS).
+type starttlsUpgrader func(conn net.Conn, sni string) error
+
+var starttlsUpgraders = map[string]starttlsUpgrader{
+	"smtp":        starttlsSMTP,
+	"submission":  starttlsSMTP,
+	"imap":        starttlsIMAP,
+	"pop3":        starttlsPOP3,
+	"xmpp-client": starttlsXMPPClient,
+	"xmpp-server": starttlsXMPPServer,
+}
+
+// starttlsSMTP implements ESMTP EHLO + STARTTLS (RFC 3207).
+func starttlsSMTP(conn net.Conn, sni string) error {
+	rw := bufio.NewReadWriter(bufio.NewReader(conn), bufio.NewWriter(conn))
+
+	if err := readSMTPGreeting(rw.Reader); err != nil {
+		return fmt.Errorf("read greeting: %w", err)
+	}
+
+	if _, err := rw.WriteString("EHLO checker.happydomain.org\r\n"); err != nil {
+		return fmt.Errorf("write ehlo: %w", err)
+	}
+	if err := rw.Flush(); err != nil {
+		return fmt.Errorf("flush ehlo: %w", err)
+	}
+	lines, err := readSMTPResponse(rw.Reader)
+	if err != nil {
+		return fmt.Errorf("read ehlo: %w", err)
+	}
+	if !hasSTARTTLSExt(lines) {
+		return fmt.Errorf("%w: EHLO did not advertise STARTTLS", errStartTLSNotOffered)
+	}
+
+	if _, err := rw.WriteString("STARTTLS\r\n"); err != nil {
+		return fmt.Errorf("write starttls: %w", err)
+	}
+	if err := rw.Flush(); err != nil {
+		return fmt.Errorf("flush starttls: %w", err)
+	}
+	resp, err := readSMTPResponse(rw.Reader)
+	if err != nil {
+		return fmt.Errorf("read starttls: %w", err)
+	}
+	if len(resp) == 0 || !strings.HasPrefix(resp[0], "220") {
+		return fmt.Errorf("server refused STARTTLS: %s", strings.Join(resp, " / "))
+	}
+	return nil
+}
+
+func readSMTPGreeting(r *bufio.Reader) error {
+	_, err := readSMTPResponse(r)
+	return err
+}
+
+// readSMTPResponse reads one multi-line SMTP response (lines with "NNN-" are
+// continuation, "NNN " terminates).
+func readSMTPResponse(r *bufio.Reader) ([]string, error) {
+	var out []string
+	for {
+		line, err := r.ReadString('\n')
+		if err != nil {
+			return out, err
+		}
+		line = strings.TrimRight(line, "\r\n")
+		out = append(out, line)
+		if len(line) < 4 || line[3] == ' ' {
+			return out, nil
+		}
+	}
+}
+
+func hasSTARTTLSExt(lines []string) bool {
+	for _, l := range lines {
+		if len(l) < 4 {
+			continue
+		}
+		rest := strings.ToUpper(strings.TrimSpace(l[4:]))
+		if rest == "STARTTLS" || strings.HasPrefix(rest, "STARTTLS ") {
+			return true
+		}
+	}
+	return false
+}
+
+// starttlsIMAP implements RFC 3501 STARTTLS.
+func starttlsIMAP(conn net.Conn, sni string) error {
+	rw := bufio.NewReadWriter(bufio.NewReader(conn), bufio.NewWriter(conn))
+
+	if _, err := rw.ReadString('\n'); err != nil {
+		return fmt.Errorf("read greeting: %w", err)
+	}
+
+	if _, err := rw.WriteString("A001 CAPABILITY\r\n"); err != nil {
+		return fmt.Errorf("write CAPABILITY: %w", err)
+	}
+	if err := rw.Flush(); err != nil {
+		return err
+	}
+
+	supportsSTARTTLS := false
+	for {
+		line, err := rw.ReadString('\n')
+		if err != nil {
+			return fmt.Errorf("read CAPABILITY: %w", err)
+		}
+		if strings.Contains(strings.ToUpper(line), "STARTTLS") {
+			supportsSTARTTLS = true
+		}
+		if strings.HasPrefix(line, "A001 ") {
+			break
+		}
+	}
+	if !supportsSTARTTLS {
+		return fmt.Errorf("%w: IMAP CAPABILITY did not advertise STARTTLS", errStartTLSNotOffered)
+	}
+
+	if _, err := rw.WriteString("A002 STARTTLS\r\n"); err != nil {
+		return err
+	}
+	if err := rw.Flush(); err != nil {
+		return err
+	}
+	for {
+		line, err := rw.ReadString('\n')
+		if err != nil {
+			return fmt.Errorf("read STARTTLS response: %w", err)
+		}
+		if strings.HasPrefix(line, "A002 OK") {
+			return nil
+		}
+		if strings.HasPrefix(line, "A002 ") {
+			return fmt.Errorf("server refused STARTTLS: %s", strings.TrimSpace(line))
+		}
+	}
+}
+
+// starttlsPOP3 implements RFC 2595 STLS.
+func starttlsPOP3(conn net.Conn, sni string) error {
+	rw := bufio.NewReadWriter(bufio.NewReader(conn), bufio.NewWriter(conn))
+
+	greeting, err := rw.ReadString('\n')
+	if err != nil {
+		return fmt.Errorf("read greeting: %w", err)
+	}
+	if !strings.HasPrefix(greeting, "+OK") {
+		return fmt.Errorf("unexpected POP3 greeting: %s", strings.TrimSpace(greeting))
+	}
+
+	if _, err := rw.WriteString("CAPA\r\n"); err != nil {
+		return err
+	}
+	if err := rw.Flush(); err != nil {
+		return err
+	}
+	first, err := rw.ReadString('\n')
+	if err != nil {
+		return fmt.Errorf("read CAPA: %w", err)
+	}
+	supportsSTLS := false
+	if strings.HasPrefix(first, "+OK") {
+		for {
+			line, err := rw.ReadString('\n')
+			if err != nil {
+				return fmt.Errorf("read CAPA body: %w", err)
+			}
+			line = strings.TrimRight(line, "\r\n")
+			if line == "." {
+				break
+			}
+			if strings.EqualFold(line, "STLS") {
+				supportsSTLS = true
+			}
+		}
+	}
+	if !supportsSTLS {
+		return fmt.Errorf("%w: POP3 CAPA did not advertise STLS", errStartTLSNotOffered)
+	}
+
+	if _, err := rw.WriteString("STLS\r\n"); err != nil {
+		return err
+	}
+	if err := rw.Flush(); err != nil {
+		return err
+	}
+	resp, err := rw.ReadString('\n')
+	if err != nil {
+		return fmt.Errorf("read STLS response: %w", err)
+	}
+	if !strings.HasPrefix(resp, "+OK") {
+		return fmt.Errorf("server refused STLS: %s", strings.TrimSpace(resp))
+	}
+	return nil
+}
+
+// starttlsXMPPClient implements RFC 6120 STARTTLS for c2s streams.
+func starttlsXMPPClient(conn net.Conn, sni string) error {
+	return starttlsXMPP(conn, sni, "jabber:client")
+}
+
+// starttlsXMPPServer implements RFC 6120 STARTTLS for s2s streams.
+func starttlsXMPPServer(conn net.Conn, sni string) error {
+	return starttlsXMPP(conn, sni, "jabber:server")
+}
+
+func starttlsXMPP(conn net.Conn, sni, ns string) error {
+	header := fmt.Sprintf(`<?xml version='1.0'?><stream:stream xmlns='%s' xmlns:stream='http://etherx.jabber.org/streams' version='1.0' to='%s'>`, ns, sni)
+	if _, err := io.WriteString(conn, header); err != nil {
+		return fmt.Errorf("write stream header: %w", err)
+	}
+
+	dec := xml.NewDecoder(conn)
+
+	// Read the inbound <stream:stream> opening and its <stream:features>.
+	hasStartTLS := false
+	for {
+		tok, err := dec.Token()
+		if err != nil {
+			return fmt.Errorf("read stream features: %w", err)
+		}
+		if se, ok := tok.(xml.StartElement); ok {
+			if se.Name.Local == "features" {
+				// Scan features children.
+				for {
+					t2, err := dec.Token()
+					if err != nil {
+						return fmt.Errorf("read features body: %w", err)
+					}
+					switch ee := t2.(type) {
+					case xml.StartElement:
+						if ee.Name.Local == "starttls" {
+							hasStartTLS = true
+						}
+						_ = dec.Skip()
+					case xml.EndElement:
+						if ee.Name.Local == "features" {
+							goto doneFeatures
+						}
+					}
+				}
+			}
+		}
+	}
+doneFeatures:
+	if !hasStartTLS {
+		return fmt.Errorf("%w: XMPP features did not advertise starttls", errStartTLSNotOffered)
+	}
+
+	if _, err := io.WriteString(conn, `<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>`); err != nil {
+		return fmt.Errorf("write starttls: %w", err)
+	}
+
+	for {
+		tok, err := dec.Token()
+		if err != nil {
+			return fmt.Errorf("read proceed: %w", err)
+		}
+		if se, ok := tok.(xml.StartElement); ok {
+			switch se.Name.Local {
+			case "proceed":
+				return nil
+			case "failure":
+				return fmt.Errorf("server refused STARTTLS (<failure/>)")
+			}
+		}
+	}
+}

checker/types.go

@@ -0,0 +1,65 @@
+// Package checker implements a TLS checker for happyDomain. See README for
+// the payload shape and consumer contract.
+package checker
+
+import "time"
+
+// ObservationKeyTLSProbes is the observation key this checker writes.
+const ObservationKeyTLSProbes = "tls_probes"
+
+// Option ids on CheckerOptions.
+const (
+	OptionEndpoints      = "endpoints"
+	OptionProbeTimeoutMs = "probeTimeoutMs"
+)
+
+// Defaults shared between the definition's Default field and the runtime
+// fallback when probeTimeoutMs is unset or invalid.
+const (
+	DefaultProbeTimeoutMs = 10000
+	// MaxConcurrentProbes caps parallel probes per collect run to avoid
+	// exhausting file descriptors on domains with many endpoints.
+	MaxConcurrentProbes = 32
+)
+
+// Severity values used in Issue.Severity (lowercase, ascii).
+const (
+	SeverityCrit = "crit"
+	SeverityWarn = "warn"
+	SeverityInfo = "info"
+)
+
+// TLSData is the full collected payload written under ObservationKeyTLSProbes.
+type TLSData struct {
+	Probes      map[string]TLSProbe `json:"probes"`
+	CollectedAt time.Time           `json:"collected_at"`
+}
+
+// TLSProbe captures the outcome of probing a single endpoint. Field names
+// mirror what consumers already parse (checker-xmpp's tlsProbeView).
+type TLSProbe struct {
+	Host          string    `json:"host"`
+	Port          uint16    `json:"port"`
+	Endpoint      string    `json:"endpoint"`
+	Type          string    `json:"type"`
+	SNI           string    `json:"sni,omitempty"`
+	TLSVersion    string    `json:"tls_version,omitempty"`
+	CipherSuite   string    `json:"cipher_suite,omitempty"`
+	HostnameMatch *bool     `json:"hostname_match,omitempty"`
+	ChainValid    *bool     `json:"chain_valid,omitempty"`
+	NotAfter      time.Time `json:"not_after,omitempty"`
+	Issuer        string    `json:"issuer,omitempty"`
+	Subject       string    `json:"subject,omitempty"`
+	DNSNames      []string  `json:"dns_names,omitempty"`
+	ElapsedMS     int64     `json:"elapsed_ms,omitempty"`
+	Error         string    `json:"error,omitempty"`
+	Issues        []Issue   `json:"issues,omitempty"`
+}
+
+// Issue is a single TLS finding surfaced to the consumer.
+type Issue struct {
+	Code     string `json:"code"`
+	Severity string `json:"severity"`
+	Message  string `json:"message,omitempty"`
+	Fix      string `json:"fix,omitempty"`
+}

contract/contract.go

@@ -0,0 +1,143 @@
+// Package contract defines the DiscoveryEntry schema that checker-tls
+// consumes. Producer checkers (checker-xmpp, checker-srv, checker-sip, …)
+// import this package to describe the TLS endpoints they want probed; the
+// SDK remains agnostic of TLS specifics because the payload is marshaled
+// here and carried as opaque bytes inside sdk.DiscoveryEntry.
+//
+// Stability: this package is the source of truth for the on-wire payload
+// under Type. Breaking changes must bump the version suffix on Type
+// (tls.endpoint.v1 → v2) and ship a new Go type; consumers that only
+// understand the previous version will then ignore the new entries
+// automatically.
+package contract
+
+import (
+	"crypto/sha1"
+	"encoding/hex"
+	"encoding/json"
+	"fmt"
+
+	sdk "git.happydns.org/checker-sdk-go/checker"
+)
+
+// Type is the value placed in sdk.DiscoveryEntry.Type for this contract.
+// Consumers match entries by comparing against this exact string.
+const Type = "tls.endpoint.v1"
+
+// TLSEndpoint describes a single (host, port) that checker-tls should probe,
+// optionally preceded by a protocol-specific STARTTLS upgrade.
+type TLSEndpoint struct {
+	// Host is the target hostname. Consumers dial it directly; trailing
+	// dots are tolerated.
+	Host string `json:"host"`
+
+	// Port is the TCP port number.
+	Port uint16 `json:"port"`
+
+	// SNI is the server name to use in the TLS handshake. Leave empty when
+	// equal to Host; consumers substitute Host in that case. Set
+	// explicitly when the upstream service is fronted by a name that does
+	// not match Host (CDN, multi-tenant host, XMPP domain vs. SRV target).
+	SNI string `json:"sni,omitempty"`
+
+	// STARTTLS names the plaintext protocol that must be upgraded before
+	// the TLS handshake. Empty means "speak TLS immediately after TCP
+	// connect". Non-empty values follow the SRV service-name convention
+	// (RFC 6335): "smtp", "submission", "imap", "pop3", "xmpp-client",
+	// "xmpp-server", "ldap", "nntp", "ftp", "sieve", "postgres", …
+	//
+	// Unknown values are surfaced by checker-tls as a handshake_failed
+	// issue; they do not prevent other endpoints from being probed.
+	STARTTLS string `json:"starttls,omitempty"`
+
+	// RequireSTARTTLS is meaningful only when STARTTLS is non-empty. It
+	// distinguishes a mandatory upgrade (server absence = crit) from an
+	// opportunistic one (server absence = warn). Producers set this from
+	// protocol knowledge: XMPP mandates STARTTLS, SMTP 25 is opportunistic,
+	// SMTP 587/submission mandates it, etc.
+	RequireSTARTTLS bool `json:"require,omitempty"`
+}
+
+// NewEntry wraps ep in an sdk.DiscoveryEntry with Type, a deterministic Ref
+// derived from ep, and a marshaled Payload. The returned entry can be
+// returned as-is from a DiscoveryPublisher implementation.
+func NewEntry(ep TLSEndpoint) (sdk.DiscoveryEntry, error) {
+	payload, err := json.Marshal(ep)
+	if err != nil {
+		return sdk.DiscoveryEntry{}, fmt.Errorf("contract: marshal TLSEndpoint: %w", err)
+	}
+	return sdk.DiscoveryEntry{
+		Type:    Type,
+		Ref:     Ref(ep),
+		Payload: payload,
+	}, nil
+}
+
+// Ref computes a stable identifier for ep. The output is deterministic:
+// two endpoints with the same Host, Port, effective SNI, STARTTLS protocol,
+// and RequireSTARTTLS flag yield the same Ref. It is meant to double as a
+// human-readable key in probe result maps.
+//
+// Format (length capped by hashing the long form):
+//
+//	sha1(host|port|sni|starttls|require)[:16]
+//
+// A 16-hex-digit (8-byte) prefix of SHA-1 is sufficient for this use case:
+// we only need stability against the single (producer, target) space,
+// where collisions are vanishingly unlikely.
+func Ref(ep TLSEndpoint) string {
+	sni := ep.SNI
+	if sni == "" {
+		sni = ep.Host
+	}
+	req := "0"
+	if ep.RequireSTARTTLS {
+		req = "1"
+	}
+	canonical := fmt.Sprintf("%s|%d|%s|%s|%s", ep.Host, ep.Port, sni, ep.STARTTLS, req)
+	sum := sha1.Sum([]byte(canonical))
+	return hex.EncodeToString(sum[:8])
+}
+
+// ParseEntry decodes e into a TLSEndpoint. Returns an error when e.Type is
+// not Type or when e.Payload fails to unmarshal.
+func ParseEntry(e sdk.DiscoveryEntry) (TLSEndpoint, error) {
+	if e.Type != Type {
+		return TLSEndpoint{}, fmt.Errorf("contract: entry type %q does not match %q", e.Type, Type)
+	}
+	var ep TLSEndpoint
+	if err := json.Unmarshal(e.Payload, &ep); err != nil {
+		return TLSEndpoint{}, fmt.Errorf("contract: unmarshal TLSEndpoint: %w", err)
+	}
+	return ep, nil
+}
+
+// Entry pairs a decoded TLSEndpoint with the Ref that appeared on the wire.
+// The wire Ref is preserved verbatim rather than recomputed so consumers
+// always match the value observed downstream (in RelatedObservation /
+// probe map keys), even if a future version of this package changes Ref's
+// derivation.
+type Entry struct {
+	Ref      string
+	Endpoint TLSEndpoint
+}
+
+// ParseEntries filters entries to those of Type and decodes each payload.
+// Entries of other types are ignored silently — they belong to other
+// contracts. Entries of this type whose Payload fails to unmarshal are
+// skipped and returned as warnings so a single malformed payload cannot
+// starve the checker of the rest of its workload.
+func ParseEntries(entries []sdk.DiscoveryEntry) (out []Entry, warnings []error) {
+	for _, e := range entries {
+		if e.Type != Type {
+			continue
+		}
+		ep, err := ParseEntry(e)
+		if err != nil {
+			warnings = append(warnings, fmt.Errorf("ref=%q: %w", e.Ref, err))
+			continue
+		}
+		out = append(out, Entry{Ref: e.Ref, Endpoint: ep})
+	}
+	return out, warnings
+}

contract/contract_test.go

@@ -0,0 +1,106 @@
+package contract
+
+import (
+	"encoding/json"
+	"testing"
+
+	sdk "git.happydns.org/checker-sdk-go/checker"
+)
+
+func TestNewEntry_RoundTrip(t *testing.T) {
+	in := TLSEndpoint{
+		Host:            "jabber.example.net",
+		Port:            5222,
+		SNI:             "example.net",
+		STARTTLS:        "xmpp-client",
+		RequireSTARTTLS: true,
+	}
+	e, err := NewEntry(in)
+	if err != nil {
+		t.Fatalf("NewEntry: %v", err)
+	}
+	if e.Type != Type {
+		t.Errorf("Type = %q, want %q", e.Type, Type)
+	}
+	if e.Ref == "" {
+		t.Error("Ref is empty")
+	}
+
+	out, err := ParseEntry(e)
+	if err != nil {
+		t.Fatalf("ParseEntry: %v", err)
+	}
+	if out != in {
+		t.Errorf("round-trip mismatch:\n got %+v\nwant %+v", out, in)
+	}
+}
+
+func TestRef_StableAcrossCalls(t *testing.T) {
+	ep1 := TLSEndpoint{Host: "a", Port: 443, STARTTLS: "smtp"}
+	ep2 := TLSEndpoint{Host: "a", Port: 443, STARTTLS: "smtp"}
+	if Ref(ep1) != Ref(ep2) {
+		t.Error("Ref is not deterministic")
+	}
+}
+
+func TestRef_SNIDefaultsToHost(t *testing.T) {
+	a := TLSEndpoint{Host: "example.net", Port: 443}
+	b := TLSEndpoint{Host: "example.net", Port: 443, SNI: "example.net"}
+	if Ref(a) != Ref(b) {
+		t.Errorf("Ref differs when SNI is omitted vs. explicit but equal: %s vs %s", Ref(a), Ref(b))
+	}
+}
+
+func TestRef_FieldsAffectOutput(t *testing.T) {
+	base := TLSEndpoint{Host: "example.net", Port: 443}
+	cases := []TLSEndpoint{
+		{Host: "other.net", Port: 443},
+		{Host: "example.net", Port: 8443},
+		{Host: "example.net", Port: 443, SNI: "alt.example.net"},
+		{Host: "example.net", Port: 443, STARTTLS: "smtp"},
+		{Host: "example.net", Port: 443, STARTTLS: "smtp", RequireSTARTTLS: true},
+	}
+	baseRef := Ref(base)
+	for i, c := range cases {
+		if Ref(c) == baseRef {
+			t.Errorf("case %d: expected Ref to change when a significant field changes, got %q", i, baseRef)
+		}
+	}
+}
+
+func TestParseEntry_WrongType(t *testing.T) {
+	e := sdk.DiscoveryEntry{Type: "something.else.v1", Ref: "x", Payload: json.RawMessage(`{}`)}
+	if _, err := ParseEntry(e); err == nil {
+		t.Error("expected error on wrong type, got nil")
+	}
+}
+
+func TestParseEntry_BadPayload(t *testing.T) {
+	e := sdk.DiscoveryEntry{Type: Type, Ref: "x", Payload: json.RawMessage(`not json`)}
+	if _, err := ParseEntry(e); err == nil {
+		t.Error("expected error on malformed payload, got nil")
+	}
+}
+
+func TestParseEntries_FiltersAndAccumulatesWarnings(t *testing.T) {
+	good, err := NewEntry(TLSEndpoint{Host: "a", Port: 443})
+	if err != nil {
+		t.Fatal(err)
+	}
+	bad := sdk.DiscoveryEntry{Type: Type, Ref: "bad", Payload: json.RawMessage(`not json`)}
+	foreign := sdk.DiscoveryEntry{Type: "other.v1", Ref: "f", Payload: json.RawMessage(`{}`)}
+
+	entries, warnings := ParseEntries([]sdk.DiscoveryEntry{good, bad, foreign})
+	if len(entries) != 1 {
+		t.Errorf("entries = %d, want 1", len(entries))
+	}
+	if entries[0].Endpoint.Host != "a" {
+		t.Errorf("decoded host = %q, want %q", entries[0].Endpoint.Host, "a")
+	}
+	if entries[0].Ref != good.Ref {
+		t.Errorf("preserved ref = %q, want %q", entries[0].Ref, good.Ref)
+	}
+	if len(warnings) != 1 {
+		t.Errorf("warnings = %d, want 1 (one malformed payload of the correct type)", len(warnings))
+	}
+}

go.mod

@@ -0,0 +1,5 @@
+module git.happydns.org/checker-tls
+
+go 1.25.0
+
+require git.happydns.org/checker-sdk-go v1.1.0

go.sum

@@ -0,0 +1,2 @@
+git.happydns.org/checker-sdk-go v1.1.0 h1:xgR39X1Mh+v481BHTDYHtGYFL1qRwldTsehazwSc67Y=
+git.happydns.org/checker-sdk-go v1.1.0/go.mod h1:aNAcfYFfbhvH9kJhE0Njp5GX0dQbxdRB0rJ0KvSC5nI=

main.go

@@ -0,0 +1,23 @@
+package main
+
+import (
+	"flag"
+	"log"
+
+	sdk "git.happydns.org/checker-sdk-go/checker"
+	tls "git.happydns.org/checker-tls/checker"
+)
+
+var Version = "custom-build"
+
+var listenAddr = flag.String("listen", ":8080", "HTTP listen address")
+
+func main() {
+	flag.Parse()
+	tls.Version = Version
+
+	server := sdk.NewServer(tls.Provider())
+	if err := server.ListenAndServe(*listenAddr); err != nil {
+		log.Fatalf("server error: %v", err)
+	}
+}

plugin/plugin.go

@@ -0,0 +1,13 @@
+package main
+
+import (
+	sdk "git.happydns.org/checker-sdk-go/checker"
+	tls "git.happydns.org/checker-tls/checker"
+)
+
+var Version = "custom-build"
+
+func NewCheckerPlugin() (*sdk.CheckerDefinition, sdk.ObservationProvider, error) {
+	tls.Version = Version
+	return tls.Definition(), tls.Provider(), nil
+}