Initial commit

Generic SRV records checker for happyDomain.

For each SRV record attached to an svcs.UnknownSRV service, the checker
resolves every target and probes reachability:

  - DNS resolution (A/AAAA), CNAME detection (RFC 2782 violation),
    null-target detection (RFC 2782 "service explicitly unavailable")
  - TCP connect to target:port for _tcp SRVs
  - UDP probe for _udp SRVs, using ICMP port-unreachable detection

The checker also publishes TLS endpoints (host, port, SNI) for every
SRV target hitting a well-known direct-TLS port (443, 465, 636, 853,
993, 995, 5061, 5223, …) via the EndpointDiscoverer SDK interface, so
a downstream TLS checker can pick them up.

The HTML report groups records as cards and surfaces the most common
failure scenarios (DNS failure, CNAME target, TCP unreachable,
null-target) at the top with remediation guidance.

checker/rules_cname.go

@@ -0,0 +1,38 @@
+package checker
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	sdk "git.happydns.org/checker-sdk-go/checker"
+)
+
+type ruleTargetNotCNAME struct{}
+
+func RuleTargetNotCNAME() sdk.CheckRule { return &ruleTargetNotCNAME{} }
+func (ruleTargetNotCNAME) Name() string { return "srv_target_not_cname" }
+func (ruleTargetNotCNAME) Description() string {
+	return "RFC 2782: SRV targets must resolve directly to A/AAAA, not through a CNAME."
+}
+func (ruleTargetNotCNAME) Evaluate(ctx context.Context, obs sdk.ObservationGetter, _ sdk.CheckerOptions) []sdk.CheckState {
+	d, cs := getData(ctx, obs)
+	if cs != nil {
+		return []sdk.CheckState{*cs}
+	}
+	var bad []string
+	for _, r := range d.Records {
+		if r.IsNullTarget {
+			continue
+		}
+		if r.IsCNAME {
+			bad = append(bad, r.Target)
+		}
+	}
+	if len(bad) == 0 {
+		return []sdk.CheckState{{Status: sdk.StatusOK, Code: "srv_targets_not_cname",
+			Message: "All SRV targets resolve directly (no CNAME)."}}
+	}
+	return []sdk.CheckState{{Status: sdk.StatusWarn, Code: "srv_targets_are_cname",
+		Message: fmt.Sprintf("RFC 2782 violation, SRV target(s) are CNAMEs: %s", strings.Join(bad, ", "))}}
+}