ctcloud: CertSpotter-backed CT cloud-resource monitor

Single stateless Go process that consumes CT logs via embedded (patched)
CertSpotter internals, classifies cloud/database/storage DNS identifiers
in-process with a zero-allocation classifier, enriches via round-robin DNS
resolvers with CNAME following, and batch-writes useful findings to PostgreSQL.

- third_party/certspotter: 2-hunk IdentifierSink patch, no stdout/JSON/files
- internal/classify: AWS/Azure/GCP/Firebase/Cloudflare R2/DigitalOcean/Supabase/OCI
- internal/dns: sharded bounded TTL cache, multi-resolver load balancing + failover
- internal/pipeline: bounded queues, dedup, direct-priority load shedding
- internal/store: pgx batch writer, Postgres CT checkpoints (no cert contents)
- internal/probe: optional non-destructive reachability checks (off by default)
- Docker: distroless non-root, read-only rootfs, no app volume, resource limits
- tests enforce 0 allocs/op on the classifier and sink hot path

internal/dns/pool.go

@@ -0,0 +1,119 @@
+package dns
+
+import (
+	"context"
+	"errors"
+	"net"
+	"strings"
+	"sync/atomic"
+	"time"
+)
+
+// ServerPool load-balances DNS lookups across multiple resolvers using
+// round-robin, and fails over to the next server on a timeout or server error.
+// Definitive answers (NXDOMAIN) are returned immediately and are not retried on
+// every server.
+// defaultAttemptTimeout bounds each individual server attempt so a dead
+// resolver costs a couple of seconds before failover, not the entire lookup
+// budget.
+const defaultAttemptTimeout = 2 * time.Second
+
+type ServerPool struct {
+	resolvers      []Resolver
+	next           atomic.Uint64
+	attemptTimeout time.Duration
+}
+
+func (p *ServerPool) withAttemptTimeout(ctx context.Context) (context.Context, context.CancelFunc) {
+	t := p.attemptTimeout
+	if t <= 0 {
+		t = defaultAttemptTimeout
+	}
+	return context.WithTimeout(ctx, t)
+}
+
+// NewServerPool builds a pool from "host", "host:port" or "system" entries.
+// An empty list uses the system resolver.
+func NewServerPool(servers []string) *ServerPool {
+	var resolvers []Resolver
+	for _, s := range servers {
+		s = strings.TrimSpace(s)
+		switch {
+		case s == "":
+			continue
+		case s == "system" || s == "local":
+			return &ServerPool{resolvers: []Resolver{&GoResolver{R: net.DefaultResolver}}}
+		default:
+			if !strings.Contains(s, ":") {
+				s += ":53"
+			}
+			resolvers = append(resolvers, &GoResolver{R: newServerResolver(s)})
+		}
+	}
+	if len(resolvers) == 0 {
+		resolvers = append(resolvers, &GoResolver{R: net.DefaultResolver})
+	}
+	return &ServerPool{resolvers: resolvers}
+}
+
+func newServerResolver(server string) *net.Resolver {
+	return &net.Resolver{
+		PreferGo: true,
+		Dial: func(ctx context.Context, network, _ string) (net.Conn, error) {
+			if network != "tcp" {
+				network = "udp"
+			}
+			d := net.Dialer{Timeout: 5 * time.Second}
+			return d.DialContext(ctx, network, server)
+		},
+	}
+}
+
+func (p *ServerPool) pick() int {
+	return int((p.next.Add(1) - 1) % uint64(len(p.resolvers)))
+}
+
+// definitive reports whether err is a real DNS answer (NXDOMAIN) that should not
+// be retried against other servers.
+func definitive(err error) bool {
+	var dnsErr *net.DNSError
+	return errors.As(err, &dnsErr) && dnsErr.IsNotFound
+}
+
+func (p *ServerPool) LookupCNAME(ctx context.Context, host string) (string, error) {
+	n := len(p.resolvers)
+	start := p.pick()
+	var last error
+	for i := 0; i < n; i++ {
+		actx, cancel := p.withAttemptTimeout(ctx)
+		cname, err := p.resolvers[(start+i)%n].LookupCNAME(actx, host)
+		cancel()
+		if err == nil {
+			return cname, nil
+		}
+		if definitive(err) {
+			return "", err
+		}
+		last = err
+	}
+	return "", last
+}
+
+func (p *ServerPool) LookupIP(ctx context.Context, host string) ([]net.IP, error) {
+	n := len(p.resolvers)
+	start := p.pick()
+	var last error
+	for i := 0; i < n; i++ {
+		actx, cancel := p.withAttemptTimeout(ctx)
+		ips, err := p.resolvers[(start+i)%n].LookupIP(actx, host)
+		cancel()
+		if err == nil {
+			return ips, nil
+		}
+		if definitive(err) {
+			return nil, err
+		}
+		last = err
+	}
+	return nil, last
+}