primeiro commit

internal/probe/dns.go

@@ -0,0 +1,52 @@
+package probe
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"net/netip"
+	"time"
+)
+
+// DNSResult contains the outcome of one DNS lookup.
+type DNSResult struct {
+	Reachable bool
+	Duration  time.Duration
+	IPs       []net.IP
+	Err       error
+}
+
+// Resolve checks whether a domain can be resolved against a specific DNS server.
+func Resolve(ctx context.Context, server string, domain string, timeout time.Duration) DNSResult {
+	start := time.Now()
+	lookupCtx, cancel := context.WithTimeout(ctx, timeout)
+	defer cancel()
+
+	dialer := net.Dialer{Timeout: timeout}
+	resolver := net.Resolver{
+		PreferGo: true,
+		Dial: func(ctx context.Context, network string, address string) (net.Conn, error) {
+			conn, err := dialer.DialContext(ctx, network, dnsServerAddress(server))
+			if err != nil {
+				return nil, fmt.Errorf("dialing dns server %s: %w", server, err)
+			}
+			return conn, nil
+		},
+	}
+	ips, err := resolver.LookupIP(lookupCtx, "ip", domain)
+	duration := time.Since(start)
+	if err != nil {
+		return DNSResult{Duration: duration, Err: fmt.Errorf("resolving %s with %s: %w", domain, server, err)}
+	}
+	return DNSResult{Reachable: len(ips) > 0, Duration: duration, IPs: ips}
+}
+
+func dnsServerAddress(server string) string {
+	if _, err := netip.ParseAddrPort(server); err == nil {
+		return server
+	}
+	if addr, err := netip.ParseAddr(server); err == nil {
+		return net.JoinHostPort(addr.String(), "53")
+	}
+	return net.JoinHostPort(server, "53")
+}