feat(health/dns): try another DNS server if one fails

internal/healthcheck/dns/dns.go

@@ -5,33 +5,60 @@ import (
 	"errors"
 	"fmt"
 	"net"
+	"net/netip"
+
+	"github.com/qdm12/dns/v2/pkg/provider"
 )
 
 // Client is a simple plaintext UDP DNS client, to be used for healthchecks.
 // Note the client connects to a DNS server only over UDP on port 53,
 // because we don't want to use DoT or DoH and impact the TCP connections
 // when running a healthcheck.
-type Client struct{}
+type Client struct {
+	serverAddrs []netip.AddrPort
+	dnsIPIndex  int
+}
 
 func New() *Client {
-	return &Client{}
+	return &Client{
+		serverAddrs: concatAddrPorts([][]netip.AddrPort{
+			provider.Cloudflare().Plain.IPv4,
+			provider.Google().Plain.IPv4,
+			provider.Quad9().Plain.IPv4,
+			provider.OpenDNS().Plain.IPv4,
+			provider.LibreDNS().Plain.IPv4,
+			provider.Quadrant().Plain.IPv4,
+			provider.CiraProtected().Plain.IPv4,
+		}),
+	}
+}
+
+func concatAddrPorts(addrs [][]netip.AddrPort) []netip.AddrPort {
+	var result []netip.AddrPort
+	for _, addrList := range addrs {
+		result = append(result, addrList...)
+	}
+	return result
 }
 
 var ErrLookupNoIPs = errors.New("no IPs found from DNS lookup")
 
 func (c *Client) Check(ctx context.Context) error {
+	dnsAddr := c.serverAddrs[c.dnsIPIndex].Addr()
 	resolver := &net.Resolver{
 		PreferGo: true,
 		Dial: func(ctx context.Context, _, _ string) (net.Conn, error) {
 			dialer := net.Dialer{}
-			return dialer.DialContext(ctx, "udp", "1.1.1.1:53")
+			return dialer.DialContext(ctx, "udp", dnsAddr.String())
 		},
 	}
 	ips, err := resolver.LookupIP(ctx, "ip", "github.com")
 	switch {
 	case err != nil:
+		c.dnsIPIndex = (c.dnsIPIndex + 1) % len(c.serverAddrs)
 		return err
 	case len(ips) == 0:
+		c.dnsIPIndex = (c.dnsIPIndex + 1) % len(c.serverAddrs)
 		return fmt.Errorf("%w", ErrLookupNoIPs)
 	default:
 		return nil