diff --git a/config.go b/config.go
index 81b1f04..e2024c4 100644
--- a/config.go
+++ b/config.go
@@ -78,6 +78,12 @@ const (
 	controlDNetDomain = "controld.net"
 	controlDDevDomain = "controld.dev"
 
+	// nextDNSDomain is the parent domain of the NextDNS DoH endpoints. Beside
+	// dns.nextdns.io, NextDNS serves alternative endpoints under it, such as
+	// ultralow.dns.nextdns.io and anycast.dns2.nextdns.io, which are the same
+	// service and take the same client info headers.
+	nextDNSDomain = "nextdns.io"
+
 	endpointPrefixHTTPS = "https://"
 	endpointPrefixQUIC  = "quic://"
 	endpointPrefixH3    = "h3://"
@@ -761,6 +767,7 @@ func (uc *UpstreamConfig) IsControlD() bool {
 	return false
 }
 
+// isNextDNS reports whether this is a NextDNS upstream.
 func (uc *UpstreamConfig) isNextDNS() bool {
 	domain := uc.Domain
 	if domain == "" {
@@ -768,7 +775,7 @@ func (uc *UpstreamConfig) isNextDNS() bool {
 			domain = u.Hostname()
 		}
 	}
-	return domain == "dns.nextdns.io"
+	return dns.IsSubDomain(nextDNSDomain, domain)
 }
 
 func (uc *UpstreamConfig) dohTransport(ctx context.Context, dnsType uint16) http.RoundTripper {
diff --git a/config_internal_test.go b/config_internal_test.go
index 24f85b6..b1f4042 100644
--- a/config_internal_test.go
+++ b/config_internal_test.go
@@ -398,6 +398,59 @@ func TestUpstreamConfig_VerifyDomain(t *testing.T) {
 	}
 }
 
+// TestUpstreamConfig_isNextDNS is a regression test for issue-610: NextDNS
+// serves its alternative DoH endpoints, the "ultralow" and "anycast" variants,
+// under nextdns.io, so recognition follows the parent domain instead of the
+// single dns.nextdns.io host. Each case runs through the Domain field and
+// through the endpoint-hostname fallback, the two ways an upstream names its
+// host.
+func TestUpstreamConfig_isNextDNS(t *testing.T) {
+	tests := []struct {
+		name   string
+		domain string
+		want   bool
+	}{
+		{"the default endpoint", "dns.nextdns.io", true},
+		{"the numbered endpoints", "dns1.nextdns.io", true},
+		{"the second numbered endpoint", "dns2.nextdns.io", true},
+		{"an ultralow endpoint", "ultralow.dns.nextdns.io", true},
+		{"a numbered ultralow endpoint", "ultralow.dns1.nextdns.io", true},
+		{"the second numbered ultralow endpoint", "ultralow.dns2.nextdns.io", true},
+		{"an anycast endpoint", "anycast.dns.nextdns.io", true},
+		{"a numbered anycast endpoint", "anycast.dns1.nextdns.io", true},
+		{"the second numbered anycast endpoint", "anycast.dns2.nextdns.io", true},
+		{"a mixed case endpoint", "UltraLow.DNS.NextDNS.IO", true},
+		{"the parent domain itself", "nextdns.io", true},
+
+		{"another TLD", "dns.nextdns.com", false},
+		{"a domain ending in the same labels", "notnextdns.io", false},
+		{"the parent domain as a prefix", "xnextdns.io", false},
+		{"the parent domain under another one", "nextdns.io.example.com", false},
+		{"an unrelated resolver", "dns.google", false},
+		{"a ControlD upstream", "freedns.controld.com", false},
+		{"no host at all", "", false},
+	}
+
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.name+" as a domain", func(t *testing.T) {
+			t.Parallel()
+			uc := &UpstreamConfig{Domain: tc.domain}
+			if got := uc.isNextDNS(); got != tc.want {
+				t.Errorf("isNextDNS() = %v for domain %q, want %v", got, tc.domain, tc.want)
+			}
+		})
+		t.Run(tc.name+" as an endpoint", func(t *testing.T) {
+			t.Parallel()
+			// Domain is empty, so the lookup falls back to the endpoint host.
+			uc := &UpstreamConfig{Endpoint: "https://" + tc.domain + "/abc123"}
+			if got := uc.isNextDNS(); got != tc.want {
+				t.Errorf("isNextDNS() = %v for endpoint %q, want %v", got, uc.Endpoint, tc.want)
+			}
+		})
+	}
+}
+
 func TestUpstreamConfig_UpstreamSendClientInfo(t *testing.T) {
 	tests := []struct {
 		name           string
@@ -414,6 +467,16 @@ func TestUpstreamConfig_UpstreamSendClientInfo(t *testing.T) {
 			&UpstreamConfig{Endpoint: "https://freedns.controld.com/p2", Type: ResolverTypeDOH3},
 			true,
 		},
+		{
+			"default with nextdns upstream DoH",
+			&UpstreamConfig{Endpoint: "https://dns.nextdns.io/abc123", Type: ResolverTypeDOH},
+			true,
+		},
+		{
+			"default with nextdns ultralow upstream DoH",
+			&UpstreamConfig{Endpoint: "https://ultralow.dns2.nextdns.io/abc123", Type: ResolverTypeDOH},
+			true,
+		},
 		{
 			"default with non-ControlD upstream",
 			&UpstreamConfig{Endpoint: "https://dns.google/dns-query", Type: ResolverTypeDOH},