diff --git a/edns.go b/edns.go
index 7a58aa9b..0550aaa3 100644
--- a/edns.go
+++ b/edns.go
@@ -128,8 +128,18 @@ func (rr *OPT) Do() bool {
 }
 
 // SetDo sets the DO (DNSSEC OK) bit.
-func (rr *OPT) SetDo() {
-	rr.Hdr.Ttl |= _DO
+// If we pass an argument, set the DO bit to that value.
+// It is possible to pass 2 or more arguments. Any arguments after the 1st is silently ignored.
+func (rr *OPT) SetDo(do ...bool) {
+	if len(do) == 1 {
+		if do[0] {
+			rr.Hdr.Ttl |= _DO
+		} else {
+			rr.Hdr.Ttl &^= _DO
+		}
+	} else {
+		rr.Hdr.Ttl |= _DO
+	}
 }
 
 // EDNS0 defines an EDNS0 Option. An OPT RR can have multiple options appended to it.
diff --git a/edns_test.go b/edns_test.go
index 5fd75abb..c290b0c8 100644
--- a/edns_test.go
+++ b/edns_test.go
@@ -7,10 +7,46 @@ func TestOPTTtl(t *testing.T) {
 	e.Hdr.Name = "."
 	e.Hdr.Rrtype = TypeOPT
 
+	// verify the default setting of DO=0
 	if e.Do() {
 		t.Errorf("DO bit should be zero")
 	}
 
+	// There are 6 possible invocations of SetDo():
+	//
+	// 1. Starting with DO=0, using SetDo()
+	// 2. Starting with DO=0, using SetDo(true)
+	// 3. Starting with DO=0, using SetDo(false)
+	// 4. Starting with DO=1, using SetDo()
+	// 5. Starting with DO=1, using SetDo(true)
+	// 6. Starting with DO=1, using SetDo(false)
+
+	// verify that invoking SetDo() sets DO=1 (TEST #1)
+	e.SetDo()
+	if !e.Do() {
+		t.Errorf("DO bit should be non-zero")
+	}
+	// verify that using SetDo(true) works when DO=1 (TEST #5)
+	e.SetDo(true)
+	if !e.Do() {
+		t.Errorf("DO bit should still be non-zero")
+	}
+	// verify that we can use SetDo(false) to set DO=0 (TEST #6)
+	e.SetDo(false)
+	if e.Do() {
+		t.Errorf("DO bit should be zero")
+	}
+	// verify that if we call SetDo(false) when DO=0 that it is unchanged (TEST #3)
+	e.SetDo(false)
+	if e.Do() {
+		t.Errorf("DO bit should still be zero")
+	}
+	// verify that using SetDo(true) works for DO=0 (TEST #2)
+	e.SetDo(true)
+	if !e.Do() {
+		t.Errorf("DO bit should be non-zero")
+	}
+	// verify that using SetDo() works for DO=1 (TEST #4)
 	e.SetDo()
 	if !e.Do() {
 		t.Errorf("DO bit should be non-zero")