`Rule.String()` method returns `.Dst`

- Return `to <all | ipNetString>` in string
- Return `all` for `from` if `rule.Src == nil`
- Return `all` for `to` if `rule.Dst == nil`

Add unit test

rule.go

@@ -28,7 +28,18 @@ type Rule struct {
 }
 
 func (r Rule) String() string {
-	return fmt.Sprintf("ip rule %d: from %s table %d", r.Priority, r.Src, r.Table)
+	from := "all"
+	if r.Src != nil && r.Src.String() != "<nil>" {
+		from = r.Src.String()
+	}
+
+	to := "all"
+	if r.Dst != nil && r.Dst.String() != "<nil>" {
+		to = r.Dst.String()
+	}
+
+	return fmt.Sprintf("ip rule %d: from %s to %s table %d",
+		r.Priority, from, to, r.Table)
 }
 
 // NewRule return empty rules.

rule_test.go

@@ -1,3 +1,4 @@
+//go:build linux
 // +build linux
 
 package netlink
@@ -352,6 +353,50 @@ func runRuleListFiltered(t *testing.T, family int, srcNet, dstNet *net.IPNet) {
 	}
 }
 
+func TestRuleString(t *testing.T) {
+	t.Parallel()
+
+	testCases := map[string]struct {
+		r Rule
+		s string
+	}{
+		"empty rule": {
+			s: "ip rule 0: from all to all table 0",
+		},
+		"rule with src and dst equivalent to <nil>": {
+			r: Rule{
+				Priority: 100,
+				Src:      &net.IPNet{IP: net.IPv4(10, 0, 0, 0)},
+				Dst:      &net.IPNet{IP: net.IPv4(20, 0, 0, 0)},
+				Table:    99,
+			},
+			s: "ip rule 100: from all to all table 99",
+		},
+		"rule with src and dst": {
+			r: Rule{
+				Priority: 100,
+				Src:      &net.IPNet{IP: net.IPv4(10, 0, 0, 0), Mask: net.IPv4Mask(255, 255, 255, 0)},
+				Dst:      &net.IPNet{IP: net.IPv4(20, 0, 0, 0), Mask: net.IPv4Mask(255, 255, 255, 0)},
+				Table:    99,
+			},
+			s: "ip rule 100: from 10.0.0.0/24 to 20.0.0.0/24 table 99",
+		},
+	}
+
+	for name, testCase := range testCases {
+		testCase := testCase
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			s := testCase.r.String()
+
+			if s != testCase.s {
+				t.Errorf("expected %q but got %q", testCase.s, s)
+			}
+		})
+	}
+}
+
 func ruleExists(rules []Rule, rule Rule) bool {
 	for i := range rules {
 		if ruleEquals(rules[i], rule) {