Refuse IPv4-mapped prefixes with an error that says so

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -162,6 +162,9 @@ All notable changes to Housecarl are documented here. The format follows
 
 ### Fixed
 
+- An IPv4-mapped prefix (`::ffff:0:0/96`) is refused with an error that
+  says so; it was refused as "has host bits set; the prefix is invalid
+  Prefix".
 - `hc forward show` shows the forward's hits and comment, and its
   hairpin as `no`, `yes` or `yes, from …` (it printed `no -`).
 - A history entry's ruleset diff showed the object it changed as moved:

internal/model/addr.go

@@ -84,7 +84,11 @@ func ParseAddress(s string) (Address, error) {
 		if err != nil {
 			return Address{}, fmt.Errorf("%q is not an address or prefix", s)
 		}
-		p = netip.PrefixFrom(p.Addr().Unmap(), p.Bits())
+		if p.Addr().Is4In6() {
+			// Unmapping would leave an IPv4 address with IPv6 bits
+			// (::ffff:0:0/96 as 0.0.0.0/96).
+			return Address{}, fmt.Errorf("%q is an IPv4-mapped prefix, which can't be used: write IPv4 prefixes as IPv4 (10.0.0.0/8)", s)
+		}
 		if p.Masked() != p {
 			return Address{}, fmt.Errorf("%q has host bits set; the prefix is %s", s, p.Masked())
 		}

internal/model/addr_test.go

@@ -0,0 +1,28 @@
+package model
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestParseAddress(t *testing.T) {
+	for _, c := range []struct{ in, want, err string }{
+		{in: "10.0.0.0/8", want: "10.0.0.0/8"},
+		{in: "2001:db8::/32", want: "2001:db8::/32"},
+		{in: "::ffff:10.0.0.1", want: "10.0.0.1"},
+		{in: "10.0.0.1/8", err: "host bits"},
+		{in: "::ffff:0:0/96", err: "IPv4-mapped"},
+		{in: "::ffff:10.0.0.0/104", err: "IPv4-mapped"},
+	} {
+		a, err := ParseAddress(c.in)
+		if c.err != "" {
+			if err == nil || !strings.Contains(err.Error(), c.err) {
+				t.Errorf("%s: error %v, want one saying %q", c.in, err, c.err)
+			}
+			continue
+		}
+		if err != nil || a.String() != c.want {
+			t.Errorf("%s: %q, %v; want %q", c.in, a.String(), err, c.want)
+		}
+	}
+}