read: fix IPv4 lookup on IPv6 databases (#49)

Author: june-ipinfo

lib/cmd_read.go

@@ -113,6 +113,7 @@ func CmdRead(f CmdReadFlags, args []string, printHelp func()) error {
 			}
 			continue
 		}
+		addr = addr.Unmap()
 		if err := db.Lookup(addr).Decode(&record); err != nil || len(record) == 0 {
 			if !requiresHdr {
 				fmt.Fprintf(os.Stderr,

lib/cmd_read_test.go

@@ -0,0 +1,90 @@
+package lib
+
+import (
+	"net"
+	"net/netip"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/oschwald/maxminddb-golang/v2"
+)
+
+// TestReadLookupOnIPv6Database verifies that both IPv4 and IPv6 lookups work
+// on IPv6 databases. The IPv4 cases are a regression test: netip.AddrFromSlice
+// on a 16-byte net.IP (as returned by net.ParseIP for IPv4) produces an
+// IPv4-mapped-IPv6 address (::ffff:x.x.x.x) which maxminddb-golang/v2 does
+// not resolve through the IPv4 subtree. The fix is to call .Unmap() before
+// lookup.
+func TestReadLookupOnIPv6Database(t *testing.T) {
+	// Create a small IPv6 MMDB with IPv4 data via import.
+	tempDir := t.TempDir()
+	inputFile := filepath.Join(tempDir, "input.csv")
+	mmdbFile := filepath.Join(tempDir, "test.mmdb")
+
+	csvData := "network,descr\n127.0.0.0/8,Loopback\n192.168.0.0/16,Private-Use\n2001:db8::/32,Documentation\nfc00::/7,Unique-Local\n"
+	if err := os.WriteFile(inputFile, []byte(csvData), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	err := CmdImport(CmdImportFlags{
+		Ip:    6,
+		Size:  32,
+		Merge: "none",
+		In:    inputFile,
+		Out:   mmdbFile,
+		Csv:   true,
+	}, []string{}, func() {})
+	if err != nil {
+		t.Fatalf("import failed: %s", err)
+	}
+
+	db, err := maxminddb.Open(mmdbFile)
+	if err != nil {
+		t.Fatalf("failed to open MMDB: %s", err)
+	}
+	defer db.Close()
+
+	if db.Metadata.IPVersion != 6 {
+		t.Fatalf("expected IPv6 database, got version %d", db.Metadata.IPVersion)
+	}
+
+	tests := []struct {
+		ip       string
+		expected string
+	}{
+		{"127.0.0.1", "Loopback"},
+		{"192.168.1.1", "Private-Use"},
+		{"2001:db8::1", "Documentation"},
+		{"fd00::1", "Unique-Local"},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.ip, func(t *testing.T) {
+			// net.ParseIP returns a 16-byte slice for IPv4, same as
+			// iputil.IPListFromAllSrcs. This is the code path that
+			// CmdRead uses.
+			ip := net.ParseIP(tc.ip)
+			if len(ip) != 16 {
+				t.Fatalf("expected 16-byte IP, got %d", len(ip))
+			}
+
+			addr, ok := netip.AddrFromSlice(ip)
+			if !ok {
+				t.Fatalf("AddrFromSlice failed for %s", tc.ip)
+			}
+			addr = addr.Unmap()
+
+			var record map[string]interface{}
+			if err := db.Lookup(addr).Decode(&record); err != nil {
+				t.Fatalf("lookup failed: %s", err)
+			}
+			if len(record) == 0 {
+				t.Fatalf("empty record for %s", tc.ip)
+			}
+			if got := record["descr"]; got != tc.expected {
+				t.Errorf("got descr=%v, want %s", got, tc.expected)
+			}
+		})
+	}
+}