From 761d7eb4217933c96d0f7e77e13bd1c2c9ec2cc0 Mon Sep 17 00:00:00 2001
From: Hayato Kiwata <dev@haytok.jp>
Date: Sun, 13 Sep 2026 22:16:00 +0900
Subject: [PATCH] refactor: use `netip.Addr.Next()` instead of `math/big` in
 `AddIPInt6`

follow-up: https://github.com/rootless-containers/rootlesskit/pull/620#discussion_r3957446613

Signed-off-by: Hayato Kiwata <dev@haytok.jp>
---
 pkg/network/iputils/iputils.go      | 22 ++++++++++++++--------
 pkg/network/iputils/iputils_test.go |  5 +++++
 2 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/pkg/network/iputils/iputils.go b/pkg/network/iputils/iputils.go
index d200018d..616b5a24 100644
--- a/pkg/network/iputils/iputils.go
+++ b/pkg/network/iputils/iputils.go
@@ -4,8 +4,8 @@ import (
 	"encoding/binary"
 	"fmt"
 	"math"
-	"math/big"
 	"net"
+	"net/netip"
 )
 
 func AddIPInt(ip net.IP, i int) (net.IP, error) {
@@ -28,12 +28,18 @@ func AddIPInt6(ip net.IP, i int) (net.IP, error) {
 	if ip.To4() != nil || ip6 == nil {
 		return nil, fmt.Errorf("expected IPv6 address, got %s", ip.String())
 	}
-	b := new(big.Int).SetBytes(ip6)
-	b.Add(b, big.NewInt(int64(i)))
-	if b.Sign() < 0 || b.BitLen() > 128 {
-		return nil, fmt.Errorf("%s + %d overflows", ip.String(), i)
+	if i < 0 {
+		return nil, fmt.Errorf("expected non-negative integer, got %d", i)
 	}
-	res := make(net.IP, net.IPv6len)
-	b.FillBytes(res)
-	return res, nil
+	addr, ok := netip.AddrFromSlice(ip6)
+	if !ok {
+		return nil, fmt.Errorf("expected IPv6 address, got %s", ip.String())
+	}
+	for n := 0; n < i; n++ {
+		addr = addr.Next()
+		if !addr.IsValid() {
+			return nil, fmt.Errorf("%s + %d overflows", ip.String(), i)
+		}
+	}
+	return net.IP(addr.AsSlice()), nil
 }
diff --git a/pkg/network/iputils/iputils_test.go b/pkg/network/iputils/iputils_test.go
index 8d6ae79d..b54ca5d8 100644
--- a/pkg/network/iputils/iputils_test.go
+++ b/pkg/network/iputils/iputils_test.go
@@ -72,6 +72,11 @@ func TestAddIPInt6(t *testing.T) {
 			0x1,
 			"fd00::1:0",
 		},
+		{
+			"fd00::ffff",
+			-1,
+			"",
+		},
 		{
 			"ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff",
 			0x1,