[PATCH net] selftests/net: Fix tun IPv6 test addresses to avoid 6to4 range

[PATCH net] selftests/net: Fix tun IPv6 test addresses to avoid 6to4 range

[Ricardo B. Marlière]

The IPv6 addresses used for the tun_vnet_udptnl fixture currently fall in
the 2002::/16 prefix, which is reserved for the 6to4 transition mechanism
(RFC 3056).

On systems where the sit module is loaded, the kernel automatically claims
2002::/16 as a 6to4 tunnel prefix. When the test assigns a 2002:: address
to a TUN interface, sit registers a competing local route for the same
address. This ambiguity breaks the GENEVE decapsulation path: packets
injected via the TUN fd are not delivered to the test socket, causing the
IPv6-outer gtgso send_gso_packet variants to fail.

Replace all four IPv6 test addresses with addresses from the fd00:db8::/32
range, which is part of the ULA space (fc00::/7, RFC 4193) and carries no
special kernel semantics.

Fixes: 24e59f26eef2 ("selftest: tun: Add helpers for GSO over UDP tunnel")
Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
---
 tools/testing/selftests/net/tun.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/net/tun.c b/tools/testing/selftests/net/tun.c
index cf106a49b55e..abe488bac50b 100644
--- a/tools/testing/selftests/net/tun.c
+++ b/tools/testing/selftests/net/tun.c
@@ -42,19 +42,19 @@ static struct in_addr param_ipaddr4_inner_src = {
 };
 
 static struct in6_addr param_ipaddr6_outer_dst = {
-	{ { 0x20, 0x02, 0x0d, 0xb8, 0x01, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1 } },
+	{ { 0xfd, 0x00, 0x0d, 0xb8, 0x01, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1 } },
 };
 
 static struct in6_addr param_ipaddr6_outer_src = {
-	{ { 0x20, 0x02, 0x0d, 0xb8, 0x01, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2 } },
+	{ { 0xfd, 0x00, 0x0d, 0xb8, 0x01, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2 } },
 };
 
 static struct in6_addr param_ipaddr6_inner_dst = {
-	{ { 0x20, 0x02, 0x0d, 0xb8, 0x02, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1 } },
+	{ { 0xfd, 0x00, 0x0d, 0xb8, 0x02, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1 } },
 };
 
 static struct in6_addr param_ipaddr6_inner_src = {
-	{ { 0x20, 0x02, 0x0d, 0xb8, 0x02, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2 } },
+	{ { 0xfd, 0x00, 0x0d, 0xb8, 0x02, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2 } },
 };
 
 #ifndef BIT

---
base-commit: 56abdaebbf0da304b860bed1f2b5a85f5a6a16a0
change-id: 20260706-b4-net_tun_addr-055d454ab810

Best regards,
--  
Ricardo B. Marlière <rbm@suse.com>

Re: [PATCH net] selftests/net: Fix tun IPv6 test addresses to avoid 6to4 range

[Simon Horman]

On Mon, Jul 06, 2026 at 01:24:04PM -0300, Ricardo B. Marlière wrote:
> The IPv6 addresses used for the tun_vnet_udptnl fixture currently fall in
> the 2002::/16 prefix, which is reserved for the 6to4 transition mechanism
> (RFC 3056).
> 
> On systems where the sit module is loaded, the kernel automatically claims
> 2002::/16 as a 6to4 tunnel prefix. When the test assigns a 2002:: address
> to a TUN interface, sit registers a competing local route for the same
> address. This ambiguity breaks the GENEVE decapsulation path: packets
> injected via the TUN fd are not delivered to the test socket, causing the
> IPv6-outer gtgso send_gso_packet variants to fail.
> 
> Replace all four IPv6 test addresses with addresses from the fd00:db8::/32
> range, which is part of the ULA space (fc00::/7, RFC 4193) and carries no
> special kernel semantics.
> 
> Fixes: 24e59f26eef2 ("selftest: tun: Add helpers for GSO over UDP tunnel")
> Signed-off-by: Ricardo B. Marlière <rbm@suse.com>

Reviewed-by: Simon Horman <horms@kernel.org>