[api] check on prefix length when slaac is enabled (#5530)

src/core/api/border_router_api.cpp

@@ -62,6 +62,10 @@ otError otBorderRouterAddOnMeshPrefix(otInstance *aInstance, const otBorderRoute
     // Thread 1.1 Specification 5.13.2 says
     // "A valid prefix MUST NOT allow both DHCPv6 and SLAAC for address configuration"
     VerifyOrExit(!aConfig->mDhcp || !aConfig->mSlaac, error = OT_ERROR_INVALID_ARGS);
+    // RFC 4944 Section 6 says:
+    // An IPv6 address prefix used for stateless autoconfiguration [RFC4862]
+    // of an IEEE 802.15.4 interface MUST have a length of 64 bits.
+    VerifyOrExit(!aConfig->mSlaac || aConfig->mPrefix.mLength == OT_IP6_PREFIX_BITSIZE, error = OT_ERROR_INVALID_ARGS);
 
     error = instance.Get<NetworkData::Local>().AddOnMeshPrefix(*config);
 #if OPENTHREAD_FTD && OPENTHREAD_CONFIG_BACKBONE_ROUTER_ENABLE

tests/toranj/test-010-on-mesh-prefix-config-gateway.py

@@ -249,7 +249,7 @@ wpan.verify_within(check_prefix4_removed_from_r1, WAIT_TIME)
 
 r1.add_prefix(
     prefix4,
-    48,
+    64,
     priority="-1",
     stable=True,
     on_mesh=False,
@@ -262,7 +262,7 @@ r1.add_prefix(
 verify_prefix(
     [r1],
     prefix4,
-    48,
+    64,
     priority="low",
     stable=True,
     on_mesh=False,