RFC3315 Section 12.1 is a little vague about adding bits to the prefix.

The correct interpretation is that we must add bits to the prefix length,
thus having a sla_id of 0 is valid because the prefix_length is always
extended.

1 files changed, 7 insertions, 2 deletions

diff --git a/ipv6.c b/ipv6.c
index 1c16e08a..3bd52068 100644
--- a/ipv6.c
+++ b/ipv6.c
@@ -510,8 +510,7 @@ ipv6_userprefix(
 	uint64_t vh, vl, user_low, user_high;
 
 	if (prefix_len < 1 || prefix_len > 128 ||
-	    result_len < 1 || result_len > 128 ||
-	    user_number == 0)
+	    result_len < 1 || result_len > 128)
 	{
 		errno = EINVAL;
 		return -1;
@@ -525,6 +524,12 @@ ipv6_userprefix(
 	       return -1;
 	}
 
+	/* If user_number is zero, just copy the prefix into the result. */
+	if (user_number == 0) {
+		*result = *prefix;
+		return 0;
+	}
+
 	/* Shift user_number so it fit's just inside result_len.
 	 * Shifting by 0 or sizeof(user_number) is undefined,
 	 * so we cater for that. */