[OpenWrt-Devel] [PATCH v2 netifd] interface-ip: Fix broadcast address when using /31 or /32 IPv4 addressing

[Baptiste Jonglez]

From: Baptiste Jonglez <git at bitsofnetworks.org>

A /31-addressed interface requires a broadcast address of 255.255.255.255,
because there is no room for a proper broadcast address.  Without this,
any packet destinated to the other end of the link is sent as broadcast,
which is incorrect.

For consistency with the Linux kernel, /32-addressed interfaces are
treated in the same way.

Signed-off-by: Baptiste Jonglez <git at bitsofnetworks.org>
---
 interface-ip.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/interface-ip.c b/interface-ip.c
index 8eb2ff3..0c72e46 100644
--- a/interface-ip.c
+++ b/interface-ip.c
@@ -473,11 +473,17 @@ interface_update_proto_addr(struct vlist_tree *tree,
 		if ((a_new->flags & DEVADDR_FAMILY) == DEVADDR_INET4 &&
 		    !a_new->broadcast) {
 
-			uint32_t mask = ~0;
-			uint32_t *a = (uint32_t *) &a_new->addr;
-
-			mask >>= a_new->mask;
-			a_new->broadcast = *a | htonl(mask);
+			/* /31 and /32 addressing need 255.255.255.255
+			 * as broadcast address. */
+			if (a_new->mask >= 31) {
+				a_new->broadcast = (uint32_t) ~0;
+			} else {
+				uint32_t mask = ~0;
+				uint32_t *a = (uint32_t *) &a_new->addr;
+
+				mask >>= a_new->mask;
+				a_new->broadcast = *a | htonl(mask);
+			}
 		}
 	}
 
-- 
2.5.1
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