diff --git a/ospfd/ospf_packet.c b/ospfd/ospf_packet.c
index ce82ee2..8bc4966 100644
--- a/ospfd/ospf_packet.c
+++ b/ospfd/ospf_packet.c
@@ -1881,7 +1881,12 @@
   unsigned int ifindex = 0;
   struct iovec iov;
   struct cmsghdr *cmsg;
-  char buff [sizeof (*cmsg) + SOPT_SIZE_CMSG_PKTINFO_IPV4()];
+#if defined(CMSG_SPACE)
+  /* Header and data both require alignment. */
+  char buff [CMSG_SPACE(SIZE_CMSG_IFINDEX_IPV4())];
+#else
+  char buff [sizeof (*cmsg) + SOPT_SIZE_CMSG_IFINDEX_IPV4()];
+#endif
   struct msghdr msgh;
 
   msgh.msg_name = NULL;