--- sysdep/bsd/krt-sock.c	2021-03-21 22:29:42 UTC
+++ sysdep/bsd/krt-sock.c	
@@ -6,6 +6,7 @@ 
  *     Can be freely distributed and used under the terms of the GNU GPL.
  */
+#include <assert.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <ctype.h>
@@ -189,7 +189,17 @@ struct ks_msg
 static inline void
 sockaddr_fill_dl(struct sockaddr_dl *sa, struct iface *ifa)
 {
-  uint len = OFFSETOF(struct sockaddr_dl, sdl_data);
+  /* The original code does not work on FreeBSD 13.0. The kernel checks
+     if the passed struct is properly initialized. It checks that len is at
+     least as large as sizeof(sockaddr_dl_short). Unfortunately,
+     sockaddr_dl_short is not defined in the user space header file.
+
+     Also note that the caller only ever allocates `sizeof(struct sockaddr*)`
+     for the `struct sockaddr_dl*` parameter, but sockaddr_dl is potentially
+     much bigger. */
+  // uint len = OFFSETOF(struct sockaddr_dl, sdl_data); // buggy
+  static_assert(sizeof(struct sockaddr) <= sizeof(struct sockaddr_dl));
+  uint len = sizeof(struct sockaddr);
   memset(sa, 0, len);
   sa->sdl_len = len;
   sa->sdl_family = AF_LINK;