Index: head/sys/net/if.c
===================================================================
--- head/sys/net/if.c
+++ head/sys/net/if.c
@@ -97,6 +97,13 @@
 
 #include <security/mac/mac_framework.h>
 
+/*
+ * Consumers of struct ifreq such as tcpdump assume no pad between ifr_name
+ * and ifr_ifru when it is used in SIOCGIFCONF.
+ */
+_Static_assert(sizeof(((struct ifreq *)0)->ifr_name) ==
+    offsetof(struct ifreq, ifr_ifru), "gap between ifr_name and ifr_ifru");
+
 #ifdef COMPAT_FREEBSD32
 #include <sys/mount.h>
 #include <compat/freebsd32/freebsd32.h>
Index: head/sys/netinet6/in6.c
===================================================================
--- head/sys/netinet6/in6.c
+++ head/sys/netinet6/in6.c
@@ -112,6 +112,14 @@
 #include <netinet6/in6_fib.h>
 #include <netinet6/in6_pcb.h>
 
+/*
+ * struct in6_ifreq and struct ifreq must be type punnable for common members
+ * of ifr_ifru to allow accessors to be shared.
+ */
+_Static_assert(offsetof(struct in6_ifreq, ifr_ifru) ==
+    offsetof(struct ifreq, ifr_ifru),
+    "struct in6_ifreq and struct ifreq are not type punnable");
+
 VNET_DECLARE(int, icmp6_nodeinfo_oldmcprefix);
 #define V_icmp6_nodeinfo_oldmcprefix	VNET(icmp6_nodeinfo_oldmcprefix)