Index: sys/net/if.h
===================================================================
--- sys/net/if.h
+++ sys/net/if.h
@@ -421,6 +421,13 @@
 #define	ifr_lan_pcp	ifr_ifru.ifru_vlan_pcp	/* VLAN priority */
 };
 
+/*
+ * Consumers such as tcpdump assume no pad between ifr_name and ifr_ifru
+ * when struct ifreq is used in SIOCGIFCONF.
+ */
+CTASSERT(sizeof(((struct ifreq *)NULL)->ifr_name) ==
+    offsetof(struct ifreq, ifr_ifru));
+
 #define	_SIZEOF_ADDR_IFREQ(ifr) \
 	((ifr).ifr_addr.sa_len > sizeof(struct sockaddr) ? \
 	 (sizeof(struct ifreq) - sizeof(struct sockaddr) + \
Index: sys/netinet6/in6_var.h
===================================================================
--- sys/netinet6/in6_var.h
+++ sys/netinet6/in6_var.h
@@ -289,6 +289,12 @@
 	} ifr_ifru;
 };
 
+/*
+ * struct in6_ifreq and struct ifreq must be type-punnable for common members
+ * of ifr_ifru to allow accessors to be shared.
+ */
+CTASSERT(offsetof(struct in6_ifreq, ifr_ifru) == offsetof(struct ifreq));
+
 struct	in6_aliasreq {
 	char	ifra_name[IFNAMSIZ];
 	struct	sockaddr_in6 ifra_addr;