Index: head/share/man/man4/netintro.4
===================================================================
--- head/share/man/man4/netintro.4
+++ head/share/man/man4/netintro.4
@@ -28,7 +28,7 @@
 .\"     @(#)netintro.4	8.2 (Berkeley) 11/30/93
 .\" $FreeBSD$
 .\"
-.Dd January 26, 2012
+.Dd October 14, 2020
 .Dt NETINTRO 4
 .Os
 .Sh NAME
@@ -349,6 +349,13 @@
 convention that specification of the default address means
 to delete the first address for the interface belonging to
 the address family in which the original socket was opened.
+.It Dv SIOCGIFALIAS
+This request provides means to get additional addresses
+together with netmask and broadcast/destination from an
+interface.
+It also uses the
+.Vt ifaliasreq
+structure.
 .It Dv SIOCGIFCONF
 Get interface configuration list.
 This request takes an
Index: head/sys/netinet/in.c
===================================================================
--- head/sys/netinet/in.c
+++ head/sys/netinet/in.c
@@ -72,6 +72,7 @@
 
 static int in_aifaddr_ioctl(u_long, caddr_t, struct ifnet *, struct thread *);
 static int in_difaddr_ioctl(u_long, caddr_t, struct ifnet *, struct thread *);
+static int in_gifaddr_ioctl(u_long, caddr_t, struct ifnet *, struct thread *);
 
 static void	in_socktrim(struct sockaddr_in *);
 static void	in_purgemaddrs(struct ifnet *);
@@ -237,6 +238,11 @@
 	case SIOCGIFDSTADDR:
 	case SIOCGIFNETMASK:
 		break;
+	case SIOCGIFALIAS:
+		sx_xlock(&in_control_sx);
+		error = in_gifaddr_ioctl(cmd, data, ifp, td);
+		sx_xunlock(&in_control_sx);
+		return (error);
 	case SIOCDIFADDR:
 		sx_xlock(&in_control_sx);
 		error = in_difaddr_ioctl(cmd, data, ifp, td);
@@ -646,6 +652,60 @@
 	    IFADDR_EVENT_DEL);
 	ifa_free(&ia->ia_ifa);		/* in_ifaddrhead */
 
+	return (0);
+}
+
+static int
+in_gifaddr_ioctl(u_long cmd, caddr_t data, struct ifnet *ifp, struct thread *td)
+{
+	struct in_aliasreq *ifra = (struct in_aliasreq *)data;
+	const struct sockaddr_in *addr = &ifra->ifra_addr;
+	struct epoch_tracker et;
+	struct ifaddr *ifa;
+	struct in_ifaddr *ia;
+
+	/*
+	 * ifra_addr must be present and be of INET family.
+	 */
+	if (addr->sin_len != sizeof(struct sockaddr_in) ||
+	    addr->sin_family != AF_INET)
+		return (EINVAL);
+
+	/*
+	 * See whether address exist.
+	 */
+	ia = NULL;
+	NET_EPOCH_ENTER(et);
+	CK_STAILQ_FOREACH(ifa, &ifp->if_addrhead, ifa_link) {
+		struct in_ifaddr *it;
+
+		if (ifa->ifa_addr->sa_family != AF_INET)
+			continue;
+
+		it = (struct in_ifaddr *)ifa;
+		if (it->ia_addr.sin_addr.s_addr == addr->sin_addr.s_addr &&
+		    prison_check_ip4(td->td_ucred, &addr->sin_addr) == 0) {
+			ia = it;
+			break;
+		}
+	}
+	if (ia == NULL) {
+		NET_EPOCH_EXIT(et);
+		return (EADDRNOTAVAIL);
+	}
+
+	ifra->ifra_mask = ia->ia_sockmask;
+	if ((ifp->if_flags & IFF_POINTOPOINT) &&
+	    ia->ia_dstaddr.sin_family == AF_INET)
+		ifra->ifra_dstaddr = ia->ia_dstaddr;
+	else if ((ifp->if_flags & IFF_BROADCAST) &&
+	    ia->ia_broadaddr.sin_family == AF_INET)
+		ifra->ifra_broadaddr = ia->ia_broadaddr;
+	else
+		memset(&ifra->ifra_broadaddr, 0,
+		    sizeof(ifra->ifra_broadaddr));
+
+	NET_EPOCH_EXIT(et);
 	return (0);
 }
 
Index: head/sys/sys/sockio.h
===================================================================
--- head/sys/sys/sockio.h
+++ head/sys/sys/sockio.h
@@ -84,6 +84,7 @@
 #define	SIOCGIFDESCR	_IOWR('i', 42, struct ifreq)	/* get ifnet descr */ 
 #define	SIOCAIFADDR	 _IOW('i', 43, struct ifaliasreq)/* add/chg IF alias */
 #define	SIOCGIFDATA	 _IOW('i', 44, struct ifreq)	/* get if_data */
+#define	SIOCGIFALIAS	_IOWR('i', 45, struct ifaliasreq)/* get IF alias */
 
 #define	SIOCADDMULTI	 _IOW('i', 49, struct ifreq)	/* add m'cast addr */
 #define	SIOCDELMULTI	 _IOW('i', 50, struct ifreq)	/* del m'cast addr */