Miquel van Smoorenburg <miquels@cistron.nl> supplied this note and patch:

[...]

However, in IPv6 mode, sshd doesn't write the utmp 'ut_addr' field.
This patch fixes that; for 4-in-6 connections, the IPv4 address
is written to ut_addr, for IPv6 connections, the IPv6 address
is written to ut_addr_v6.

This basically is a security fix, though a minor one; Linux doesn't
have the ut_addr / ut_addr_v6 field just to leave them empty !

The next sysvinit (2.85) will include a 'last' that knows how
to read ut_addr_v6 from the wtmp file (I'm the maintainer).

diff -ruN t/openssh-3.5p1/loginrec.c openssh-3.5p1/loginrec.c
--- t/openssh-3.5p1/loginrec.c	2002-09-26 02:38:49.000000000 +0200
+++ openssh-3.5p1/loginrec.c	2002-11-05 13:14:33.000000000 +0100
@@ -609,6 +609,9 @@
 construct_utmp(struct logininfo *li,
 		    struct utmp *ut)
 {
+# ifdef HAVE_ADDR_V6_IN_UTMP
+	struct sockaddr_in6 *sa6;
+#  endif
 	memset(ut, '\0', sizeof(*ut));
 
 	/* First fill out fields used for both logins and logouts */
@@ -661,6 +664,19 @@
 	if (li->hostaddr.sa.sa_family == AF_INET)
 		ut->ut_addr = li->hostaddr.sa_in.sin_addr.s_addr;
 # endif
+# ifdef HAVE_ADDR_V6_IN_UTMP
+	/* this is just a 128-bit IPv6 address */
+	if (li->hostaddr.sa.sa_family == AF_INET6) {
+		sa6 = ((struct sockaddr_in6 *)&li->hostaddr.sa);
+		memcpy(ut->ut_addr_v6, sa6->sin6_addr.s6_addr, 16);
+		if (IN6_IS_ADDR_V4MAPPED(&sa6->sin6_addr)) {
+			ut->ut_addr_v6[0] = ut->ut_addr_v6[3];
+			ut->ut_addr_v6[1] = 0;
+			ut->ut_addr_v6[2] = 0;
+			ut->ut_addr_v6[3] = 0;
+		}
+	}
+# endif
 }
 #endif /* USE_UTMP || USE_WTMP || USE_LOGIN */
 
@@ -689,6 +705,9 @@
 void
 construct_utmpx(struct logininfo *li, struct utmpx *utx)
 {
+# ifdef HAVE_ADDR_V6_IN_UTMP
+	struct sockaddr_in6 *sa6;
+#  endif
 	memset(utx, '\0', sizeof(*utx));
 # ifdef HAVE_ID_IN_UTMPX
 	line_abbrevname(utx->ut_id, li->line, sizeof(utx->ut_id));
@@ -725,6 +744,19 @@
 	if (li->hostaddr.sa.sa_family == AF_INET)
 		utx->ut_addr = li->hostaddr.sa_in.sin_addr.s_addr;
 # endif
+# ifdef HAVE_ADDR_V6_IN_UTMP
+	/* this is just a 128-bit IPv6 address */
+	if (li->hostaddr.sa.sa_family == AF_INET6) {
+		sa6 = ((struct sockaddr_in6 *)&li->hostaddr.sa);
+		memcpy(ut->ut_addr_v6, sa6->sin6_addr.s6_addr, 16);
+		if (IN6_IS_ADDR_V4MAPPED(&sa6->sin6_addr)) {
+			ut->ut_addr_v6[0] = ut->ut_addr_v6[3];
+			ut->ut_addr_v6[1] = 0;
+			ut->ut_addr_v6[2] = 0;
+			ut->ut_addr_v6[3] = 0;
+		}
+	}
+# endif
 # ifdef HAVE_SYSLEN_IN_UTMPX
 	/* ut_syslen is the length of the utx_host string */
 	utx->ut_syslen = MIN(strlen(li->hostname), sizeof(utx->ut_host));
diff -ruN t/openssh-3.5p1/session.c openssh-3.5p1/session.c
--- t/openssh-3.5p1/session.c	2002-09-26 02:38:50.000000000 +0200
+++ openssh-3.5p1/session.c	2002-11-05 13:11:30.000000000 +0100
@@ -730,8 +730,8 @@
 	 * the address be 0.0.0.0.
 	 */
 	memset(&from, 0, sizeof(from));
+	fromlen = sizeof(from);
 	if (packet_connection_is_on_socket()) {
-		fromlen = sizeof(from);
 		if (getpeername(packet_get_connection_in(),
 		    (struct sockaddr *) & from, &fromlen) < 0) {
 			debug("getpeername: %.100s", strerror(errno));
diff -ruN t/openssh-3.5p1/sshlogin.c openssh-3.5p1/sshlogin.c
--- t/openssh-3.5p1/sshlogin.c	2002-09-04 08:45:11.000000000 +0200
+++ openssh-3.5p1/sshlogin.c	2002-11-05 13:12:25.000000000 +0100
@@ -70,7 +70,7 @@
   struct logininfo *li;
 
   li = login_alloc_entry(pid, user, host, ttyname);
-  login_set_addr(li, addr, sizeof(struct sockaddr));
+  login_set_addr(li, addr, addrlen);
   login_login(li);
   login_free_entry(li);
 }