Index: tests/sys/netinet/libalias/2_natout.c
===================================================================
--- /dev/null
+++ tests/sys/netinet/libalias/2_natout.c
@@ -0,0 +1,100 @@
+#include <atf-c.h>
+#include <alias.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#include <arpa/inet.h>
+#include <netinet/ip.h>
+
+static struct ip *
+ip_packet(struct in_addr src, struct in_addr dst, u_char protocol, size_t len) {
+	struct ip * p;
+
+	ATF_REQUIRE(len >= 64 && len <= IP_MAXPACKET);
+
+	p = malloc(len);
+	ATF_REQUIRE(p != NULL);
+
+	p->ip_v = IPVERSION;
+	p->ip_hl = sizeof(*p)/4;
+	p->ip_len = htons(len);
+	p->ip_ttl = IPDEFTTL;
+	p->ip_src = src;
+	p->ip_dst = dst;
+	p->ip_p = protocol;
+	ATF_REQUIRE(p->ip_hl == 5);
+
+	return (p);
+}
+
+static inline bool
+addr_eq(struct in_addr a, struct in_addr b) {
+	return a.s_addr == b.s_addr;
+}
+
+#define a2h(a)	ntohl(a.s_addr)
+
+#define NAT_CHECK(pip, src, dst)	do {	\
+	int res;				\
+	res = LibAliasOut(la, pip, 64);		\
+	ATF_CHECK(res == PKT_ALIAS_OK);		\
+	ATF_CHECK(addr_eq(src, pip->ip_src));	\
+	ATF_CHECK(addr_eq(dst, pip->ip_dst));	\
+} while(0)
+
+ATF_TC_WITHOUT_HEAD(1_simplemasq);
+ATF_TC_BODY(1_simplemasq, dummy)
+{
+	struct libalias *la = LibAliasInit(NULL);
+	struct in_addr masq = { htonl(0x01020304) };
+	struct in_addr src  = { htonl(0x0a010203) };
+	struct in_addr dst  = { htonl(0x12345678) };
+	struct ip *pip;
+
+	ATF_REQUIRE(la != NULL);
+	LibAliasSetAddress(la, masq);
+
+	pip = ip_packet(src, dst, 254, 64);
+	NAT_CHECK(pip, masq, dst);
+	free(pip);
+
+	LibAliasUninit(la);
+}
+
+ATF_TC_WITHOUT_HEAD(2_unregistered);
+ATF_TC_BODY(2_unregistered, dummy)
+{
+	struct libalias *la = LibAliasInit(NULL);
+	struct in_addr masq = { htonl(0x01020304) };
+	struct in_addr src1 = { htonl(0x0a010203) };
+	struct in_addr src2 = { htonl(0xa0102030) };
+	struct in_addr dst  = { htonl(0x12345678) };
+	struct ip *pip;
+
+	ATF_REQUIRE(la != NULL);
+	LibAliasSetAddress(la, masq);
+	LibAliasSetMode(la, PKT_ALIAS_UNREGISTERED_ONLY, ~0);
+
+	pip = ip_packet(src1, dst, 254, 64);
+	NAT_CHECK(pip, masq, dst);
+
+	pip->ip_src = src2;
+	NAT_CHECK(pip, src2, dst);
+
+	LibAliasSetMode(la, 0, PKT_ALIAS_UNREGISTERED_ONLY);
+	NAT_CHECK(pip, masq, dst);
+
+	free(pip);
+	LibAliasUninit(la);
+}
+
+ATF_TP_ADD_TCS(natout)
+{
+	/* Use "dd if=/dev/random bs=2 count=1 | od -x" to reproduce */
+	srand(0x0b61);
+
+	ATF_TP_ADD_TC(natout, 1_simplemasq);
+	ATF_TP_ADD_TC(natout, 2_unregistered);
+
+	return atf_no_error();
+}
Index: tests/sys/netinet/libalias/Makefile
===================================================================
--- tests/sys/netinet/libalias/Makefile
+++ tests/sys/netinet/libalias/Makefile
@@ -7,7 +7,8 @@
 TESTSDIR=	${TESTSBASE}/sys/netinet/libalias
 BINDIR=		${TESTSDIR}
 
-ATF_TESTS_C+=	1_instance
+ATF_TESTS_C+=	1_instance	\
+		2_natout	\
 
 LIBADD+=	alias