Only ip6_setmoptions() needs to use cached value. Nothing critical if
the destination interface was changed and ip6_output() will load
`ip6_defmcasthlim' once again to re-initialize `ip6' header.

Index: sys/netinet6/in6_proto.c
===================================================================
RCS file: /cvs/src/sys/netinet6/in6_proto.c,v
retrieving revision 1.141
diff -u -p -r1.141 in6_proto.c
--- sys/netinet6/in6_proto.c	24 Jul 2025 18:02:19 -0000	1.141
+++ sys/netinet6/in6_proto.c	24 Jul 2025 18:31:27 -0000
@@ -352,9 +352,9 @@ int	ip6_forwarding = 0;	/* [a] no forwar
 int	ip6_mforwarding = 0;	/* no multicast forwarding unless ... */
 int	ip6_multipath = 0;	/* [a] no using multipath routes unless ... */
 int	ip6_sendredirects = 1;	/* [a] */
-int	ip6_defhlim = IPV6_DEFHLIM;	/* [a] */
-int	ip6_defmcasthlim = IPV6_DEFAULT_MULTICAST_HOPS;
-int	ip6_maxfragpackets = 200;	/* [a] */
+int	ip6_defhlim = IPV6_DEFHLIM;			/* [a] */
+int	ip6_defmcasthlim = IPV6_DEFAULT_MULTICAST_HOPS; /* [a] */
+int	ip6_maxfragpackets = 200;			/* [a] */
 int	ip6_maxfrags = 200;
 int	ip6_log_interval = 5;	/* [a] */
 int	ip6_hdrnestlimit = 10;	/* [a] appropriate? */
Index: sys/netinet6/ip6_input.c
===================================================================
RCS file: /cvs/src/sys/netinet6/ip6_input.c,v
retrieving revision 1.285
diff -u -p -r1.285 ip6_input.c
--- sys/netinet6/ip6_input.c	24 Jul 2025 18:02:19 -0000	1.285
+++ sys/netinet6/ip6_input.c	24 Jul 2025 18:31:27 -0000
@@ -1452,10 +1452,10 @@ const struct sysctl_bounded_args ipv6ctl
 	{ IPV6CTL_HDRNESTLIMIT, &ip6_hdrnestlimit, 0, 100 },
 	{ IPV6CTL_DAD_COUNT, &ip6_dad_count, 0, 10 },
 	{ IPV6CTL_AUTO_FLOWLABEL, &ip6_auto_flowlabel, 0, 1 },
+	{ IPV6CTL_DEFMCASTHLIM, &ip6_defmcasthlim, 0, 255 },
 };
 
 const struct sysctl_bounded_args ipv6ctl_vars[] = {
-	{ IPV6CTL_DEFMCASTHLIM, &ip6_defmcasthlim, 0, 255 },
 	{ IPV6CTL_USE_DEPRECATED, &ip6_use_deprecated, 0, 1 },
 	{ IPV6CTL_MAXFRAGS, &ip6_maxfrags, 0, 1000 },
 	{ IPV6CTL_MFORWARDING, &ip6_mforwarding, 0, 1 },
@@ -1575,6 +1575,7 @@ ip6_sysctl(int *name, u_int namelen, voi
 	case IPV6CTL_HDRNESTLIMIT:
 	case IPV6CTL_DAD_COUNT:
 	case IPV6CTL_AUTO_FLOWLABEL:
+	case IPV6CTL_DEFMCASTHLIM:
 		return (sysctl_bounded_arr(
 		    ipv6ctl_vars_unlocked, nitems(ipv6ctl_vars_unlocked),
 		    name, namelen, oldp, oldlenp, newp, newlen));
Index: sys/netinet6/ip6_output.c
===================================================================
RCS file: /cvs/src/sys/netinet6/ip6_output.c,v
retrieving revision 1.301
diff -u -p -r1.301 ip6_output.c
--- sys/netinet6/ip6_output.c	8 Jul 2025 00:47:41 -0000	1.301
+++ sys/netinet6/ip6_output.c	24 Jul 2025 18:31:27 -0000
@@ -420,7 +420,7 @@ reroute:
 		if (im6o != NULL)
 			ip6->ip6_hlim = im6o->im6o_hlim;
 		else
-			ip6->ip6_hlim = ip6_defmcasthlim;
+			ip6->ip6_hlim = atomic_load_int(&ip6_defmcasthlim);
 	}
 
 #ifdef IPSEC
@@ -1879,6 +1879,7 @@ ip6_setmoptions(int optname, struct ip6_
 	struct ip6_moptions *im6o = *im6op;
 	struct in6_multi_mship *imm;
 	struct proc *p = curproc;	/* XXX */
+	int ip6_defmcasthlim_local = atomic_load_int(&ip6_defmcasthlim);
 
 	if (im6o == NULL) {
 		/*
@@ -1890,7 +1891,7 @@ ip6_setmoptions(int optname, struct ip6_
 			return (ENOBUFS);
 		*im6op = im6o;
 		im6o->im6o_ifidx = 0;
-		im6o->im6o_hlim = ip6_defmcasthlim;
+		im6o->im6o_hlim = ip6_defmcasthlim_local;
 		im6o->im6o_loop = IPV6_DEFAULT_MULTICAST_LOOP;
 		LIST_INIT(&im6o->im6o_memberships);
 	}
@@ -1937,7 +1938,7 @@ ip6_setmoptions(int optname, struct ip6_
 		if (optval < -1 || optval >= 256)
 			error = EINVAL;
 		else if (optval == -1)
-			im6o->im6o_hlim = ip6_defmcasthlim;
+			im6o->im6o_hlim = ip6_defmcasthlim_local;
 		else
 			im6o->im6o_hlim = optval;
 		break;
@@ -2137,7 +2138,7 @@ ip6_setmoptions(int optname, struct ip6_
 	 * structure.
 	 */
 	if (im6o->im6o_ifidx == 0 &&
-	    im6o->im6o_hlim == ip6_defmcasthlim &&
+	    im6o->im6o_hlim == ip6_defmcasthlim_local &&
 	    im6o->im6o_loop == IPV6_DEFAULT_MULTICAST_LOOP &&
 	    LIST_EMPTY(&im6o->im6o_memberships)) {
 		free(*im6op, M_IPMOPTS, sizeof(**im6op));
@@ -2169,7 +2170,7 @@ ip6_getmoptions(int optname, struct ip6_
 		hlim = mtod(m, u_int *);
 		m->m_len = sizeof(u_int);
 		if (im6o == NULL)
-			*hlim = ip6_defmcasthlim;
+			*hlim = atomic_load_int(&ip6_defmcasthlim);
 		else
 			*hlim = im6o->im6o_hlim;
 		return (0);
@@ -2178,7 +2179,7 @@ ip6_getmoptions(int optname, struct ip6_
 		loop = mtod(m, u_int *);
 		m->m_len = sizeof(u_int);
 		if (im6o == NULL)
-			*loop = ip6_defmcasthlim;
+			*loop = atomic_load_int(&ip6_defmcasthlim);
 		else
 			*loop = im6o->im6o_loop;
 		return (0);