As the corresponding IPv4 `ip_defttl', `ip6_defhlim' used at
initialization time.

traceroute(8) is the only user of IPV6CTL_DEFHLIM, but it doesn't
included in the ramdisk.

Index: sys/netinet/ip_ipip.c
===================================================================
RCS file: /cvs/src/sys/netinet/ip_ipip.c,v
retrieving revision 1.110
diff -u -p -r1.110 ip_ipip.c
--- sys/netinet/ip_ipip.c	8 Jul 2025 00:47:41 -0000	1.110
+++ sys/netinet/ip_ipip.c	17 Jul 2025 19:23:35 -0000
@@ -483,7 +483,7 @@ ipip_output(struct mbuf **mp, struct tdb
 		ip6o->ip6_vfc &= ~IPV6_VERSION_MASK;
 		ip6o->ip6_vfc |= IPV6_VERSION;
 		ip6o->ip6_plen = htons(m->m_pkthdr.len - sizeof(*ip6o));
-		ip6o->ip6_hlim = ip6_defhlim;
+		ip6o->ip6_hlim = atomic_load_int(&ip6_defhlim);
 		in6_embedscope(&ip6o->ip6_src, &tdb->tdb_src.sin6, NULL, NULL);
 		in6_embedscope(&ip6o->ip6_dst, &tdb->tdb_dst.sin6, NULL, NULL);
 
Index: sys/netinet/tcp_subr.c
===================================================================
RCS file: /cvs/src/sys/netinet/tcp_subr.c,v
retrieving revision 1.215
diff -u -p -r1.215 tcp_subr.c
--- sys/netinet/tcp_subr.c	8 Jul 2025 00:47:41 -0000	1.215
+++ sys/netinet/tcp_subr.c	17 Jul 2025 19:23:35 -0000
@@ -461,7 +461,7 @@ tcp_newtcpcb(struct inpcb *inp, int wait
 #ifdef INET6
 	if (ISSET(inp->inp_flags, INP_IPV6)) {
 		tp->pf = PF_INET6;
-		inp->inp_ipv6.ip6_hlim = ip6_defhlim;
+		inp->inp_ipv6.ip6_hlim = atomic_load_int(&ip6_defhlim);
 	} else
 #endif
 	{
Index: sys/netinet/udp_usrreq.c
===================================================================
RCS file: /cvs/src/sys/netinet/udp_usrreq.c,v
retrieving revision 1.348
diff -u -p -r1.348 udp_usrreq.c
--- sys/netinet/udp_usrreq.c	14 Jul 2025 09:06:17 -0000	1.348
+++ sys/netinet/udp_usrreq.c	17 Jul 2025 19:23:35 -0000
@@ -1134,7 +1134,8 @@ udp_attach(struct socket *so, int proto,
 		return error;
 #ifdef INET6
 	if (ISSET(sotoinpcb(so)->inp_flags, INP_IPV6))
-		sotoinpcb(so)->inp_ipv6.ip6_hlim = ip6_defhlim;
+		sotoinpcb(so)->inp_ipv6.ip6_hlim =
+		    atomic_load_int(&ip6_defhlim);
 	else
 #endif
 		sotoinpcb(so)->inp_ip.ip_ttl = atomic_load_int(&ip_defttl);
Index: sys/netinet6/icmp6.c
===================================================================
RCS file: /cvs/src/sys/netinet6/icmp6.c,v
retrieving revision 1.269
diff -u -p -r1.269 icmp6.c
--- sys/netinet6/icmp6.c	17 Jul 2025 03:00:45 -0000	1.269
+++ sys/netinet6/icmp6.c	17 Jul 2025 19:23:35 -0000
@@ -1158,7 +1158,7 @@ icmp6_reflect(struct mbuf **mp, size_t o
 	ip6->ip6_vfc &= ~IPV6_VERSION_MASK;
 	ip6->ip6_vfc |= IPV6_VERSION;
 	ip6->ip6_nxt = IPPROTO_ICMPV6;
-	ip6->ip6_hlim = ip6_defhlim;
+	ip6->ip6_hlim = atomic_load_int(&ip6_defhlim);
 
 	icmp6->icmp6_cksum = 0;
 	m->m_pkthdr.csum_flags = M_ICMP_CSUM_OUT;
Index: sys/netinet6/in6_proto.c
===================================================================
RCS file: /cvs/src/sys/netinet6/in6_proto.c,v
retrieving revision 1.135
diff -u -p -r1.135 in6_proto.c
--- sys/netinet6/in6_proto.c	8 Jul 2025 00:47:41 -0000	1.135
+++ sys/netinet6/in6_proto.c	17 Jul 2025 19:23:35 -0000
@@ -352,7 +352,7 @@ 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;
+int	ip6_defhlim = IPV6_DEFHLIM;	/* [a] */
 int	ip6_defmcasthlim = IPV6_DEFAULT_MULTICAST_HOPS;
 int	ip6_maxfragpackets = 200;
 int	ip6_maxfrags = 200;
Index: sys/netinet6/in6_src.c
===================================================================
RCS file: /cvs/src/sys/netinet6/in6_src.c,v
retrieving revision 1.103
diff -u -p -r1.103 in6_src.c
--- sys/netinet6/in6_src.c	8 Jul 2025 00:47:41 -0000	1.103
+++ sys/netinet6/in6_src.c	17 Jul 2025 19:23:35 -0000
@@ -377,7 +377,7 @@ in6_selecthlim(const struct inpcb *inp)
 	if (inp && inp->inp_hops >= 0)
 		return (inp->inp_hops);
 
-	return (ip6_defhlim);
+	return (atomic_load_int(&ip6_defhlim));
 }
 
 /*
Index: sys/netinet6/ip6_input.c
===================================================================
RCS file: /cvs/src/sys/netinet6/ip6_input.c,v
retrieving revision 1.279
diff -u -p -r1.279 ip6_input.c
--- sys/netinet6/ip6_input.c	8 Jul 2025 00:47:41 -0000	1.279
+++ sys/netinet6/ip6_input.c	17 Jul 2025 19:23:35 -0000
@@ -1446,10 +1446,10 @@ const struct sysctl_bounded_args ipv6ctl
 #ifdef MROUTING
 	{ IPV6CTL_MRTPROTO, &ip6_mrtproto, SYSCTL_INT_READONLY },
 #endif
+	{ IPV6CTL_DEFHLIM, &ip6_defhlim, 0, 255 },
 };
 
 const struct sysctl_bounded_args ipv6ctl_vars[] = {
-	{ IPV6CTL_DEFHLIM, &ip6_defhlim, 0, 255 },
 	{ IPV6CTL_MAXFRAGPACKETS, &ip6_maxfragpackets, 0, 1000 },
 	{ IPV6CTL_LOG_INTERVAL, &ip6_log_interval, 0, INT_MAX },
 	{ IPV6CTL_HDRNESTLIMIT, &ip6_hdrnestlimit, 0, 100 },
@@ -1569,6 +1569,7 @@ ip6_sysctl(int *name, u_int namelen, voi
 #ifdef MROUTING
 	case IPV6CTL_MRTPROTO:
 #endif
+	case IPV6CTL_DEFHLIM:
 		return (sysctl_bounded_arr(
 		    ipv6ctl_vars_unlocked, nitems(ipv6ctl_vars_unlocked),
 		    name, namelen, oldp, oldlenp, newp, newlen));