diff --git a/sys/netinet/ip_icmp.c b/sys/netinet/ip_icmp.c
index fd1a3cd24..61f71b6ca 100644
--- a/sys/netinet/ip_icmp.c
+++ b/sys/netinet/ip_icmp.c
@@ -124,6 +124,20 @@ SYSCTL_UINT(_net_inet_icmp, OID_AUTO, maskfake, CTLFLAG_VNET | CTLFLAG_RW,
 	&VNET_NAME(icmpmaskfake), 0,
 	"Fake reply to ICMP Address Mask Request packets");
 
+/*
+ * XXX TEST/DEBUG ONLY -- not for upstream submission.
+ *
+ * Truncates outgoing ICMP_TSTAMPREPLY/ICMP_MASKREPLY packets by this many
+ * bytes before transmit, to reproduce ping(8) pr_pack() short-reply
+ * handling for local testing.  0 (default) disables and is a no-op.
+ */
+VNET_DEFINE_STATIC(int, icmp_debug_trunclen) = 0;
+#define	V_icmp_debug_trunclen		VNET(icmp_debug_trunclen)
+SYSCTL_INT(_net_inet_icmp, OID_AUTO, debug_trunclen, CTLFLAG_VNET | CTLFLAG_RW,
+	&VNET_NAME(icmp_debug_trunclen), 0,
+	"XXX TEST ONLY: truncate outgoing ICMP timestamp/mask replies by "
+	"this many bytes; 0 disables (default)");
+
 VNET_DEFINE(int, drop_redirect) = 0;
 #define	V_drop_redirect			VNET(drop_redirect)
 SYSCTL_INT(_net_inet_icmp, OID_AUTO, drop_redirect, CTLFLAG_VNET | CTLFLAG_RW,
@@ -655,6 +669,18 @@ icmp_input(struct mbuf **mp, int *offp, int proto)
 			    ip->ip_src = satosin(&ia->ia_dstaddr)->sin_addr;
 		}
 reflect:
+		/* XXX TEST/DEBUG ONLY -- see V_icmp_debug_trunclen above. */
+		if (V_icmp_debug_trunclen > 0 &&
+		    (icp->icmp_type == ICMP_TSTAMPREPLY ||
+		     icp->icmp_type == ICMP_MASKREPLY)) {
+			int trunclen = min(V_icmp_debug_trunclen,
+			    icmplen - ICMP_MINLEN);
+
+			if (trunclen > 0) {
+				m_adj(m, -trunclen);
+				ip->ip_len = htons(ntohs(ip->ip_len) - trunclen);
+			}
+		}
 		ICMPSTAT_INC(icps_reflect);
 		ICMPSTAT_INC2(icps_outhist, icp->icmp_type);
 		icmp_reflect(m);
@@ -753,6 +779,10 @@ icmp_input(struct mbuf **mp, int *offp, int proto)
 	}
 
 raw:
+	if (icmplen <= 32)
+		printf("XXXDBG icmp_input: raw: type=%d icmplen=%d "
+		    "hlen=%d m_len=%d m_pkthdr.len=%d\n", icp->icmp_type,
+		    icmplen, hlen, m->m_len, m->m_pkthdr.len);
 	*mp = m;
 	rip_input(mp, offp, proto);
 	return (IPPROTO_DONE);