Suggested rule set addition:
icmp_types = "{ echoreq, unreach }"
pass inet proto icmp all icmp-type $icmp_types keep state
See RFC792, RFC950, RFC1191, RFC1256, RFC2521, RFC2765 (ICMP4),
RFC1885, RFC2463, RFC2466, RFC4443 (ICMP6)
ietf.org or faqs.org + PF man pages