npf NAT stops working on external interface IP changed

["Jeremy C. Reed" <reed%reedmedia.net@localhost>]

Last week, my NetBSD NPF router got a new IP address via DHCP.

npfctl list showed many entries with the nat-addr:port with the old 
address.

I did a npfctl reload and my NAT started working again.

Today it happened again.

"npfctl show" shows the current IP address in the map.

Part of my /etc/npf.conf follows:

$ext_if = "re1"
$int_if = "re0"
$ext_addrs = { ifaddrs($ext_if) }
$localnet = { 172.16.1.0/24 }

# Allow pings
alg "icmp"

# Perform IPv4 NAT
map inet4($ext_if) dynamic $localnet -> inet4($ext_if)

group "external" on $ext_if {
    # Allow all outbound traffic
    pass stateful out all

    # Block all incoming traffic
    block in all
}

group "internal" on $int_if {
    # Pass everything to internal networks,
    # should be ok, because we are nat'ed.
    pass final all
}

# default group is mandatory
group default {
    # Loopback interface should allows packets to traverse it.
    pass final on lo0 all
    # Block everything by default.
    block all
}


When the problem began my logs had:

Jan 16 18:28:24 t1 unbound: [210:0] error: event_add failed. in cpsl.
Jan 16 18:28:25 t1 syslogd[189]: last message repeated 2 times
Jan 16 18:28:25 t1 unbound: [210:0] error: could not event_del on close
Jan 16 18:28:25 t1 unbound: [210:0] error: event_add failed. in cpsl.
...
Jan 16 18:28:49 t1 unbound: [210:0] error: could not event_del on close
Jan 16 18:28:49 t1 unbound: [210:0] error: event_add failed. in cpsl.

Jan 16 18:28:50 t1 dhcpcd[152]: re1: probing for an IPv4LL address
Jan 16 18:28:50 t1 dhcpcd[152]: re1: using IPv4LL address 169.254.77.128
Jan 16 18:28:50 t1 dhcpcd[152]: re1: DHCP lease expired

Then it was offered a new IP, added route, changed default route.

I did a "sudo npfctl reload" to get NAT to work again.

How can I get it to automatically reload on external interface changes?