Reliably detecting the absence of IPv6

[Oldřich Jedlička]

st 17. 12. 2025 v 19:26 odesílatel Philip Prindeville
<philipp_subx at redfish-solutions.com> napsal:
>
> > On Dec 12, 2025, at 6:48 AM, Oldřich Jedlička <oldium.pro at gmail.com> wrote:
> >
> > čt 11. 12. 2025 v 4:45 odesílatel Philip Prindeville via openwrt-devel
> > <openwrt-devel at lists.openwrt.org> napsal:
> >> I previously tested with -z “$(ip -6 -o route show default)” but as someone pointed out, if at the time that Bind starts up, someone later brings up an IPv6 default route (say via DHCP or a tunnel), then this caused a denial of service.
> >
> > If you have issues with network connectivity changes during startup,
> > there is a way for the service to react. This is how I tried to solve
> > similar startup issue (network not up or changed) for fwknopd:
> >
> > https://github.com/openwrt/packages/blob/master/net/fwknop/files/fwknopd.init#L16-L45
> >
> > The procd options for service startup are described here, although I
> > had to check the source code to really verify what is expected as input
> > (network name or linux interface name):
> >
> > https://openwrt.org/docs/guide-developer/procd-init-scripts
> >
> > So when you correctly define your network dependencies, you should get
> > restarted by procd automatically.
> >
> > Cheers,
> > Oldrich.
>
>
> I’ve noticed that if /etc/init.d/network restart is called, it makes the LAN interfaces go away (especially the VLAN or bridge ones) which can make dhcpd quit.
>
> What’s the “ubus” command to get the ifnames on the group “lan”?  Or the wrapper for that?  Is it network_get_device()?
>
> I’m wondering if I need to do:
>
> procd_set_param netdev $dhcp_ifs
>
> in /etc/init.d/dhcpd …
>

The fwknopd knows which network interface (in the sense of
/etc/config/network interface name) it is listening to - this is
configured. This one goes to procd_add_reload_interface_trigger (yeah,
I feel the interface vs interface confusion). Then you need to find
the existing physical interface (i.e. the Linux kernel one), which can
be done by a call to network_get_device (function from the imported
network.sh):

https://github.com/openwrt/packages/blob/master/net/fwknop/files/fwknopd.init#L153

That physical interface (if found!) is the parameter to
procd_set_param netdev. If the interface is not yet known, it means
that it does not exist yet and the procd_add_reload_interface_trigger
should handle that situation and reload the service. Reloading means
that the start_service will be called again.

One gotcha in the fwknopd script is that you actually let procd start
and stop the application instances for you based on the procd_*
definitions. This means that when you find the physical interface is
missing, you do not define any procd_open_instance (the fwknopd needs
an existing physical interface to start listening on). When you know
the physical interface, that is the first time you call
procd_open_instance. Procd is able to start/stop/restart application
based on the definitions you provide, so just configure the instances
(command line arguments, netdev...) you are sure about and procd will
handle the rest for you.

You can even define more instances too if necessary. I am using the
following script for hostapd hotplug functionality with multiple
daemons running:

#!/bin/sh /etc/rc.common

START=90

USE_PROCD=1
PROG=/usr/sbin/hostapd_cli

start_service() {
        local ifnames
        ifnames=$(ubus call network.wireless status | jsonfilter -e
"@[@.up=true].interfaces[*].ifname")

        for ifname in $ifnames; do
                procd_open_instance
                procd_set_param respawn
                procd_set_param command "$PROG" -a
/usr/libexec/hotplug/hostapd -i $ifname
                procd_set_param netdev $ifname
                procd_close_instance
        done
}

service_triggers() {
        procd_add_reload_trigger "wireless"
}

Get the inspiration from the actual scripts in the packages repo, this
one uses the same logic as fwknopd:

https://github.com/openwrt/packages/blob/master/net/mini_snmpd/files/mini_snmpd.init

Some scripts just use procd_add_reload_interface_trigger and not the
procd_set_param netdev. I am not sure whether that is enough.

Hope this helps :-)

Cheers,
Oldrich.