[RFC] netifd: add support for dhcp fallback to static ip

[Felix Fietkau]

On 14.07.24 02:34, Roman Yeryomin wrote:
> It is pretty common use case for a network device to be configured
> as DHCP client but having some fallback static IP address where it
> would be reachable for, e.g., configuration.
> 
> This reacts on udhcpc events and sets/removes ipaddr which is
> configured on an iface from network config.
> This means that for an interface with proto set to dhcp but with
> fallback static IP disabled (ipaddr parameter is not set in config)
> cost of this feature is one uci call.
> If static fallback is enabled (ipaddr is present) then the cost is
> 3 uci calls and one ip call for each deconfig/renew/bound udhcpc
> event.
> It is not nothing but to me it seem like it's a very small overhead
> for such a convenience and I think it should be OpenWrt default at
> least for targets with one ethernet port.
> 
> Signed-off-by: Roman Yeryomin <roman at advem.lv>
> ---
> diff --git a/package/network/config/netifd/files/etc/udhcpc.user.d/fallbackip.sh b/package/network/config/netifd/files/etc/udhcpc.user.d/fallbackip.sh
> new file mode 100644
> index 0000000000..79984e9a2f
> --- /dev/null
> +++ b/package/network/config/netifd/files/etc/udhcpc.user.d/fallbackip.sh
> @@ -0,0 +1,23 @@
> +#!/bin/sh
> +
> +[ -z "$1" ] && echo "Error: should be run by udhcpc" && exit 1
> +
> +ipaddr=$(uci get network.$INTERFACE.ipaddr)
> +[ -z "$ipaddr" ] && exit 0
> +
> +ifname=$(uci get network.$INTERFACE.ifname)
> +[ -z "$ifname" ] && exit 0
> +
> +netmask=$(uci get network.$INTERFACE.netmask)
> +[ -z "$netmask" ] && netmask=24
> +
> +case "$1" in
> +	deconfig)
> +		ip ad add $ipaddr/$netmask dev $ifname
> +	;;
> +	renew|bound)
> +		ip ad del $ipaddr dev $ifname
> +	;;
> +esac
> +
> +exit 0

Here's my take on this patch:

1. The feature should be opt-in via an extra config option, since it 
changes the behavior in a meaningful way that might be problematic for 
some users.
2. IP addresses should be set from within netifd, e.g. via the same 
mechanism that the dhcp script uses to bring up the interface.
3. IP settings should not be read directly from uci, but passed down 
from the netifd proto handler, e.g. via env vars.

- Felix