[dhcrelay] Unsupported device type 512 for "pptp-vpn"

Maintainer: @<github-user> (find it by checking history of the package Makefile)
Environment: (lantiq, xrx200 (FB7360-SL), 19-07)

Description:
as defined in the config (uci) we cant use ppp0 interfaces as upstream interfaces ex: for wan.

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All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/
Listening on LPF/eth0.4/9c:c7:a6:e8:63:28
Sending on   LPF/eth0.4/9c:c7:a6:e8:63:28
Listening on LPF/eth0.3/9c:c7:a6:e8:63:28
Sending on   LPF/eth0.3/9c:c7:a6:e8:63:28
Listening on LPF/eth0.2/9c:c7:a6:e8:63:28
Sending on   LPF/eth0.2/9c:c7:a6:e8:63:28
Listening on LPF/eth0.1/9c:c7:a6:e8:63:28
Sending on   LPF/eth0.1/9c:c7:a6:e8:63:28
Unsupported device type 512 for "pptp-vpn"

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than a configuration issue please read the section on submitting
bugs on either our web page at www.isc.org or in the README file
before submitting a bug.  These pages explain the proper
process and the information we find helpful for debugging.

exiting.
root@fb7360:~#

Check the package Makefile history to identify the maintainer and locate the dhcrelay package entry point. Reproduce the configuration with a ppp0 upstream interface on the reported OpenWrt environment, then determine whether the unsupported device type is handled; done means dhcrelay starts and relays traffic through ppp0.