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just for the completeness of the picture:
proxying a request from freeradius-v4 to MTik-UM-v7b5 seems to be a nogo
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after setting up a new instance for MTik-UM in new radius_rlm of FRv4; FRv4 tries something like a check or hello or something, before the rlm is fully instantiated … which fails … dropped … unsupported
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###### mods-available/radius #####
#
# The module adds a Proxy-State attribute to all proxied packets.
# This `Proxy-State` contains a 32-bit random number, which is unique
# to this module. This unique number helps to detect proxy loops.
#
proxy-hlandtikrad - Status check packet type will be Status-Server
(proxy-hlandtikrad) Event-Timestamp = "Jan 1 1970 00:00:00 UTC"
(proxy-hlandtikrad) NAS-Identifier = "status check - are you alive?"
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but I’ve been warned … so it what it is … anti-corona-boreout-sit-ups 
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Info : FreeRADIUS Version 4.0.0
Info : Copyright 1999-2019 The FreeRADIUS server project and contributors
Info : There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
Info : PARTICULAR PURPOSE
.
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freerad_v4_debug.txt (101 KB)