http://ru.wikipedia.org/wiki/PPP_(сетевой_протокол)

root@gate:~# cat /etc/ppp/chap-secrets

user1 * password1 192.168.100+X.101
user2 * password2 192.168.100+X.102

[gate:~] # cat /etc/ppp/ppp.secret

user1 ppassword1 192.168.100+X.101
user2 ppassword2 192.168.100+X.102

[gate:~] # cat /etc/ppp/ppp.conf

inet:
 enable mschapv2
 set ifaddr 192.168.100+X.1 192.168.100+X.2-192.168.100+X.2
# set dns 192.168.X.10
# accept dns
 set timeout 0

[root@gate ~]# cat /etc/ppp/pppoe-server-options

require-mschap-v2
# ms-dns 192.168.X.10

Сервис FreeRADIUS

gate# apt install libradcli4

gate# cat /etc/ppp/pppoe-server-options

...
plugin radius.so
radius-config-file /etc/radcli/radiusclient.conf

gate# cat /etc/radcli/radiusclient.conf

seqfile /var/run/radius.seq
mapfile /etc/radcli/port-id-map
authserver      server
acctserver      server
servers         /etc/radcli/servers
dictionary      /etc/radcli/dictionary
default_realm
radius_timeout  10
radius_retries  3
bindaddr        *

gate# cat /etc/radcli/servers

server testing123

gate# cat /etc/hosts

...
192.168.X.10 server

gate# rm /etc/ppp/chap-secrets

ubuntu14# apt install radiusclient1

ubuntu16# apt install libfreeradius-client-dev
или хватит
ubuntu16# apt install libfreeradius-client2

root@gate:~# cat /etc/radiusclient/radiusclient.conf

...
authserver    server.corpX.un
...
acctserver    server.corpX.un
...
#radius_deadtime       0
...

root@gate:~# cat /etc/radiusclient/servers

server.corpX.un testing123

ubuntu16# touch /etc/radiusclient/port-id-map

root@gate:~# cat /etc/ppp/pppoe-server-options

+chap
plugin radius.so

[gate:~] # rm /etc/ppp/ppp.secret

[gate:~] # cat /etc/radius.conf

auth server.corpX.un testing123 3 2
acct server.corpX.un testing123 3 2

[gate:~] # cat /etc/ppp/ppp.conf

...
 set timeout 0
 set radius /etc/radius.conf