Strange behavior with aliased ethernet port

[Alan Madill]

We are setting up a new radius server and replacing several existing
servers.  During the transition we set a second ip address on eth0:1 on
one of the new servers so that it would respond to radius and DNS
queries in place of the retired server at its old ip address.

We have been trying to debug an authentication issue with our vpop
provider.  Looking at the debug trace all appeared to be well.

rad_recv: Access-Request packet from host 216.113.192.27:51209, id=131,
length=170
        User-Name = "name at domain.com"
        User-Password = "secret"
        NAS-IP-Address = 207.194.241.33
        NAS-Identifier = "PGRGBC01AS01.provider.com"
        NAS-Port = 1189
        NAS-Port-Type = Async
        Service-Type = Framed-User
        Framed-Protocol = PPP
        State = 0x
        Calling-Station-Id = "2505674819"
        Called-Station-Id = "5674831"
        Acct-Session-Id = "425788824"
        Ascend-Data-Rate = 31200
        Ascend-Xmit-Rate = 46667
  Processing the authorize section of radiusd.conf
......
rlm_ldap: user name authenticated succesfully
  modcall[authenticate]: module "ldap" returns ok for request 2
modcall: leaving group LDAP (returns ok) for request 2
Login OK: [name] (from client uniserv.2 port 1189 cli 2505674819)
Sending Access-Accept of id 131 to 216.113.192.27 port 51209
Finished request 2
Going to the next request

But the Windows DUN client would fail with a Error 691

Looking at a tcpdump of the traffic between the servers we see...

[root at host raddb]# tcpdump -nn host 216.113.192.27
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
10:04:02.675842 IP 216.113.192.27.51209 > 204.244.116.2.1645: RADIUS,
Access Request (1), id: 0x7e length: 171
10:04:02.680874 IP 204.244.116.14.1645 > 216.113.192.27.51209: RADIUS,
Access Accept (2), id: 0x7e length: 20
10:05:20.101253 IP 216.113.192.27.51209 > 204.244.116.2.1645: RADIUS,
Access Request (1), id: 0x7f length: 171
10:05:20.106329 IP 204.244.116.14.1645 > 216.113.192.27.51209: RADIUS,
Access Accept (2), id: 0x7f length: 20

Notice the request comes to the aliased address but the reply comes from
the real address of the port.  Is this a configuration error, normal
behavior, or a bug?  Looking at other services such as DNS the reply
comes back from the target ip address and port.

Issue was resolved by having our vpop provider put in the proper addresses.