[nSLUG] [OT] Net connectivity problem (Attn. sys admins)

Gerard MacNeil macneil.g at gmail.com
Sun Apr 28 09:46:01 ADT 2013

Being a "Been there, fixed that" guy, the important facts are both
stated and unstated:

On Sun, 28 Apr 2013 04:12:51 -0300
mspencer at tallships.ca (Mike Spencer) wrote:

>     + traceroute fails to reach the POP3 server, dying after one hop,
>       the response from the modem terminal server itself.
Clue 1. Modem terminal server cannot find POP3 server!?!

> Connection to the ISP's SMTP and HTTP servers (different machines)
> work as expected. Connection to the rest of the net works as expected.
Clue 2: And which machine, pray tell, real or virtualized, is the POP3

OK. I'll just guess that we have a network of PCs, all servers on
separate boxes, old style (I'll call it the LAN, expecting several
Internet Connected Router
Terminal Server
POP3 Server
SMTP Server
HTTP Server
Authentication Server
Dialup Client 

> Once connected to the net via OtherISP, I can always access
> the POP3 server at MyISP without problem
Clue 3: All routing works coming from a separate network via the
Internet Connected Router. Thus confirming that we are "on the
Internet" ... I never could figure out what that phrase really meant.

> Anybody with sys admin experience have a notion, however weird, of
> what's going on here?  Packet of death? Known hardware failure mode?
> Known routing or firewall configuration error? 

Yeah, Chebucto, Supercity, and more. The problem is in clue 1.  The
random times you connect to the POP3 server are likely routed through
one of the other server's in the LAN. Unknown variables like when you
have mail to send, you connect some other LAN server before the POP3
request or maybe the time after that sort of thing happens or some
other consequence of the routing implementation on the Terminal Server.
That's implied by Clue 3.

The Default Route from the Terminal Server is NOT to the POP3
server. The Default Route is to the Internet Connected Router. So when
the Dialup Client, which MUST be considered part of the LAN in this
context, tries to access the POP3 server via it's Default Route, the
POP3 Server cannot be found. 

After the one hop from the Dialup Client to the Terminal Server, the
next hop to the POP3 server probably needs to be to another subnet on
the LAN. The Terminal Server does not seem to know how to get there.

IIRC, it has been some years, POP3 was different than other
servers. Something about the ISP giving away the family jewels if the
Email in their possession was sent to the wrong customer. Again, IIRC,
the POP3 server itself tries to verify the reverse connection to the

I thought the 'iproute2' command set ('ip route ...' or 'ip neigh ...')
was supposed to address this sort of situation. It was brand new when I
left the industry.


ps. My solution back in the day was to put the Terminal Server on the
same subnet as the POP3/SMTP Servers. Then again, I had lots of IP
addresses to play with.

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