[nSLUG] Switched to Fibreop, telnet backdoor on Bell router

D G Teed donald.teed at gmail.com
Tue Nov 18 09:53:11 AST 2014


I now have Fibreop, and there are some changes, and one surprise.

I could previously relay email from my own domain using
postfix and the Eastlink SMTP.  Bell allows email only by
authentication and the email must be from the Bell Aliant
email address assigned.  (Can't disagree, I've done
the same sort of restriction at my work's SMTP.)

To resolve this, I used smtpcorp.com free SMTP
service for relaying less than 20 emails a day.  Works
for my domain's needs.

The firewall on the Actiontec is OK, but not very flexible for
dyndns users.  There doesn't seem to be a way to reproduce
the Postrouting I had before with Linux so my dyndns domain
works from inside the house.  I can see where replacing the
Actiontec router can be useful.

At least the thing is stable.  Half the consumer routers out there
can't keep a ssh connection up over night.

The surprise is the Actiontec router has what seems to be a backdoor
of telnet open to the Internet.  I have set its firewall to not allow 23
in or out, and I can still telnet to it from outside:

$ telnet 47.XXX.YYY.ZZZ
Trying 47.XXX.YYY.ZZZ...
Connected to 47.XXX.YYY.ZZZ.
Escape character is '^]'.
===Actiontec xDSL Router===
Login:

Nmap of the 2-254 range around my IP reveals hundreds of routers also open
telnet access to the Internet.

If you don't have access to nmap from outside your home, you can test
with Gibson Research Corp's "Shields Up" website, and scan common ports.

I called Bell support on this, and the guy was earnestly trying to help,
but said they are not trained on the router's firewall and all he
could suggest was a reset, to eliminate the possibility my port
forwarding rules are the problem.  So its looking like I will reuse
my Linux firewall with the vlan tweaks someone posted links to
last year.

The post #5 on this page seems to cover what I'd need to add
on a basic (no wireless) Linux router (in my case,
an Atom system with 2 NICs).

http://digitalhome.ca/forum/showthread.php?t=134496

vconfig comes with the package vlan on Debian.

I remember DSL router backdoors as something from the 90's.
Very surprised to see it still around today.

Of course the speed is nice and my kids won't let me even talk
about going back.
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