Re: HELP: Linux telnet smtp server fails, Works from MS Windows

From: Gonzo (gamazono_at_yahoo.com)
Date: 07/23/03 

Date: 23 Jul 2003 08:13:00 -0700

"Peter T. Breuer" <ptb@oboe.it.uc3m.es> wrote in message news:<jlhlfb.7cg.ln@news.it.uc3m.es>...
> Gonzo <gamazono@yahoo.com> wrote:
> > Trying to get sendmail working. On linux box, cannot telnet to my
> > ISP's smtp server.
>
> Then you have a firewall up on port 25 outgoing. Take it away.
> Either that or they have one up and you are sending from a wrong IP
> address to get through.

I'm not sure that's it. I think it may be something else with my
network setup on my Linux box. If it is a firewall thing, it must be
on the linux box itself. Any suggestions where I should look there?
Since it works from a windows box on the same LAN, it's *not* the
wireless router (or it's firewall).

>
> > From linux box: telnet my-isp-smtp-server 25
> > I get: telnet: Unable to connect to remote host: Connection refused
>
> Well, refused is a bit strong ...
>
> > I can ping it fine though.
>
> But not on port 25!

Obviously, since 25 is talking SMTP. The point being that the linux
box CAN see the the smtp server and can translate the name to it's IP
address.

>
> > From my windows box, when I try the same telnet, it works fine and I
> > can do a manual email out, and it gets delivered fine.
>
> > Any help would be greatly appreciated. I've been working on it for a
> > few evenings now, and I'm out of ideas.
>
> THe first idea you should have would be to tell us about your
> networking.
>
> > Here's my network setup, just in case:
>
> "in case"?
>
> > RoadRunner cable modem --> wireless router --> linux box & windows
>
> Uh ... you surely don't mnean wireless router, but wireless HUB. A
> basestation, no, attached to one of the sockets of the *cable modem
> and router*.

Not to be contrary but it is a wireless router. A hub is a shared
network device that only connects devices on the _same_ network. A
router, which this is, routes traffic from one network to another
network, which this does. The wireless router has TWO IP addresses, in
this case one is the IP address for the wan (via the cable modem), the
other IP address is for my lan (in this case a 192.168.x.x address).
Then computers on one side of the router can speak with computers on
the other side of the router. It also does NAT, so that my lan side
can be non-routable IP addresses and all go out looking as if they
came from the routable address given to me by my ISP.

I'm not real sure about the inner workings of cable modems or the
ISP's network, but I'm pretty sure that the cable modem is not a
router. It allows ONE routable IP address through it's ONE port. It
seems to be just a dhcp enabled device on their network. It certainly
does NOT seem to route traffic. Meaning, I can't hook up a hub or
switch to it and have it as my gateway to route other traffic. I may
be wrong on that point though. Irregardless though, this wireless
device of mine is a router which routes traffic from my 192.x side to
the ISP side of the network.

>
> > Now, my linux box does NOT use dhcp, it has a fixed 192.168.x.x
> > address.
>
> Well, that's not going to even get out. That's a private address.
> You are going to have to set your cable modem router to do NAT
> to make the packets look like they are coming from its approved
> range - let's suppose it does that. Apparently it's picky about who it
> does it for, so you get to play with the cable modem router to
> tell it to pass more stuff out.

The wireless router does do NAT. And it handles it fine. My linux box
has no problems doing anything else that I need. I *can* telnet from
the linux box to other machines, including port 25 on another smtp
server.

The problem is that other smtp server can't be used (it won't send
outside of its domain). So the smtp server that I need to use, I can't
connect to port on 25 from my linux box (even though I *can* connect
to it from a windows box that's on the same internal network).

>
> > My windows box DOES use dhcp. The wireless router is the dhcp server
> > too.
>
> No, the "cable modem router" is! The wireless basestation is just a
> hub. Talk to the wireless router .. there's no reason why it shouldn't
> do NAT for you if you ask it, and there's also no reason why you
> shouldn't allow yourself to get a local address by dhcp from it too.

The wireless thingy really is a router, not a hub.

It does do NAT. I *could* get an address from it using DHCP, but since
I allow my wireless router to route inbound ssh traffic to my linux
box, the linux box needs a fixed address.

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