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

From: Peter T. Breuer (ptb_at_oboe.it.uc3m.es)
Date: 07/23/03 

Date: Wed, 23 Jul 2003 18:04:40 +0200

Gonzo <gamazono@yahoo.com> wrote:
> "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

No it isn't - unless your routing is wrong, and you PROVED it is not
wrong by pinging successfully. Therefore it is what I said.

> 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?

At the firewall. But please PAY ATTENTION to what I say. The
indication is that the oruter is firewalling or not doing NAT for you.

> Since it works from a windows box on the same LAN, it's *not* the
> wireless router (or it's firewall).

Nonsense. What kind of misbegotten thinking causes you that brainfart?
The windows box has gotten its IP by dhcp, so will certainly have had a
hole in the firewall opened for it by the router. Something stopping
you looking at the routers 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.

But it can't talk on port 25, therefore

   Then you have a firewall up on port 25 outgoing.
   Either that or they have one up and you are sending from a wrong IP
   address to get through.

>> 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

That's correct.

> 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

The cable modem will normally have a WAN address and a localnet
address and will be the router between yoru intranet and your ISPs
intranet.

> 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.

What you are saying is that the cable modem is a hub and the wireless
station is the router. That the cable modem is not a router is nigh on
incredible. Maybe they exist. I've not seen one. I can imagine one - it
would be a terminal device, essentially, an extension of your isp's
net.

> 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

That's exactly right. It normally has an external WAN address (the
cable side), and it routes between that and the port(s) on the internal
net side. It usually does NAT translation for the intranet, but you can
configure it any which way.

> 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

It normally would if you configured it to do so. As I said, it normally
does NAT translation from an intranet. A more convenient way to set it
up is as a port forwarder, however, for a chosen machine on your
intranet. That machine then acts as your gateway.

> switch to it and have it as my gateway to route other traffic. I may

You normally should be able to!

> 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.

Well, what's its ISP side address? How have you managed to get two
addresses out of your ISP? One for the cable modem and one for your
router?

>> > 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.

Then outgoing 25 is not firewalled by you or your router, but incoming
25 from you is firewalled by your ISP. That is hard to imagine.

> 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

Please run tcpdump while making the connection. Compare with the dump
when doing it from windows.

> 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.

Peter

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