Welcome to the most active Linux Forum on the web.
Go Back > Forums > Linux Forums > Linux - Newbie
User Name
Linux - Newbie This Linux forum is for members that are new to Linux.
Just starting out and have a question? If it is not in the man pages or the how-to's this is the place!


  Search this Thread
Old 02-22-2009, 08:44 AM   #1
LQ Newbie
Registered: Feb 2009
Posts: 3

Rep: Reputation: 0
problem telnet to WAN IP address from LAN side


I built my first Linux server yesterday. My network consists of 1 Linux server and several Windows PC behind ADSl router/modem with ISP providing a static IP address. I have setup postfix and POP3 accounts. I can send email (relay via my ISP SMTP server) but have some problems receiving when i'm on my home network (i can receive emails fine when connected to 'other networks' eg my neighbours unprotected wifi ;-))

Whenever i'm on my home network i receive error 'could not open connection to host'

Since then i have noticed from my linux machine (Ubuntu 8.10) i can
$ netcat localhost 25


$ netcat <my_WAN_IP_Address> 25


i can telnet from my windows laptop when connected to my neighbours wifi:

>telnet (my_WAN_IP_Address> 25


Any ideas? I have port forwarding configured in my router to direct all incoming WAN traffic on port 25, 110 to my Linux IP Address on LAN side. But if i try to access the Linux server via my static WAN IP FROM LAN side it never works.

Pleaes help!

Old 02-22-2009, 08:51 AM   #2
Registered: Jun 2001
Location: UK
Distribution: Gentoo, RHEL, Fedora, Centos
Posts: 43,417

Rep: Reputation: 1975Reputation: 1975Reputation: 1975Reputation: 1975Reputation: 1975Reputation: 1975Reputation: 1975Reputation: 1975Reputation: 1975Reputation: 1975Reputation: 1975
I don't quite understand the description of the scenario you gave, but the examples with telnet are the correct behaviour. You would normally only be able to hit services on the wan IP from outside of your network. in your example you connect to wan:25 from the client. This is then forwarded into your server. the server then sees a packet addressed to it from the client, and so replies *directly* to the client to establish a connection. however your client is expecting to talk to the wan ip, not the server ip, so disregards the response. Compare this to you wardriving and hitting wan:25 then that also hits the server, but the response is sent back to the internet ip address of your neighbors router, so goes back through your router, which then translates the ip back to the wan address and you then continue to establish a connection.

Last edited by acid_kewpie; 02-22-2009 at 08:52 AM.
Old 02-22-2009, 09:05 AM   #3
LQ Newbie
Registered: Feb 2009
Posts: 3

Original Poster
Rep: Reputation: 0
Thanks for the reply, maybe if i describe what i need to do you can help me !

I have created email accounts within postfix running on my Ubuntu server which i want to access from my windows laptop with outlook express.

So i configure outlook express to receive emails through pop3.
ie outlook POP3 server address is: points to my static IP address given by my ISP.

Problem is outlook express cant open port 110 to from my LAN. It DOES work when i'm not conneced to my home network eg neighbours wifi.

I have same issue with telnet also as i tried to describe previously.

Hopefully this explanation is clearer.

Thanks !
Old 02-22-2009, 09:12 AM   #4
LQ Newbie
Registered: Feb 2009
Posts: 3

Original Poster
Rep: Reputation: 0

if i change the pop3 server address in Outlook Express from:



It works OK.

However, now i can only download emails when i'm on my home LAN. Obviously if im on a different network the linux-server-hostname can not be found.

Is there a single configuration that can work whether im on my home network or another network?


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Trying to run ftp server on Suse 10: Problem with WAN side access cornfusedlinuxuser Linux - Networking 1 03-07-2006 05:34 PM
SSH WAN/LAN problem Hangdog42 Linux - Security 6 10-18-2005 09:51 PM
Cant get WAN side ip...... hkl8324 Linux - Wireless Networking 1 06-08-2005 06:03 AM
Connectivity problem with network on Mandrake 10.1 LAN/WAN/Samba BiO|Happy Linux - Wireless Networking 0 04-29-2005 04:11 PM
LAN/WAN IP address and http to virtual server behind DSL modem E-Rick Linux - Networking 3 10-19-2004 10:01 AM > Forums > Linux Forums > Linux - Newbie

All times are GMT -5. The time now is 09:09 PM.

Main Menu
Write for LQ is looking for people interested in writing Editorials, Articles, Reviews, and more. If you'd like to contribute content, let us know.
Main Menu
RSS1  Latest Threads
RSS1  LQ News
Twitter: @linuxquestions
Facebook: linuxquestions Google+: linuxquestions
Open Source Consulting | Domain Registration