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Old 09-06-2004, 08:35 AM   #1
Registered: Oct 2003
Location: Karachi, PAKISTAN
Distribution: Redhat, Fedora, Open BSD, FreeBSD, SlackWare
Posts: 109

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Question For Linux GURU's: route configuration in complex network

Dear All,
I am in big trouble and tried since from 2 weeks, but unsucsesfull PLEASE HELP ME.

I configure a Linux (mandrake 10 ) router with 4 NICs.

eth0 = (attached with my Local LAN)
eth1 = (attached with Internet DSL for internet browsing)
eth1:0 = (attachec with DSL for my e-mail server by NAT)
eth2 = (attached with my second LAN )
eth3 = (attached with my second DSL Link for internet access)

my route command shows.
[root@router root]# route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface * U 0 0 0 eth3 * U 0 0 0 eth2 * U 0 0 0 eth0 * U 0 0 0 eth1 * U 0 0 0 lo
default UG 0 0 0 eth3
[root@router root]#

now I want to NAT my e-mail server traffice from IP to my Local LAN PC IP,HOW CAN I DO THIS PLEASSSSSSSS HELP ME
If I change default route then internet not work. and I dont know how my e-mail server send traffic on network.

Help me soon Its already tooooooooo LATE.
Thanks in advance
Old 09-06-2004, 09:00 AM   #2
Senior Member
Registered: Jul 2004
Distribution: Ubuntu 7.04
Posts: 1,990

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Look at the /etc/services file. It lists the port numbers for all the various “standard” protocols.
SMTP is outgoing email (pushing to a mailbox from a client).
POP3 is incoming email (i.e. fetching email from a mailbox to a client).
IMAP is also incoming email (i.e. fetching email from a mailbox to a client). Most setups use only one of IMAP or POP3.

You need to redirect these ports only to do what you're asking.

But there may be other ways. Can you set up to use the Linux router as a gateway? Then it could just connect directly to to fetch or send email, and the gateway should take care of it by the non-default route (the default route is only ever used as a fallback).

You could also look at port forwarding and tunneling programs. Personally, I use stunnel on my computer to tunnel and secure the POP3 socket on my local machine to the POP3S socket on my mail server. The mail client then fetches my email from the local machine — at which point, neither the email client nor NAT cares about the fact that the email actually comes from a different computer.

Hope that helps,

— Robert J. Lee


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