Re: What is a 'default route'??
From: David Haggett (news-spam_at_haggett.demon.co.uk)
Date: 03/10/04
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Date: Wed, 10 Mar 2004 08:31:21 +0000
On Tue, 09 Mar 2004 21:18:57 -0800, Arthur wrote:
This is not a linux question, and the answer would apply whichever OS you
use.
> I have a Linux box (Mandrake 9.2). It has 2 NIC's. I have 2
> different DSL providers. Thus I have 2 routers, 1 switch and 2 DSL
> modems.
OOoooh....nice:)
>
> Below is my routing table. What is a default route? Basically, does
> this mean that ALL traffic on your network goes out to the internet
The default route is the next-hop used to send traffic with a destination
address not on your local network. It is used as a "last resort" if there
is no specific route to a destination in the hosts routing table.
The IP stack will allow you to set more than 1 default gateway but in your
set-up it wouldn't work. If you set both, default behavior is (probably) to
load-balance (i.e. alternate) packets between gateways. Because you use
NAT, alternate packets arrive at the destination server with a different
source address, and a TCP connection cannot be reliably maintained.
Packet 1 (SYN) SRC: 206.126.232.53 DST <Server on Internet>
Reply 1 (SYN-ACK) SRC: <Server on Internet> DST: 206.126.232.53
Packet 2 (ACK) SRC: 64.36.28.114 DST: <Server on Internet>
Because the Internet server hasn't started the TCP negotiation with
host206.126.232.53 64.36.28.114, it drops the 2nd packet and the
connection is broken.
> If the second diagram is true, then I should have automatic failover.
> I should be able to use both routes regardless.
>
> Should the same not be true for incoming connections? Why can't I
> SSH or TELNET into both like this:
Because you reply packets also get translated as they leave your routers.
> eth0 / eth1 <======> 206.126.232.53 OR 64.36.28.114
>
> Can someone please, please explain this in dingbat terms for me?
>
> Basically I have 2 DSL lines and 2 NIC's. I should be able to go out
> over either one and come in over either one. And, if one goes down, the
> other should take over and I can stil get in and out over that
> one.....that is what I really want.
To do this you will need to take the following steps
1. Purchase a valid (non-RFC1918) range of IP addresses.
2. Negotiate with BOTH your ISPs to provide BGP peering.
3. Configure BGP peering on your two routers to advertise your
public address space to the internet.
4. Purchase a third router to sit between you and your edge routers and
configure dynamic routing between all three.
Good Luck
-- David Haggett (CCNP) Linux user since 01/01/2003 Email: david<at>haggett<dot>demon<dot>co<dot>uk
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