Using Linux as a Router

[Rick Stevens]

Waldher, Travis R wrote:
>  
> 
> 
>>-----Original Message-----
>>From: Rick Stevens [mailto:rstevens at vitalstream.com] 
>>Sent: Wednesday, September 22, 2004 10:01 AM
>>To: Getting started with Red Hat Linux
>>Subject: Re: Using Linux as a Router
>>
>>Waldher, Travis R wrote:
>>
>>>Anyone got a how to sitting around?
>>>
>>>I'de like to use Redhat for my home firewall/router, but can't find 
>>>any good info on how to forward IP from one NIC to the other.
>>>
>>>I have a machine on the private side NIC(eth1) of the 
>>
>>router that can 
>>
>>>ping the public NIC(eth0) of the router.  But nothing beyond eth0.
>>>:confused:
>>
>>You need to set the "ip_forward" flag in the kernel:
>>
>>	echo 1 >/proc/sys/net/ipv4/ip_forward
>>
>>To make that permanent, edit /etc/sysctl.conf and look for this line:
>>
>>	net.ipv4.ip_forward = 0
>>
>>Change the "0" to "1" and save it.  ip_forward will be set on 
>>the next reboot.
>>
>>If you're going to do a full-up router, don't forget about 
>>your NAT rules in iptables (a.k.a. "Masquerade").  For info 
>>on that, see:
>>
>>	http://www.tldp.org/HOWTO/IP-Masquerade-HOWTO/index.html
>>
> 
> 
> Thanks, was looking for ip forward everywhere and never wandered my way
> across it.
> 
> Been having a problem with comcast getting ssslloooowwww from time to
> time.  Got them to agree that if a ping to a comcast internal IP addy
> showed periods of slowness compared to others.  They would actually come
> out and fix everything no cost to me, and no more arguing that it's my
> equipment causing the problem. :)

Good deal.  Remember that broadband is a weird beastie.  Download speeds
are pretty fast as it's a direct feed, but they poll your cable modem
periodically to get the upload stuff.  This is called TDM (time division
multiplexing).  I can explain it as follows:

The ISP polls the entire loop N times a second and they have to service
X users per poll.  If you have 1000 users and they poll 100 times a
second, each user has a 500 microsecond window to get the attention of
the poll and can only move as much data as will fit in that 500uS
window.  At T-1 speeds (1.54Mbps), that's about 770 bits (96 bytes) and
the total upload speed becomes about 77.6 kilobits per second.  Faster
than dialup and many DSLs, but not by much.  The more users there are,
the smaller the window, the less data anyone can upload per query and
the lower the effective baud rate.

Many companies don't really do any capacity planning.  They keep putting
people on one of these poll loops until someone bitches about the
performance.  They then set up a new loop and move some of the people
over to the new loop.  Suddenly the upload window gets longer and
performance goes up.  Voila!

The behaviour looks like what happens when one puts too many computers
on a shared network segment--although the cause of slow access in this
situation is caused by collisions as a true LAN doesn't do polling.

Hope that makes it a bit clearer.
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- Rick Stevens, Senior Systems Engineer     rstevens at vitalstream.com -
- VitalStream, Inc.                       http://www.vitalstream.com -
-                                                                    -
-   Errors have occurred. We won't tell you where or why.  We have   -
-                         lazy programmers.                          -
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