[CentOS] network question

Robert Moskowitz rgm at htt-consult.com
Tue Jan 27 21:12:05 UTC 2009


John R Pierce wrote:
> ann kok wrote:
>   
>> How can I clear the arp cache?
>>
>>   
>>     
>
> man arp      shows the options.   arp caches expire fairly quickly 
> though, I don't think I've ever needed to manually expire one.
>   

Per Dr. Larry Peterson who was 'there' as a grad student at Purdue under 
Dr. Comer....

ARP time outs were set at 10 min as Dr. Comer set up 2 teams of grad 
students that had to power down one of the DECs, swap the ethernet card, 
and bring the system back up. The fastest team did this in 10 min, thus 
ARP time out was set to 10 min...

Oh, Dr. Noel Chiappa 'created' ARP as a hack while he was a post-doc 
researcher at MIT.

>   
>> and 
>>
>> ls it possible to setup 2 gateways in centos
>>   
>>     
>
> you can have various static routes to different gateways for specific 
> subnets.    only one 'default' gateway will effectively work, if more 
> than one is defined, its somewhat indeterminate as to what happens.
>   

There is extensive work in the IETF around multihoming systems. This is 
becoming more critical as we are interfacing systems to multiple network 
types, and standards like IEEE 802.21 handoff.

Basically it is HARD and current stuff does it wrong, and it will be a 
while before we agree on how to do something that has a chance of success.

Tread lightly here unless you are a researcher or are desperate....

And my HIP protocol is one of the technologies being used as a model for 
how we COULD get multihoming working.

> now, you -can- play games with 'ip route' and 'ip table' and have 
> alternate route tables, and use tagging to identify packets you want to 
> route through an alternate gateway, this gets kinda messy.
>
> example...      3 interfaces, eth0 -> one internet connection with a 
> static subnet, eth1 -> another internet connection with a static subnet, 
> and eth2 -> LAN as 10.0.0.0/16
>
> The regular CentOS networknig is setup to use eth0's default gateway, 
> and have all 3 subnets properly configured.    NAT rules are setup so 
> any host on the LAN on 10.0.0.* is to be routed to eth0, and any host on 
> 10.0.1.* is to be routed to eth1.  futher, a bunch of IPs on eth1 are 
> mapped to specific server hosts on 10.0.1.* ... that part is fairly 
> straight forward.   the actually tricky part is to ensure that packets 
> to/from these eth1 mapped hosts only exit via eth1...  thats done as 
> follows...
>
>     # network of eth1 interface
>     eth1net=100.100.100.0/24
>     # internet gateway of eth1 interface
>     eth1gate=100.100.100.1
>     ip rule add from 10.0.1.0/24 table 200
>     ip route add default via $eth1gate dev eth1 table 200
>     ip route flush cache
>
> (where 100.100.100.* is the internet address of this eth1 subnet) 
> the ip rule command 'tags' any packets from 10.0.1.0/24 to use this 
> alternate table '200' (if you prefer, you can name these tables via 
> /etc/iproute2/rt_tables)
> the ip route add command sets an alternate gateway only for packets 
> using this special table, and the flush command makes sure any route 
> caching is cleaned.   this `ip` command (actually /sbin/ip) is part of 
> the iproute2 package.  
>
> this stuff gets quite tricky fast, and requires you to really understand 
> what you're doing.
>
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>   


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