Date: Thu, 29 Nov 2001 15:39:33 -0800 From: justin@mac.com To: <freebsd-net@FreeBSD.ORG> Subject: Re: netmask for aliased ip Message-ID: <5756E9F8-E522-11D5-A7BB-0003934474AC@mac.com> In-Reply-To: <001301c178da$90108550$0100a8c0@ahsanalikh>
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On Thursday, November 29, 2001, at 05:01 , Ahsan Ali wrote:
>> For TCP, that is what is always used by default when creating an
>> outbound connection. For incoming connections, the machine will of
>> course reply using the IP address the connection came in on. And a
>> program can always request to use a specific address if it wants to.
>>
>> I am not sure where you see a problem.
>
> What I am saying is that if you have (for instance) 192.168.0.0/24 as a
> network.
>
> Interface A has the IP 192.168.0.10 with a netmask of 255.255.255.0
> (/24)
> Alias A:1 has the IP 192.168.0.11 with a netmask of 255.255.255.255
> (/32)
>
> Now Host B (192.168.0.20 mask 255.255.255.0) tries to access Alias A:1
> which
> is 192.168.0.11/32 so B sends to A:1 which it (correctly) assumes to be
> on
> its own network, Alias A:1 cannot however reach B without sending the
> data
> to its configured gateway. If routing is enabled on this host then it
> may be
> able to send the reply routed through Interface A only...
I think you are confusing interfaces and addresses. In your case, B
will want to send to ...11 (it doesn't see the mask that's installed on
A), so it will ARP for it, and A will reply. B then sends the packet to
A's MAC address (which is the same for both the original and the aliased
address). When A replies to B, it will send to B through the same
physical interface through which B's packet arrived. There's no issue
with reachability: both systems, and all three addresses, are on the
same link.
The netmask tom-foolery is just a
local-to-the-system-installing-the-alias trick; it has nothing to do
with the on-the-wire behavior, or how the remote site sees the site
installing the alias. With a /32 mask, the system can keep track of the
various addresses without a lot of routing tricks which would otherwise
be necessary (more work for the admin to install and remove aliases).
It is confusing, but it has to do with the way the routing
infrastructure works on *BSD systems, not with the way networking works
in general.
Regards,
Justin
---
Justin C. Walker, Curmudgeon-At-Large *
Institute for General Semantics | It's not whether you win or
lose...
| It's whether *I* win or lose.
*--------------------------------------*-------------------------------*
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