[CentOS] Network configuration issue with second public ip on CentOS 6

[Valeri Galtsev]

On Wed, December 23, 2015 5:00 pm, Louis Lagendijk wrote:
> On Wed, 2015-12-23 at 19:04 +0100, Patrick Bervoets wrote:
>>
>> > Op 23 dec. 2015 om 18:31 heeft Valeri Galtsev <galtsev at kicp.uchicag
>> > o.edu> het volgende geschreven:
>> >
>> >
>> > >
>> >
>> > If I understand IP networking correctly, you only can have
>> > "aliases" of
>> > the interface appear on the _same_ network segment (I'm tempted to
>> > say
>> > same class C network) as the main IP of interface, say you have:
>> >
>> > DEVICE=eth0
>> > IPADDR=x.y.z.w
>> > NETMASK=255.255.255.0
>> > GATEWAY=x.y.z.254
>> >
>> > Then with the restriction I mentioned you can have alias:
>> >
>> > DEVICE=eth0:0
>> > IPADDR=x.y.z.a
>> > NETMASK=255.255.255.255
>> >
>> > Note that "x.y.z." part is the same in both IPs.
>> >
>> > Somebody may correct me if I'm wrong.
>> >
>> > Valeri
>> >
>> > >
>> > >
>> I have used an 10. Alias on a 192. Interface so it is possible
>> Patrick
>
> I believe you are right. A netmask of 255.255.255.255 however seems
> wrong. With that netmask a broadcast for address resolution will not
> work... 

NETMASK=255.255.255.255 for alias interface serves very simple purpose. It
allows to avoid processing of all broadcasts on particular segment of the
network through alias interface as well. As main interface already does
it, you don't want duplication of the same through alias interface. In
general, if you make NETMASK=255.255.255.0 for alias, all should work (on
Linux I'm sure it will, on FreeBSD you will get an error), but with this
setting you do bizarre thing I described above. All these packets are sent
to kernel, and the difference is in kernel network stack of Linux and
FreeBSD, I figure.

Valeri

> Louis
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++++++++++++++++++++++++++++++++++++++++
Valeri Galtsev
Sr System Administrator
Department of Astronomy and Astrophysics
Kavli Institute for Cosmological Physics
University of Chicago
Phone: 773-702-4247
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