Desired behaviour of "ifconfig -alias"
Oliver Fromme
olli at lurza.secnetix.de
Tue Feb 13 20:36:33 UTC 2007
Freddie Cash wrote:
> Oliver Fromme wrote:
> > Freddie Cash wrote:
> > > For a set of IPs in the same subnet on the same interface, wouldn't
> > > the primary IP be the one with the proper netmask, and all IPs with
> > > netmasks of /32 be secondary?
> >
> > That's historic. :-) Old versions of FreeBSD indeed
> > required the netmask of the "aliases" to be /32 in that
> > case. But it's no longer the case.
>
> Hmmm, if this is the case, then the man page for ifconfig(8) is
> out-of-date wrt this as well:
>
> alias Establish an additional network address for this interface. This
> is sometimes useful when changing network numbers, and one wishes
> to accept packets addressed to the old interface. If the address
> is on the same subnet as the first network address for this
> interface, a non-conflicting netmask must be given. Usually
> 0xffffffff is most appropriate.
Well, yes, the ifconfig(8) manual page is lacking in
several aspects, it seems.
> > # ifconfig re0
> > re0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
> > options=1b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING>
> > inet 88.198.44.136 netmask 0xffffffe0 broadcast 88.198.44.159
> > inet 88.198.173.154 netmask 0xfffffff8 broadcast 88.198.173.159
> > inet 88.198.173.155 netmask 0xfffffff8 broadcast 88.198.173.159
> > inet 88.198.173.156 netmask 0xfffffff8 broadcast 88.198.173.159
> > inet 88.198.173.157 netmask 0xfffffff8 broadcast 88.198.173.159
> > inet 88.198.173.158 netmask 0xfffffff8 broadcast 88.198.173.159
> >
> > > In that situation, wouldn't deleting the primary IP
> > > cause connection issues for the rest of the IPs?
> >
> > No. I can delete _any_ of the above IP addresses, and the
> > others would still work perfectly fine. I already did
> > things like that (on a different machine).
>
> Yes, but each of the IPs is on their own subnet.
No, please look closer. The addresses above are all in the
same subnet (except for the first one). It's a /29 subnet
in this case, but it works exactly the same with /24 or any
other subnet masks.
> I'm talking about a
> situation where one IP on the interface has a /24 netmask, and all the
> other IPs on the interface have /32 netmasks. Would removing the IP with
> a /24 netmask cause connection issues for the other IPs on that
> interface?
I'm not sure. I think they should just continue to work,
but I would have to try that. But why would you want to
use /32 netmasks? That was just a hack for the historic
limitation that you couldn't use real netmasks for IPs
within the same subnet. There's no reason to use that
hack anymore.
> If you add the following IPs to an interface:
> x.x.x.2/24
> x.x.x.3/32
> x.x.x.4/32
> x.x.x.5/32
> Then remove x.x.x.2, and re-add it as x.x.x.2/24 so it appears at the
> bottom of the list of IPs, what IP is used for outgoing connections?
As I said, I would have to try that because I haven't used
the /32 netmask hack for quite some time. I think it would
indeed use the first address, i.e. x.x.x.2.
> My gut tells me it'll be x.x.x.2, but I'll have to check that when I get
> home.
Best regards
Oliver
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$ dd if=/dev/urandom of=test.pl count=1
$ file test.pl
test.pl: perl script text executable
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