Date: Mon, 21 Sep 1998 14:23:57 +0300 From: Ruslan Ermilov <ru@ucb.crimea.ua> To: Graham Wheeler <gram@cdsec.com>, hackers@FreeBSD.ORG Subject: Re: ifconfig alias weirdness Message-ID: <19980921142357.A1023@relay.ucb.crimea.ua> In-Reply-To: <199809210953.LAA04029@cdsec.com>; from Graham Wheeler on Mon, Sep 21, 1998 at 11:53:49AM %2B0200 References: <199809210953.LAA04029@cdsec.com>
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Hi!
On Mon, Sep 21, 1998 at 11:53:49AM +0200, Graham Wheeler wrote:
> Hi all
>
> On a different note to the hang/reboot, I've noticed the following which
> may be a bug, or perhaps I'm just missing something (or maybe I'm just
> being pedantic!).
Yes, you are missing the FAQ article "How can I setup Ethernet aliases".
>
> If I want to set up an alias on an interface with a subnet, there are
> two different behaviours I observe depending on whether or not the
> netmask is specified with the alias.
>
> If I use:
>
> ifconfig ed0 inet 196.37.78.96 netmask 255.255.255.240 up
> ifconfig ed0 inet 196.37.78.99 alias
>
> then the routes get messed up. After the first ifconfig the routes
> are correct:
>
> 196.37.78.96/26 link#1
^^
Are you sure it is equal 26? if you use 255.255.255.240, it should be 28.
>
> But after the alias we have:
>
> 196.37.78 link#1
> 196.37.78.96/26 link#1
>
> which indicates that an implicit netmask of 255.255.255.0 has been used.
Yes, you're absolutely right.
This is a class C address, so the default netmask for it will be 255.255.255.0.
Imagine, if you'd have 2 different class C networks hosted on your machine:
ifconfig ed0 inet 192.168.1.1 netmask 255.255.255.0
ifconfig ed0 inet 192.168.2.1 netmask 255.255.255.0 alias
> On the other hand, if I use:
>
> ifconfig ed0 inet 196.37.78.99 netmask 255.255.255.240 alias
>
> then the alias is right, and the bad route doesn't get added; however,
> ifconfig prints an error: `SIOCIFADDR: file exists'.
This is what your kernel should do, because when you issue ``ifconfig''
command, your kernel implicitly adjusts its routing table with "interface"
route, which is marked as "link#N" in the ``netstat -r'' output.
In your case, when you issue
ifconfig ed0 inet 196.37.78.99 netmask 255.255.255.240 alias
this "interface" route will be 196.37.78.96/28, and it's already exists.
>
> I can live with the error message, but it is a bit disturbing.
>
In your specific case, when you want an alias from an already defined
subnet, you should specify netmask as 255.255.255.255:
ifconfig ed0 inet 196.37.78.99 netmask 255.255.255.255 alias
Best regards,
--
Ruslan Ermilov Sysadmin and DBA of the
ru@ucb.crimea.ua United Commercial Bank
+380.652.247.647 Simferopol, Ukraine
http://www.FreeBSD.org The Power To Serve
http://www.oracle.com Enabling The Information Age
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