Re: Deleting an alias causes rest to get deleted

	Hello,

On Mon, 2 Jul 2012, Volkan Yazıcı wrote:

Hi!

I observe an IP aliasing anomaly that occurs when I try to delete an IP alias
from an interface. That is, when I delete the first address in a set of IP
aliased addresses assigned according to a particular subnet, rest of the
aliases get deleted as well. Check out the below snippet.

	This is in Linux may be from the 2.3/2.4 times

   $ *for I in `seq 1 6`; do sudo ip addr add 192.168.2.$I/29 dev eth0;
   done*
   $ ip addr list
   1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN
        link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
        inet 127.0.0.1/8 scope host lo
        inet6 ::1/128 scope host
           valid_lft forever preferred_lft forever
   2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast
   state UP qlen 1000
        link/ether 00:24:54:b9:1c:f8 brd ff:ff:ff:ff:ff:ff
   *inet 192.168.1.200/24 brd 192.168.1.255 scope global eth0**
        inet 192.168.2.1/29 scope global eth0
        inet 192.168.2.2/29 scope global secondary eth0
        inet 192.168.2.3/29 scope global secondary eth0
        inet 192.168.2.4/29 scope global secondary eth0
        inet 192.168.2.5/29 scope global secondary eth0
        inet 192.168.2.6/29 scope global secondary eth0*
        inet6 fe80::224:54ff:feb9:1cf8/64 scope link
           valid_lft forever preferred_lft forever
   3: wlan0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN qlen 1000
        link/ether e8:39:df:6a:21:2a brd ff:ff:ff:ff:ff:ff
   $ *sudo ip addr del 192.168.2.1/29 dev eth0*
   $ ip addr list
   1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN
        link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
        inet 127.0.0.1/8 scope host lo
        inet6 ::1/128 scope host
           valid_lft forever preferred_lft forever
   2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast
   state UP qlen 1000
        link/ether 00:24:54:b9:1c:f8 brd ff:ff:ff:ff:ff:ff
   *inet 192.168.1.200/24 brd 192.168.1.255 scope global eth0*
        inet6 fe80::224:54ff:feb9:1cf8/64 scope link
           valid_lft forever preferred_lft forever
   3: wlan0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN qlen 1000
        link/ether e8:39:df:6a:21:2a brd ff:ff:ff:ff:ff:ff

Per see, deleting 192.168.2.1/29 causes the rest of the aliased interfaces get
deleted as well. This is something that is slightly documented in the ifconfig
manual: /for every scope (i.e. same net with address/netmask combination) all
aliases are deleted, if you delete the first (primary)/. So what is the right
way of just deleting the first (primary) alias without affecting the rest? If
this is a scoping issue, is it possible to assign each alias as primary within
its own dedicated scope?

	There is (yet) undocumented feature for the
interfaces:

/proc/sys/net/ipv4/conf/*/promote_secondaries

	You set it for specific interface _or_ for "all".
It defaults to 0. When you enable it, deleting a primary
address will not delete all secondary addresses but will
change the next secondary as primary. The term alias may
refer for addresses on same interface while here the
problem is for a subset of addresses - from same subnet.
Primary is the first address added for the configured
subnet, all next addresses in subnet are added as
secondaries as shown by ip addr list.

As a side note, when I first asked this question to Stephen Hemminger (he
forwarded me to this mailing list) he also told me that "/In Linux the
interface aliases are really a legacy from the BSD style addressing, and don't
act the same. It is not common practice to use them./" Is that really the
case? Because, as you know, IP aliasing is the heart of a majority of the
high-availability and clustering solutions in Linux. Is IP aliasing a really
deprecated technology in Linux? Should we avoid using it? If so, what do you
recommend as an alternative?

	Please do not stop using aliases!

Regards

--
Julian Anastasov [off-list ref]