netdev - Re: iproute2: removing primary address removes secondaries

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Date:	Fri, 11 Jan 2008 18:13:15 +0100
From:	Daniel Lezcano <dlezcano@...ibm.com>
To:	netdev discussion list <netdev@...r.kernel.org>
Subject: Re: iproute2: removing primary address removes secondaries

martin f krafft wrote:
> Dear list,
> 
> When I add an address to an interface whose network prefix is the
> same as that of an address already bound to the interface, the new
> address becomes a secondary address. As per
> http://www.policyrouting.org/iproute2.doc.html:
> 
>   "secondary --- this address is not used when selecting the default
>   source address for outgoing packets. An IP address becomes
>   secondary if another address within the same prefix (network)
>   already exists. The first address within the prefix is primary and
>   is the tag address for the group of all the secondary addresses.
>   When the primary address is deleted all of the secondaries are
>   purged too."
> 
> In the following, I want to argue that this is not necessary.
> I think that removal of a primary address should cause the next
> address to be promoted to be the default source address and the
> link-scoped route to be retained. This is basically out of
> http://bugs.debian.org/429689, the maintainer asked me to turn
> directly to this list.
> 
> If I add an address to a device with 'ip add', ip also implicitly
> adds a link-scoped route according to the netmask. It only does this
> for primary addresses, so if I add a second address within the same
> network, the route is not duplicated.
> 
> Thus, the net effect on the routing table is the same for the
> following two commands:
> 
>   ip a a 172.16.0.100/12 dev eth0 && ip a a 172.16.0.200/12 dev eth0
>   ip a a 172.16.0.100/12 dev eth0 && ip a a 172.16.0.200/32 dev eth0
>                                                         ^^^^
> In the first case, the .200 address becomes a secondary of the .100
> address. In the second case, they are both primaries. In both cases,
> only one /12 link-scoped route will be created.
> 
> However, in both cases, if I remove the .100 address, the .200 is
> affected: if it's secondary, it ceases to exist, and if it's
> primary (i.e. in the /32 case), then the host can no longer use it
> to communicate to hosts in the same link segment, only to hosts on
> the other side of the default gateway.
> 
> I thus question the point of purging secondary addresses. Obviously,
> only one address can be primary (it is used as source address for
> packets leaving the machine by the respective route). But if the
> primary address is removed, the next secondary should be promoted
> and the route should *not* be deleted.
> 
> Comments?
> 
> Cheers,

There is a tweak in /proc/sys which activate secondaries promotion when 
a primary is deleted.

/proc/sys/net/ipv4/conf/all/promote_secondaries

I think it changes the behavior to the one you wish.

Regards
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