Mailing List Archive

On Thu, 15 Jan 2004, Gilad Arnold wrote:

> When I said rougher, I referred to the implication on source address
> selection: it seems that a single address is used to access all
> connected networks on a given interface.

No.. its whatever the route table determines for source address
selection, iirc its typically the primary address for that subnet, or at
least the ip util would lead one to believe so. And src selection can be
influenced by adding routes, eg:

# ip addr add 192.168.24.1/24 dev dummy0
# ip addr add 192.168.25.1/24 dev dummy0
# ip addr add 192.168.24.2/24 dev dummy0
# ip -4 ad sho dev dummy0
17: dummy0: <BROADCAST,NOARP,UP> mtu 1500 qdisc noqueue
link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
inet 192.168.24.1/24 scope global dummy0
inet 192.168.25.1/24 scope global dummy0
inet 192.168.24.2/24 scope global secondary dummy0

# ip ro get 192.168.25.0/24
broadcast 192.168.25.0 dev dummy0 src 192.168.25.1
cache <local,brd> mtu 1500 advmss 1460

# ip ro get 192.168.24.0/24
broadcast 192.168.24.0 dev dummy0 src 192.168.24.1
cache <local,brd> mtu 1500 advmss 1460

# # add a route to specify a different source:
# ip ro add 192.168.25.100/32 dev dummy0 src 192.168.24.2

# ip ro get 192.168.25.100
192.168.25.100 dev dummy0 src 192.168.24.2
cache mtu 1500 advmss 1460
# ip ro ge 192.168.25.10
192.168.25.10 dev dummy0 src 192.168.25.1
cache mtu 1500 advmss 1460

> neighbor 10.0.1.5/24, although the natural choice, and the one taken
> in Linux kernels, would be 10.0.0.1/24.

sure? that'd mean the kernel doesnt do what ip would lead one to
believe. (which'd be annoying).

> Didn't ever check, however I believe it does (otherwise can't tell how
> source address ambiguity problem is resolved).

It doesnt appear to. It has the secondary flag, but it isnt set. Dont
know if that's intentional or whether its a buglet wrt IPv6
implementation.

There doesnt appear to be a way to influence src address selection
either. (then again this may be intentional. It was my vague
understanding IPv6 possibly does not allow multiple subnets/addresses
per link. But I cant remember why I have this vague understanding, nor
can i find any handy reference to answer the question.).

> Gilad

regards,

--paulj

Paul Jakma wrote:

>>When I said rougher, I referred to the implication on source address
>>selection: it seems that a single address is used to access all
>>connected networks on a given interface.
>
> No.. its whatever the route table determines for source address
> selection, iirc its typically the primary address for that subnet, or at
> least the ip util would lead one to believe so.

In fact, I was referring to Greg's example of a BSD stack (and you
probably figured I was talking about Linux?)


> And src selection can be influenced by adding routes, eg:

[ snip iproute2 trace ]

Yes, you can do that, however in order to change the src for the whole
connected network (ie, replace the current, kernel derived connected
route with one of your own) you'll need to explicitly remove the current
connected route and install a new one, otherwise you get an error adding
this new route. But anyway, you don't really want to do that in most
cases, as the kernel still considers the first address of a subnet to be
the primary one, with all the associated implications (like secondary
chain flushing, etc).


> sure? that'd mean the kernel doesnt do what ip would lead one to
> believe. (which'd be annoying).

Don't be annoyed! (I was talking about BSD, see above ;->)

Gilad

Paul Jakma wrote:

> # ip -6 addr add 2001:770:105:1::22/64 dev eth0
> # ip -6 ad sho dev eth0
> 7: eth0: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast qlen 1000
> inet6 2001:770:105:1::22/64 scope global tentative
> inet6 fe80::260:97ff:fe54:1ec9/64 scope link
> inet6 2001:770:105:1:260:97ff:fe54:1ec9/64 scope global

Seems like your new address is added with that "tentative" flag --
what's that? Could it be another way of saying "secondary", in IPv6-ish?

Gilad

On Sun, 18 Jan 2004, Gilad Arnold wrote:

>
> Paul Jakma wrote:
>
> > # ip -6 addr add 2001:770:105:1::22/64 dev eth0
> > # ip -6 ad sho dev eth0
> > 7: eth0: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast qlen 1000
> > inet6 2001:770:105:1::22/64 scope global tentative
> > inet6 fe80::260:97ff:fe54:1ec9/64 scope link
> > inet6 2001:770:105:1:260:97ff:fe54:1ec9/64 scope global
>
> Seems like your new address is added with that "tentative" flag --
> what's that? Could it be another way of saying "secondary", in IPv6-ish?

No. The tentative flag meens that this address is being checked for
availability - if there is no other host with this address. After check,
which takes about 2 sec, this flag is removed.


best regards,

Krzysztof Olêdzki

> # ip -6 addr add 2001:770:105:1::22/64 dev eth0
> # ip -6 ad sho dev eth0
> 7: eth0: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast qlen 1000
> inet6 2001:770:105:1::22/64 scope global tentative
> inet6 fe80::260:97ff:fe54:1ec9/64 scope link
> inet6 2001:770:105:1:260:97ff:fe54:1ec9/64 scope global

Seems like your new address is added with that "tentative" flag --
what's that? Could it be another way of saying "secondary", in IPv6-ish?

No, tentative means that Duplicate Address Detection has not yet
completed.
It is interesting that the new address shows up earlier, and in
particular that it is ahead of the Link Local address.