Re: [PATCH iproute2] ip-address: properly display zero IPv4 peer address

On Tue, 2015-11-24 at 12:44 +0100, Phil Sutter wrote:

On Mon, Nov 23, 2015 at 04:04:50PM -0800, Stephen Hemminger wrote:

quoted

On Thu, 22 Oct 2015 10:34:28 +0200
Thomas Haller [off-list ref] wrote:

quoted

Kernel allows for zero IPv4 peer addresses (IFA_ADDRESS):

   ip address add 192.168.5.1 peer 0.0.0.0/24 dev dummy

which is distinct from a usual address like:

   ip address add 192.168.5.1/24 dev dummy
   ip address add 192.168.5.1 peer 192.168.5.1/24 dev dummy

For IPv4, a missing IFA_ADDRESS attribute means that the peer
is 0.0.0.0. See inet_fill_ifaddr(), which does:

  if ((ifa->ifa_address &&
       nla_put_in_addr(skb, IFA_ADDRESS, ifa->ifa_address)) ||

Signed-off-by: Thomas Haller <redacted>

I would prefer that this apply to both IPv4 and IPv6.

The case that patch handles does not happen in IPv6.

quoted

If the kernel sends back an address, then display it.

It's rather "if the kernel *does not* send back an address ...".

When reviewing this patch, I tried to find an easier (and less ugly)
solution, but failed.  Here's the result from testing all variants:

Thank you Phil for evaluating this.

I also tried to change the patch to come up with a cleaner/unified way,
but the result wasn't subjectively better.


Thomas






For reference, the relevant lines of code from kernel:



ipv4: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/net/ipv4/devinet.c?id=6a13feb9c82803e2b815eca72fa7a9f5561d7861#n1542

        if ((ifa->ifa_address &&
             nla_put_in_addr(skb, IFA_ADDRESS, ifa->ifa_address)) ||
            (ifa->ifa_local &&
             nla_put_in_addr(skb, IFA_LOCAL, ifa->ifa_local)) ||



ipv6: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/net/ipv6/addrconf.c?id=6a13feb9c82803e2b815eca72fa7a9f5561d7861#n4310

        if (!ipv6_addr_any(&ifa->peer_addr)) {
                if (nla_put_in6_addr(skb, IFA_LOCAL, &ifa->addr) < 0 ||
                    nla_put_in6_addr(skb, IFA_ADDRESS, &ifa->peer_addr) < 0)
                        goto error;
        } else
                if (nla_put_in6_addr(skb, IFA_ADDRESS, &ifa->addr) < 0)
                        goto error;