(partly) SOLVED: tun0 not responding to ping

[perryh at pluto.rain.com]

Ian Smith <nimnet.asn.au!smithi at agora.rdrop.com> wrote:

> On Fri, 2 Jan 2009, perryh at pluto.rain.com wrote:
>
>  > Why would a local interface, reported as up in ifconfig, not respond
>  > to a ping of its own IP address?  The tun0 reported below doesn't,
...
>  >   $ ifconfig -a
...
>  >   tun0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1412
>  >           inet6 fe80::2b0:d0ff:fe28:ad4f%tun0 prefixlen 64 scopeid 0x4
>  >           inet ZZZ.ZZZ.233.42 --> ZZZ.ZZZ.233.42 netmask 0xffffffff
>  >           Opened by PID 24635
>
> I don't know if this is relevant or not, but I've never seen a point to 
> point interface use the same IP address on both ends of its link before.

It turns out to be normal -- or at least tolerable -- for a tun(4)
interface used by vpnc to have the same IP address at both ends.
It started working when I added

  NAT Traversal Mode cisco-udp

to vpnc.conf.  (Presumably not all configurations of the Cisco 3000
will need that, else it would be the default, but it seems to be
correct for the one involved here.)

I never did figure out why that kept the interface from responding
to a ping of its own address :(