Using the dummy0 interface for a local-only service to be broadcasted by Avahi

[Till Kamppeter]

On 12/29/2016 04:31 PM, Stéphane Graber wrote:
> On Thu, Dec 29, 2016 at 04:14:52PM -0200, Till Kamppeter wrote:
>> On 12/29/2016 02:37 PM, Stéphane Graber wrote:
>>>> How can I assign a different name to a dummy interface? Can I freely choose
>>>> a name somehow, for example "ippusbxd"? Or have I to use "dummy1", "dummy2",
>>>> ... (loading the dummy kernel module with an option to support more than one
>>>> interface)?
>>>
>>> root at castiana:~# ip link add ippusbxd type dummy
>>> root at castiana:~# ip link set ippusbxd up
>>> root at castiana:~# ifconfig ippusbxd
>>> ippusbxd: flags=195<UP,BROADCAST,RUNNING,NOARP>  mtu 1500
>>>         inet6 fe80::3004:2dff:feb6:b5c7  prefixlen 64  scopeid 0x20<link>
>>>         ether 32:04:2d:b6:b5:c7  txqueuelen 1000  (Ethernet)
>>>         RX packets 0  bytes 0 (0.0 B)
>>>         RX errors 0  dropped 0  overruns 0  frame 0
>>>         TX packets 2  bytes 140 (140.0 B)
>>>         TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
>>>
>>>
>>> Which gets you your own dummy device with its IPv6 link-local address.
>>
>> Thank you very much. I copied and pasted the commands and got an ifconfig
>> output similar to yours, only with different IP and MAC addresses and
>> different values in the statistics.
>>
>> Then I tried to bind to the IPv6 IP address of this entry, on port 60000 and
>> this did not work.
>>
>> Do I have to create an additional IP address? If yes, how? Do I have to run
>> additional commands (route?)? Which ones?
>>
>>    Till
>
> Link-local addresses are slightly special in that they are indeed link local.
>
> So you can't bind fe80::3004:2dff:feb6:b5c7 as you could in theory have
> the same address on multiple interfaces. Instead, you need to tell
> bind() what interface to bind on. This is typically indicated as
> fe80::3004:2dff:feb6:b5c7%ippusbxd.
>
>
> For example:
>
> stgraber at castiana:~$ nc -l fe80::3004:2dff:feb6:b5c7 1234
> nc: Invalid argument
>
> ^ Fails because the kernel doesn't know what interface you want.
>
> stgraber at castiana:~$ nc -l fe80::3004:2dff:feb6:b5c7%ippusbxd 1234
>
> ^ Works
>

Thank you. I want to bind with the bind(2) function in C. How do I 
supply the interface here or what function do I need to call instead?

    Till