Thursday, 29 December 2016

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

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)?
>>>
>>> [email protected]:~# ip link add ippusbxd type dummy
>>> [email protected]:~# ip link set ippusbxd up
>>> [email protected]:~# 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:
>
> [email protected]:~$ nc -l fe80::3004:2dff:feb6:b5c7 1234
> nc: Invalid argument
>
> ^ Fails because the kernel doesn't know what interface you want.
>
> [email protected]:~$ 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


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