[Dnsmasq-discuss] Method to get Dnsmasq serve address of a host from interface address

[0zl]

Thank you Simon for the quick reply,

I did indeed miss `interface-name` when skimming through the manual page 
and from the looks of it other users on this thread did too.

My suggestion is to have interface-name and dynamic-host mention each 
other in the manual page and to modify the error message `warning: no 
addresses found for interface dsl-provider` to something less cryptic.

The only reason I used dynamic-host is because it was first in the 
manual page (I know it's not an excuse for not reading thoroughly, but 
it seemed to do what I wanted and I didn't think that there was 
something better).

Thanks again!

On 3/13/23 19:01, Simon Kelley wrote:
> Thanks for the bug report, it's definitely not noise.
>
> The history of line 363 is interesting.
>
> When the dynamic-host option was added, it looked like
>
> if (netmask.s_addr == 0xffff)
>
>
> and stayed like that until October 2022 when it got changed to the 
> current version
>
> if (netmask.s_addr == 0xffffffff)
>
> That's what was released as part of 2.88
>
> So that explains why it used to work for you, and no longer does in 
> 2.88 and 2.89. If your PPoe interface had netmask 255.255.0.0 then it 
> would not have worked previously, but would work now.
>
> The change was prompted by this email to the list.
>
> https://lists.thekelleys.org.uk/pipermail/dnsmasq-discuss/2022q4/016607.html 
>
>
> and the explanation that it looks like the intention was to skip 
> interfaces with /32 prefixes looks pretty sensible, so the patch went 
> in without too much thought.
>
> My dilemma is that I can't see the reason to do that and I have no 
> recollection of why I might done it in early 2021 when dynamic-host 
> was first implemented. Someone should invent a way to core-dump human 
> brain state to git for future reference.
>
> The best I can come up with is that what you are trying to do is not 
> what I expected dynamic-host to be used for. I expected it to be used 
> to name hosts on an internal network whose prefix was supplied by the 
> ISP, typically in IPv6-land with prefix delegation. So the host is not 
> the one running dnsmasq but another host connected to the same network 
> as the interface but with a suffix. Clearly that can only exist when 
> the prefix length is smaller than 32(IPv4) or 128(IPv6).
>
> Actually, it's reasonable not to have considered your "weird hack", 
> because dnsmasq has had an explicit mechanism to do what you want for 
> a long time, interface-name. You could just use.
>
> interface-name=myip.my.local.domain,dsl-provider
>
> So.
>
> There's an easier way to do what you want to do.
> Your "weird hack" used to work because the code that was supposed to 
> stop it used to be buggy and didn't.
> The bug got fixed in 2.88 and now does stop your weird hack.
> There's a very limited need for the once-buggy, now fixed code and I 
> can't really remember what it was for.
>
> Going forward we have three options for line 363
>
> 1) revert the October 2022 change. That would be stupid.
> 2) leave as is and educate people to use interface-name for this and 
> not hack dynamic-host
> 3) remove it so that people skimming the man page who find 
> dynamic-host before they find interface-name and rediscover your 
> "weird hack" are not bewildered. This would also avoid people who 
> found and used the "weird hack" in pre-2.88 releases from suffering a 
> regression when they upgrade.
>
>
> For that last reason alone, I propose option 3.
>
>
> Comments?
>
>
>
> Cheers,
>
> Simon.
>
> On 13/03/2023 00:31, 0zl wrote:
>> Well, it seems like dnsmasq just ignores addresses with /32s. I see 
>> in src/network.c for some reason if an interface has 255.255.255.255 
>> it just skips it (line 363)?
>>
>> This might possibly the cause for the issue, I'm not sure if this is 
>> intentional or some sort of bug.
>>
>> Thanks again and sorry for the noise!
>>
>>
>> On 3/13/23 02:08, 0zl wrote:
>>> Hi everyone,
>>>
>>> Previously on Debian Stable, I was using dynamic-host as a weird 
>>> hack to get Dnsmasq to serve my PPPOE interface's IP address however 
>>> this no longer seems to work with the Dnsmasq supplied in Debian 
>>> Testing.
>>>
>>> It was OK to just specify 0.0.0.0 as my ISP would allocate for me a 
>>> /32 and so this thankfully matched my interface address and worked 
>>> properly.
>>>
>>> Now that this no longer works on 2.89, are there any alternatives I 
>>> could use?
>>>
>>> (What no longer works is 
>>> `dynamic-host=myip.my.local.domain,0.0.0.0,dsl-provider` and Dnsmasq 
>>> fails with `warning: no addresses found for interface dsl-provider`)
>>>
>>> TIA
>>>
>>>
>>>
>>
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>>
>
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