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

[Simon Kelley]

On 13/03/2023 17:01, 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.



Replying to myself, there's further evidence of the intention behind 
line 363 in the analogous code for IPv6, around line 401.

                     /* No sense in doing /128. */
		    if (prefixlen == 128)
		      continue;

If 363 goes, 401 should go too.

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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