[LEDE-DEV] umdns - TTL restricted to 255

[Philipp Meier]

My question is about query (not response). LEDE is ignoring query when 
TTL != 255.

Philipp


On 09/25/2017 05:16 PM, John Crispin wrote:
> Hi,
>
> rfc6762 has the following ...
>
> 11.  Source Address Check
>
>    All Multicast DNS responses (including responses sent via unicast)
>    SHOULD be sent with IP TTL set to 255.  This is recommended to
>    provide backwards-compatibility with older Multicast DNS queriers
>    (implementing a draft version of this document, posted in February
>    2004) that check the IP TTL on reception to determine whether the
>    packet originated on the local link.  These older queriers discard
>    all packets with TTLs other than 255.
>
>     John
>
>
>
> On 25/09/17 16:44, Philipp Meier wrote:
>> Hi,
>>
>> When using umdns I was wondering why my mDNS query did not get any 
>> answer.
>>
>> I found the following reason:
>>
>> My mDNS query has TTL field set to 1. But LEDE umdns package expects 
>> a TTL value of 255 (see interface.c function read_socket4).
>>
>> According 
>> https://www.systutorials.com/docs/linux/man/5-avahi-daemon.conf/ (see 
>> check-response-ttl) this was necessary security behavior in older 
>> mDNS versions but will be incompatible with newer versions of mDNS 
>> implementations.
>>
>> Therefore my question here is: Is this by purpose restricted to 255 
>> by LEDE umdns package or could this be changed?
>>
>> Regards
>> Philipp
>>
>>
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>
>
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