[LEDE-DEV] umdns - TTL restricted to 255

[John Crispin]

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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