Please add an option to specify TCP maximum segment size (MSS) for listening TCP socket.

It is known that in some circumstance PMTUD doesn't work correctly, for example under anycast deployment or ECMP server load-balancing [RFC7690]. [RFC7690] suggests to deploy an proxy which receives ICMP Toobig and broadcasts it to all nodes but I think that just setting small TCP MSS for avoiding PMTUD is easier and suitable for DNS.

I submitted similar patch to NSD/Unbound and has been accepted. https://www.nlnetlabs.nl/bugs-script/show_bug.cgi?id=731 https://www.nlnetlabs.nl/bugs-script/show_bug.cgi?id=732