Hi, I anyway wrote a patch for disabling PMTUD for UDP socket (for both IPv4/IPv6), and posted to issue tracker: https://gitlab.labs.nic.cz/labs/knot/issues/467#note_24304 This patch includes extra bonus for mitigating DNS fragmentation attack for IPv4 UDP, by using Linux's newer sockopt IP_PMTUDISC_OMIT. I tried to illustrate how PMTUD badly interact with DNS over UDP, which draft-andrews-dnsext-udp-fragmentation is addressing. Assuming that there is a small MTU link between DNS requester and DNS responder. Responder is serving large response, which size is exceeding that small MTU (1454): Apparently many DNS operator don't want timeout and retransmission. MTU MTU MTU Requester--1500--[router1]--1454--[router2]--1500-- Responder | . . | | . . | o --------------------------------------------->| 1. Requester initiates | . . | DNS query. | . . | | . .<-------------o 2. Responder generates | . . | large DNS resnponse | . . | (packet size > 1454) | . . | | . o------------->| 3. Intermediate router | . . | drops large packet | . . | due to small MTU, and | . . | generates ICMPv6 | . . | "Packet Too Big", | . . | since the response | . . | packet can't go | . . | through the link. | . . | | . . | Responder would learn | . . | smaller path MTU toward | . . | requester, but | . . | does not resend | . . | the response. (It's UDP!) ~ ~ ~ ~ | . . | o---------------------------------------------->| 4. After timeout | . . | requester retries | . . | query. | . . | |<----------------------------------------------| 5. Responder generates | . . | fragmented DNS response | . . | which can go throgh | . . | the "MTU 1454" link. | . . | Regards, -- Daisuke Higashi