As per Stefan Tatschner from Debian Bug #903390:
"TCP fast open can save a roundtrip on subsequent TCP connects 1. Since systemd-resolved supports DNSoverTLS with TFO out of the box 3 and TFO support is included in recent kernels, it might be worth enabling. There are configure switches availble in the build system 2."
/proc/sys/net/ipv4/tcp_fastopen must have a value of 3 for this to be effective.
More DNS queries will eventually go through TCP given that the EDNS buffer size is suggested to be reduced 4.
Signed-off-by: Jose Olivera oliverajeo@gmail.com