AFAIK Nginx already includes support for TFO. The define that is found all over the internet isn't needed on archlinux, as glibc defines TCP_FASTOPEN=23 already.

nginx needs to be compiled with -DTCP_FASTOPEN=23 in order to provide support for TCP Fast Open. Our nginx does not provide that and I think that it would be nice addition.

Did you bother to read what JGC wrote?

sure, but did anyone test if TCP Fast Open actually works? You can enable it in nginx but connection will not be handled that way.

Alright, but that's not what you said.

glibc defines TCP_FASTOPEN=23 and the configure script detects that, so adding -DTCP_FASTOPEN=23 will not change anything in the nginx package. The client also needs to support TFO, both kernel and browser.

OK, I am using latest kernel with:
[root@s5 ~]# cat /proc/sys/net/ipv4/tcp_fastopen
3

nginx config contains:
listen 80 fastopen=256;
listen [::]:80 fastopen=256;
listen 443 ssl http2 fastopen=256;
listen [::]:443 ssl http2 fastopen=256;

and yet, I am failing to verity that TFO is active:

[root@s5 ~]# grep '^TcpExt:' /proc/net/netstat | cut -d ' ' -f 87-92 | column -t
TCPOFOMerge TCPChallengeACK TCPSYNChallenge TCPFastOpenActive TCPFastOpenActiveFail TCPFastOpenPassive
686 15121 786 0 0 0

And how does that relate to what JGC said? "The client also needs to support TFO, both kernel and browser."