Is TCP_FASTOPEN supported on Debian 12 (bookworm) ?

[Hacksign]

What version of shadowsocks-libev are you using?

shadowsocks-libev 3.3.5

What operating system are you using?

# uname -an
Linux xxhost 6.1.0-25-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.106-3 (2024-08-26) x86_64 GNU/Linux
# cat /etc/os-release 
PRETTY_NAME="Debian GNU/Linux 12 (bookworm)"
NAME="Debian GNU/Linux"
VERSION_ID="12"
VERSION="12 (bookworm)"
VERSION_CODENAME=bookworm
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"
# cat /proc/sys/net/ipv4/tcp_fastopen 
3

What did you do?

Setup a ss-server with --fast-open option.

What did you expect to see?

The ss-server would enable tcp fast open feature.

What did you see instead?

I saw a log:

ERROR: fast open is not supported on this platform

Question

Is Debian 12 bookworm support fast open or not ?

I investgated the source code of this project and found :

setfastopen(int fd)
{
    int s = 0;
#ifdef TCP_FASTOPEN
    if (fast_open) {
#if defined(__APPLE__) || defined(__MINGW32__)
        int opt = 1;
#else
        int opt = 5;
#endif
        s = setsockopt(fd, IPPROTO_TCP, TCP_FASTOPEN, &opt, sizeof(opt));

        if (s == -1) {
            if (errno == EPROTONOSUPPORT || errno == ENOPROTOOPT) {
                LOGE("fast open is not supported on this platform");
                fast_open = 0;
            } else {
                ERROR("setsockopt");
            }
        }
    }
#endif
    return s;
}

It seems the system call setsockopt gave the error result ? I guess this is the Debian system problem (?) ...

I run ss-server in docker container BTW.