valyala/fasthttp · error

cannot disable nagle's algorithm: %w

Error message

cannot disable nagle's algorithm: %w

What it means

fdSetup set TCP_NODELAY on the listening socket (inherited by accepted connections to disable Nagle's algorithm) and setsockopt returned an error. The library treats this as fatal because low-latency behavior is part of its contract; users wanting Nagle can re-enable it later via TCPConn.SetNoDelay(false).

Source

Thrown at tcplisten/tcplisten.go:101

	if err = file.Close(); err != nil {
		ln.Close()
		return nil, err
	}

	return ln, nil
}

func (cfg *Config) fdSetup(fd int, sa unix.Sockaddr, addr string) error {
	var err error

	if err = unix.SetsockoptInt(fd, unix.SOL_SOCKET, unix.SO_REUSEADDR, 1); err != nil {
		return fmt.Errorf("cannot enable so_reuseaddr: %w", err)
	}

	// This should disable Nagle's algorithm in all accepted sockets by default.
	// Users may enable it with net.TCPConn.SetNoDelay(false).
	if err = unix.SetsockoptInt(fd, unix.IPPROTO_TCP, unix.TCP_NODELAY, 1); err != nil {
		return fmt.Errorf("cannot disable nagle's algorithm: %w", err)
	}

	if cfg.ReusePort {
		if err = unix.SetsockoptInt(fd, unix.SOL_SOCKET, soReusePort, 1); err != nil {
			return fmt.Errorf("cannot enable so_reuseport: %w", err)
		}
	}

	if cfg.DeferAccept {
		if err = enableDeferAccept(fd); err != nil {
			return err
		}
	}

	if cfg.FastOpen {
		if err = enableFastOpen(fd); err != nil {
			return err
		}

View on GitHub (pinned to c96f600972)

Solutions

1. Check the wrapped errno; ENOPROTOOPT means the platform lacks TCP_NODELAY
2. Ensure the network is "tcp"/"tcp4"/"tcp6" so the IPPROTO_TCP option is valid
3. Relax sandbox/seccomp restrictions on setsockopt
4. Vendor/patch the library to skip TCP_NODELAY if your platform does not support it

Example fix

// before
ln, err := cfg.NewListener("tcp", ":8080")
// after (callers re-enabling Nagle per-connection)
ln, err := cfg.NewListener("tcp", ":8080")
// per conn:
// conn.(*net.TCPConn).SetNoDelay(false)

// verify TCP_NODELAY support before enabling the listener
s, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_STREAM, syscall.IPPROTO_TCP)
if err == nil {
    if serr := syscall.SetsockoptInt(s, syscall.IPPROTO_TCP, syscall.TCP_NODELAY, 1); serr != nil {
        log.Printf("TCP_NODELAY unsupported here: %v", serr)
    }
    syscall.Close(s)
}

null

ln, err := cfg.NewListener("tcp", addr)
if err != nil {
    if strings.Contains(err.Error(), "cannot disable nagle") {
        var errno syscall.Errno
        if errors.As(err, &errno) && errno == syscall.ENOPROTOOPT {
            log.Printf("TCP_NODELAY unsupported on this stack; proceeding without")
            // vendor a patched fdSetup that tolerates ENOPROTOOPT
        }
        return err
    }
}

When it happens

Trigger: Calling NewListener with network "tcp4"/"tcp6" (IPPROTO_TCP level) where setsockopt(TCP_NODELAY) fails: ENOPROTOOPT (stack lacking the option), EBADF, EPERM in sandboxes.

Common situations: Non-TCP socket families accidentally routed through fdSetup; minimal/exotic TCP stacks (embedded, some BSD variants, older z/OS) lacking TCP_NODELAY; restricted container policies.

Related errors

• fasthttp: dialing to the given tcp address timed out
• couldn't find dns entries for the given domain: try using du
• only tcp, tcp4, or tcp6 is supported
• value is negative, cannot convert to uintptr
• cannot enable so_reuseaddr: %w

AI-assisted analysis of valyala/fasthttp@c96f600972 (2026-08-31). Data as JSON: /api/errors/67c7460673ca3159. Report an issue: GitHub.