but I can share the like 4 relevant lines : In an InboundHandler :

ctx.channel().attr(KEEP_ALIVE_KEY).set(HttpHeaders.isKeepAlive(req));

In your OutboundHandler :

if (ctx.channel().attr(KEEP_ALIVE_KEY).get()) {
            HttpHeaders.setKeepAlive(res, true);

            ctx.writeAndFlush(response);
        } else {
            ctx.writeAndFlush(response).addListener(ChannelFutureListener.CLOSE);
        }