socks proxy server with shadowsocks on ubuntu

"copied from https://www.linode.com/docs/networking/vpn/create-a-socks5-proxy-server-with-shadowsocks-on-ubuntu-and-centos7/"

ubuntu 16.04

#update 
apt update && apt upgrade -yuf
apt install -y --no-install-recommends gettext build-essential autoconf libtool libpcre3-dev \
                                       asciidoc xmlto libev-dev libudns-dev automake libmbedtls-dev \
                                       libsodium-dev git python-m2crypto libc-ares-dev
                                       

cd /opt
git clone https://github.com/shadowsocks/shadowsocks-libev.git
cd shadowsocks-libev
git submodule update --init --recursive

./autogen.sh
./configure
make && make install

adduser --system --no-create-home --group shadowsocks
mkdir -m 755 /etc/shadowsocks
sudo vi /etc/shadowsocks/shadowsocks.json
contents are:
#########################
{
    "server":"server_ip",
    "server_port":server_port,
    "password":"server password",
    "timeout":300,
    "method":"aes-256-gcm",
    "fast_open": true
}
############################

#optimize shadowsocks
sudo vi /etc/sysctl.d/local.conf
contents are:
############################################
# max open files
fs.file-max = 51200
# max read buffer
net.core.rmem_max = 67108864
# max write buffer
net.core.wmem_max = 67108864
# default read buffer
net.core.rmem_default = 65536
# default write buffer
net.core.wmem_default = 65536
# max processor input queue
net.core.netdev_max_backlog = 4096
# max backlog
net.core.somaxconn = 4096
# resist SYN flood attacks
net.ipv4.tcp_syncookies = 1
# reuse timewait sockets when safe
net.ipv4.tcp_tw_reuse = 1
# turn off fast timewait sockets recycling
net.ipv4.tcp_tw_recycle = 0
# short FIN timeout
net.ipv4.tcp_fin_timeout = 30
# short keepalive time
net.ipv4.tcp_keepalive_time = 1200
# outbound port range
net.ipv4.ip_local_port_range = 10000 65000
# max SYN backlog
net.ipv4.tcp_max_syn_backlog = 4096
# max timewait sockets held by system simultaneously
net.ipv4.tcp_max_tw_buckets = 5000
# turn on TCP Fast Open on both client and server side
net.ipv4.tcp_fastopen = 3
# TCP receive buffer
net.ipv4.tcp_rmem = 4096 87380 67108864
# TCP write buffer
net.ipv4.tcp_wmem = 4096 65536 67108864
# turn on path MTU discovery
net.ipv4.tcp_mtu_probing = 1
# for high-latency network
net.ipv4.tcp_congestion_control = hybla
# for low-latency network, use cubic instead
net.ipv4.tcp_congestion_control = cubic
###################################################

#apply optimizations
sysctl --system

#create a shadowsocks systemd service
sudo vi /etc/systemd/system/shadowsocks.service
contents are:
###########################################
[Unit]
Description=Shadowsocks proxy server

[Service]
User=root
Group=root
Type=simple
ExecStart=/usr/local/bin/ss-server -c /etc/shadowsocks/shadowsocks.json -a shadowsocks -v start
ExecStop=/usr/local/bin/ss-server -c /etc/shadowsocks/shadowsocks.json -a shadowsocks -v stop

[Install]
WantedBy=multi-user.target
#############################################

#enable and start shadowsocks.service
systemctl daemon-reload
systemctl enable shadowsocks
systemctl start shadowsocks

#open firewall port for shadowsocks client
iptables -4 -A INPUT -p tcp --dport 8388 -m comment --comment "Shadowsocks server listen port" -j ACCEPT

firewall-cmd --permanent --zone=public --add-rich-rule='
    rule family="ipv4"
    port protocol="tcp" port="8388" accept'

firewall-cmd --reload