ipset-cidr

# For CIDR IP range
ipset create bypass hash:net hashsize 4096 maxelem 16384

mark_packets_to_route_through_tun_Linux

## https://serverfault.com/a/389004

iptables -t mangle -A PREROUTING -s 10.1.1.3 ! --destination 10.1.1.0/24 -j MARK --set-mark 3
iptables -t nat -A POSTROUTING -o tun0 -j SNAT --to-source 192.168.255.6 (openvpn and not necessary for sing-box)

(
	iptables -A FORWARD -j ACCEPT
	OR
	iptables -A FORWARD -s 10.1.1.0/24 -j ACCEPT
	iptables -A FORWARD -d 10.1.1.0/24 -j ACCEPT
)

ip rule add fwmark 3 table 5
ip route add default via 192.168.255.5 table 5
ip route flush cache

# Not sure
sysctl -w net.ipv4.conf.tap0.rp_filter=2

specify_different_route

# Specify different route for some IPs

echo "100 custom" >> /etc/iproute2/rt_tables
ip rule add to 8.8.8.8 table custom
ip route add default via 192.168.122.1 dev enp1s0 table custom

specify_different_route_using_ipset

# Specify different route for some IPs (Better version, using ipset)

echo "100 custom" >> /etc/iproute2/rt_tables

ipset create bypass hash:ip
ipset add bypass {IP}

# Mark packets in both PREROUTING and OUTPUT chains
iptables -t mangle -A PREROUTING -m set --match-set bypass dst -j MARK --set-mark 1
iptables -t mangle -A OUTPUT -m set --match-set bypass dst -j MARK --set-mark 1

# Ensure conntrack marks the connection
#iptables -t mangle -A PREROUTING -d 8.8.8.8 -j CONNMARK --save-mark
#iptables -t mangle -A PREROUTING -j CONNMARK --restore-mark

# Add the rule and route
ip rule add fwmark 1 table custom
ip route add default via 192.168.122.1 dev enp8s0 table custom

ip route flush cache