ubuntu@ubuntu:~$ cat /etc/netplan/50-cloud-init.yaml
# This file is generated from information provided by the datasource.  Changes
# to it will not persist across an instance reboot.  To disable cloud-init's
# network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
network:
    ethernets:
        eth0:
            addresses:
              - 10.7.1.1/24
            gateway4: 10.7.1.1
            dhcp4: false
            dhcp6: false
        eth1:
            dhcp4: true
            dhcp4-overrides:
              use-routes: false
            optional: true
    version: 2

root@ubuntu:~# cat /etc/dhcp/dhcpd.conf | grep -iv '#'

option domain-name "example.org";
option domain-name-servers ns1.example.org, ns2.example.org;

default-lease-time 600;
max-lease-time 7200;

ddns-update-style none;

authoritative;

subnet 10.7.1.0 netmask 255.255.255.0 {
  option routers 10.7.1.1;
  option subnet-mask 255.255.255.0;
  option domain-name-servers 119.29.29.29, 223.5.5.5;
  range 10.7.1.100 10.7.1.254;
}

root@ubuntu:~# cat /etc/default/isc-dhcp-server
# Defaults for isc-dhcp-server (sourced by /etc/init.d/isc-dhcp-server)

# Path to dhcpd's config file (default: /etc/dhcp/dhcpd.conf).
#DHCPDv4_CONF=/etc/dhcp/dhcpd.conf
#DHCPDv6_CONF=/etc/dhcp/dhcpd6.conf

# Path to dhcpd's PID file (default: /var/run/dhcpd.pid).
#DHCPDv4_PID=/var/run/dhcpd.pid
#DHCPDv6_PID=/var/run/dhcpd6.pid

# Additional options to start dhcpd with.
#   Don't use options -cf or -pf here; use DHCPD_CONF/ DHCPD_PID instead
#OPTIONS=""

# On what interfaces should the DHCP server (dhcpd) serve DHCP requests?
#   Separate multiple interfaces with spaces, e.g. "eth0 eth1".
INTERFACESv4="eth0"
INTERFACESv6="eth0"

systemctl enable isc-dhcp-server
systemctl restart isc-dhcp-servere

apt install pppoeconf -y
pppoeconf  # 开始配置，然后该怎么填怎么填了

5: ppp0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1492 qdisc fq_codel state UNKNOWN group default qlen 3
    link/ppp
    inet 8.8.8.8 peer 8.8.8.1/32 scope global ppp0
       valid_lft forever preferred_lft forever

iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE

# 临时开下 IPv4 的转发，如果你要持久生效自行改下 sysctl 配置
sysctl -w net.ipv4.ip_forward=1

root@ubuntu:~# cat /etc/nftables.conf
#!/usr/sbin/nft -f

flush ruleset

table inet filter {
    chain input {
        type filter hook input priority 0;
    }
    chain forward {
        type filter hook forward priority 0;
    }
    chain output {
        type filter hook output priority 0;
    }
}

table ip nat {
    chain postrouting {
        type nat hook postrouting priority 100; policy accept;
        oifname "ppp0" masquerade
    }
}