Why to Do This?

Essentially it is a user case that:

• sharing network between a host and a guest
• testing how a host and guest respond to each other

Setup DHCP Server and Share the Networking to Clients Emulated by QEMU

Setup DHCP Server

Add Tap Device

In our case, we use a new tap device to connect the server and client. Let's create the device by:

$ ip tuntap add tap0 mode tap user root
$ ip addr add dev tap0 192.10.0/24
$ ip link set tap0 up

• tap0 is the name of your tap device. It could be any name you want.
• the subnet 192.10.0/24 is also up to you. Specify a range of IP you are interested in.

If everything is smooth, ip a should show your new tap device and its status is UP.

Get the Server Ready

There are different DHCP servers available on Ubuntu. In this case we choose ISC DHCP to run the server.

Firstly install the package:

sudo apt install -y isc-dhcp-server

Then edit the dhcp table so your dhcp server knows how to arrange the IPs:

cat > /etc/dhcp/dhcpd.conf <<EOF
subnet 192.10.0.0 netmask 255.255.255.0 {
        authoritative;
        range 192.10.0.1 192.10.0.254;
        default-lease-time 3600;
        max-lease-time 3600;
        option subnet-mask 255.255.255.0;
        option broadcast-address 192.10.0.255;
        option routers 192.10.0.0;
        option domain-name-servers 8.8.8.8;
        option domain-name "example.com";
}

host your-client {
   hardware ethernet <your mac address>;
   #fixed-address 192.10.0.100;
   option host-name "your-client";
}

EOF

Again, the IP like 192.10.0.0 etc. are something you customize. If you have no idea which value they should be, you may read more knowledge about basic networking like broadcast address, subnet mask...etc.

Optionally, touch /var/lib/dhcp/dhcpd.leases only if there is no such file. You can find the descripton of its manual by man dhcpd.leases:

       When dhcpd is first installed, there is no lease database.   However, dhcpd requires that a lease database be present before  it  will  start.   To  make  the  initial  lease  database,  just  create  an  empty  file  called
       /var/lib/dhcp/dhcpd.leases.   You can do this with:

            touch /var/lib/dhcp/dhcpd.leases

Restart the dhcpd by:

sudo systemctl restart isc-dhcp-server.service

and feed the table to the interface tap0 and force the dhcpd runs in foreground for debugging.

dhcpd -f tap0

Troubleshooting

• If you have permission issue of dhcpd.leases when running the above dhcpd -f command, you can make it work by:

chmod +777 /var/lbi/dhcp/dhcpd.leases

• In some cases that you may not launch the process with sudo permission, you may need to make the following tweaks:

chown $(whoami) /etc/dhcp
chown $(whoami) /var/lib/dhcp/

• In dhcp table, this line fixed-address 192.10.0.100 with the overlapped IP with your subnet is unnecessary. If there is such a line, you will get message in your syslog like Remove host declaration. See this email thread.

Set up the Client with QEMU

This Ubuntu Wiki page is very useful. Read it before you move forward.

Edit the -netdev with the specified option ifname:

    -netdev type=tap,id=net0,ifname=tap0,script=no,downscript=no

Again, note tap0 is the tap device of the host.

Ready and Testing

By monitoring syslog of the host, you can see the dhcpd begins to get the broadcast of packt from the qemu client when booting the instance. If you see such log and the guest gets the IP from your subnet range. Your dhcp cluster is working properly now.

Testing host-name

In the dhcp table, there is an option host-name. If you want to see if your guest instance a.k.a. the dhcp client works properly to get the host-name dispatched by the host, edit the content of /etc/hostname to make the content is localhost. Rebooting your guest instance, if the instance fetches dhcp data properly during boot process, you will see the hostname changes.