Setting up Qemu with a tap interface

por | 16 octubre, 2020
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There are two parts to networking within QEMU:

  • The virtual network device that is provided to the guest (e.g. a PCI network card).
  • The network backend that interacts with the emulated NIC (e.g. puts packets onto the host’s network).

Example: User mode network

User mode networking allows the guest to connect back to the outside world through TCP, UDP etc. ICMP Ping is not allowed. Also connections from host to guest are not allowed unless using port forwarding.

$ qemu-system-i386 -cdrom Core-current.iso -boot d -netdev user,id=mynet0,hostfwd=tcp::8080-:80 -device e1000,netdev=mynet0
-netdev user,id=mynet0,hostfwd=tcp::8080-:80

Create the user mode network backend having id mynet0. Redirect incoming tcp connections on host port 8080 to guest port 80. The syntax is hostfwd=[tcp|udp]:[hostaddr]:hostport-[guestaddr]:guestport

-device e1000,netdev=mynet0

Create a NIC (model e1000) and connect to mynet0 backend created by the previous parameter

Example: Tap network

TAP network overcomes all of the limitations of user mode networking, but requires a tap to be setup before running qemu. Also qemu must be run with root privileges.

$ sudo qemu-system-i386 -cdrom Core-current.iso -boot d -netdev tap,id=mynet0,ifname=tap0,script=no,downscript=no -device e1000,netdev=mynet0,mac=52:55:00:d1:55:01
-netdev tap,id=mynet0,ifname=tap0,script=no,downscript=no

Create a tap network backend with id mynet0. This will connect to a tap interface tap0 which must be already setup. Do not use any network configuration scripts.

-device e1000,netdev=mynet0,mac=52:55:00:d1:55:01

Create a NIC (model e1000) and connect to mynet0 backend created by the previous parameter. Also specify a mac address for the NIC.

Setup

  • Create a bridge brctl addbr br0
  • Clear IP of eth0 ip addr flush dev eth0
  • Add eth0 to bridge brctl addif br0 eth0
  • Create tap interface tunctl -t tap0 -u `whoami`
  • Add tap0 to bridge brctl addif br0 tap0
  • Make sure everything is up ifconfig eth0 up ifconfig tap0 up ifconfig br0 up
  • Check if properly bridged brctl show
  • Assign ip to br0 dhclient -v br0

Cleanup

  • Remove tap interface tap0 from bridge br0brctl delif br0 tap0
  • Delete tap0tunctl -d tap0
  • Remove eth0 from bridgebrctl delif br0 eth0
  • Bring bridge downifconfig br0 down
  • Remove bridgebrctl delbr br0
  • Bring eth0 upifconfig eth0 up
  • Check if an IP is assigned to eth0, if not request onedhclient -v eth0

dhclient – Auto configuration using a DHCP server

  • Release IPdhclient -v -r <interface>
  • Request IPdhclient -v <interface>

References