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qemu-networking.md

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Setting up Qemu with a tap interface

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 br0

    brctl delif br0 tap0

  • Delete tap0

    tunctl -d tap0

  • Remove eth0 from bridge

    brctl delif br0 eth0

  • Bring bridge down

    ifconfig br0 down

  • Remove bridge

    brctl delbr br0

  • Bring eth0 up

    ifconfig eth0 up

  • Check if an IP is assigned to eth0, if not request one

    dhclient -v eth0

dhclient - Auto configuration using a DHCP server

  • Release IP

    dhclient -v -r <interface>

  • Request IP

    dhclient -v <interface>

References