3 Configuring virtual machines • To remove an attachment, select it and click on the “remove” icon at the bottom (or right-click on it and select the menu item). Removable media (CD/DVDs and floppies) can be changed while the guest is running. Since the “Settings” dialog is not available at that time, you can also access these settings from the “Devices” menu of your virtual machine window. 3.7 Audio settings The “Audio” section in a virtual machine’s Settings window determines whether the VM will see a sound card connected, and whether the audio output should be heard on the host system. If audio is enabled for a guest, you can choose between the emulation of an Intel AC’97 controller, an Intel HD Audio controller6 or a SoundBlaster 16 card. In any case, you can select what audio driver VirtualBox will use on the host. On a Linux host, depending on your host configuration, you can also select between the OSS, ALSA or the PulseAudio subsystem. On newer Linux distributions (Fedora 8 and above, Ubuntu 8.04 and above) the PulseAudio subsystem should be preferred. 3.8 Network settings The “Network” section in a virtual machine’s Settings window allows you to configure how VirtualBox presents virtual network cards to your VM, and how they operate. When you first create a virtual machine, VirtualBox by default enables one virtual network card and selects the “Network Address Translation” (NAT) mode for it. This way the guest can connect to the outside world using the host’s networking and the outside world can connect to services on the guest which you choose to make visible outside of the virtual machine. This default setup is good for probably 95% of VirtualBox users. However, VirtualBox is ex- tremely flexible in how it can virtualize networking. It supports many virtual network cards per virtual machine, the first four of which can be configured in detail in the Manager window. Additional network cards can be configured on the command line with VBoxManage. Because of the vast array of options available, we have dedicated an entire chapter of this manual to discussing networking configuration please see chapter 6, Virtual networking, page 83. 3.9 Serial ports VirtualBox fully supports virtual serial ports in a virtual machine in an easy-to-use manner.7 Ever since the original IBM PC, personal computers have been equipped with one or two serial ports (also called COM ports by DOS and Windows). Most importantly, computer mice used to be connected to serial ports before USB became commonplace. While serial ports are no longer as important as they used to be, there are still some important uses left for them. For example, serial ports can be used to set up a primitive network over a null-modem cable, in case Ethernet is not available. Also, serial ports are indispensable for system programmers needing to do kernel debugging, since kernel debugging software usually interacts with developers over a serial port. With virtual serial ports, system programmers can do kernel debugging on a virtual machine instead of needing a real computer to connect to. If a virtual serial port is enabled, the guest operating system sees a standard 16450-type device. Both receiving and transmitting data is supported. How this virtual serial port is then connected to the host is configurable, and the details depend on your host operating system. 6Intel HD Audio support was added with VirtualBox 4.0 because Windows 7 (32-bit and 64-bit versions) as well as 64-bit Windows Vista do not support the Intel AC’97 controller. 7Serial port support was added with VirtualBox 1.5. 48