There are two main ways to set up networking on OS X / macOS, as it does not
function with what QEMU defaults to for network settings:

-----------------------------------
User Mode Networking (Easier Setup)
-----------------------------------

By default, QEMU uses user mode networking (SLiRP). This networking mode is the
slowest and is not visible via the outside network, but requires no host-side
setup, so it's perfect if you just want internet but don't care about latency
or about connecting to the VM from an external source.

The problem that we run into here is that OS X is nitpicky about what emulated
networking devices it is willing to accept, so we need to specify the network
configuration to use the e1000 emulated card with SLiRP networking.

In order to do this, change the line in your qemu-system-x86_64 command (found
in boot-macOS.sh) to the following:

-netdev user,id=net0 -device e1000-82545em,netdev=net0,id=net0,mac=52:54:00:c9:18:27 \

No further setup is required; your internet should Just Werk™ in your virtual machine!

For further information on detailed configuration options, see QEMU's
documentation on networking ( http://wiki.qemu.org/Documentation/Networking )

-----------------------------------
Tap Networking (Better Performance)
-----------------------------------

Installing "virt-manager" automagically creates the "virbr0" local private bridge :-)

sudo apt-get install uml-utilities virt-manager
sudo ip tuntap add dev tap0 mode tap
sudo ip link set tap0 up promisc on
sudo brctl addif virbr0 tap0

Add "-netdev tap,id=net0,ifname=tap0,script=no,downscript=no -device e1000-82545em,netdev=net0,id=net0,mac=52:54:00:c9:18:27 \"
to your qemu-system-x86_64 command.

QEMU networking tip
-------------------

# printf '52:54:00:AB:%02X:%02X\n' $((RANDOM%256)) $((RANDOM%256))  # generates QEMU compatible mac addresses!

------------------
Bridged Networking
------------------

QEMU defaults to using NAT for its guests. It has a built-in DHCP server that
provides addresses from the 192.168.12x.0 subnet. However, this configuration
makes file sharing, printer sharing, and other common networking activities
harder to use in a home network.

Bridged networking allows your QEMU guest to get an address on the same subnet
as the host computer. For example, many home networks let the wireless router
handle IP assignment via DHCP. Here are the steps for setting up the bridge.

To setup bridged networking from the command line, refer to this documentation
at the Ubuntu website. https://help.ubuntu.com/community/KVM/Networking

Ultimately, the script for booting the QEMU guest will need a line similar to
the following to enable bridged networking in the guest:

    -netdev bridge,id=net0,br=virbr0,"helper=/usr/lib/qemu/qemu-bridge-helper"

On some systems the `qemu-bridge-helper` file has incorrect permissions. For it
to work, it needs to be setuid root. This can be accomplished with this command:

    $ sudo chmod u+s /usr/lib/qemu/qemu-bridge-helper

Note that this is sometimes viewed as a security hole. Be careful and understand
what you are doing before running this command.