Sheep Bridge

The SheepShaver and Basilisk II classic Mac emulators implement a simple, UDP-based network protocol that allows two virtual machines to communicate. Sheep Bridge allows a physical, Linux-based machine to communicate with the virtual machines using a TAP interface.

This allows the Linux machine to provide Internet access to the virtual machines, or other services like AppleTalk.

Requirements

A Linux machine with Python 3.

General usage

Let’s suppose you have two machines on the 192.168.1.0/24 network:

• 192.168.1.2 will run Sheep Bridge
• 192.168.1.3 will run SheepShaver or Basilisk II.

On 192.168.1.3, edit the emulator configuration file (~/.basilisk_ii_prefs for Basilisk II) and set the following options:

udptunnel true
udpport 6066

(you can specify another port than 6066 -- in that case you will need to adjust Sheep Bridge’s configuration)

Make sure that the configuration file does not contain any ether ... option.

On 192.168.1.2, start Sheep Bridge:

# ./sheep_bridge.py 192.168.1.2/24 6066

Sheep Bridge needs to be started as root in order to create the TAP interface. It will drop its privileges once it’s done. You should now have a sheep_bridge Ethernet interface.

You can assign an IP and netmask to the sheep_bridge interface. (Do not use a network range already used on your network). For instance:

# ip addr add 10.42.42.1/24 dev sheep_bridge
# ip link set sheep_bridge up

Configure Mac OS in your Sheep Shaver/Basilisk II to use an address on the same network (for instance 10.42.42.2) and you should be able to communicate between the machines.

systemd integration

Sheep Bridge can be used with systemd:

• Copy sheep-bridge.defaults to /etc/default/sheep-bridge and edit the NET_ADDR and PORT parameters.
• Copy sheep-bridge.service to /etc/systemd/system/sheep-bridge.service.
• Copy sheep_bridge.py to /usr/local/sbin/sheep_bridge.py.
• Run systemctl enable sheep-bridge and systemctl start sheep-bridge as root.

Sheep Bridge will be started before network configuration tools, so you can use them to configure the sheep_bridge interface the same way physical network interfaces are configured.

For instance, on Debian, you can use the following configuration in /etc/network/interfaces:

allow-hotplug sheep_bridge
iface sheep_bridge inet static
	address 10.42.42.1
	netmask 255.255.255.0

You can also install a DHCP server on the Sheep Bridge machine and use it to assign addresses to the virtual machines.

Internet access

You can configure Internet sharing to allow the virtual machines to access the Internet. See your distribution’s documentation for details. It usually boils down to:

• Enable IP forwarding (net.ipv4.ip_forward=1 in /etc/sysctl.conf or equivalent)
• Enable masquerading (iptables -t nat -A POSTROUTING -j MASQUERADE)

AppleTalk

The netatalk v2 (not v3) tools can be used to share files between the virtual machines and the Linux host.

• This requires support for the AppleTalk protocol in the Linux kernel. It is generally available, but there are some caveats (see this gist for details)
• Make sure that the sheep_bridge interface is enabled (up) on boot (no need to assign it an IP address, though)
• The atalkd daemon must be enabled, and must be configured to use the sheep_bridge interface.
• The afpd daemon must be configured to allow file sharing over AppleTalk (use -ddp or -transall in afpd.conf)
• AppleTalk routing can be used to allow communication between SheepShaver/Basilisk II and physical Mac computers. Configure atalkd to configure routing between the sheep_bridge interface, and the interface whose Macs are connected to. Unfortunately, there is an issue with Linux’s AppleTalk implementation that will prevent router discovery from working properly unless you recompile the kernel; see the aforementioned gist for more info.

Network issues

Sheep Bridge perform checks on packets received from the VMs, and packets received on the virtual Ethernet interface, before transmitting them.

• Packets sent to multicast destinations are disallowed, with the exception of the standard broadcast address (FF:FF:FF:FF:FF:FF) and the AppleTalk broadcast address (09:00:07:FF:FF:FF). IPv6 auto-configuration packets are silently dropped if they are sent on the virtual Ethernet interface.
• SheepShaver/Basilisk II VMs (and Sheep Bridge) use local MAC addresses that are derived from the host computer’s IP address. Sheep Bridge checks the coherency between MAC addresses and IP addresses before forwarding a packet. Unfortunately, SheepShaver/Basilisk II may use incoherent addresses if the computer it’s running on has multiple interfaces active at the same time (for instance, Ethernet and Wi-Fi). In that case, you will have to disable one of the interfaces and try again.

When an unexpected/invalid packet is received, Sheep Bridge will log a message on the standard output, so you can directly debug issues if you started it manually. If it is started using systemd, the output will usually be redirected to journal or the /var/log/syslog file.