Allocate a /29 network instead of /30 to work around bootpd bug

README.md

@@ -23,7 +23,7 @@ Softnet solves two problems:
 2. DHCP exhaustion
     * macOS built-in DHCP-server allocates a `/24` subnet with 86400 seconds lease time by default, which only allows for ~253 VMs a day (or 1 VM every ~6 minutes) to be spawned without causing a denial-of-service, which is pretty limiting for CI services like Cirrus CI
 
-Each VM gets a separate [`vmnet` network](https://developer.apple.com/documentation/vmnet/vmnet_network_create(_:_:)) with a private `/30` subnet selected to avoid overlap with existing host interface subnets. Softnet reserves an IP-address for the VM's MAC-address, delivers it through its own DHCP-server, and uses the reserved address for packet filtering.
+Each VM gets a separate [`vmnet` network](https://developer.apple.com/documentation/vmnet/vmnet_network_create(_:_:)) with a private `/29` subnet selected to avoid overlap with existing host interface subnets. Softnet reserves an IP-address for the VM's MAC-address, delivers it through its own DHCP-server, and uses the reserved address for packet filtering.
 
 ### Stateful flow authorization
 

lib/host.rs

@@ -42,8 +42,8 @@ impl Host {
         vm_mac: EthernetAddress,
         enable_isolation: bool,
     ) -> Result<Host> {
-        // Find an unused /30 subnet for the gateway and VM
-        let (gateway_ip, vm_ip, subnet) = subnet_finder::find_available_subnet(30)?;
+        // Find an unused /29 subnet for the gateway and VM
+        let (gateway_ip, vm_ip, subnet) = subnet_finder::find_available_subnet(29)?;
 
         // Create vmnet's network configuration
         let mut configuration = NetworkConfiguration::new(match vm_net_type {

lib/subnet_finder/mod.rs

@@ -76,11 +76,11 @@ mod tests {
     #[serial_test::serial]
     fn finds_two_usable_addresses() {
         // Find a subnet on the current host
-        let (gateway_ip, vm_ip, subnet) = find_available_subnet(30).unwrap();
+        let (gateway_ip, vm_ip, subnet) = find_available_subnet(29).unwrap();
 
-        // Check the subnet and its two usable addresses
-        assert_eq!(subnet.prefix_len(), 30);
+        // Check the subnet and its first two usable addresses
+        assert_eq!(subnet.prefix_len(), 29);
         assert!(subnet.network().is_private());
-        assert_eq!(subnet.hosts().collect::<Vec<_>>(), [gateway_ip, vm_ip]);
+        assert_eq!(subnet.hosts().collect::<Vec<_>>()[..2], [gateway_ip, vm_ip]);
     }
 }