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Allocate a /29 network instead of /30 to work around bootpd bug (#214)

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edi-oai
2026-10-09 16:15:19 +01:00
committed by GitHub
parent e1ecab195b
commit 27fe8a8567
3 changed files with 7 additions and 7 deletions
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README.md
+1 -1
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@@ -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
+2 -2
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@@ -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
+4 -4
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@@ -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]);
}
}