run: add `--ip-address` option by tofugarden · Pull Request #261 · nirs/vmnet-helper

tofugarden · 2026-07-03T04:01:43Z

Objective

Allow assigning a static IP to a VM started with run. Closes #260.

Implementation notes

The argument validation is pretty strict, requiring the requested IP to be in the network subnet, but outside the DHCP range. Addresses inside the range route just fine, but this helps avoid conflicts.

Since the same VM can be reused across subnets, the cloud-init image is rebuilt if the contents of the network-config file are not as specified on the command line.

cloud-init supports setting multiple IP addresses, but we only support one command line argument. It wouldn't be that difficult to implement, it's just not needed.

The IP must be on the same subnet as the start address, but outside the DHCP range. If the IP changes between runs, or --static-ip is no longer set, the cidata.iso disk is re-provisioned.

nirs

Tested locally:

Switch from dhcp to ip address

% ./run ubuntu --ip-address 192.168.200.128 --start-address 192.168.200.1 --end-address 192.168.200.127 --subnet-mask 255.255.255.0
[   0.042] INFO Starting vmnet-helper for 'ubuntu' with interface id '6bbc6ce9-5e26-4b39-bc9c-28c88776d519'
[   0.213] INFO Creating cloud-init iso '/Users/nir/.vmnet-helper/vms/ubuntu/cidata.iso'
[   0.235] INFO Starting 'vfkit' virtual machine 'ubuntu' with mac address '82:e9:dd:3d:68:1f'
[   5.080] INFO VM is ready at ubuntu-vmnet-helper.local

% ssh ubuntu -- ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host noprefixroute 
       valid_lft forever preferred_lft forever
2: enp0s1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 82:e9:dd:3d:68:1f brd ff:ff:ff:ff:ff:ff
    altname enx82e9dd3d681f
    inet 192.168.200.128/24 brd 192.168.200.255 scope global enp0s1
       valid_lft forever preferred_lft forever
    inet6 fdd3:28eb:866d:8319:80e9:ddff:fe3d:681f/64 scope global dynamic mngtmpaddr noprefixroute 
       valid_lft 2591905sec preferred_lft 604705sec
    inet6 fe80::80e9:ddff:fe3d:681f/64 scope link proto kernel_ll 
       valid_lft forever preferred_lft forever

Switch back to dhcp

% ./run ubuntu --start-address 192.168.200.1 --end-address 192.168.200.127 --subnet-mask 255.255.255.0 
[   0.041] INFO Starting vmnet-helper for 'ubuntu' with interface id '6bbc6ce9-5e26-4b39-bc9c-28c88776d519'
[   0.148] INFO Starting 'vfkit' virtual machine 'ubuntu' with mac address '82:e9:dd:3d:68:1f'
[   4.646] INFO VM is ready at ubuntu-vmnet-helper.local

We did not detect the change (ip-address change to None), and did not generate the ciabatta.iso.

% ssh ubuntu -- ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host noprefixroute 
       valid_lft forever preferred_lft forever
2: enp0s1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 82:e9:dd:3d:68:1f brd ff:ff:ff:ff:ff:ff
    altname enx82e9dd3d681f
    inet 192.168.200.128/24 brd 192.168.200.255 scope global enp0s1
       valid_lft forever preferred_lft forever
    inet6 fdd3:28eb:866d:8319:80e9:ddff:fe3d:681f/64 scope global dynamic mngtmpaddr noprefixroute 
       valid_lft 2591992sec preferred_lft 604792sec
    inet6 fe80::80e9:ddff:fe3d:681f/64 scope link proto kernel_ll 
       valid_lft forever preferred_lft forever

So we got the same IP address in the guest.

We need to generate the network config, read the previous one, and compare. If network-config changed, we to regenerate the iso. The same way will work or user-data later if we want to change packages or mDNS implementation.

Switch back to dhcp by deleting the iso

% rm ~/.vmnet-helper/vms/ubuntu/cidata.iso 

% ./run ubuntu --start-address 192.168.200.1 --end-address 192.168.200.127 --subnet-mask 255.255.255.0
[   0.041] INFO Starting vmnet-helper for 'ubuntu' with interface id '6bbc6ce9-5e26-4b39-bc9c-28c88776d519'
[   0.166] INFO Creating cloud-init iso '/Users/nir/.vmnet-helper/vms/ubuntu/cidata.iso'
[   0.178] INFO Starting 'vfkit' virtual machine 'ubuntu' with mac address '82:e9:dd:3d:68:1f'
[   5.083] INFO VM is ready at ubuntu-vmnet-helper.local

% ssh ubuntu -- ip a                     
Warning: Permanently added 'ubuntu-vmnet-helper.local' (ED25519) to the list of known hosts.
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host noprefixroute 
       valid_lft forever preferred_lft forever
2: enp0s1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 82:e9:dd:3d:68:1f brd ff:ff:ff:ff:ff:ff
    altname enx82e9dd3d681f
    inet 192.168.200.2/24 metric 100 brd 192.168.200.255 scope global dynamic enp0s1
       valid_lft 576sec preferred_lft 576sec
    inet6 fdd3:28eb:866d:8319:80e9:ddff:fe3d:681f/64 scope global dynamic mngtmpaddr noprefixroute 
       valid_lft 2591993sec preferred_lft 604793sec
    inet6 fe80::80e9:ddff:fe3d:681f/64 scope link proto kernel_ll 
       valid_lft forever preferred_lft forever

nirs · 2026-07-04T19:10:53Z

+
+```console
+./run test 
+    --start-address 192.168.55.1 \


This network is likely to be used by vment-helper or Apple container (both try to get the next available network and will get 192.168.64.0/24 or 192.168.65.0/24. It is better to show network which is unlikely to be used by the system like 192.168.200.0/24.

nirs · 2026-07-04T19:11:59Z

 cp ~/src/socket_vmnet/test/perf.out/socket_vmnet out/bench/
 ```
+
+## Static IP addresses in `run`


This is nice but we are missing the basic section about using the run scripts without static ip. Do you want to add this?

nirs · 2026-07-04T19:17:43Z

+            logging.debug("Static IP changed, deleting old cloud-init iso")
+            os.remove(cidata)
+    except FileNotFoundError:
+        pass


Move this to a helper function like network_config_changed(), so we can have clear flow here without unwanted details:

if network_config_changed(vm): silent_remove(cidata)

Or:

if os.path.exists(cidata) and !network_config_changed(vm): logging.debug("Reusing cloud-init iso '%s'", cidata) return cidata

We don't really need to delete the network config file, creating new file will overwrite it.

Keep exceptions scope narrow as possible - FileNotFoundError can raise only from the open call, so the code should be:

try: with open(network_config_path, "r") as f: network_config = yaml.safe_load(f.read(4096)) except FileNotFoundError: pass

This ensures we handle expected exceptions correctly, and we don't handle unexpected exceptions.

nirs · 2026-07-04T19:18:58Z

+        with open(network_config_path, "r") as f:
+            network_config = yaml.safe_load(f.read(4096))
+        addresses = network_config["ethernets"]["eth0"].get("addresses", [])
+        ip_address = ip_with_prefixlen(vm)


This call is relevant only if vm.ip_address is not None

nirs · 2026-07-04T19:25:42Z

    return keys
+
+
+def ip_with_prefixlen(vm):


Let's add testing/net.py for networking related functions. We need this for validating command line options and creating cidata.

nirs · 2026-07-04T19:33:22Z

    )

+    p.add_argument(
+        "--ip-address",


We can specify the type to function parsing IPv4 address in the private IP range (RFC 1918), and returning a IPv4 address type that can be used later for more validations. We have the same issue with other addresses. It will be best to add the missing validation to other address options before this change.

The advantage is that the validations of all the address options becomes shorter, and we fail quickly for invalid input.

If we need subnet-mask to validate the address, keep string type and validate after finishing parsing the options.

nirs · 2026-07-04T19:37:27Z

+        ip_address = IPv4Interface(f"{args.ip_address}/{args.subnet_mask}")
+        if ip_address not in subnet:
+            p.error("--ip-address must be in the requested subnet")
+        if ip_address > start_address and ip_address < end_address:


Not sure that <= works with IPv4Interface but want to ensure that you ip_address is not start_address or end_address, or in the range between them.

if start_address <= ip_address <= end_address:

nirs · 2026-07-04T19:44:16Z

        if args.enable_isolation:
            p.error("--enable-isolation not compatible with --operation-mode=bridged")

+    if args.ip_address:


Should we do this validation in testing/net.py?

net.validate_network_options(p, args)

The advantage is keeping run script simple, and make it easy to write unit tests for the delicate validations which are easy to get wrong. It is easier to write correct code when you have small function with less context to load into your head.

nirs · 2026-07-04T19:47:04Z

 """

 import argparse
+from ipaddress import IPv4Interface


We typically import modules, not single types. This makes it easier to tell where the type came from. The code will use ipaddress.IPv4Interface - more verbose but also more clear.

nirs · 2026-07-04T19:50:46Z

+        start_address = IPv4Interface(f"{args.start_address}/{args.subnet_mask}")
+        end_address = IPv4Interface(f"{args.end_address}/{args.subnet_mask}")
+        subnet = start_address.network
+        ip_address = IPv4Interface(f"{args.ip_address}/{args.subnet_mask}")


We are missing validation for private IP range (RFC 1918) for all addresses.

run: add --ip-address option to set vm static ip

fce38d3

The IP must be on the same subnet as the start address, but outside the DHCP range. If the IP changes between runs, or --static-ip is no longer set, the cidata.iso disk is re-provisioned.

nirs requested changes Jul 4, 2026

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tofugarden commented Jul 3, 2026

Objective

Implementation notes

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Switch from dhcp to ip address

Switch back to dhcp

Switch back to dhcp by deleting the iso

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