What does "Cannot connect to compute 'GNS3 VM'" mean?

GNS3's controller tried to send a request, such as POST /projects, to the GNS3 server inside the GNS3 VM and got no usable answer. The VM may not have started, may have no IP address, may run a different GNS3 version, or may be blocked by a firewall.

The GNS3 VM is running in VMware. Why is the indicator still red?

A running VM is not enough. GNS3 must also read its IP address through VMware Tools and reach port 3080 on it. If the VM has no IP on the host-only network, or the host's VMnet adapter is disabled, the VM runs but GNS3 cannot connect.

Do I have to start the GNS3 VM from GNS3?

No. A GNS3 moderator recommends the opposite on Windows: start the GNS3 VM yourself and set it as the remote main server in GNS3's server preferences. You lose the automatic start, and many local-server problems go away.

Can a VPN break the connection to the GNS3 VM?

Yes. Some VPN clients route or block all traffic, including traffic to host-only networks. If GNS3 connects with the VPN off and fails with it on, add an exception for the host-only subnet in the VPN client, or disconnect while you lab.

GNS3 "Cannot connect to compute 'GNS3 VM'": how to fix it

GNS3 starts, the GNS3 VM stays red or grey in the Servers Summary, and every action fails with Cannot connect to compute 'GNS3 VM (GNS3 VM)' with request POST /projects (or GET /dynamips/images, or another request). It means GNS3's controller, the local server on your computer, could not talk to the GNS3 server running inside the GNS3 VM.

Why it happens

When you enable the GNS3 VM, the local server does three things, as you can follow in the gns3server/controller/gns3vm code of the GNS3 server:

It starts the VM through your hypervisor (vmrun for VMware, VBoxManage for VirtualBox, PowerShell for Hyper-V).
It waits for the VM's IP address. With VMware it reads it through VMware Tools. With VirtualBox it reads a guest property, or asks the VM through a NAT port forward.
It connects to the GNS3 server on that IP, port 3080 by default, and checks the version.

The error appears when any of these steps fails. The usual reasons:

The hypervisor didn't start the VM, or GNS3 can't control it.
The VM has no address on its host-only network, so there is nothing to connect to.
The host-only adapter on your computer is disabled or missing.
A firewall, antivirus or VPN client blocks the traffic.
The GNS3 VM runs a different GNS3 version than the app.

How to fix it

1. Read the real error

Look at the console at the bottom of the GNS3 window. The line before or after the error often names the cause, for example Could not find guest IP address for GNS3 VM or VMware tools are not installed in GNS3 VM. If it says GNS3 version 2.2.61 is not the same as the GNS3 VM version 2.2.59. Please upgrade the GNS3 VM., stop here and read GNS3 server version mismatch.

2. Read the GNS3 VM's screen

Open the GNS3 VM's window in VMware, VirtualBox or Hyper-V. Its information screen shows a line like IP: 192.168.56.101 PORT: 3080. Note both values.

If it says eth0 is not configured instead, the VM got no address on its first adapter. Go to step 4.

3. Test the connection from your computer

From a terminal on your computer, using the IP and port from step 2:

ping 192.168.56.101
curl http://192.168.56.101:3080/v2/version

A working GNS3 2.2 server answers with JSON such as {"local": false, "version": "2.2.61"}. You can also open that URL in a browser.

No ping, no answer: a network problem between your computer and the VM. Go to step 4.
Ping works, curl fails: the GNS3 service in the VM is down, or something blocks port 3080. Check the server log with Log in the GNS3 VM menu, then go to step 5.
Both work: the VM is fine. The problem is in GNS3's settings. Go to step 6.

4. Repair the host-only network

The GNS3 VM's first adapter must sit on a host-only network with DHCP. Its second adapter uses NAT for internet access.

VMware Workstation or Fusion. In the VM's settings, adapter 1 should be Host-only. In Workstation's Virtual Network Editor, check that VMnet1 is Host-only with DHCP enabled. On Windows, open Network Connections and make sure VMware Network Adapter VMnet1 is enabled. Disabling and enabling it again has fixed this for several users in gns3-gui issue #2969.

VirtualBox. GNS3 checks this itself and tells you what is missing, with errors such as VM "GNS3 VM" must have a network adapter attached to a host-only adapter in order to start or Please add a VirtualBox host-only network with DHCP enabled and attached it to network adapter 1 for "GNS3 VM". Create the host-only network in VirtualBox's network manager, enable its DHCP server, and attach adapter 1 to it. On macOS with VirtualBox 7, that adapter must be attached to a Host-only Network rather than the older host-only adapter.

Restart the GNS3 VM and check its screen again for an IP.

5. Let the traffic through

The GNS3 VM setup guide recommends exceptions in your antivirus for the gns3server, ubridge and dynamips executables, and checking that Windows Firewall allows them. Also disconnect any VPN client and try again. Some route all traffic, including the host-only network, through the tunnel.

6. Check GNS3's own settings

Open Edit, Preferences (GNS3, Preferences on macOS):

GNS3 VM: "Enable the GNS3 VM" is ticked, the virtualization engine matches your hypervisor, and the VM name is the one you imported. Click Refresh if the list is empty.
Server: if the console shows The GNS3 VM (IP=..., NETWORK=...) is not on the same network as the local server (IP=..., NETWORK=...), please make sure the local server binding is in the same network as the GNS3 VM, links between nodes on your computer and nodes in the VM will fail. Set Host binding to your computer's address on the host-only network, then restart GNS3.

With VMware Workstation Player, GNS3 also needs the VMware VIX API to control the VM. Run GNS3 and VMware as the same user, and don't start GNS3 with "Run as administrator": VMware then can't see your VMs, as the GNS3 troubleshooting page explains.

How to check it worked

Restart GNS3. In the Servers Summary panel, the GNS3 VM should turn green and show its CPU and RAM use. Create a new project, drag a VPCS node onto the canvas, choose the GNS3 VM as its server, and start it. If it goes green, the controller and the VM are talking.

If it still fails

Use the VM as the remote main server. A GNS3 moderator's advice in Solving 90% of GNS3 issues on Windows is to stop using the local server: start the GNS3 VM yourself, untick "Enable the GNS3 VM", and in Edit, Preferences, Server, set the remote main server to the VM's IP and port. GNS3 then talks to the VM directly. You start and stop the VM yourself, and VMware or VirtualBox VMs can no longer be added as nodes from your computer.
Hyper-V. GNS3 reads the IP address that Hyper-V reports for the VM. If Could not find guest IP address for GNS3 VM keeps coming back, open the VM's Networking tab in Hyper-V Manager. No address there means the integration services in the VM aren't reporting it; in gns3-server issue #1813 a maintainer fixed it by installing the linux-virtual, linux-cloud-tools-virtual and linux-tools-virtual packages in the VM and rebooting.
Apple Silicon. The usual x86 GNS3 VM can't run there at all. See GNS3 on Apple Silicon.
The VM connects, but QEMU nodes don't start. That is a different problem, often KVM support available: False.

For more GNS3 errors, see the GNS3 troubleshooting guide.

A lighter way to build the lab

If you mostly need Linux machines and services on a few subnets, you can build that lab without a GNS3 VM in the middle. Isothrum's editor runs in your browser, with no account, and exports what you draw as Docker Compose or Vagrant files that run on the tools you already have. It doesn't repair a GNS3 VM, but a lab of containers on Docker has no host-only network to break.