Qrexec is built on top of *vchan*, a Xen library providing data links between VMs.
During domain creation, a process named `qrexec-daemon` is started in dom0, and a process named `qrexec-agent` is started in the VM.
They are connected over a **vchan** channel.
`qrexec-daemon` listens for connections from a dom0 utility named `qrexec-client`.
Let's say we want to start a process (call it `VMprocess`) in a VM (`someVM`).
Typically, the first thing that a `qrexec-client` instance does is to send a request to the `qrexec-daemon`, which in turn relays it to `qrexec-agent` running in `someVM`.
`qrexec-daemon` assigns unique vchan connection details and sends them to both `qrexec-client` (in dom0) and `qrexec-agent` (in `someVM`).
`qrexec-client` starts a vchan server, which `qrexec-agent` then connects to.
Once this channel is established, stdin/stdout/stderr from the VMprocess is passed between `qrexec-agent` and the `qrexec-client` process.
The string before the colon specifies what user to run the command as.
The `-e` flag tells `qrexec-client` to exit immediately after sending the execution request and receiving a status code from `qrexec-agent` (whether the process creation succeeded).
With this option, no further data is passed between the domains.
There can be almost arbitrary number of `qrexec-client` processes for a domain (so, connected to the same `qrexec-daemon`, same domain) -- their data is multiplexed independently.
Number of available vchan channels is the limiting factor here, it depends on the underlying hypervisor.
Some common tasks (like copying files between VMs) have an RPC-like structure: a process in one VM (say, the file sender) needs to invoke and send/receive data to some process in other VM (say, the file receiver).
The Qubes RPC framework was created to securely facilite a range of such actions.
Additionally, disposable VMs are tightly integrated -- RPC to a DisposableVM is identical to RPC to a normal domain, all one needs is to pass `$dispvm` as the remote domain name.
You can specify srcvm and destvm by name or by one of three reserved keywords: `$anyvm`, `$dispvm`, and `dom0` (without the `$`).
Only `$anyvm` keyword makes sense in srcvm field.
(Service calls from dom0 are currently always allowed, `$dispvm` means "new VM created for this particular request," so it is never a source of request.)
Whenever a RPC request for an action is received, the domain checks the first matching line of the relevant file in `/etc/qubes-rpc/policy/` to determine access:
whether to allow the request, what VM to redirect the execution to, and what user account the program should run under.
Note that if the request is redirected (`target=` parameter), policy action remains the same - even if there is another rule which would otherwise deny such request.
In the target VM, the `/etc/qubes-rpc/RPC_ACTION_NAME` must exist, containing the file name of the program that will be invoked, or being that program itself - in which case it must have executable permission set (`chmod +x`).
Be very careful when coding and adding a new RPC service.
Unless the offered functionality equals full control over the target (it is the case with e.g. `qubes.VMShell` action), any vulnerability in an RPC server can be fatal to Qubes security.
On the other hand, this mechanism allows to delegate processing of untrusted input to less privileged (or disposable) AppVMs, thus wise usage of it increases security.
This will look for a `firefox.desktop` file in a standard location in a DisposableVM based on `work`, then launch the application described by that file.
The practical difference is that the bare `qvm-run` command uses the `qubes.VMShell` service, which allows you to run an arbitrary command with arbitrary arguments, essentially providing full control over the target VM.
By contrast, the `qubes.StartApp` service allows you to run only applications that are advertised in `/usr/share/applications` (or other standard locations) *without* control over the arguments, so giving a VM access to `qubes.StartApp` is much safer.
While there isn't much practical difference between the two commands above when starting an application from dom0 in Qubes 4.0, there is a significant security risk when launching applications from a domU (e.g., from a separate GUI domain).
This is why `qubes.StartApp` uses our standard `qrexec` argument grammar to strictly filter the permissible grammar of the `Exec=` lines in `.desktop` files that are passed from untrusted domUs to dom0, thereby protecting dom0 from command injection by maliciously-crafted `.desktop` files.
In Qubes 4.0, target VM can be specified also as `$dispvm:DISP_VM`, which is very similar to `$dispvm` but forces using a particular VM (`DISP_VM`) as a base VM to be started as DisposableVM.
Adding such policy itself will not force usage of this particular `DISP_VM` - it will only allow it when specified by the caller.
But `$dispvm:DISP_VM` can also be used as target in request redirection, so _it is possible_ to force particular `DISP_VM` usage, when caller didn't specify it:
Note that without redirection, this rule would allow using default Disposable VM (`default_dispvm` VM property, which itself defaults to global `default_dispvm` property).
Also note that the request will be allowed (`allow` action) even if there is no second rule allowing calls to `$dispvm:anon-whonix-dvm`, or even if there is a rule explicitly denying it.
This is because the redirection happens _after_ considering the action.
Target VM can be also specified as `$default`, which matches the case when calling VM didn't specified any particular target (either by using `$default` target, or empty target).
The first rule allow call from `work-mail` to `work-archive`, without any confirmation.
The second rule will ask the user about calls from `work-mail` VM to any VM with tag `work`.
And the confirmation dialog will have `work-files` VM chosen by default, regardless of the VM specified by the caller (`work-mail` VM).
The third rule allow the caller to not specify target VM at all and let the user choose, still - from VMs with tag `work` (and `work-archive`, regardless of tag), and with `work-files` as default.
Sometimes just service name isn't enough to make reasonable qrexec policy.
One example of such a situation is [qrexec-based USB passthrough](https://github.com/qubesos/qubes-issues/issues/531) - using just service name isn't possible to express the policy "allow access to device X and deny to others".
It also isn't feasible to create a separate service for every device...
For this reason, starting with Qubes 3.2, it is possible to specify a service argument, which will be subject to policy.
Besides the above example of USB passthrough, a service argument can make many service policies more fine-grained and easier to write precise policy with "allow" and "deny" actions, instead of "ask" (offloading additional decisions to the user).
And generally the less choices the user must make, the lower the chance to make a mistake.
Then create a policy as usual, including the argument (`/etc/qubes-rpc/policy/RPC_ACTION_NAME+ARGUMENT`).
If the policy for the specific argument is not set (file does not exist), then the default policy for this service is loaded (`/etc/qubes-rpc/policy/RPC_ACTION_NAME`).
This will prompt the user whether given RPC call should be allowed.
That prompt window also has a "Yes to All" option, which will allow the action and add a new entry to the policy file, which will unconditionally allow further calls for the given service-srcVM-dstVM tuple.
As a demonstration, we can create an RPC service that adds two integers in a target domain (the server, call it "anotherVM") and returns back the result to the invoker (the client, "someVM").
In someVM, create a file with the following contents and save it with the path `/usr/bin/our_test_add_client`:
**Note:** For a real world example of writing a qrexec service, see this [blog post](https://blog.invisiblethings.org/2013/02/21/converting-untrusted-pdfs-into-trusted.html).
Besides really naive storage, it may be a very simple password manager.
Additionally, in this example a simplified workflow will be used - server code placed directly in the service definition file (in `/etc/qubes-rpc` directory).