The qrexec framework consists of a number of processes communicating with each other using common IPC protocol (described in detail below).
Components residing in the same domain (`qrexec-client-vm` to `qrexec-agent`, `qrexec-client` to `qrexec-daemon`) use local sockets as the underlying transport medium.
Components in separate domains (`qrexec-daemon` to `qrexec-agent`, data channel between `qrexec-agent`s) use vchan links.
Because of [vchan limitation](https://github.com/qubesos/qubes-issues/issues/951), it is not possible to establish qrexec connection back to the source domain.
*`-d target-domain-name`: Specifies the target for the execution/service request.
*`-l local-program`: Optional. If present, `local-program` is executed and its stdout/stdin are used when sending/receiving data to/from the remote peer.
*`-e`: Optional. If present, stdout/stdin are not connected to the remote peer. Only process creation status code is received.
*`-c <request-id,src-domain-name,src-domain-id>`: used for connecting a VM-VM service request by `qrexec-policy`. Details described below in the service example.
*`cmdline`: Command line to pass to `qrexec-daemon` as the execution/service request. Service request format is described below in the service example.
**Note:** None of the above tools are designed to be used by users directly.
`user` may be substituted with the literal `DEFAULT`. In that case, default Qubes user will be used to execute `some_command`.
- **dom0**: `qrexec-client` sets `QREXEC_REMOTE_DOMAIN` environment variable to **domX**.
- **dom0**: If `local_program` is set, `qrexec-client` executes it and uses that child's stdin/stdout in place of its own when exchanging data with `qrexec-agent` later.
- **dom0**: `qrexec-client` connects to **domX**'s `qrexec-daemon`.
- **dom0**: `qrexec-daemon` sends `MSG_HELLO` header followed by `peer_info` to `qrexec-client`.
- **dom0**: `qrexec-client` replies with `MSG_HELLO` header followed by `peer_info` to `qrexec-daemon`.
- **dom0**: `qrexec-client` sends `MSG_EXEC_CMDLINE` header followed by `exec_params` to `qrexec-daemon`.
/* variable size */
struct exec_params {
uint32_t connect_domain; /* target domain id */
uint32_t connect_port; /* target vchan port for i/o exchange */
char cmdline[0]; /* command line to execute, size = msg_header.len - sizeof(struct exec_params) */
};
In this case, `connect_domain` and `connect_port` are set to 0.
- **dom0**: `qrexec-daemon` replies to `qrexec-client` with `MSG_EXEC_CMDLINE` header followed by `exec_params`, but with empty `cmdline` field. `connect_domain` is set to Qubes ID of **domX** and `connect_port` is set to a vchan port allocated by `qrexec-daemon`.
- **dom0**: `qrexec-daemon` sends `MSG_EXEC_CMDLINE` header followed by `exec_params` to the associated **domX**`qrexec-agent` over vchan. `connect_domain` is set to 0 (**dom0**), `connect_port` is the same as sent to `qrexec-client`. `cmdline` is unchanged except that the literal `DEFAULT` is replaced with actual user name, if present.
- **dom0**: `qrexec-client` disconnects from `qrexec-daemon`.
- **dom0**: `qrexec-client` starts a vchan server using the details received from `qrexec-daemon` and waits for connection from **domX**'s `qrexec-agent`.
- **domX**: `qrexec-agent` receives `MSG_EXEC_CMDLINE` header followed by `exec_params` from `qrexec-daemon` over vchan.
- **domX**: `qrexec-agent` connects to `qrexec-client` over vchan using the details from `exec_params`.
- **domX**: `qrexec-agent` executes `some_command` in **domX** and connects the child's stdin/stdout to the data vchan. If the process creation fails, `qrexec-agent` sends `MSG_DATA_EXIT_CODE` to `qrexec-client` followed by the status code (**int**) and disconnects from the data vchan.
- Data read from `some_command`'s stdout is sent to the data vchan using `MSG_DATA_STDOUT` by `qrexec-agent`. `qrexec-client` passes data received as `MSG_DATA_STDOUT` to its own stdout (or to `local_program`'s stdin if used).
-`qrexec-client` sends data read from local stdin (or `local_program`'s stdout if used) to `qrexec-agent` over the data vchan using `MSG_DATA_STDIN`. `qrexec-agent` passes data received as `MSG_DATA_STDIN` to `some_command`'s stdin.
-`MSG_DATA_STDOUT` or `MSG_DATA_STDIN` with data `len` field set to 0 in `msg_header` is an EOF marker. Peer receiving such message should close the associated input/output pipe.
- When `some_command` terminates, **domX**'s `qrexec-agent` sends `MSG_DATA_EXIT_CODE` header to `qrexec-client` followed by the exit code (**int**). `qrexec-agent` then disconnects from the data vchan.
### domY: invoke execution of qubes service `qubes.SomeRpc` in domX and pass stdin/stdout
- **domY**: `qrexec-client-vm` is invoked as follows:
- **domY**: `qrexec-client-vm` connects to `qrexec-agent` (via local socket/named pipe).
- **domY**: `qrexec-client-vm` sends `trigger_service_params` data to `qrexec-agent` (without filling the `request_id` field):
struct trigger_service_params {
char service_name[64];
char target_domain[32];
struct service_params request_id; /* service request id */
};
struct service_params {
char ident[32];
};
- **domY**: `qrexec-agent` allocates a locally-unique (for this domain) `request_id` (let's say `13`) and fills it in the `trigger_service_params` struct received from `qrexec-client-vm`.
- **domY**: `qrexec-agent` sends `MSG_TRIGGER_SERVICE` header followed by `trigger_service_params` to `qrexec-daemon` in **dom0** via vchan.
- **dom0**: `qrexec-policy` evaluates if the RPC should be allowed or denied. If the action is allowed it returns `0`, if the action is denied it returns `1`.
- **dom0**: **domY**'s `qrexec-daemon` checks the exit code of `qrexec-policy`.
- If `qrexec-policy` returned **not**`0`: **domY**'s `qrexec-daemon` sends `MSG_SERVICE_REFUSED` header followed by `service_params` to **domY**'s `qrexec-agent`. `service_params.ident` is identical to the one received. **domY**'s `qrexec-agent` disconnects its `qrexec-client-vm` and RPC processing is finished.
- If `qrexec-policy` returned `0`, RPC processing continues.
- **dom0**: if `qrexec-policy` allowed the RPC, it executed `qrexec-client -d domX -c 13,domY,domY_id user:QUBESRPC qubes.SomeRpc domY`.
- **dom0**: `qrexec-client` sets `QREXEC_REMOTE_DOMAIN` environment variable to **domX**.
- **dom0**: `qrexec-client` connects to **domX**'s `qrexec-daemon`.
- **dom0**: **domX**'s `qrexec-daemon` sends `MSG_HELLO` header followed by `peer_info` to `qrexec-client`.
- **dom0**: `qrexec-client` replies with `MSG_HELLO` header followed by `peer_info` to **domX**'s`qrexec-daemon`.
- **dom0**: `qrexec-client` sends `MSG_EXEC_CMDLINE` header followed by `exec_params` to **domX**'s`qrexec-daemon`
/* variable size */
struct exec_params {
uint32_t connect_domain; /* target domain id */
uint32_t connect_port; /* target vchan port for i/o exchange */
char cmdline[0]; /* command line to execute, size = msg_header.len - sizeof(struct exec_params) */
};
In this case, `connect_domain` is set to id of **domY** (from the `-c` parameter) and `connect_port` is set to 0. `cmdline` field contains the RPC to execute, in this case `user:QUBESRPC qubes.SomeRpc domY`.
- **dom0**: **domX**'s `qrexec-daemon` replies to `qrexec-client` with `MSG_EXEC_CMDLINE` header followed by `exec_params`, but with empty `cmdline` field. `connect_domain` is set to Qubes ID of **domX** and `connect_port` is set to a vchan port allocated by **domX**'s `qrexec-daemon`.
- **dom0**: **domX**'s `qrexec-daemon` sends `MSG_EXEC_CMDLINE` header followed by `exec_params` to **domX**'s `qrexec-agent`. `connect_domain` and `connect_port` fields are the same as in the step above. `cmdline` is set to the one received from `qrexec-client`, in this case `user:QUBESRPC qubes.SomeRpc domY`.
- **dom0**: `qrexec-client` disconnects from **domX**'s `qrexec-daemon` after receiving connection details.
- **dom0**: `qrexec-client` connects to **domY**'s `qrexec-daemon` and exchanges `MSG_HELLO` as usual.
- **dom0**: `qrexec-client` sends `MSG_SERVICE_CONNECT` header followed by `exec_params` to **domY**'s `qrexec-daemon`. `connect_domain` is set to ID of **domX** (received from **domX**'s `qrexec-daemon`) and `connect_port` is the one received as well. `cmdline` is set to request ID (`13` in this case).
- **dom0**: **domY**'s `qrexec-daemon` sends `MSG_SERVICE_CONNECT` header followed by `exec_params` to **domY**'s `qrexec-agent`. Data fields are unchanged from the step above.
- **domY**: `qrexec-agent` starts a vchan server on the port received in the step above. It acts as a `qrexec-client` in this case because this is a VM-VM connection.
- **domX**: `qrexec-agent` connects to the vchan server of **domY**'s `qrexec-agent` (connection details were received before from **domX**'s `qrexec-daemon`).
- After that, connection follows the flow of the previous example (dom0-VM).
