RFD - Static host users (#43666)

* Add very rough draft

* First pass

* Another pass

* Add details about user deletion

* Add who and why

* Add reviewer

* Delete deletion

* Add missing sections

* Address comments

* Rename teleport-created to teleport-static

rfd/0175-static-host-users.md

@@ -0,0 +1,215 @@
+---
+author: Andrew Burke (andrew.burke@goteleport.com)
+state: draft
+---
+
+# RFD 175 - Static Host Users
+
+## Required Approvers
+
+- Engineering: @rosstimothy && @lxea && @@espadolini
+
+## What
+
+Teleport nodes will be able to create host users statically, i.e. independently
+of a Teleport user creating one when SSHing with the current host user creation.
+
+## Why
+
+Host users can be created and used (potentially by third-party services) without
+a Teleport user needing to log in first.
+
+## Details
+
+### UX
+
+To create a static host user, an admin will create a `static_host_user` resource:
+
+```yaml
+# foo-dev.yaml
+kind: static_host_user
+metadata:
+    name: foo-dev
+spec:
+    login: foo
+    node_labels:
+        env: dev
+```
+
+Then create it with `tctl`:
+
+```code
+$ tctl create foo-dev.yaml
+```
+
+The user `foo` will eventually appear on nodes with label `env: dev` once the
+`foo-dev` resource makes it through the cache.
+
+To update an existing static host user, an admin will update update `foo-dev.yaml`,
+then update the resource in Teleport with `tctl`:
+
+```code
+$ tctl create -f foo-dev.yaml
+```
+
+### Resource
+
+We will add a new resource to Teleport called `static_host_user`. This resource defines
+a single host user, including groups, sudoers entitlements, uid, and gid, as well as labels
+to select specific nodes the user should be created on.
+
+```yaml
+kind: static_host_user
+metadata:
+    name: hostuser
+spec:
+    login: user1
+    # groups and sudoers are identical to their role counterparts
+    groups: [abc, def]
+    sudoers: [
+        # ...
+    ]
+    # same as from user traits
+    uid: "1234"
+    gid: "5678"
+    # same as allow rules in roles
+    node_labels:
+        # ...
+    node_labels_expression: # ...
+```
+
+```proto
+message StaticHostUser {
+    string kind = 1;
+    string sub_kind = 2;
+    string version = 3;
+    teleport.header.v1.Metadata metadata = 4;
+
+    StaticHostUserSpec spec = 5;
+}
+
+message StaticHostUserSpec {
+    string login = 1;
+    repeated string groups = 2;
+    repeated string sudoers = 3;
+    string uid = 4;
+    string gid = 5;
+
+    wrappers.LabelValues node_labels = 6;
+    string node_labels_expression = 7;
+}
+
+service UsersService {
+  rpc GetStaticHostUser(GetStaticHostUserRequest) returns (GetStaticHostUserResponse);
+  rpc ListStaticHostUsers(ListStaticHostUsersRequest) returns (ListStaticHostUsersResponse);
+  rpc CreateStaticHostUser(CreateStaticHostUserRequest) returns (CreateStaticHostUserResponse);
+  rpc UpdateStaticHostUser(UpdateStaticHostUserRequest) returns (UpdateStaticHostUserResponse);
+  rpc UpsertStaticHostUser(UpsertStaticHostUserRequest) returns (UpsertStaticHostUserResponse);
+  rpc DeleteStaticHostUser(DeleteStaticHostUserRequest) returns (google.protobuf.Empty);
+}
+
+message GetStaticHostUserRequest {
+    string name = 1;
+}
+
+message GetStaticHostUserResponse {
+    types.StaticHostUserV1 user = 1;
+}
+
+message ListStaticHostUsersRequest {
+    int32 page_size = 1;
+    string page_token = 2;
+}
+
+message ListStaticHostUsersResponse {
+    repeated types.StaticHostUserV1 users = 1;
+    string next_page_token = 2;
+}
+
+message CreateStaticHostUserRequest {
+    types.StaticHostUserV1 user = 1;
+}
+
+message CreateStaticHostUserResponse {
+    types.StaticHostUserV1 user = 1;
+}
+
+message UpdateStaticHostUserRequest {
+    types.StaticHostUserV1 user = 1;
+}
+
+message UpdateStaticHostUserResponse {
+    types.StaticHostUserV1 user = 1;
+}
+
+message UpsertStaticHostUserRequest {
+    types.StaticHostUserV1 user = 1;
+}
+
+message UpsertStaticHostUserResponse {
+    types.StaticHostUserV1 user = 1;
+}
+
+message DeleteStaticHostUserRequest {
+    string name = 1;
+}
+```
+
+### Propagation
+
+On startup, nodes will apply all available `static_host_user`s in the cache,
+then watch the cache for new and updated users. Nodes will use the labels in the
+`static_host_user`s to filter out those that don't apply to them, with the same
+logic that currently determines access with roles. Updated `static_host_user`s
+override the existing user. When a `static_host_user` is deleted, any host users
+created by it are *not* deleted (same behavior as `keep` mode for current host
+user creation).
+
+Nodes that disable host user creation (by setting `ssh_service.disable_create_host_user`
+to true in their config) will ignore `static_host_user`s entirely.
+
+### Audit events
+
+The `session.start` audit event will be extened to include a flag
+indicating whether or not the host user for an SSH session was
+created by Teleport (for both static and non-static host users).
+
+Two new audit events, `host_user.create` and `host_user.update`, will be added
+and emitted by nodes when they create or update a host user, respectively.
+
+### Product usage
+
+The session start PostHog event will be extended to include the
+same flag described in [Audit events](#audit-events).
+
+### Security
+
+CRUD operations on `static_host_user`s can be restricted with verbs
+in allow/deny rules like any other resource.
+
+We want to minimize the ability of Teleport users to mess with existing host users
+via `static_host_user`s. To that end, all host users created from `static_host_user`s
+will be in the `teleport-static` group (similar to the `teleport-system` group, which
+we currently use to mark users that Teleport should clean up). New users will not override
+existing users that are not in `teleport-static`.
+
+### Backward compatibility
+
+Consider nodes that do not support static host users but are connected to an
+auth server that does. These nodes will silently ignore static
+host users. When these nodes are upgraded to a supporting
+version, they will create static host users as normal.
+
+### Test plan
+
+Integration test for:
+- nodes create/update nodes in response to `static_host_user` updates from the cache
+
+Manual test for:
+- create static host user with `tctl` and verify it's applied to nodes
+
+### Future work
+
+Extend server heartbeats to include static host users. This will allow Teleport
+users to spot incorrect propagation of host users due to misconfiguration, nodes
+that don't support them, etc.