[v10] Allow connections to nodes when Auth is offline #18914
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`tsh ssh user@foo` currently fails if Auth is unreachable. Prior to
connecting to the target, `tsh` sends a `proto.AuthService/IsMFARequired`
request to determine if per-session MFA is required. If necessary, the MFA
ceremony then occurs before `tsh` attempts to connect to the node.
However, nodes ultimately makes the decision on whether the user has
access. To both increase availability of nodes and reduce initial
connection latency, `tsh` can first attempt to connect to a node and
fallback to checking if per-session MFA is required and then performing
the ceremony if necessary followed by attempting another connection to the
node. The major tradeoff is now when per-session MFA is required, `tsh`
will connect to the target **twice**. The added latency is likely to be
minimized by the amount of time it takes to perform the MFA ceremony.
## Connection flows prior to this change:
### Auth online and per-session MFA required
```mermaid
sequenceDiagram
participant tsh
participant Auth
participant Node
tsh->>+Auth: IsMFARequired
Auth-->>-tsh: yes
tsh->>+Auth: perform mfa ceremony
Auth-->>+tsh: issue challenge
tsh-->>+Auth: challenge response
Auth-->>+tsh: issue certificates
tsh->>+Node: Connect
Node-->>+tsh: Success
```
### Auth online and per-session MFA not required
```mermaid
sequenceDiagram
participant tsh
participant Auth
participant Node
tsh->>+Auth: IsMFARequired
Auth-->>-tsh: no
tsh->>+Node: Connect
Node-->>+tsh: Success
```
### Auth offline and per-session MFA not required
```mermaid
sequenceDiagram
participant tsh
participant Auth
participant Node
tsh->>+Auth: IsMFARequired
Auth-->>-tsh: Error
```
### Auth offline and per-session MFA required
```mermaid
sequenceDiagram
participant tsh
participant Auth
participant Node
tsh->>+Auth: IsMFARequired
Auth-->>-tsh: Error
```
## Connection flows after this change:
### Auth online and per-session MFA required
```mermaid
sequenceDiagram
participant tsh
participant Auth
participant Node
tsh->>+Node: Connect
Node-->>+tsh: Access Denied
tsh->>+Auth: IsMFARequired
Auth-->>-tsh: yes
tsh->>+Auth: perform mfa ceremony
Auth-->>+tsh: issue challenge
tsh-->>+Auth: challenge response
Auth-->>+tsh: issue certificates
tsh->>+Node: Connect
Node-->>+tsh: Success
```
### Auth online and per-session MFA not required
```mermaid
sequenceDiagram
participant tsh
participant Auth
participant Node
tsh->>+Node: Connect
Node-->>+tsh: Success
```
### Auth offline and per-session MFA not required
```mermaid
sequenceDiagram
participant tsh
participant Auth
participant Node
tsh->>+Node: Connect
Node-->>+tsh: Success
```
### Auth offline and per-session MFA required
```mermaid
sequenceDiagram
participant tsh
participant Auth
participant Node
tsh->>+Node: Connect
Node-->>+tsh: Access Denied
tsh->>+Auth: IsMFARequired
Auth-->>-tsh: Error
```
rosstimothy
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espadolini
approved these changes
Dec 1, 2022
|
PTAL @zmb3 |
zmb3
approved these changes
Dec 3, 2022
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Backport #18302 to branch/v10