This document was automatically generated from the exports/exports.go file

# About the API
package main implements the main exported API to be used by other languages

Some notes:

- Errors are returned as JSON c strings. The JSON type is defined in
`types/error/error.go Error`. Free them using `FreeString`. Same is the case for
other string types, you should also free them. The errors are always localized

- Types are converted from the Go representation to C using JSON strings

- Cookies are used for cancellation, just fancy contexts. Create a cookie using
`CookieNew`, pass it to the function that needs one as the first argument. To
cancel the function, call `CookieCancel`, passing in the same cookie as argument

- Cookies must also be freed, by using the CookieDelete function if the cookie
is no longer needed

- The state machine is used to track the state of a client. It is mainly used
for asking for certain data from the client, e.g. asking for profiles and
locations. But a client may also wish to build upon this state machine to build
the whole UI around it. The SetState and InState functions are useful for this


# Functions
## AddServer
Signature:

```go
func AddServer(c C.uintptr_t, _type C.int, id *C.char, ot *C.longlong) *C.char
```

AddServer adds a server to the eduvpn-common server list `c` is the cookie
that is used for cancellation. Create a cookie first with CookieNew.
This same cookie is also used for replying to state transitions.

`_type` is the type of server that needs to be added. This type is defined
in `types/server/server.go Type`

`id` is the identifier of the string:

  - In case of secure internet: The organization ID
  - In case of custom server: The base URL
  - In case of institute access: The base URL

`ni` stands for non-interactive. If non-zero, any state transitions will not
be run.

This `ot` flag is useful for preprovisioned servers; set this to non-null to
non-interactively add a server. This flag represents the Unix time OAuth was
last triggered, if the server needs to be added non-interactively but there
is no token structure, set this to zero (integer) or the current Unix time.
This value will be overwritten once OAuth is triggered.

If the server cannot be added it returns the error as `types/error/error.go
Error`. Note that the server is removed when an error has occured

The following state callbacks are mandatory to handle:

  - OAUTH_STARTED: This indicates that the OAuth procedure has been started,
    it returns the URL as the data. The client should open the webbrowser
    with this URL and continue the authorization process. Note: For mobile
    platforms this returns a Cookie and data (json: `{"cookie": x, "data":
    url}`). This `url` should also be opened in the browser like desktop
    platforms. But these platforms also need to reply to the library to give
    back the full authorization code URI with `CookieReply(x, uri)`. E.g.
    `CookieReply(x, "/callback?code=...&state=...&iss=...")` this is the
    path of the request that the apps get back when the user clicks approve.
    For this, apps need to register an app url or sorts. For the valid
    values for app URLs, see the redirect URIs for mobile platforms here
    https://git.sr.ht/~fkooman/vpn-user-portal/tree/v3/item/src/OAuth/VpnClientDb.php

Example Input (3=custom server): ```AddServer(mycookie, 3,
"https://demo.eduvpn.nl", 0)```

Example Output:

    {
      "message": {
        "en": "failed to add server"
      },
      "misc": false
    }

## CalculateGateway
Signature:

```go
func CalculateGateway(subnet *C.char) (*C.char, *C.char)
```

CalculateGateway calculates the gateway for a subnet, it can take IPv4 or
IPv6 networks with CIDR notation as inputs and returns the gateway address.

This is useful to pass to `StartFailover`.
It returns an error if it fails to calculate a gateway.
The function is implemented according to: [the eduVPN
docs](https://docs.eduvpn.org/server/v3/client-implementation-notes.html#fail-over).

Example Input: ```CalculateGateway("10.10.0.5/24")```

Example Output: ```"10.10.0.1", null```

## Cleanup
Signature:

```go
func Cleanup(c C.uintptr_t) *C.char
```

Cleanup sends a `/disconnect` to cleanup the connection.

This MUST be called when disconnecting, see [the eduVPN
docs](https://docs.eduvpn.org/server/v3/api.html#application-flow). `c` is
the Cookie that needs to be passed. Create a new Cookie using `CookieNew`.

If it was unsuccessful, it returns an error.

Example Input: ```Cleanup(myCookie)```

Example Output:

    {
      "message": {
        "en": "cleanup was not successful"
      },
      "misc": false
    }

## CookieCancel
Signature:

```go
func CookieCancel(c C.uintptr_t) *C.char
```

CookieCancel cancels the cookie.

This means that functions which take this as first argument,
return if they're still running. The error cause is always
`context.Canceled` for that cancelled function: [see the Go
docs](https://pkg.go.dev/context#pkg-variables).

This CookieCancel function can also return an error if cancelling was
unsuccessful. Example Input: ```CookieCancel(myCookie)```

Example Output: null

## CookieDelete
Signature:

```go
func CookieDelete(c C.uintptr_t) *C.char
```

CookieDelete deletes the cookie by cancelling it and deleting the underlying
cgo handle.

This function MUST be called when the cookie that is created using
`CookieNew` is no longer needed. Example Input: ```CookieDelete(myCookie)```

Example Output: null

## CookieNew
Signature:

```go
func CookieNew() C.uintptr_t
```

CookieNew creates a new cookie and returns it. Functions that take a cookie
have it as the first argument.

This value should not be parsed or converted somehow by the client. This
value is simply to pass back to the Go library. This value has two purposes:

  - Cancel a long running function
  - Send a reply to a state transition (ASK_PROFILE and ASK_LOCATION)

Example Input: ```CookieNew()```

Example Output: ```5```

## CookieReply
Signature:

```go
func CookieReply(c C.uintptr_t, data *C.char) *C.char
```

CookieReply replies to a state transition using the cookie.

  - `c` is the Cookie
  - `data` is the data to send, e.g. a profile ID

Example Input: ```CookieReply(myCookie, "split-tunnel-profile")```

Example Output: ```null```

## CurrentServer
Signature:

```go
func CurrentServer() (*C.char, *C.char)
```

CurrentServer gets the current server from eduvpn-common

In eduvpn-common, a server is marked as 'current' if you have gotten a VPN
configuration for it

It returns the server as JSON, defined in `types/server/server.go Current`.

If there is no current server or some other, e.g. there is no current state,
an error is returned with a nil string.

Example Input: ```CurrentServer()```

Example Output:

    {
      "institute_access_server": {
        "display_name": {
          "en": "Demo"
        },
        "identifier": "https://demo.eduvpn.nl/",
        "profiles": {
          "map": {
            "internet": {
              "display_name": {
                "en": "Internet"
              },
              "supported_protocols": [
                1,
                2
              ]
            },
            "internet-split": {
              "display_name": {
                "en": "No rfc1918 routes"
              },
              "supported_protocols": [
                1,
                2
              ]
            }
          },
          "current": "internet"
        },
        "support_contacts": [
          "mailto:eduvpn@surf.nl"
        ],
        "delisted": false
      },
      "server_type": 1
    }, null

## Deregister
Signature:

```go
func Deregister() *C.char
```

Deregister cleans up the state for the client.

This function SHOULD be called when the application exits such that the
configuration file is saved correctly. Note that saving of the configuration
file also happens in other cases, such as after getting a VPN configuration.
Thus it is often not problematic if this function cannot be called due to a
client crash.

If no client is available or deregistering fails, it returns an error.

Example Input: ```Deregister()```

Example Output:

    {
      "message": {
        "en": "failed to deregister"
      },
      "misc": false
    }

## DiscoOrganizations
Signature:

```go
func DiscoOrganizations(c C.uintptr_t, search *C.char) (*C.char, *C.char)
```

DiscoOrganizations gets the organizations from discovery, returned as
`types/discovery/discovery.go Organizations` marshalled as JSON.

  - `c` is the Cookie that needs to be passed. Create a new Cookie using
    `CookieNew`
  - `search` is the search string for filtering the list.

If any of the words in the `search` query is not contained in
any of the display names or keywords, the candidate is filtered.
Otherwise they are ranked based on the levenshtein distance: [Levenshtein
Wikipedia](https://en.wikipedia.org/wiki/Levenshtein_distance). If `search`
is empty it returns ALL organizations currently known in common

If it was unsuccessful, it returns an error. Note that when the lib was
built in release mode the data is almost always non-nil, even when an error
has occurred This means it has just returned the cached list, the error
should then not be handled in a fatal way. E.g. show the returned cache list
but log the error or show the error with a warning.

Example Input: ```DiscoOrganizations(myCookie, "")```

Example Output:

    {
     "organization_list": [
       {
         "display_name": {
           "en": "Academic Network of Albania - RASH"
         },
         "org_id": "https://idp.rash.al/simplesaml/saml2/idp/metadata.php",
       },
       {
         "display_name": {
           "da": "Dansk Sprognævn",
           "en": "Danish Language Council"
         },
         "org_id": "http://idp.dsn.dk/adfs/services/trust",
       },
       {
         "display_name": {
           "da": "Erhvervsakademi Aarhus",
           "en": "Business Academy Aarhus"
         },
         "org_id": "http://adfs.eaaa.dk/adfs/services/trust",
    }, null

Example Input: ```DiscoOrganizations(myCookie, "rash")```

Example Output:

    	{
    	 "organization_list": [
    	   {
    	     "display_name": {
    	       "en": "Academic Network of Albania - RASH"
    	     },
    	     "org_id": "https://idp.rash.al/simplesaml/saml2/idp/metadata.php",
    	   },
          ]
    	}, null

## DiscoServers
Signature:

```go
func DiscoServers(c C.uintptr_t, search *C.char) (*C.char, *C.char)
```

DiscoServers gets the servers from discovery, returned as
`types/discovery/discovery.go Servers` marshalled as JSON

  - `c` is the Cookie that needs to be passed. Create a new Cookie using
    `CookieNew`
  - `search` is the search string for filtering the list.

If any of the words in the search query is not contained in any of
the display names or keywords, the candidate is filtered. Otherwise
they are ranked based on the levenshtein distance: [Levenshtein
Wikipedia](https://en.wikipedia.org/wiki/Levenshtein_distance). If `search`
is empty it returns ALL servers currently known in common

If it was unsuccessful, it returns an error. Note that when the lib was
built in release mode the data is almost always non-nil, even when an error
has occurred. This means it has just returned the cached list, the error
should then not be handled in a fatal way. E.g. show the returned cache list
but log the error or show the error with a warning.

Example Input: ```DiscoServers(myCookie, "")```

Example Output:

    	{
    	 "server_list": [
    	   {
    	     "base_url": "https://eduvpn.rash.al/",
              "country_code": "AL",
    	     "server_type": "secure_internet",
    	   },
    	   {
    	     "base_url": "https://eduvpn.deic.dk/",
              "country_code": "DK",
    	     "server_type": "secure_internet",
    	} , null

Example Input: ```DiscoServers(myCookie, "heanet")```

Example Output:

    	{
    	 "server_list": [
             {
               "base_url": "https://eduvpn.heanet.ie/",
               "display_name": {
                 "en": "HEAnet Staff"
                },
               "server_type": "institute_access",
             },
           ]
    	} , null

## DiscoveryStartup
Signature:

```go
func DiscoveryStartup(refresh C.RefreshList) *C.char
```

DiscoveryStartup does a discovery request in the background.

  - The `refresh` argument is a callback that is called when the refreshing
    is done.

When this callback is thus called, the app SHOULD refresh the server list
of the already configured servers. This DiscoveryStartup function MUST be
called after calling `Register`.

## ExpiryTimes
Signature:

```go
func ExpiryTimes() (*C.char, *C.char)
```

ExpiryTimes gets the expiry times for the current server

Expiry times are just fields that represent unix timestamps at which to
do certain events regarding expiry, e.g. when to show the renew button,
when to show expiry notifications

The expiry times structure is defined in `types/server/server.go Expiry` If
some error occurs, it is returned as `types/error/error.go Error`

Example Input: ```ExpiryTimes()```

Example Output (1...4 are unix timestamps):

    {
         "start_time": 1,
         "end_time": 2,
         "button_time": 3,
         "countdown_time": 4,
         "notification_times": [
             1,
             2,
         ],
    }, null

## FreeString
Signature:

```go
func FreeString(addr *C.char)
```

FreeString frees a string that was allocated by the eduvpn-common Go
library.

This happens when we return strings, such as errors from the Go lib back to
the client. The client MUST thus ensure that this memory is freed using this
function. Simply pass the pointer to the string in here.

Example Input: ```FreeString(strPtr)```

## GetConfig
Signature:

```go
func GetConfig(c C.uintptr_t, _type C.int, id *C.char, pTCP C.int, startup C.int) (*C.char, *C.char)
```

GetConfig gets a configuration for the server.

`c` is the cookie that is used for cancellation. Create a cookie first with
CookieNew, this same cookie is also used for replying to state transitions

`_type` is the type of server that needs to be added. This type is defined
in `types/server/server.go Type`

`id` is the identifier of the string

  - In case of secure internet: The organization ID
  - In case of custom server: The base URL
  - In case of institute access: The base URL

`pTCP` is if we prefer TCP or not to get the configuration, non-zero means
yes

`startup` is if the client is just starting up, set this to true (non-zero)
if you autoconnect to a server on startup. If this startup value is true
(non-zero) then any authorization or other callacks (profile/location) are
not triggered

After getting a configuration, the FSM moves to the GOT_CONFIG state
The return data is the configuration, marshalled as JSON and defined in
`types/server/server.go Configuration`

If the config cannot be retrieved it returns an error as
`types/error/error.go Error`.

The current state callbacks MUST be handled:

### ASK_PROFILE

This asks the client for profile.

This is called when the user/client has not set a profile for this server
before, or the current profile is invalid

When the user has selected a profile, reply with the choice using the
`CookieReply` function and the profile ID e.g. CookieReply(cookie,
"wireguard"). CookieReply can be done in the background as the Go library
waits for a reply

The data for this transition is defined in `types/server/server.go
RequiredAskTransition` with embedded data `Profiles` in
`types/server/server.go`. Note that `RequiredAskTransition` contains the
cookie to be used for the `CookieReply`.

So a client would:

  - Parse the data to get the cookie and data
  - get the cookie
  - get the profiles from the data
  - show it in the UI and then reply with CookieReply using the choice

### ASK_LOCATION

This asks the client for a location. Note that under normal circumstances,
this callback is not actually called as the home organization for the
secure internet server is set as the current if for some reason, an invalid
location has been configured, the library will ask the client for a new one

When the user has selected a location, reply with the choice using the
`CookieReply` function and the location ID e.g. CookieReply(cookie, "nl")

CookieReply can be done in the background as the Go library waits for a
reply The data for this transition is defined in `types/server/server.go
RequiredAskTransition` with embedded data a list of strings (`[]string`)

Note that `RequiredAskTransition` contains the cookie to be used for the
`CookieReply` function,

So a client would:

  - Parse the data to get the cookie and data
  - get the cookie
  - get the list of locations from the data
  - show it in the UI and then reply with CookieReply using the choice

### OAUTH_STARTED

  - OAUTH_STARTED: This indicates that the OAuth procedure has been started,
    it returns the URL as the data. The client should open the webbrowser
    with this URL and continue the authorization process. Note: For mobile
    platforms this returns a Cookie and data (json: `{"cookie": x, "data":
    url}`). This `url` should also be opened in the browser like desktop
    platforms. But these platforms also need to reply to the library to give
    back the full authorization code URI with `CookieReply(x, uri)`. E.g.
    `CookieReply(x, "/callback?code=...&state=...&iss=...")` this is the
    path of the request that the apps get back when the user clicks approve.
    For this, apps need to register an app url or sorts. For the valid
    values for app URLs, see the redirect URIs for mobile platforms here
    https://git.sr.ht/~fkooman/vpn-user-portal/tree/v3/item/src/OAuth/VpnClientDb.php

The client should open the webbrowser with this URL and continue the
authorization process. This is only called if authorization needs to be
retriggered

Example Input (3=custom server): ```GetConfig(myCookie, 3,
"https://demo.eduvpn.nl/", 0, 0)```

Example Output (2=WireGuard):

    {
     "config": "[Interface]\nPrivateKey = ...\nAddress = ...\nDNS = ...\n\n[Peer]\nPublicKey = ...=\nAllowedIPs = 0.0.0.0/0,::/0\nEndpoint = ...",
     "protocol": 2,
     "default_gateway": true,
     "should_failover": true, <- whether or not the failover procedure should happen
    }

## InState
Signature:

```go
func InState(fsmState C.int) (C.int, *C.char)
```

InState checks if the FSM is in `fsmState`.

Example Input: ```InState(5)```

Example Output: ```1, null```

## NewProxyguard
Signature:

```go
func NewProxyguard(c C.uintptr_t, lp C.int, tcpsp C.int, peer *C.char, proxySetup C.ProxySetup) (C.uintptr_t, *C.char)
```

NewProxyguard creates the 'proxyguard' procedure in eduvpn-common. If the
proxy cannot be created it returns an error.

This function proxies WireGuard UDP connections over HTTP: [ProxyGuard on
Codeberg](https://codeberg.org/eduvpn/proxyguard).

These input variables can be gotten from the configuration that is retrieved
using the `proxy` JSON key

  - `c` is the cookie. Note that if you cancel/delete the cookie,
    ProxyGuard gets cleaned up. Common automatically cleans up ProxyGuard
    when `Cleanup` is called, but it is good to cleanup yourself too.
  - `lp` is the `port` of the local udp ProxyGuard connection, this is what
    is set to the WireGuard endpoint
  - `tcpsp` is the TCP source port. Pass 0 if you do not route based on
    source port, so far only the Linux client has to pass non-zero.
  - `peer` is the `ip:port` of the remote server
  - `proxySetup` is a callback which is called when the socket is setting
    up, this can be used for configuring routing in the client. It takes
    two arguments: the file descriptor (integer) and a JSON list of IPs the
    client connects to

Example Input: ```NewProxyguard(myCookie, 1337, 0, "5.5.5.5:51820",
proxySetupCB)```

Example Output: ```null```

## ProxyguardPeerIPs
Signature:

```go
func ProxyguardPeerIPs(proxyH C.uintptr_t) (*C.char, *C.char)
```

ProxyguardPeerIPs gets the Peer IPs configured by ProxyGuard Example Input:
```ProxyguardPeerIPs(handle)```

Example Output: ```["1.1.1.1"], null```

## ProxyguardTunnel
Signature:

```go
func ProxyguardTunnel(c C.uintptr_t, proxyH C.uintptr_t, wglisten C.int) *C.char
```

ProxyguardTunnel starts the tunneling for ProxyGuard `c` is the cookie
`proxyH` is the proxy handle `wglisten` is the port WireGuard is listening
on

## Register
Signature:

```go
func Register(
	name *C.char,
	version *C.char,
	configDirectory *C.char,
	cb C.StateCB,
	debug C.int,
) *C.char
```

Register creates a new client and also registers the FSM to go to the
initial state

`Name` is the name of the client, must be a valid client ID.

`Version` is the version of the client. This version field is used for the
user agent in all HTTP requests.

`cb` is the state callback. It takes three arguments: The old state, the new
state and the data for the state as JSON.

  - Note that the states are defined in client/fsm.go, e.g. `Main` (in Go:
    `StateMain`), `ASK_PROFILE` (in Go: `StateAskProfile`)

  - This callback returns non-zero if the state transition is handled.
    This is used to check if the client handles the needed transitions

`debug`, if non-zero, enables debugging mode for the library, this means:

  - Log everything in debug mode, so you can get more detail of what is
    going on

  - Write the state graph to a file in the configDirectory. This can be used
    to create a FSM png file with mermaid https://mermaid.js.org/

After registering, the FSM is initialized and the state transition `MAIN`
should have been completed If some error occurs during registering, it is
returned as a `types/error/error.go Error`

Example Input: ```Register("org.eduvpn.app.linux", "0.0.1",
"/tmp/eduvpn-common", myCallbackFunc, 1)```

Example Output:

    {
      "message": {
        "en": "failed to register, a VPN state is already present"
      },
      "misc": false
    }

## RemoveServer
Signature:

```go
func RemoveServer(_type C.int, id *C.char) *C.char
```

RemoveServer removes a server from the eduvpn-common server list

`_type` is the type of server that needs to be added. This type is defined
in `types/server/server.go Type`

`id` is the identifier of the string:

  - In case of secure internet: The organization ID
  - In case of custom server: The base URL
  - In case of institute access: The base URL

If the server cannot be removed it returns the error `types/error/error.go
Error`.

Example Input (3=custom server): ```RemoveServer(3, "bogus")```

Example Output:

    {
      "message": {
        "en": "failed to remove server"
      },
      "misc": false
    }

## RenewSession
Signature:

```go
func RenewSession(c C.uintptr_t) *C.char
```

RenewSession renews the session of the VPN

This essentially means that the OAuth tokens are deleted. And it also
possibly re-runs every state callback you need when getting a config.
So least you MUST handle the OAuth started transition

It returns an error if unsuccessful. Example Input:
```RenewSession(myCookie)```

Example Output:

    {
      "message": {
        "en": "could not renew session"
      },
      "misc": false
    }

## ServerList
Signature:

```go
func ServerList() (*C.char, *C.char)
```

ServerList gets the list of servers that are currently added

This is NOT the discovery list, but the servers that have previously been
added with `AddServer`.

It returns the server list as a JSON string defined in
`types/server/server.go List`. If the server list cannot be retrieved it
returns a nil string and an error.

Example Input: ```ServerList()```

Example Output (current profile here is empty as none has been chosen yet):

    {
      "institute_access_servers": [
        {
          "display_name": {
            "en": "Demo"
          },
          "identifier": "https://demo.eduvpn.nl/",
          "profiles": {
            "current": ""
          },
          "support_contacts": [
            "mailto:eduvpn@surf.nl"
          ],
          "delisted": false
        }
      ]
    }, null

## SetProfileID
Signature:

```go
func SetProfileID(data *C.char) *C.char
```

SetProfileID sets the profile ID of the current serrver.

This MUST only be called if the user/client wishes to manually set a profile
instead of the common lib asking for one using a transition.

  - `data` is the profile ID.

It returns an error if unsuccessful. Example Input:
```SetProfileID("splittunnel")```

Example Output:

    {
      "message": {
        "en": "profile does not exist"
      },
      "misc": false
    }

## SetSecureLocation
Signature:

```go
func SetSecureLocation(orgID *C.char, cc *C.char) *C.char
```

SetSecureLocation sets the location for the secure internet server if it
exists.

This MUST only be called if the user/client wishes to manually set a
location instead of the common lib asking for one using a transition.

  - `orgID` is the organisation ID for the secure internet server
  - `cc` is the location ID/country code

It returns an error if unsuccessful. Example Input:
```SetSecureLocation("http://idp.geant.org/", "nl")```

Example Output:

    {
      "message": {
        "en": "location does not exist"
      },
      "misc": false
    }

## SetState
Signature:

```go
func SetState(fsmState C.int) *C.char
```

SetState sets the state of the state machine.

Note: this transitions the FSM into the new state without passing any data
to it. Example Input: ```SetState(5)```

Example Output: ```null```

## SetTokenHandler
Signature:

```go
func SetTokenHandler(getter C.TokenGetter, setter C.TokenSetter) *C.char
```

SetTokenHandler sets the token getters and token setters for OAuth.

Because the data that is saved does not contain OAuth tokens for server,
the common lib asks and sets the tokens using these callback functions.
The client can thus pass callbacks to this function so that the tokens can
be securely stored in a keyring.

The client must pass two callback arguments to this function:

1. `getter` is the void function that gets tokens from the client. It takes
three arguments:
  - The `server` for which to get the tokens for, marshalled as JSON and
    defined in `types/server/server.go Current`
  - The `output` buffer
  - The `length` of the output buffer. This 'output buffer' must contain the
    tokens, marshalled as JSON that is defined in `types/server/server.go
    Tokens`

2. `setter` is the void function that sets tokens. It takes two arguments:

  - The `server` for which to get the tokens for, marshalled as JSON and
    defined in `types/server/server.go Current`
  - The `tokens`, defined in `types/server/server.go Tokens` marshalled as
    JSON

It returns an error when the tokens cannot be set. Example Input:
```SetTokenHandler(getterFunc, setterFunc)```

Example Output: ```null```

## StartFailover
Signature:

```go
func StartFailover(c C.uintptr_t, gateway *C.char, mtu C.int, readRxBytes C.ReadRxBytes) (C.int, *C.char)
```

StartFailover starts the 'failover' procedure in eduvpn-common.

Failover has one primary goal: check if the VPN can reach the gateway.
This can be used to check whether or not the client needs to 'failover' to
prefer TCP (if currently using UDP). Which is useful to go from a broken
WireGuard connection to OpenVPN over TCP.

  - `c` is the cookie that is passed for cancellation. To create a cookie,
    use the `CookieNew` function
  - `gateway` is the gateway IP of the VPN. You MAY calculate this with the
    `CalculateGateway` function
  - `readRxBytes` is a function that returns the current rx bytes of the VPN
    interface, this should return a `long long int` in c

It returns a boolean whether or not the common lib has determined
that it cannot reach the gateway. Non-zero=dropped, zero=not dropped.
It also returns an error, if it fails to indicate if it has dropped or not.
In this case, dropped is also set to zero.

Example Input: ```StartFailover(myCookie, "10.10.10.1", 1400,
myRxBytesReader)```

Example Output: ```1, null```