diff --git a/gvm/connections.py b/gvm/connections.py
index e1a1fb38a..cf77cfccd 100644
--- a/gvm/connections.py
+++ b/gvm/connections.py
@@ -13,9 +13,10 @@
 import ssl
 import sys
 import time
+from abc import ABC, abstractmethod
 from os import PathLike
 from pathlib import Path
-from typing import Optional, Union
+from typing import Optional, Protocol, Union, runtime_checkable
 
 import paramiko
 import paramiko.ssh_exception
@@ -41,6 +42,19 @@
 Data = Union[str, bytes]
 
 
+@runtime_checkable
+class GvmConnection(Protocol):
+    def connect(self) -> None: ...
+
+    def disconnect(self) -> None: ...
+
+    def send(self, data: Data) -> None: ...
+
+    def read(self) -> str: ...
+
+    def finish_send(self): ...
+
+
 class XmlReader:
     """
     Read a XML command until its closing element
@@ -77,7 +91,7 @@ def feed_xml(self, data: Data) -> None:
             ) from None
 
 
-class GvmConnection:
+class AbstractGvmConnection(ABC):
     """
     Base class for establishing a connection to a remote server daemon.
 
@@ -97,6 +111,7 @@ def _read(self) -> bytes:
 
         return self._socket.recv(BUF_SIZE)
 
+    @abstractmethod
     def connect(self) -> None:
         """Establish a connection to a remote server"""
         raise NotImplementedError
@@ -164,7 +179,7 @@ def finish_send(self):
             self._socket.shutdown(socketlib.SHUT_WR)
 
 
-class SSHConnection(GvmConnection):
+class SSHConnection(AbstractGvmConnection):
     """
     SSH Class to connect, read and write from GVM via SSH
 
@@ -174,7 +189,7 @@ class SSHConnection(GvmConnection):
             127.0.0.1.
         port: Port of the remote SSH server. Default is port 22.
         username: Username to use for SSH login. Default is "gmp".
-        password: Passwort to use for SSH login. Default is "".
+        password: Password to use for SSH login. Default is "".
     """
 
     def __init__(
@@ -188,8 +203,7 @@ def __init__(
         known_hosts_file: Optional[Union[str, PathLike]] = None,
         auto_accept_host: Optional[bool] = None,
     ) -> None:
-        super().__init__(timeout=timeout)
-
+        super().__init__(timeout)
         self.hostname = hostname if hostname is not None else DEFAULT_HOSTNAME
         self.port = int(port) if port is not None else DEFAULT_SSH_PORT
         self.username = (
@@ -414,11 +428,11 @@ def connect(self) -> None:
     def _read(self) -> bytes:
         return self._stdout.channel.recv(BUF_SIZE)
 
-    def send(self, data: Union[bytes, str]) -> int:
+    def send(self, data: Data) -> None:
         if isinstance(data, str):
-            return self._send_all(data.encode())
-
-        return self._send_all(data)
+            self._send_all(data.encode())
+        else:
+            self._send_all(data)
 
     def finish_send(self) -> None:
         # shutdown socket for sending. only allow reading data afterwards
@@ -439,7 +453,7 @@ def disconnect(self) -> None:
             del self._socket, self._stdin, self._stdout, self._stderr
 
 
-class TLSConnection(GvmConnection):
+class TLSConnection(AbstractGvmConnection):
     """
     TLS class to connect, read and write from a remote GVM daemon via TLS
     secured socket.
@@ -524,7 +538,7 @@ def disconnect(self):
         return super().disconnect()
 
 
-class UnixSocketConnection(GvmConnection):
+class UnixSocketConnection(AbstractGvmConnection):
     """
     UNIX-Socket class to connect, read, write from a daemon via direct
     communicating UNIX-Socket
diff --git a/tests/connections/test_gvm_connection.py b/tests/connections/test_gvm_connection.py
index fea552073..48b415062 100644
--- a/tests/connections/test_gvm_connection.py
+++ b/tests/connections/test_gvm_connection.py
@@ -6,7 +6,13 @@
 import unittest
 from unittest.mock import patch
 
-from gvm.connections import DEFAULT_TIMEOUT, GvmConnection, XmlReader
+from gvm.connections import (
+    DEFAULT_TIMEOUT,
+    AbstractGvmConnection,
+    DebugConnection,
+    GvmConnection,
+    XmlReader,
+)
 from gvm.errors import GvmError
 
 
@@ -19,39 +25,49 @@ def test_is_end_xml_false(self):
         self.assertFalse(false)
 
 
+class TestConnection(AbstractGvmConnection):
+    def connect(self) -> None:
+        pass
+
+
 class GvmConnectionTestCase(unittest.TestCase):
     # pylint: disable=protected-access
     def test_init_no_args(self):
-        connection = GvmConnection()
+        connection = TestConnection()
         self.check_for_default_values(connection)
 
     def test_init_with_none(self):
-        connection = GvmConnection(timeout=None)
+        connection = TestConnection(timeout=None)
         self.check_for_default_values(connection)
 
     def check_for_default_values(self, gvm_connection: GvmConnection):
         self.assertIsNone(gvm_connection._socket)
         self.assertEqual(gvm_connection._timeout, DEFAULT_TIMEOUT)
 
-    def test_connect_not_implemented(self):
-        connection = GvmConnection()
-        with self.assertRaises(NotImplementedError):
-            connection.connect()
-
-    @patch("gvm.connections.GvmConnection._read")
+    @patch("gvm.connections.AbstractGvmConnection._read")
     def test_read_no_data(self, _read_mock):
         _read_mock.return_value = None
-        connection = GvmConnection()
+        connection = TestConnection()
         with self.assertRaises(GvmError, msg="Remote closed the connection"):
             connection.read()
 
-    @patch("gvm.connections.GvmConnection._read")
+    @patch("gvm.connections.AbstractGvmConnection._read")
     def test_read_trigger_timeout(self, _read_mock):
         # mocking the response into two parts, so we run into the timeout
         # check in the loop
         _read_mock.side_effect = [b"<foo>xyz<bar></bar>", b"</foo>"]
-        connection = GvmConnection(timeout=0)
+        connection = TestConnection(timeout=0)
         with self.assertRaises(
             GvmError, msg="Timeout while reading the response"
         ):
             connection.read()
+
+    def test_is_gvm_connection(self):
+        connection = TestConnection()
+        self.assertTrue(isinstance(connection, GvmConnection))
+
+
+class DebugConnectionTestCase(unittest.TestCase):
+    def test_is_gvm_connection(self):
+        connection = DebugConnection(TestConnection())
+        self.assertTrue(isinstance(connection, GvmConnection))
diff --git a/tests/connections/test_ssh_connection.py b/tests/connections/test_ssh_connection.py
index 029e7922d..75d5fe70b 100644
--- a/tests/connections/test_ssh_connection.py
+++ b/tests/connections/test_ssh_connection.py
@@ -17,6 +17,7 @@
     DEFAULT_SSH_PASSWORD,
     DEFAULT_SSH_PORT,
     DEFAULT_SSH_USERNAME,
+    GvmConnection,
     SSHConnection,
 )
 from gvm.errors import GvmError
@@ -177,7 +178,7 @@ def test_connect_adding_and_save_hostkey(self, input_mock, _print_mock):
         )
 
         with self.assertLogs("gvm.connections", level="INFO") as cm:
-            hostkeys = paramiko.HostKeys(filename=self.known_hosts_file)
+            hostkeys = paramiko.HostKeys(filename=str(self.known_hosts_file))
             ssh_connection._ssh_authentication_input_loop(
                 hostkeys=hostkeys, key=key
             )
@@ -229,7 +230,7 @@ def test_connect_adding_and_dont_save_hostkey(
         )
 
         with self.assertLogs("gvm.connections", level="INFO") as cm:
-            hostkeys = paramiko.HostKeys(filename=self.known_hosts_file)
+            hostkeys = paramiko.HostKeys(filename=str(self.known_hosts_file))
             ssh_connection._ssh_authentication_input_loop(
                 hostkeys=hostkeys, key=key
             )
@@ -274,7 +275,7 @@ def test_connect_wrong_input(self, stdout_mock, input_mock):
         ssh_connection._socket = paramiko.SSHClient()
 
         with self.assertLogs("gvm.connections", level="INFO") as cm:
-            hostkeys = paramiko.HostKeys(filename=self.known_hosts_file)
+            hostkeys = paramiko.HostKeys(filename=str(self.known_hosts_file))
             ssh_connection._ssh_authentication_input_loop(
                 hostkeys=hostkeys, key=key
             )
@@ -323,7 +324,7 @@ def test_user_denies_auth(self, input_mock):
         with self.assertRaises(
             SystemExit, msg="User denied key. Host key verification failed."
         ):
-            hostkeys = paramiko.HostKeys(filename=self.known_hosts_file)
+            hostkeys = paramiko.HostKeys(filename=str(self.known_hosts_file))
             ssh_connection._ssh_authentication_input_loop(
                 hostkeys=hostkeys, key=key
             )
@@ -441,8 +442,7 @@ def test_send(self):
             )
 
             ssh_connection.connect()
-            req = ssh_connection.send("blah")
-            self.assertEqual(req, 4)
+            ssh_connection.send("blah")
             ssh_connection.disconnect()
 
     def test_send_error(self):
@@ -473,8 +473,7 @@ def test_send_and_slice(self):
             )
 
             ssh_connection.connect()
-            req = ssh_connection.send("blah")
-            self.assertEqual(req, 4)
+            ssh_connection.send("blah")
 
             stdin.channel.send.assert_called()
             with self.assertRaises(AssertionError):
@@ -495,3 +494,7 @@ def test_read(self):
             recved = ssh_connection._read()
             self.assertEqual(recved, b"foo bar baz")
             ssh_connection.disconnect()
+
+    def test_is_gvm_connection(self):
+        ssh_connection = SSHConnection(known_hosts_file=self.known_hosts_file)
+        self.assertTrue(isinstance(ssh_connection, GvmConnection))
diff --git a/tests/connections/test_tls_connection.py b/tests/connections/test_tls_connection.py
index f479f16da..7b1cf9dd6 100644
--- a/tests/connections/test_tls_connection.py
+++ b/tests/connections/test_tls_connection.py
@@ -10,6 +10,7 @@
     DEFAULT_GVM_PORT,
     DEFAULT_HOSTNAME,
     DEFAULT_TIMEOUT,
+    GvmConnection,
     TLSConnection,
 )
 
@@ -62,3 +63,7 @@ def test_connect_auth(self):
             context_mock.load_cert_chain.assert_called_once()
             context_mock.wrap_socket.assert_called_once()
             self.assertFalse(context_mock.check_hostname)
+
+    def test_is_gvm_connection(self):
+        connection = TLSConnection()
+        self.assertTrue(isinstance(connection, GvmConnection))
diff --git a/tests/connections/test_unix_socket_connection.py b/tests/connections/test_unix_socket_connection.py
index 478928edd..4568f78e6 100644
--- a/tests/connections/test_unix_socket_connection.py
+++ b/tests/connections/test_unix_socket_connection.py
@@ -14,6 +14,7 @@
 from gvm.connections import (
     DEFAULT_TIMEOUT,
     DEFAULT_UNIX_SOCKET_PATH,
+    GvmConnection,
     UnixSocketConnection,
 )
 from gvm.errors import GvmError
@@ -65,8 +66,7 @@ def test_unix_socket_connection_connect_send_bytes_read(self):
             path=self.socketname, timeout=DEFAULT_TIMEOUT
         )
         connection.connect()
-        req = connection.send(bytes("<gmp/>", "utf-8"))
-        self.assertIsNone(req)
+        connection.send(bytes("<gmp/>", "utf-8"))
         resp = connection.read()
         self.assertEqual(resp, '<gmp_response status="200" status_text="OK"/>')
         connection.disconnect()
@@ -76,8 +76,7 @@ def test_unix_socket_connection_connect_send_str_read(self):
             path=self.socketname, timeout=DEFAULT_TIMEOUT
         )
         connection.connect()
-        req = connection.send("<gmp/>")
-        self.assertIsNone(req)
+        connection.send("<gmp/>")
         resp = connection.read()
         self.assertEqual(resp, '<gmp_response status="200" status_text="OK"/>')
         connection.disconnect()
@@ -120,6 +119,6 @@ def check_default_values(self, connection: UnixSocketConnection):
         self.assertEqual(connection._timeout, DEFAULT_TIMEOUT)
         self.assertEqual(connection.path, DEFAULT_UNIX_SOCKET_PATH)
 
-
-if __name__ == "__main__":
-    unittest.main()
+    def test_is_gvm_connection(self):
+        connection = UnixSocketConnection()
+        self.assertTrue(isinstance(connection, GvmConnection))