diff --git a/server/resources/nodeResource.ts b/server/resources/nodeResource.ts
index 12a2a49..d8ac067 100644
--- a/server/resources/nodeResource.ts
+++ b/server/resources/nodeResource.ts
@@ -10,7 +10,7 @@ import {forConstraint, forConstraints} from "../validation/validator";
 import * as Resources from "../utils/resources";
 import {Entity} from "../utils/resources";
 import {Request, Response} from "express";
-import {Node} from "../types";
+import {EnhancedNode, Node} from "../types";
 
 const nodeFields = ['hostname', 'key', 'email', 'nickname', 'mac', 'coords', 'monitoring'];
 
@@ -97,7 +97,7 @@ async function doGetAll(req: Request): Promise<{ total: number; pageNodes: any }
     const macs = _.map(realNodes, (node: Node): string => node.mac);
     const nodeStateByMac = await MonitoringService.getByMacs(macs);
 
-    const enhancedNodes: Entity[] = _.map(realNodes, (node: Node) => {
+    const enhancedNodes: EnhancedNode[] = _.map(realNodes, (node: Node): EnhancedNode => {
         const nodeState = nodeStateByMac[node.mac];
         if (nodeState) {
             return deepExtend({}, node, {
@@ -107,10 +107,10 @@ async function doGetAll(req: Request): Promise<{ total: number; pageNodes: any }
             });
         }
 
-        return node;
+        return node as EnhancedNode;
     });
 
-    const filteredNodes = Resources.filter(
+    const filteredNodes = Resources.filter<EnhancedNode>(
         enhancedNodes,
         [
             'hostname',
diff --git a/server/services/monitoringService.test.ts b/server/services/monitoringService.test.ts
index 1edf951..6159071 100644
--- a/server/services/monitoringService.test.ts
+++ b/server/services/monitoringService.test.ts
@@ -1,6 +1,6 @@
 import moment from 'moment';
 import {ParsedNode, parseNode, parseNodesJson, parseTimestamp} from "./monitoringService";
-import {NodeState} from "../types";
+import {OnlineState} from "../types";
 import Logger from '../logger';
 import {MockLogger} from "../__mocks__/logger";
 
@@ -242,7 +242,7 @@ test('parseNode() should succeed parsing node without site and domain', () => {
     const expectedParsedNode: ParsedNode = {
         mac: "12:34:56:78:90:AB",
         importTimestamp: importTimestamp,
-        state: NodeState.ONLINE,
+        state: OnlineState.ONLINE,
         lastSeen: parseTimestamp(TIMESTAMP_VALID_STRING),
         site: '<unknown-site>',
         domain: '<unknown-domain>'
@@ -274,7 +274,7 @@ test('parseNode() should succeed parsing node with site and domain', () => {
     const expectedParsedNode: ParsedNode = {
         mac: "12:34:56:78:90:AB",
         importTimestamp: importTimestamp,
-        state: NodeState.ONLINE,
+        state: OnlineState.ONLINE,
         lastSeen: parseTimestamp(TIMESTAMP_VALID_STRING),
         site: 'test-site',
         domain: 'test-domain'
@@ -463,7 +463,7 @@ test('parseNodesJson() should parse valid nodes', () => {
     const expectedParsedNode: ParsedNode = {
         mac: "12:34:56:78:90:AB",
         importTimestamp: parseTimestamp(TIMESTAMP_VALID_STRING),
-        state: NodeState.ONLINE,
+        state: OnlineState.ONLINE,
         lastSeen: parseTimestamp(TIMESTAMP_VALID_STRING),
         site: 'test-site',
         domain: 'test-domain'
diff --git a/server/services/monitoringService.ts b/server/services/monitoringService.ts
index efe86c6..c3f3184 100644
--- a/server/services/monitoringService.ts
+++ b/server/services/monitoringService.ts
@@ -16,7 +16,7 @@ import {normalizeMac} from "../utils/strings";
 import {monitoringDisableUrl} from "../utils/urlBuilder";
 import CONSTRAINTS from "../validation/constraints";
 import {forConstraint} from "../validation/validator";
-import {MAC, MailType, Node, NodeId, NodeState, NodeStateData, UnixTimestampSeconds} from "../types";
+import {MAC, MailType, Node, NodeId, OnlineState, NodeStateData, UnixTimestampSeconds} from "../types";
 
 const MONITORING_STATE_MACS_CHUNK_SIZE = 100;
 const NEVER_ONLINE_NODES_DELETION_CHUNK_SIZE = 20;
@@ -37,7 +37,7 @@ const DELETE_OFFLINE_NODES_AFTER_DURATION: {amount: number, unit: unitOfTime.Dur
 export type ParsedNode = {
     mac: string,
     importTimestamp: Moment,
-    state: NodeState,
+    state: OnlineState,
     lastSeen: Moment,
     site: string,
     domain: string,
@@ -212,7 +212,7 @@ export function parseNode(importTimestamp: Moment, nodeData: any): ParsedNode {
     return {
         mac: mac,
         importTimestamp: importTimestamp,
-        state: isOnline ? NodeState.ONLINE : NodeState.OFFLINE,
+        state: isOnline ? OnlineState.ONLINE : OnlineState.OFFLINE,
         lastSeen: lastSeen,
         site: site || '<unknown-site>',
         domain: domain || '<unknown-domain>'
@@ -520,7 +520,7 @@ async function retrieveNodeInformationForUrls(urls: string[]): Promise<RetrieveN
         'SET state = ?, modified_at = ?' +
         'WHERE import_timestamp < ?',
         [
-            NodeState.OFFLINE, moment().unix(),
+            OnlineState.OFFLINE, moment().unix(),
             minTimestamp.unix()
         ]
     );
diff --git a/server/types/index.ts b/server/types/index.ts
index bc0e85e..dc82434 100644
--- a/server/types/index.ts
+++ b/server/types/index.ts
@@ -1,49 +1,15 @@
+import {Domain, MonitoringToken, OnlineState, Site} from "./shared";
+
 export * from "./config";
 export * from "./logger";
 export * from "./shared";
 
-// TODO: Token type.
-export type Token = string;
-export type FastdKey = string;
-export type MAC = string;
-
-export type UnixTimestampSeconds = number;
-export type UnixTimestampMilliseconds = number;
-
-export type MonitoringToken = string;
-export enum MonitoringState {
-    ACTIVE = "active",
-    PENDING = "pending",
-    DISABLED = "disabled",
-}
-
-export type NodeId = string;
-
-export enum NodeState {
-    ONLINE = "ONLINE",
-    OFFLINE = "OFFLINE",
-}
-
 export type NodeStateData = {
-    site: string,
-    domain: string,
-    state: NodeState,
+    site: Site,
+    domain: Domain,
+    state: OnlineState,
 }
 
-export type Node = {
-    token: Token;
-    nickname: string;
-    email: string;
-    hostname: string;
-    coords?: string; // TODO: Use object with longitude and latitude.
-    key?: FastdKey;
-    mac: MAC;
-    monitoring: boolean;
-    monitoringConfirmed: boolean;
-    monitoringState: MonitoringState;
-    modifiedAt: UnixTimestampSeconds;
-};
-
 // TODO: Complete interface / class declaration.
 export type NodeSecrets = {
     monitoringToken?: MonitoringToken,
diff --git a/server/types/shared.ts b/server/types/shared.ts
index a977a1c..ffa4134 100644
--- a/server/types/shared.ts
+++ b/server/types/shared.ts
@@ -1,12 +1,13 @@
 import {ArrayField, Field, RawJsonField} from "sparkson";
 
 // Types shared with the client.
+export type TypeGuard<T> = (arg: unknown) => arg is T;
 
 export function isObject(arg: unknown): arg is object {
     return arg !== null && typeof arg === "object";
 }
 
-export function isArray<T>(arg: unknown, isT: (arg: unknown) => arg is T): arg is Array<T> {
+export function isArray<T>(arg: unknown, isT: TypeGuard<T>): arg is Array<T> {
     if (!Array.isArray(arg)) {
         return false;
     }
@@ -22,12 +23,18 @@ export function isString(arg: unknown): arg is string {
     return typeof arg === "string"
 }
 
+export function toIsArray<T>(isT: TypeGuard<T>): TypeGuard<T[]> {
+    return (arg): arg is T[] => isArray(arg, isT);
+}
+
+export function toIsEnum<E>(enumDef: E): TypeGuard<E> {
+    return (arg): arg is E => Object.values(enumDef).includes(arg as [keyof E]);
+}
+
 export type Version = string;
 
-export function isVersion(arg: unknown): arg is Version {
-    // Should be good enough for now.
-    return typeof arg === "string";
-}
+// Should be good enough for now.
+export const isVersion = isString;
 
 export type NodeStatistics = {
     registered: number;
@@ -229,3 +236,95 @@ export function isClientConfig(arg: unknown): arg is ClientConfig {
         typeof cfg.rootPath === "string"
     );
 }
+
+// TODO: Token type.
+export type Token = string;
+export const isToken = isString;
+
+export type FastdKey = string;
+export const isFastdKey = isString;
+
+export type MAC = string;
+export const isMAC = isString;
+
+export type UnixTimestampSeconds = number;
+export type UnixTimestampMilliseconds = number;
+
+export type MonitoringToken = string;
+export enum MonitoringState {
+    ACTIVE = "active",
+    PENDING = "pending",
+    DISABLED = "disabled",
+}
+
+export const isMonitoringState = toIsEnum(MonitoringState);
+
+export type NodeId = string;
+export const isNodeId = isString;
+
+export interface Node {
+    token: Token;
+    nickname: string;
+    email: string;
+    hostname: string;
+    coords?: string; // TODO: Use object with longitude and latitude.
+    key?: FastdKey;
+    mac: MAC;
+    monitoring: boolean;
+    monitoringConfirmed: boolean;
+    monitoringState: MonitoringState;
+    modifiedAt: UnixTimestampSeconds;
+}
+
+export function isNode(arg: unknown): arg is Node {
+    if (!isObject(arg)) {
+        return false;
+    }
+    const node = arg as Node;
+    // noinspection SuspiciousTypeOfGuard
+    return (
+        isToken(node.token) &&
+        typeof node.nickname === "string" &&
+        typeof node.email === "string" &&
+        typeof node.hostname === "string" &&
+        (node.coords === undefined || typeof node.coords === "string") &&
+        (node.key === undefined || isFastdKey(node.key)) &&
+        isMAC(node.mac) &&
+        typeof node.monitoring === "boolean" &&
+        typeof node.monitoringConfirmed === "boolean" &&
+        isMonitoringState(node.monitoringState) &&
+        typeof node.modifiedAt === "number"
+    );
+}
+
+export enum OnlineState {
+    ONLINE = "ONLINE",
+    OFFLINE = "OFFLINE",
+}
+export const isOnlineState = toIsEnum(OnlineState);
+
+export type Site = string;
+export const isSite = isString;
+
+export type Domain = string;
+export const isDomain = isString;
+
+export interface EnhancedNode extends Node {
+    site?: Site,
+    domain?: Domain,
+    onlineState?: OnlineState,
+}
+
+export function isEnhancedNode(arg: unknown): arg is EnhancedNode {
+    if (!isNode(arg)) {
+        return false;
+    }
+    const node = arg as EnhancedNode;
+    return (
+        (node.site === undefined || isSite(node.site)) &&
+        (node.domain === undefined || isDomain(node.domain)) &&
+        (node.onlineState === undefined || isOnlineState(node.onlineState))
+    );
+}
+
+export const isEnhancedNodes = toIsArray(isEnhancedNode);
diff --git a/server/utils/resources.ts b/server/utils/resources.ts
index 3a34fde..bc92fe4 100644
--- a/server/utils/resources.ts
+++ b/server/utils/resources.ts
@@ -152,7 +152,7 @@ export async function getValidRestParams(
     return restParams as RestParams;
 }
 
-export function filter (entities: ArrayLike<Entity>, allowedFilterFields: string[], restParams: RestParams) {
+export function filter<E>(entities: ArrayLike<E>, allowedFilterFields: string[], restParams: RestParams): E[] {
     let query = restParams.q;
     if (query) {
         query = _.toLower(query.trim());