diff --git a/polkadot/Cargo.lock b/polkadot/Cargo.lock
index 08df727cb1..37946dac1a 100644
--- a/polkadot/Cargo.lock
+++ b/polkadot/Cargo.lock
@@ -3982,6 +3982,7 @@ dependencies = [
  "polkadot-runtime 0.7.11",
  "polkadot-validation 0.7.11",
  "sc-authority-discovery 2.0.0 (git+https://github.com/paritytech/substrate?branch=polkadot-master)",
+ "sc-chain-spec 2.0.0 (git+https://github.com/paritytech/substrate?branch=polkadot-master)",
  "sc-client 2.0.0 (git+https://github.com/paritytech/substrate?branch=polkadot-master)",
  "sc-client-api 2.0.0 (git+https://github.com/paritytech/substrate?branch=polkadot-master)",
  "sc-client-db 2.0.0 (git+https://github.com/paritytech/substrate?branch=polkadot-master)",
@@ -3993,6 +3994,7 @@ dependencies = [
  "sc-service 2.0.0 (git+https://github.com/paritytech/substrate?branch=polkadot-master)",
  "sc-telemetry 2.0.0 (git+https://github.com/paritytech/substrate?branch=polkadot-master)",
  "sc-transaction-pool 2.0.0 (git+https://github.com/paritytech/substrate?branch=polkadot-master)",
+ "serde 1.0.102 (registry+https://github.com/rust-lang/crates.io-index)",
  "slog 2.5.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "sp-api 2.0.0 (git+https://github.com/paritytech/substrate?branch=polkadot-master)",
  "sp-authority-discovery 2.0.0 (git+https://github.com/paritytech/substrate?branch=polkadot-master)",
diff --git a/polkadot/service/Cargo.toml b/polkadot/service/Cargo.toml
index 39efab8e52..c7ae4ea45f 100644
--- a/polkadot/service/Cargo.toml
+++ b/polkadot/service/Cargo.toml
@@ -6,6 +6,7 @@ edition = "2018"
 
 [dependencies]
 parking_lot = "0.9.0"
+serde = { version = "1.0.102", features = ["derive"] }
 lazy_static = "1.4.0"
 log = "0.4.8"
 futures = "0.3.1"
@@ -27,6 +28,7 @@ primitives = { package = "sp-core", git = "https://github.com/paritytech/substra
 client = { package = "sc-client", git = "https://github.com/paritytech/substrate", branch = "polkadot-master" }
 client-api = { package = "sc-client-api", git = "https://github.com/paritytech/substrate", branch = "polkadot-master" }
 client-db = { package = "sc-client-db", git = "https://github.com/paritytech/substrate", branch = "polkadot-master" }
+sc-chain-spec = { git = "https://github.com/paritytech/substrate", branch = "polkadot-master" }
 sc-executor = { git = "https://github.com/paritytech/substrate", branch = "polkadot-master" }
 sc-network = { git = "https://github.com/paritytech/substrate", branch = "polkadot-master" }
 consensus_common = { package = "sp-consensus", git = "https://github.com/paritytech/substrate", branch = "polkadot-master" }
diff --git a/polkadot/service/res/kusama.json b/polkadot/service/res/kusama.json
index ed21130a9b..12ea502915 100644
--- a/polkadot/service/res/kusama.json
+++ b/polkadot/service/res/kusama.json
@@ -25,6 +25,7 @@
     "tokenSymbol": "KSM"
   },
   "consensusEngine": null,
+  "forkBlocks": null,
   "genesis": {
     "raw": [
       {
diff --git a/polkadot/service/src/chain_spec.rs b/polkadot/service/src/chain_spec.rs
index 9466c0733b..1bace1edad 100644
--- a/polkadot/service/src/chain_spec.rs
+++ b/polkadot/service/src/chain_spec.rs
@@ -20,7 +20,9 @@ use primitives::{Pair, Public, crypto::UncheckedInto, sr25519};
 use polkadot_primitives::{AccountId, AccountPublic, parachain::ValidatorId};
 use polkadot_runtime as polkadot;
 use polkadot_runtime::constants::currency::DOTS;
+use sc_chain_spec::ChainSpecExtension;
 use sp_runtime::{traits::IdentifyAccount, Perbill};
+use serde::{Serialize, Deserialize};
 use telemetry::TelemetryEndpoints;
 use hex_literal::hex;
 use babe_primitives::AuthorityId as BabeId;
@@ -32,11 +34,25 @@ use pallet_staking::Forcing;
 const STAGING_TELEMETRY_URL: &str = "wss://telemetry.polkadot.io/submit/";
 const DEFAULT_PROTOCOL_ID: &str = "dot";
 
+/// Node `ChainSpec` extensions.
+///
+/// Additional parameters for some Substrate core modules,
+/// customizable from the chain spec.
+#[derive(Default, Clone, Serialize, Deserialize, ChainSpecExtension)]
+#[serde(rename_all = "camelCase")]
+pub struct Extensions {
+	/// Block numbers with known hashes.
+	pub fork_blocks: client::ForkBlocks<polkadot_primitives::Block>,
+}
+
 /// The `ChainSpec`.
 ///
 /// We use the same `ChainSpec` type for Polkadot and Kusama. As Kusama
 /// is only loaded from a file, the `GenesisConfig` type is not used.
-pub type ChainSpec = service::ChainSpec<polkadot::GenesisConfig>;
+pub type ChainSpec = service::ChainSpec<
+	polkadot::GenesisConfig,
+	Extensions,
+>;
 
 pub fn kusama_config() -> Result<ChainSpec, String> {
 	ChainSpec::from_json_bytes(&include_bytes!("../res/kusama.json")[..])
@@ -214,7 +230,7 @@ pub fn staging_testnet_config() -> ChainSpec {
 		Some(TelemetryEndpoints::new(vec![(STAGING_TELEMETRY_URL.to_string(), 0)])),
 		Some(DEFAULT_PROTOCOL_ID),
 		None,
-		None,
+		Default::default(),
 	)
 }
 
@@ -361,7 +377,7 @@ pub fn development_config() -> ChainSpec {
 		None,
 		Some(DEFAULT_PROTOCOL_ID),
 		None,
-		None,
+		Default::default(),
 	)
 }
 
@@ -386,6 +402,6 @@ pub fn local_testnet_config() -> ChainSpec {
 		None,
 		Some(DEFAULT_PROTOCOL_ID),
 		None,
-		None,
+		Default::default(),
 	)
 }
diff --git a/polkadot/service/src/lib.rs b/polkadot/service/src/lib.rs
index 22945b8eac..6e5d4833ea 100644
--- a/polkadot/service/src/lib.rs
+++ b/polkadot/service/src/lib.rs
@@ -84,7 +84,11 @@ pub struct CustomConfiguration {
 /// Configuration type that is being used.
 ///
 /// See [`ChainSpec`] for more information why Polkadot `GenesisConfig` is safe here.
-pub type Configuration = service::Configuration<CustomConfiguration, polkadot_runtime::GenesisConfig>;
+pub type Configuration = service::Configuration<
+	CustomConfiguration,
+	polkadot_runtime::GenesisConfig,
+	chain_spec::Extensions,
+>;
 
 impl Default for CustomConfiguration {
 	fn default() -> Self {