inspector: add readme entry, decoding option

README.md

@@ -36,6 +36,7 @@ You can check where the beacon chain fits in the Ethereum ecosystem our Two-Poin
     - [State transition simulation](#state-transition-simulation)
     - [Local network simulation](#local-network-simulation)
     - [Visualising simulation metrics](#visualising-simulation-metrics)
+    - [Network inspection](#network-inspection)
   - [For developers](#for-developers)
     - [Windows dev environment](#windows-dev-environment)
     - [Linux, MacOS](#linux-macos)
@@ -182,6 +183,25 @@ The dashboard you need to import in Grafana is "tests/simulation/beacon-chain-si
 
 ![monitoring dashboard](./media/monitoring.png)
 
+### Network inspection
+
+The [inspector tool](./beacon_chain/inspector.nim) can help monitor the libp2p network and the various channels where blocks and attestations are being transmitted, showing message and connectivity metadata. By default, it will monitor all ethereum 2 gossip traffic.
+
+```bash
+. ./env.sh
+# Build inspector for minimal config:
+./env.sh nim c -d:const_preset=minimal -o:build/inspector_minimal beacon_chain/inspector.nim
+
+# Build inspector for mainnet config:
+./env.sh nim c -d:const_preset=mainnet -o:build/inspector_mainnet beacon_chain/inspector.nim
+
+# See available options
+./env.sh build/inspector_minimal --help
+
+# Connect to a network from eth2 testnet repo bootstrap file - --decode option attempts to decode the messages as well
+./env.sh build/inspector_minimal --decode -b:$(curl -s https://raw.githubusercontent.com/eth2-clients/eth2-testnets/master/nimbus/testnet0/bootstrap_nodes.txt | head -n1)
+```
+
 ## For developers
 
 Latest updates happen in the `devel` branch which is merged into `master` every week on Tuesday before deploying a new testnets

beacon_chain/inspector.nim

@@ -8,7 +8,7 @@ import strutils, os, tables
 import confutils, chronicles, chronos, libp2p/daemon/daemonapi,
        libp2p/multiaddress
 import stew/byteutils as bu
-import spec/network
+import spec/[crypto, datatypes, network, digest], ssz
 
 const
   InspectorName* = "Beacon-Chain Network Inspector"
@@ -88,6 +88,11 @@ type
       abbr: "b"
       name: "bootnodes" }: seq[string]
 
+    decode* {.
+      desc: "Try to decode message using SSZ"
+      abbr: "d"
+      defaultValue: false }: bool
+
 proc getTopic(filter: TopicFilter): string {.inline.} =
   case filter
   of TopicFilter.Blocks:
@@ -176,6 +181,22 @@ proc run(conf: InspectorConf) {.async.} =
                              mtopics = $message.topics,
                              message = bu.toHex(message.data),
                              zpeers = len(pubsubPeers)
+
+    if conf.decode:
+      try:
+        if ticket.topic.startsWith(topicBeaconBlocks):
+          info "BeaconBlock", msg = SSZ.decode(message.data, BeaconBlock)
+        elif ticket.topic.startsWith(topicAttestations):
+          info "Attestation", msg = SSZ.decode(message.data, Attestation)
+        elif ticket.topic.startsWith(topicVoluntaryExits):
+          info "VoluntaryExit", msg = SSZ.decode(message.data, VoluntaryExit)
+        elif ticket.topic.startsWith(topicProposerSlashings):
+          info "ProposerSlashing", msg = SSZ.decode(message.data, ProposerSlashing)
+        elif ticket.topic.startsWith(topicAttesterSlashings):
+          info "AttesterSlashing", msg = SSZ.decode(message.data, AttesterSlashing)
+      except CatchableError as exc:
+        info "Unable to decode message", msg = exc.msg
+
     result = true
 
   if len(conf.topics) > 0: