Why Does eth.blockNumber Return Zero During Geth Fast Sync?

Problem Scenario

When using Ethereum (ETH) via an RPC console, calling eth.blockNumber sometimes returns 0 even after part of the chain has been synchronized, which is unexpected because the block height should be non‑zero once blocks exist.

Software Environment

Geth client version 1.7 running on Linux, connected to the public chain.

Root Cause

Geth 1.6 and 1.7 default to fast sync. During fast sync the node only downloads the latest state and does not retain full data for intermediate blocks. Therefore, until fast sync completes, the node reports a block number of 0 because the latest full block is unavailable. Once fast sync finishes and the header information is built, the node can return the correct block number.

Relevant Commands

Use eth.syncing to view synchronization progress; it returns an object such as:

{
  currentBlock: 290584,
  highestBlock: 891836,
  knownStates: 465886,
  pulledStates: 361434,
  startingBlock: 0
}

This output shows the current block being synced, the highest known block, and state download statistics.

After synchronization, use eth.getBlock("latest").number to query the latest block height.