[Ethereum] sync parity or geth starting from block x

For point number one, this should work:
geth --rpc --rpccorsdomain="*" console 2>> geth.log
Running --rpccorsdomain="*" is dangerous though, especially if you have unlocked accounts. Put your domain name instead of *. For localhost, add --rpcaddr="localhost"

From my experience #2 is the best way to check the status of a node.

eth.syncing returns false even when the blocks are not synced. When geth is starting it's false, and often while not all blocks have synced it still returns false.

> web3.eth.syncing
false
> I0607 08:09:04.878618 eth/downloader/downloader.go:320] Block synchronisation started
> web3.eth.syncing
{
  currentBlock: 1086723,
  highestBlock: 1087298,
  knownStates: 0,
  pulledStates: 0,
  startingBlock: 1086723
}

later

I0607 08:09:14.008694 core/blockchain.go:964] imported 1 block(s) (0 queued 0 ignored) including 0 txs in 4.647406ms. #1087304 [501df7da / 501df7da]
> web3.eth.syncing
false

Note the false, but the block number isn't at the syncing.highestBlock yet.

There is a callback for when sync starts and stops.. just remember that the highestBlock will change as new blocks are mined.

https://github.com/ethereum/wiki/wiki/JavaScript-API#web3ethissyncing