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Evening if all ports are closed geth can still sync with other nodes, what channels exactly are used for that ?

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In most set ups, even when ports are closed, the responses for outgoing connections are allowed through.

When started, geth contacts a series of bootnodes to retrieve a peer list. It will then reach out to peers from that list, until it finds one which is synced and available to provide data. Over the span of a few minutes (usually) it will connect to several such peers, and start syncing and participating in the network.

In such a scenario, other nodes will not be able to discover you, since your port is still closed. Only connections you initiated will work.

If your network setup is even more restrictive, and only allows outgoing connections to certain IPs, or filters out responses, you will not be able to sync.

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  • what ports need to be open to make out going transactions ?
    – Nabil Sham
    Aug 2 '18 at 1:23
  • 1
    30303 for peer discovery Aug 2 '18 at 1:24

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