IBC#030: Bitcoin Core Initialization, Step 12: Network Options

NAT-PMP port forwarding and connection options.

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Today, we continue our deep dive into step 12 of the initialization of Bitcoin Core, discussing about port forwarding and connection options. [Code Link]

Let’s start!

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NAT-PMP

First, the program tries to automatically forward the node listening port through your router using NAT-PMP, so other peers on the internet can reach you without manual port forwarding.

Briefly, NAT-PMP allows a software to ask the router to open a port and forward it back to the computer automatically. In simpler terms, your node will ask the router to forward every connection coming to the external router port to the node local port.

Thus, the code retrieves the value of the -natpmp argument, which can be either 0 or 1. The GetBoolArg() function transforms the integer number into a boolean value, which is then fed to the StartMapPort(). In case the bool resolves to true, the function will launch the automatic port-forwarding process.

// Map ports with NAT-PMP
StartMapPort(args.GetBoolArg(”-natpmp”, DEFAULT_NATPMP));

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Connection Manager Options

Then, the software initializes a CConnman::Options object. connOptions holds all the settings needed by the connection manager.

CConnman::Options connOptions;

Here is the list:

• m_local_services: It advertises what services this node offers, contained in g_local_services, discussed in IBC#024.

  connOptions.m_local_services = g_local_services;

• m_max_automatic_connections: It sets the max amount of peers that can be automatically connected at once. This is set through the -maxconnections input argument.

  connOptions.m_max_automatic_connections = nMaxConnections;

• uiInterface: A pointer to the CClientUIInterface object to allow the connection manager to send updates to the UI.

  connOptions.uiInterface = &uiInterface;

• m_banman: A pointer to the ban manager, so that connection manager can deal with banned peers.

  connOptions.m_banman = node.banman.get();

• m_msgproc: A pointer to the interface handling messages, NetEventsInterface.

  connOptions.m_msgproc = node.peerman.get();

• nSendBufferMaxSize: It sets per-peer send buffer limits in bytes, a defined by the input argument -maxsendbuffer.

  connOptions.nSendBufferMaxSize = 1000 * args.GetIntArg(”-maxsendbuffer”,
      DEFAULT_MAXSENDBUFFER);

• nReceiveFloodSize: It sets per-peer receive buffer limits in bytes, a defined by the input argument -maxreceivebuffer.

  connOptions.nReceiveFloodSize = 1000 * args.GetIntArg(”-maxreceivebuffer”,
      DEFAULT_MAXRECEIVEBUFFER);

• m_added_node: A std::vector of IP addresses of nodes to connect to defined by the -addonode input argument.

  connOptions.m_added_nodes = args.GetArgs(”-addnode”);

• nMaxOutboundLimit: It sets the limit to outbound traffic as defined by the
  -maxuploadtarget input argument.

  connOptions.nMaxOutboundLimit = *opt_max_upload;

• m_peer_contact_timeout: It specifies the amount of time a peer may be inactive before the connection to it is dropped, as defined by the -peertimeout input argument.

  connOptions.m_peer_connect_timeout = peer_connect_timeout;

• whitelist_forcerelay: It forces the node to accept relayed transactions received from whitelisted peers. This is set to false by default, since the

  -whitelistforcerelay configuration parameter has been removed in v0.20.0.

  connOptions.whitelist_forcerelay = args.GetBoolArg(”-whitelistforcerelay”,
      DEFAULT_WHITELISTFORCERELAY);

• whitelist_relay: It sets whether accepting relayed transactions received from whitelisted peers even when not relaying transactions, as defined by the
  -whitelistrelay input argument.

   connOptions.whitelist_relay = args.GetBoolArg(”-whitelistrelay”,
      DEFAULT_WHITELISTRELAY);

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