IBC#035: Bitcoin Core Initialization, Step 12: Setting Outgoing Connections

The -connect input argument

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We concluded the previous episode with discussing about port bindings. Today, we continue our deep dive into step 12 of the initialization of Bitcoin Core, discussing about the .connect input argument. [Code Link]

Let’s start!

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-seednode and -connect

First of all, the code retrieves the -seednode input argument. This allows to connect directly to specific trusted nodes, defined by their IP address, to gather initial peer address when the node starts up:

seednode=192.168.0.1

Since, the input can be defined multiple times with different IP addresses, the GetArgs() function will return a vector.

 connOptions.vSeedNodes = args.GetArgs("-seednode");

Then, the -connect input argument is retrieved. This allows a user to specify to which peers the node must connect to. No other connection will be opened.

const auto connect = args.GetArgs("-connect");

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Specify Outgoing Connections

After that, the code enters an if statement. The program checks whether the connect vector is not empty() — no -connect input as been provided — or the argument was originally passed as a negated option — -noconnect.

if (!connect.empty() || args.IsArgNegated("-connect")) {
    .
    .
    .
}

If the clause resolves to true, the node is instructed to not initiate any other outgoing connection when connecting to trusted nodes, disabling the automatic peers discovery system.

connOptions.m_use_addrman_outgoing = false;

Then, the program checks that the connect vector has more than 1 element (remember, it cannot be 0, since we checked that it is not empty!), and that the first element of the provided input is not “0” (which means no connections at all).
If the clause resolves to true, connect is stored inside the m_specified_outgoing vector.

if (connect.size() != 1 || connect[0] != "0") {
    connOptions.m_specified_outgoing = connect;
}

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Warnings

When defining inputs for the node, we can have some arguments conflicting with others. Thus, there are some checks that make sure that only the correct ones are used.

Here, the programs checks if both -connect and -seednode have been defined. If so,
-seednode will be ignored, informing the user about this automatic decision. This is done because we explicitly defined which nodes to connect to, so, there is no need for seed nodes to bootstrap peer discovery.

if (!connOptions.m_specified_outgoing.empty() 
    &&!connOptions.vSeedNodes.empty()) {
        LogInfo("-seednode is ignored when -connect is used");
}

The same happens in case -dnsseed is set, but at the same time both -connect and
-proxy are specified. If so, -dnseed will be ignored, informing the user about this automatic decision. This is done because the node is already configured to use explicit outgoing connections and a proxy, making automatic DNS-based discovery unnecessary or conflicting.

if (args.IsArgSet("-dnsseed")
    && args.GetBoolArg("-dnsseed", DEFAULT_DNSSEED)
    && args.IsArgSet("-proxy")) {
       LogInfo("-dnsseed is ignored when -connect is used and -proxy is specified");
}

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