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Today we are going to continue our trip inside the initialization of Bitcoin Core with proxy setting for outgoing network traffic.
Let’s start!
Proxy
First of all, what is a proxy1? A proxy can be seen as middleman that routes your internet traffic through another server. Basically, it acts as an intermediary between a client requesting a resource to a server, and the server providing the resource to the client. This allows a user to gain privacy or to bypass restrictions on a network.
At the beginning, different Proxy objects are declared, one for each available network type:
Proxy ipv4_proxy;
Proxy ipv6_proxy;
Proxy onion_proxy;
Proxy name_proxy;
Proxy cjdns_proxy;It is possible to set:
IPv42: Regular internet connection on the v4 standard.
IPv63: Regular internet connection on the v6 standard.
Onion: For anonymous connection on the Tor4 network.
Name: For DNS lookups.
Cjdns5: For connection on a distributed network.
By default, the proxy setting is removed, which is the same as passing the argument as -noproxy, -proxy=0 or -proxy="".
After declaration, a for loop iterates through all the inputs coming from the -proxy argument. The GetArgs function returns a string, which gets stored inside param_value. This iteration goes on until there is no more data in the -proxy list.
for (const std::string& param_value : args.GetArgs("-proxy"))Each param_value will specify a network address to a proxy server and, if needed, the type of network to which it applies. If no network is specified, it is applied to all network types. Here is an example:
127.0.0.1:9050=ipv4Network string
Once the input is in the param_value string, the = character is looked for in it. This is done to later separate the address from the type of network.
const auto eq_pos{param_value.rfind('=')};
const std::string proxy_str{param_value.substr(0, eq_pos)};eq_pos represents the position in the string at which the character is found. This variable is used to extrapolate the address string from param_value using the substr function. This function takes a starting point, 0 in our case, and a length (eq_pos) and stores the result in proxy_str.
Then, the code checks whether or not the network type was set in the input argument, and, in case, stores it in net_str string. To do so, the code checks that the eq_pos variable is different than npos, which is a member of the string class that literally says “end of the string”. In other words, it means that the = character was not found in the param_value string.
std::string net_str;
if (eq_pos != std::string::npos) {
if (eq_pos + 1 == param_value.length()) {
return InitError(strprintf(_("Invalid -proxy address or hostname, ends with '=': '%s'"), param_value));
}
net_str = ToLower(param_value.substr(eq_pos + 1)); // e.g. 127.0.0.1:9050=ipv4 -> ipv4
}In case the if clause returns true, another check is made to be sure something has been defined after =. If true, an error is returned. This input, for example, would return an error:
127.0.0.1:9050=In case everything is setup correctly, the network type is saved into the net_str variable.
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