net: introduce a new config option to enable CJDNS

CJDNS is set up in the host OS, outside of the application. When the
routing is configured properly then connecting to fc00::/8 results in
connecting to the CJDNS network.

Introduce an option so that Bitcoin Core knows whether this is the case.

Author: vasild

src/init.cpp

@@ -422,6 +422,7 @@ void SetupServerArgs(ArgsManager& argsman)
     argsman.AddArg("-asmap=<file>", strprintf("Specify asn mapping used for bucketing of the peers (default: %s). Relative paths will be prefixed by the net-specific datadir location.", DEFAULT_ASMAP_FILENAME), ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
     argsman.AddArg("-bantime=<n>", strprintf("Default duration (in seconds) of manually configured bans (default: %u)", DEFAULT_MISBEHAVING_BANTIME), ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
     argsman.AddArg("-bind=<addr>[:<port>][=onion]", strprintf("Bind to given address and always listen on it (default: 0.0.0.0). Use [host]:port notation for IPv6. Append =onion to tag any incoming connections to that address and port as incoming Tor connections (default: 127.0.0.1:%u=onion, testnet: 127.0.0.1:%u=onion, signet: 127.0.0.1:%u=onion, regtest: 127.0.0.1:%u=onion)", defaultBaseParams->OnionServiceTargetPort(), testnetBaseParams->OnionServiceTargetPort(), signetBaseParams->OnionServiceTargetPort(), regtestBaseParams->OnionServiceTargetPort()), ArgsManager::ALLOW_ANY | ArgsManager::NETWORK_ONLY, OptionsCategory::CONNECTION);
+    argsman.AddArg("-cjdnsreachable", "If set then this host is configured for CJDNS (connecting to fc00::/8 addresses would lead us to the CJDNS network) (default: 0)", ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
     argsman.AddArg("-connect=<ip>", "Connect only to the specified node; -noconnect disables automatic connections (the rules for this peer are the same as for -addnode). This option can be specified multiple times to connect to multiple nodes.", ArgsManager::ALLOW_ANY | ArgsManager::NETWORK_ONLY, OptionsCategory::CONNECTION);
     argsman.AddArg("-discover", "Discover own IP addresses (default: 1 when listening and no -externalip or -proxy)", ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
     argsman.AddArg("-dns", strprintf("Allow DNS lookups for -addnode, -seednode and -connect (default: %u)", DEFAULT_NAME_LOOKUP), ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
@@ -1294,6 +1295,14 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
         }
     }
 
+    if (!args.IsArgSet("-cjdnsreachable")) {
+        SetReachable(NET_CJDNS, false);
+    }
+    // Now IsReachable(NET_CJDNS) is true if:
+    // 1. -cjdnsreachable is given and
+    // 2.1. -onlynet is not given or
+    // 2.2. -onlynet=cjdns is given
+
     // Check for host lookup allowed before parsing any network related parameters
     fNameLookup = args.GetBoolArg("-dns", DEFAULT_NAME_LOOKUP);