net: Allow DNS lookups on nodes with IPV6 lo only

AI_ADDRCONFIG prevents ::1 from being considered a valid address on hosts that have a IPV6 loopback IP address but no other IPV6 interfaces.

Author: m3dwards

src/netbase.cpp

@@ -48,6 +48,7 @@ std::vector<CNetAddr> WrappedGetAddrInfo(const std::string& name, bool allow_loo
     ai_hint.ai_protocol = IPPROTO_TCP;
     // We don't care which address family (IPv4 or IPv6) is returned
     ai_hint.ai_family = AF_UNSPEC;
+
     // If we allow lookups of hostnames, use the AI_ADDRCONFIG flag to only
     // return addresses whose family we have an address configured for.
     //
@@ -59,7 +60,17 @@ std::vector<CNetAddr> WrappedGetAddrInfo(const std::string& name, bool allow_loo
     addrinfo* ai_res{nullptr};
     const int n_err{getaddrinfo(name.c_str(), nullptr, &ai_hint, &ai_res)};
     if (n_err != 0) {
-        return {};
+        if ((ai_hint.ai_flags & AI_ADDRCONFIG) == AI_ADDRCONFIG) {
+            // AI_ADDRCONFIG on some systems may exclude loopback-only addresses
+            // If first lookup failed we perform a second lookup without AI_ADDRCONFIG
+            ai_hint.ai_flags = (ai_hint.ai_flags & ~AI_ADDRCONFIG);
+            const int n_err_retry{getaddrinfo(name.c_str(), nullptr, &ai_hint, &ai_res)};
+            if (n_err_retry != 0) {
+                return {};
+            }
+        } else {
+            return {};
+        }
     }
 
     // Traverse the linked list starting with ai_trav.