footcannon prevention: check for invalid address family.

throw if we pass in a bogus af value when getting a sockaddr for all interfaces

llarp/net/net.cpp

@@ -591,11 +591,15 @@ namespace llarp
           addr.sin_port = htons(0);
           return SockAddr{addr};
         }
-        sockaddr_in6 addr6{};
-        addr6.sin6_family = AF_INET6;
-        addr6.sin6_port = htons(0);
-        addr6.sin6_addr = IN6ADDR_ANY_INIT;
-        return SockAddr{addr6};
+        if (af == AF_INET6)
+        {
+          sockaddr_in6 addr6{};
+          addr6.sin6_family = AF_INET6;
+          addr6.sin6_port = htons(0);
+          addr6.sin6_addr = IN6ADDR_ANY_INIT;
+          return SockAddr{addr6};
+        }
+        throw llarp::make_exception<std::invalid_argument>(af, " is not a valid address family");
       }
     }  // namespace
 

llarp/util/str.hpp

@@ -35,6 +35,17 @@ namespace llarp
     return o.str();
   }
 
+  /// util for constructing an exception with a message constructed from a set of whatever passed
+  /// into stringify
+  /// E must be derived from std::exception here
+  template <typename E, typename... T>
+  E
+  make_exception(T&&... stuff)
+  {
+    static_assert(std::is_base_of_v<std::exception, E>);
+    return E{stringify(std::forward<T>(stuff)...)};
+  }
+
   using namespace std::literals;
 
   /// Returns true if the first string is equal to the second string, compared case-insensitively.