**TL;DR**: When upgrading from `Bitcoin Core 0.20.1` to `Bitcoin Core 0.21.0`, a downstream application using the HTTP JSON-RPC API started getting HTTP 403 "Forbidden".

Turns out it is because I have `rpcallowip=::/0` in `bitcoin.conf` which used to work but does not anymore.
As a workaround, `rpcallowip=0.0.0.0/0` works.

-----

I run bitcoind in docker container1, and some application using the HTTP JSON-RPC API of bitcoind is in docker container2. They are on the same "docker network".

In `bitcoin.conf` I have:
```
rpcauth=<...>
rpcbind=0.0.0.0:8332
rpcallowip=::/0
```
(bitcoind RPC is not actually exposed to the public internet; it is up to the docker layer and above to ensure this.)

The application in container2 is connecting through HTTP accessing an URL such as `http://user:pass@172.19.1.10:8332/`.

This setup used to work when running `Bitcoin Core 0.20.1` in container1.

It no longer works with `Bitcoin Core 0.21.0`.

-----

If I change my `bitcoin.conf` as below, it starts working again using 0.21:
```
rpcauth=<...>
rpcbind=0.0.0.0:8332
rpcallowip=0.0.0.0/0
```

So `rpcallowip=::/0` used to work but does not anymore; `rpcallowip=0.0.0.0/0` works.

-----

Easy way to test:
`curl --data-binary '{"jsonrpc":"1.0","id":"curltext","method":"getblockchaininfo","params":[]}' -H 'content-type:text/plain;' http://user:pass@172.19.1.10:8332/`
* Initialize global loggers.
 *
 * Note that this is called very early in the process lifetime, so you should be
 * careful about what global state you rely on here.
 *
void InitLogging(const ArgsManager& args)
{
    init::SetLoggingOptions(args);
    init::LogPackageVersion();
}
namespace { 
int nMaxConnections;
int nUserMaxConnections;
int nFD;
ServiceFlags nLocalServices = ServiceFlags(NODE_NETWORK_LIMITED | NODE_WITNESS);
int64_t peer_connect_timeout;
std::set<BlockFilterType> g_enabled_filter_types;
} 
[[noreturn]] static void new_handler_terminate()
{
    
    
    
    
    std::set_new_handler(std::terminate);
    LogPrintf("Error: Out of memory. Terminating.\n");
    
    std::terminate();
};
bool AppInitBasicSetup(const ArgsManager& args, std::atomic<int>& exit_status)