**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/`
if (IsBadPort(port_arg)) {
            InitWarning(BadPortWarning("-port", port_arg));
        }
    }
    CService onion_service_target;
    if (!connOptions.onion_binds.empty()) {
        onion_service_target = connOptions.onion_binds.front();
    } else {
        onion_service_target = DefaultOnionServiceTarget();
        connOptions.onion_binds.push_back(onion_service_target);
    }
    if (args.GetBoolArg("-listenonion", DEFAULT_LISTEN_ONION)) {
        if (connOptions.onion_binds.size() > 1) {
            InitWarning(strprintf(_("More than one onion bind address is provided. Using %s "
                                    "for the automatically created Tor onion service."),
                                  onion_service_target.ToStringAddrPort()));
        }
        StartTorControl(onion_service_target);
    }
    if (connOptions.bind_on_any) {
         Only 
         any a
        Discover();
    }
    for (const auto& net : args.GetArgs("-whitelist")) {
        NetWhitelistPermissions subnet;
        bilingual_str error;
        if (!NetWhitelistPermissions::TryParse(net, subnet, error)) return InitError(error);
        connOptions.vWhitelistedRange.push_back(subnet);
    }