**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 (nFD < MIN_CORE_FILEDESCRIPTORS)
        return InitError(_("Not enough file descriptors available."));
    nMaxConnections = std::min(nFD - MIN_CORE_FILEDESCRIPTORS - MAX_ADDNODE_CONNECTIONS - NUM_FDS_MESSAGE_CAPTURE, nMaxConnections);
    if (nMaxConnections < nUserMaxConnections)
        InitWarning(strprintf(_("Reducing -maxconnections from %d to %d, because of system limitations."), nUserMaxConnections, nMaxConnections));
    
    auto result = init::SetLoggingCategories(args);
    if (!result) return InitError(util::ErrorString(result));
    result = init::SetLoggingLevel(args);
    if (!result) return InitError(util::ErrorString(result));
    nConnectTimeout = args.GetIntArg("-timeout", DEFAULT_CONNECT_TIMEOUT);
    if (nConnectTimeout <= 0) {
        nConnectTimeout = DEFAULT_CONNECT_TIMEOUT;
    }
    peer_connect_timeout = args.GetIntArg("-peertimeout", DEFAULT_PEER_CONNECT_TIMEOUT);
    if (peer_connect_timeout <= 0) {
        return InitError(Untranslated("peertimeout must be a positive integer."));
    }
    
    
    if (args.IsArgSet("-blockmintxfee")) {
        if (!ParseMoney(args.GetArg("-blockmintxfee", ""))) {
            return InitError(AmountErrMsg("blockmintxfee", args.GetArg("-blockmintxfee", "")));
        }
    }
    nBytesPerSigOp = args.GetIntArg("-bytespersigop", nBytesPerSigOp);
    if (!g_wallet_init_interface.ParameterInteraction()) return false;
    
    SetMockTime(args.GetIntArg("-mocktime", 0)); 
    if (args.GetBoolArg("-peerbloomfilters", DEFAULT_PEERBLOOMFILTERS))