**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/`
connOptions.m_max_outbound_full_relay = std::min(MAX_OUTBOUND_FULL_RELAY_CONNECTIONS, connOptions.nMaxConnections);
    connOptions.m_max_outbound_block_relay = std::min(MAX_BLOCK_RELAY_ONLY_CONNECTIONS, connOptions.nMaxConnections-connOptions.m_max_outbound_full_relay);
    connOptions.nMaxAddnode = MAX_ADDNODE_CONNECTIONS;
    connOptions.nMaxFeeler = MAX_FEELER_CONNECTIONS;
    connOptions.uiInterface = &uiInterface;
    connOptions.m_banman = node.banman.get();
    connOptions.m_msgproc = node.peerman.get();
    connOptions.nSendBufferMaxSize = 1000 * args.GetIntArg("-maxsendbuffer", DEFAULT_MAXSENDBUFFER);
    connOptions.nReceiveFloodSize = 1000 * args.GetIntArg("-maxreceivebuffer", DEFAULT_MAXRECEIVEBUFFER);
    connOptions.m_added_nodes = args.GetArgs("-addnode");
    connOptions.nMaxOutboundLimit = *opt_max_upload;
    connOptions.m_peer_connect_timeout = peer_connect_timeout;
     Port to bind to if `-bind=addr` is provided without a `:port` suffix.
    const uint16_t default_bind_port =
        static_cast<uint16_t>(args.GetIntArg("-port", Params().GetDefaultPort()));
    const auto BadPortWarning = [](const char* prefix, uint16_t port) {
        return strprintf(_("%s request to listen on port %u. This port is considered \"bad\" and "
                           "thus it is unlikely that any peer will connect to it. See "
                           "docp2p-bad-ports.md for details and a full list."),
                         prefix,
                         port);
    };
    for (const std::string& bind_arg : args.GetArgs("-bind")) {
        std::optional<CService> bind_addr;
        const size_t index = bind_arg.rfind('=');
        if (index == std::string::npos) {
            bind_addr = Lookup(bind_arg, default_bind_port, *fAllow
            if (bind_addr.has_value()) {
                connOptions.vBinds.push_back(bind_addr.value());
                if (IsBadPort(bind_addr.value().GetPort())) {