**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/`
assert(IsReachable(net));
            if (!nets.count(net))
                SetReachable(net, false);
        }
    }
    if (!args.IsArgSet("-cjdnsreachable")) {
        if (args.IsArgSet("-onlynet") && IsReachable(NET_CJDNS)) {
            return InitError(
                _("Outbound connections restricted to CJDNS (-onlynet=cjdns) but "
                  "-cjdnsreachable is not provided"));
        }
        SetReachable(NET_CJDNS, false);
    }
     
     
     
     
     
     
     
    if (args.GetBoolArg("-dnsseed") == true && !IsReachable(NET_IPV4) && !IsReachable(NET_IPV6)) {
        return InitError(strprintf(_("Incompatible options: -dnsseed=1 was explicitly specified, but -onlynet forbids connections to IPv4IPv6")));
    };
     
    fNameLookup = args.GetBoolArg("-dns", DEFAULT_NAME_LOOKUP);
    Proxy onion_proxy;
    bool proxyRandomize = args.GetBoolArg("-proxyrandomize", DEFAULT_PROXYRANDOMIZE);