If an IPv6 address has square brackets it will be rejected by libnftables JSON parsing. The CLI (bison) parser happily accepts the square bracket syntax. Square brackets are also accepted by ipset and iptables.

Found when working on bug 1779835. Also see RFC 2732 [1].

[1] https://tools.ietf.org/rfc/rfc2732.txt

-->8--

# nft delete table ip foobar; cat /tmp/json | nft -j -f -
internal:0:0-0: Error: Could not resolve hostname: Name or service not known

# nft delete table ip foobar; sed -e 's/[[]1234::[]]/1234::/' /tmp/json | nft -j -f -
** no error **

# nft add element ip foobar myset '{ [4321::]/64 }'
** no error **

# cat /tmp/json  | python3 -m json.tool
{
    "nftables": [
        {
            "metainfo": {
                "version": "0.9.3",
                "release_name": "Topsy",
                "json_schema_version": 1
            }
        },
        {
            "table": {
                "family": "ip",
                "name": "foobar"
            }
        },
        {
            "set": {
                "family": "ip",
                "name": "myset",
                "table": "foobar",
                "type": "ipv6_addr",
                "flags": [
                    "interval"
                ],
                "elem": [
                    {
                        "prefix": {
                            "addr": "[1234::]",
                            "len": 64
                        }
                    }
                ]
            }
        }
    ]
}