Debugging ideas for Redis connection (external service)

I have a redis connection to an external redis service (… i can easily connect virtually everywhere else, but within a Fly instance, its not working… simply “not found”… error looks like this:

[ioredis] Unhandled error event: Error: getaddrinfo ENOTFOUND

2023-01-01T09:34:01.919 app[f2b75536] lax [info] at GetAddrInfoReqWrap.onlookup [as oncomplete] (node:dns:107:26)

and the connection code looks like this:

 const connection = new Redis({
        port: Number.parseInt(url.port),
        host: url.hostname,
        username: url.username,
        password: url.password,
        connectTimeout: 12_000,
        maxRetriesPerRequest: 2,
        family: 6,

any ideas to debug this? i suspect its network config, but im not sure what fly is doing here.