Just a heads up for those who might run into the same issue:

I have an environment with a redis container named redis. In another container in the same environment, I had a nodejs server trying to connect to the redis server via the ioredis library. Basically I had a file like

import { Redis } from "ioredis";
export const redis = new Redis("redis://redis:6379");

On server start, I was seeing a stream of errors along the lines of

[ioredis] Unhandled error event: Error: getaddrinfo ENOTFOUND redis
    at GetAddrInfoReqWrap.onlookup [as oncomplete] (node:dns:109:26)
    at GetAddrInfoReqWrap.callbackTrampoline (node:internal/async_hooks:130:17)
[ioredis] Unhandled error event: Error: getaddrinfo ENOTFOUND redis
    at GetAddrInfoReqWrap.onlookup [as oncomplete] (node:dns:109:26)
    at GetAddrInfoReqWrap.callbackTrampoline (node:internal/async_hooks:130:17)
[ioredis] Unhandled error event: Error: getaddrinfo ENOTFOUND redis
    at GetAddrInfoReqWrap.onlookup [as oncomplete] (node:dns:109:26)
    at GetAddrInfoReqWrap.callbackTrampoline (node:internal/async_hooks:130:17)

This was very strange as the two containers were on the same network and redis should have been (and turns out, was) a valid hostname. Furthermore I was able to run via SSH on the node server instance:

# redis-cli -h redis PING
PONG

So the hostname was valid. What was going on?

After some discussion with the cycle team (thanks!!), it turns out the issue is that the internal networking within the environment is all done via IPv6, and by default, the ioredis client doesn't allow DNS resolution into IPv6. For whatever reason 🤷🏻‍♂️. But the fix was very simple. This works:

import { Redis } from "ioredis";
export const redis = new Redis({ host: "redis", family: 6 });

Explicitly tell the client to use IPv6, and it will.

Perhaps this will come in handy for someone later 😄 again, thanks to the cycle team for finding the solution to this!