Non-existing domain with domain that resolves to IPv6 address

[Rodeoclash]

A quick (likely stupid) question that I couldn't find an answer for in the repo or Elixir community forums Does Redix support IPV6?

I'm attempting to connect to a Redis instance on fly.io and while I seem to be able to resolve the address:

# ping vodon-redis.internal
PING vodon-redis.internal(fdaa:0:3924:a7b:ead:0:84e5:2 (fdaa:0:3924:a7b:ead:0:84e5:2)) 56 data bytes
64 bytes from fdaa:0:3924:a7b:ead:0:84e5:2 (fdaa:0:3924:a7b:ead:0:84e5:2): icmp_seq=1 ttl=62 time=39.4 ms
64 bytes from fdaa:0:3924:a7b:ead:0:84e5:2 (fdaa:0:3924:a7b:ead:0:84e5:2): icmp_seq=2 ttl=62 time=34.3 ms

and have connectivity to the Redis instance:

# telnet vodon-redis.internal 6379
Trying fdaa:0:3924:a7b:ead:0:84e5:2...
Connected to vodon-redis.internal.
Escape character is '^]'.
AUTH 5cX*******************
+OK

Things don't want to play nicely once I'm attempting to connect from within Elixir:

06:17:52.643 [error] Failed to connect to Redis (redis://vodon-redis.internal/1:6379): non-existing domain

My naïve assumption would be to configure the connection as follows:

Redix.start_link(
  host: "redis://vodon-redis.internal/1",
  socket_opts: [:inet6]
)

However I seem to be missing some nuance around passing the configuration to the underlying Erlang connection.

If I can get this resolved, happy to update your docs and also feed this back up to the Fly team to add to the page they have on configuring Redis 🙂

[whatyouhide]

Hey @Rodeoclash, thanks for the report.

I think we should support IPv6 out of the box. We are not doing anything in Redix to resolve addresses manually or anything.

I don't have an easy way right now to play with IPv6 DNS addresses. Can you try playing around with inet:resolve/5 and see if it shows any issues?

Alternatively, do you have a minimal reproducing case using Docker or a well-known accessible address that I can use to easily reproduce this locally on my machine?

Thanks! 💟

[Rodeoclash]

Sounds good. I'll dig in the next couple of days and see if I can identify how we might resolve it.

If I can't get to the bottom of it I'll put together a test repo with docker-compose.yml which will at least give you a test harness 🙂

[whatyouhide]

@Rodeoclash that sounds great. Redix already has a docker-compose.yml file so if you prefer you can go and play with this in your Redix fork itself 🙃

[Rodeoclash]

Hmm, I think I have a workaround of sorts but I'm not sure how correct it is. Firstly, I can resolve the domain:

:inet_res.resolve('vodon-redis.internal', :in, :aaaa)

{:ok,
 {:dns_rec, {:dns_header, 4, true, :query, false, false, true, false, false, 0},
  [{:dns_query, 'vodon-redis.internal', :aaaa, :in, false}],
  [
    {:dns_rr, 'vodon-redis.internal', :aaaa, :in, 0, 5,
     {64938, 0, 14628, 2683, 3757, 0, 34021, 2}, :undefined, [], false}
  ], [], []}}

However if I attempt to directly use the hostname with :gen_tcp then I get the same nxdomain error when I use Redix directly:

:gen_tcp.connect('vodon-redis.internal', 6379, [:binary, active: false])

{:error, :nxdomain}

Likewise, if I use the IPv6 address as a binary with it

:gen_tcp.connect('fdaa:0:3924:a7b:232f:54c4:a4a2:2', 6379, [:binary, active: false])

{:error, :nxdomain}

However, if I use the IPv6 tuple then the connection is established

:gen_tcp.connect({64938, 0, 14628, 2683, 3757, 0, 34021, 2}, 6379, [:binary, active: false])

{:ok, #Port<0.2111>}

So my work around could be to simply lookup the IPv6 address using :inet_res.resolve on boot and pass that to the connection string.

Either way, not a Redix issue but a :gen_tcp issue! I'll close the ticket 🙂

[whatyouhide]

@Rodeoclash that's a super interesting find though, thanks for taking the time to share it and report back! I'm egoistically glad it's not a Redix issue 😉

[Rodeoclash]

For anyone else that finds this, my final configuration looked like this:

redix_connection: [
  name: :redix,
  host: "vodon-redis.internal",
  password: redis_password,
  port: 6379,
  socket_opts: [:inet6]
]

I couldn't use the URL parsing method but specify the options individually worked with aplomb.