What version of Go are you using (go version)?

1.20

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

Amazon Linux 2 (arm64) (linux/arm64)

What did you do?

I'm trying to connect to my PostgreSQL cluster on Amazon RDS. The cluster has dual-stack mode enabled.

What did you expect to see?

Successful connection to the cluster.

What did you see instead?

 failed to connect to `host=collector.cluster-c3tdcjzw8by1.us-east-1.rds.amazonaws.com <redacted>`: dial error (dial tcp [2600:1f18:4769:c800:c1c6:ffd:fdbb:2045]:5432: connect: network is unreachable)

When running dig collector.cluster-c3tdcjzw8by1.us-east-1.rds.amazonaws.com, I get the following result:

; <<>> DiG 9.11.4-P2-RedHat-9.11.4-26.P2.amzn2.5.2 <<>> collector.cluster-c3tdcjzw8by1.us-east-1.rds.amazonaws.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 24492
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;collector.cluster-c3tdcjzw8by1.us-east-1.rds.amazonaws.com. IN A

;; ANSWER SECTION:
collector.cluster-c3tdcjzw8by1.us-east-1.rds.amazonaws.com. 5 IN CNAME collector-instance-1.c3tdcjzw8by1.us-east-1.rds.amazonaws.com.
collector-instance-1.c3tdcjzw8by1.us-east-1.rds.amazonaws.com. 5 IN A 10.0.0.28

;; Query time: 2 msec
;; SERVER: 20.0.0.2#53(20.0.0.2)
;; WHEN: Wed Feb 08 06:19:18 UTC 2023
;; MSG SIZE  rcvd: 151

Which gives the IPv4 10.0.0.28. When using this IP address instead of the hostname (host=10.0.0.28) the connection is successful.

There seems to be a similar issue here: https://groups.google.com/g/golang-nuts/c/j7ylMOg1qc4

Before anyone says "you've clearly misconfigured your AWS setup"; my setup is correct and I have tested it. The RDS security group allows IPv6 connections and the connector (EC2 instance in this case) has a security group which allows outbound access to port 5432 for IPv6. I've even added "all traffic" rules to both security groups to be 100% sure. My route tables are also correctly setup. Reachability Analyzer also confirms the network is reachable.