How to get PostgreSQL to use internal DNS resolution

Question

I have two PostgreSQL RDS databases in different regions and would like database A (us-south-1) to be able to query database B (us-west-1) using PostgreSQL's [dblink](https://www.postgresql.org/docs/current/contrib-dblink-function.html) functionality.

I have set up a VPC peering connection between the two VPCs, updated the route tables, enabled DNS resolution over the peering connection, and updated the security groups accordingly.

However when I try to connect to database B from database A using `dblink` the connection times out:

```
select dblink_connect('database-b','host=database-b.xxxyyyzzz.us-west-1.rds.amazonaws.com port=5432 dbname=dbname user=username password=password);
```

I have verified that the issue is not with the peering connection or the security groups, because the connection works fine when I connect using database B's internal IP address:

```
select dblink_connect('database-b','host=10.1.2.3 port=5432 dbname=dbname user=username password=password);
```

So the issue appears to be that within the PostgreSQL instance, it is not resolving database-b.xxxyyyzzz.us-west-1.rds.amazonaws.com to the internal VPC IP address. Strangely enough, if I connect to an EC2 instance in the same ap-south-1 VPC, it **does** resolve this correctly to the internal VPC IP address.

Is there anything I can do to make PostgreSQL resolve using the internal address like the EC2 instance does?

Leo K · Answer

One option might be to add an RDS proxy in your VPC and connect to it from PostgreSQL. The proxy's DNS name would always resolve to private IPs, while the host database would remain publicly accessible. https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/rds-proxy.html

Leo K · Answer

Is the RDS database publicly accessible? If so, the DNS name will resolve to its public IPs when queried from the outside. Since you're accessing it from inside your VPC or otherwise internally via VPC peering connections, the simplest option would be to disable the "publicly accessible" option for the RDS database. That would cause its DNS name to resolve to its private IPs regardless of where the name is queried from.

How to get PostgreSQL to use internal DNS resolution

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