Docker image has DNS problems

[gvlasov]

Describe the bug
I couldn't have it connect to mysql host, it would not resolve it despite being able to ping the container by IP.

Had to switch to https://hub.docker.com/r/justwatch/sql_exporter just because it has working DNS.

To Reproduce

services:
  mysql:
    # Some generic mysql configuration
  sql-exporter:
    image: burningalchemy/sql_exporter
    environment:
      - CONFIG=/config.yml
    volumes:
      - ./sql_exporter.yml:/config.yml:ro
    networks:
      - backend

$ docker compose exec sql-exporter ping mysql -c 1
ping: bad address 'mysql'

---
jobs:
  - name: "offers_count"
    interval: '10s'
    connections:
      - 'mysql://login:password@(mysql:3306)/whatever'
    queries:
      - name: "record_count"
        help: "Records count"
#        labels:
#          - "datname"
#          - "usename"
        values:
          - "count"
        query:  |
          SELECT count(*) AS row_count FROM records
        allow_zero_rows: false

Expected behavior
ping mysql reaches mysql service by hostname

Additional context
Otherwise works fine, but I can't rely on an IP for DSN, need a hostname.

[burningalchemist]

Hey @gvlasov,

The DSN format you're using is incorrect for this particular implementation. The proper DSN would be something like that: mysql://login:password@mysql:3306/whatever. Please check the readme section here.

Also, the configuration file you've posted is incompatible with sql_exporter. You can find working examples in the repository. 👍

The exporter is actively tested before releasing for the common databases, so I bet it's related to the configuration on the user end.

Not a bug, but I'll turn the issue into a discussion if you want some help further down.

[gvlasov]

@burningalchemist Thanks for answering

Whatever the actual format, I had your solution work with an IP. The problem is that pings from container by hostname (as opposed to by ip) not working, while pings by IP do work. So it looks like a DNS issue (not to be mixed up with DSN). Problem seems to be not in the actual sql-exporter, but in how the container is built. nslookup mysql is working though.

[burningalchemist]

Hey @gvlasov,

Based on the description and the context you've posted, it's impossible to reproduce the issue as it's wrong on many levels:

• image: burningalchemy/sql_exporter - there is no such an image (also the version is not specified);
• CONFIG environment variable is not expected, it has to be SQLEXPORTER_CONFIG instead for sql_exporter to be able to pick the configuration file;
• the configuration file posted is incorrect and simply won't get parsed: for a single target mode you're supposed to use target stanza whereas for multi-target mode jobs stanza requires static_configs list and not connections (check the configuration snippet below from the readme.md):

This gives an impression that you might be using either the wrong project/version, or the context is ai-generated, which brings no value to me in order to fix the potential issue.

---

As for the DNS resolution and the container issue: if you check Dockerfile or Dockerfile.multi-arch in the repository, you may find that the runtime container is the busybox image, built and officially maintained by the Prometheus Community which is used for the other common exporters like node_exporter (https://github.com/prometheus/node_exporter/blob/master/Dockerfile). There are no custom adjustments made on my end.

The main reason that ping command returns bad address likely relates to the fact that busybox's flavour of ping command expects a FQDN domain name (which mysql is not). This is the way ping command operates for busybox (https://busybox.net) as it's built-in (like other core utilities there). In turn, justwatch/sql_exporter is using alpine as a base image, so the behaviour of ping is different because it's a separate utility.

However, as you mentioned yourself, nslookup command works just fine, which means that DNS resolution is functional, and so it negates the original claim.

Given that ping doesn't affect the functionality of sql_exporter application at all (if it's properly configured), there is nothing for me to fix here.

At the same time, I'm glad that you found a solution that works for your goals. 😃👍

[burningalchemist]

@gvlasov You can use non-fqdn domain names from the golang applications (e.g. sql_exporter) as these apps are statically built and do not rely upon any OS components, but golang's standard library, which works well. So sql_exporter works independently from the base image.

Just to make sure, I've tested the docker compose stuff with non-fqdn name and it works properly (as it does for years since I started maintaining and developing the project). Otherwise it'd be a huge issue, indeed, as many container-based environments rely upon such names.

[gvlasov]

the configuration file posted is incorrect and simply won't get parsed

I might have copied a wrong and broken snippet. I did have your solution working if I used ip instead of a domain.

I believe ping and sql-exporter couldn't reach the destination for the same reason: DNS implementation in system libraries, which is not used by nslookup. Using non-fully qualified domain names is extremely common in docker compose networking: by default services get assigned a domain that is the same as the service name, which is usually something like mysql, nginx or something like that. So not being able to connect to non-FQDN would be a big problem.

[gvlasov]

@burningalchemist I see, I guess I messed up somewhere else

[burningalchemist]

As I said, I'm happy to help finding the root cause, but the snippets posted won't help sadly.

Anyways, given that you found your way out with the implementation from justwatch, I think there is no need anymore. 😃

But if at some point you would like to switch back or something, I'd be glad to assist. 👍