[Bug] Affected alerts are not returned on the page of a silence in 0.9.0

[benclapp]

The 'Affected alerts' field of an active silence is displaying 'Network error' rather than the alerts affected by this silence.

It looks like the request to get affected alerts has dropped the web.external-url value from the request.

Version 0.8.0 was requesting: http://alertmanager.foo.bar:9093/api/v1/alerts?silenced=true&filter=action%3D%22scale-in%22

Version 0.9.0 is requesting: http://alerts?silenced=true&filter=action%3D%22scale-in%22

[stuartnelson3]

I remember seeing this issue in the past when first adding this feature, but I haven't so far been able to reproduce it.

Running alertmanager:

# ./alertmanager -web.external-url http://localhost:3000/alertmanager/

Running local nginx proxy that forwards localhost:3000/alertmanager to localhost:9093, my requests have had the correct url

  server { # simple reverse-proxy
    listen       3000;
    server_name  domain2.com www.domain2.com;

    location /alertmanager/ {
      proxy_pass      http://localhost:9093;
    }
  }

How are you starting your alertmanager?

./alertmanager -web.external-url='http://alertmanager.foo.bar:9093'

[benclapp]

I discovered it with the alertmanager docker image. So starting with one of the command flags set like below. It was working OK in 0.8.0 and now, after the fix you made in #1008. Below is a snippet of a docker-compose I was able to reproduce the issue with:

alertmanager:
image: quay.io/prometheus/alertmanager:latest
container_name: alertmanager
restart: always
ports:
- 9093:9093
volumes:
- ./alertmanager/:/etc/alertmanager/
- /opt/alertmanager:/alertmanager
command:
- '-config.file=/etc/alertmanager/config.yml'
- '-storage.path=/alertmanager'
- '-web.external-url=http://<hostname>:9093'