Allow alertmanager to be run behind a reverse proxy

[grobie]

The -web.external-url flag is no longer respected when loading assets. It's no longer possible to run alertmanager behind a reverse proxy.

[camerondavison]

FYI I just did https://github.com/novilabs/alertmanager/commit/50937b09100abcc1d28f933baa38e2b22aa0ef01 to be able to use a prefix path of /alerts and run it behind my reverse proxy. It seems to be working well. I do not know a way to pass it through as configuration to the html and js files though. If anybody wants to know what files would need to be touched.

[hryamzik]

-web.external-url does seem to be broken (again?). @grobie, any comments on this?

@a86c6f7964 did you take a look on the pull request mentioned here? It seems to be a better solution.

[grobie]

It never got fixed so far unfortunately. Pull requests very welcome, I think all core contributors are very overworked right now.

[juliusv]

Just looking at it, I got it almost working, except for one thing: the index.html is a static HTML page (not a Go template), so we can't inject the path prefix into it to fix up the JS and CSS asset links. Should we serve the web pages (at least index.html) through the Go template engine, like we do in Prometheus?

[fabxc]

Without a closer look – that doesn't seem like something that should depend on passing through a template engine.
If it comes with complexity I cannot think of right now, sure – it doesn't have a relevant overhead.

[juliusv]

@fabxc True. The alternative is to make all the paths between the assets relative. Trying that out now, it seems to work. It would mean that one has to be careful to not introduce any absolute paths anywhere, ever. But requiring a path prefix everywhere in the HTML+JS would have the same problem. Should I go for relative paths then?

[fabxc]

Sure, it's not that many files and won't change frequently.

[mrwacky42]

+1 to relative paths.

[juliusv]

Cool. Fix is out in https://github.com/prometheus/alertmanager/pull/289/files.

[sdwerwed]

I made it work like this (--web.external-url not -web.external-url)
/usr/local/bin/alertmanager --config.file /etc/alertmanager/alertmanager.yml --web.external-url=http://example.com:9093/alertmanager then add to nginx location /alertmanager {proxy_pass http:localhost:9093/alertmanager/;}