Bad Host header when using ingress nginx with custom errors

[lahsivjar]

Is this a request for help? (If yes, you should use our troubleshooting guide and community support channels, see https://kubernetes.io/docs/tasks/debug-application-cluster/troubleshooting/.): No

What keywords did you search in NGINX Ingress controller issues before filing this one? (If you have found any duplicates, you should instead reply there.): upstream_balancer, bad host

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Is this a BUG REPORT or FEATURE REQUEST? (choose one): BUG REPORT

NGINX Ingress controller version: 0.18.0

Kubernetes version (use kubectl version):

Client Version: version.Info{Major:"1", Minor:"8", GitVersion:"v1.8.4", GitCommit:"9befc2b8928a9426501d3bf62f72849d5cbcd5a3", GitTreeState:"clean", BuildDate:"2017-11-20T05:28:34Z", GoVersion:"go1.8.3", Compiler:"gc", Platform:"linux/amd64"}
Server Version: version.Info{Major:"1", Minor:"8", GitVersion:"v1.8.4", GitCommit:"9befc2b8928a9426501d3bf62f72849d5cbcd5a3", GitTreeState:"clean", BuildDate:"2017-11-20T05:17:43Z", GoVersion:"go1.8.3", Compiler:"gc", Platform:"linux/amd64"}

Environment:

• Cloud provider or hardware configuration: AWS (custom installation)
• OS (e.g. from /etc/os-release): Ubuntu 16.04.3 LTS
• Kernel (e.g. uname -a): Linux ip-10-7-65-14 4.4.0-1066-aws #76-Ubuntu SMP Thu Aug 16 16:21:21 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
• Install tools: Custom ansible scripts

What happened: ingress-nginx with custom-http-errors and Java + Tomcat based default backend fails with 400 bad request because nginx rewrites the Host header with upstream_balancer as per https://github.com/kubernetes/ingress-nginx/blob/master/rootfs/etc/nginx/template/nginx.tmpl#L715

What you expected to happen: Custom error pages are rendered

How to reproduce it (as minimally and precisely as possible): Use ingress-nginx with custom error pages and spring boot based default backend

Anything else we need to know:

The issue seems to happen because of Host header contains invalid characters (in this case the underscore as per [RFC952]: https://www.rfc-editor.org/rfc/rfc952.txt) and tomcat seems to not take it well. The issue can be fixed by one of the following:

1. By modifying the upstream name to not contain the underscore
2. By setting host header

If we can decide on the solution I can create a PR.

[lahsivjar]

As a temporary solution, I have edited my nginx.tmpl to set Host header for custom errors. It will be really helpful if someone can take a look at the issue and suggest the correct approach to fix it (if required I can create PR to fix the issue)