Custom Error Pages: Do not write status code too soon.

[jfyuen]

Remove duplicate WriteHeader call if requested file not found.

What this PR does / why we need it:

The main purpose of this PR is to fix a dual call to WriteHeader if the requested file was not found and a http.NotFound() as this function will also call WriteHeader under the hood.

It could even panic if a wrong http code was sent via curl -H "X-Code: 0" http://localhost:8080/:

2025/11/11 13:52:58 format not specified. Using text/html
2025/11/11 13:52:58 http: panic serving 192.168.65.1:32712: invalid WriteHeader code 0
goroutine 10 [running]:
net/http.(*conn).serve.func1()
	/usr/local/go/src/net/http/server.go:1943 +0xd3
panic({0x7eb760?, 0xc0003cb2b0?})
	/usr/local/go/src/runtime/panic.go:783 +0x132
net/http.checkWriteHeaderCode(...)
	/usr/local/go/src/net/http/server.go:1162
net/http.(*response).WriteHeader(0xc0003aa3c0, 0x0)
	/usr/local/go/src/net/http/server.go:1196 +0x839
main.errorHandler.func1({0x929188, 0xc0003aa3c0}, 0xc0003e6000)
	/go/src/k8s.io/ingress-nginx/images/custom-error-pages/main.go:152 +0xfeb
net/http.HandlerFunc.ServeHTTP(0xc18520?, {0x929188?, 0xc0003aa3c0?}, 0x62ad36?)
	/usr/local/go/src/net/http/server.go:2322 +0x29
net/http.(*ServeMux).ServeHTTP(0x474df9?, {0x929188, 0xc0003aa3c0}, 0xc0003e6000)
	/usr/local/go/src/net/http/server.go:2861 +0x1c7
net/http.serverHandler.ServeHTTP({0xc000025b40?}, {0x929188?, 0xc0003aa3c0?}, 0x1?)
	/usr/local/go/src/net/http/server.go:3340 +0x8e
net/http.(*conn).serve(0xc0000e4240, {0x929978, 0xc0003cf620})
	/usr/local/go/src/net/http/server.go:2109 +0x665
created by net/http.(*Server).Serve in goroutine 1
	/usr/local/go/src/net/http/server.go:3493 +0x485

The fix is just to move the call near the io.copy() and be more robust.
Includes minor cosmetic changes.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• CVE Report (Scanner found CVE and adding report)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation only

Which issue/s this PR fixes

How Has This Been Tested?

By building the docker image manually and sending it a custom query:

• Build the image: cd images/custom-error-pages/rootfs && docker build --build-arg GOLANG_VERSION=1.25.4 . -t custom-error-pages
• Start the container: docker run --rm -p 8080:8080 custom-error-pages
• Send the query with a wrong code:

$ curl -H "X-Code: 0" http://localhost:8080/
404 page not found

Before:

$ docker run --rm -p 8080:8080 custom-error-pages
2025/11/11 13:52:58 format not specified. Using text/html
2025/11/11 13:52:58 http: panic serving 192.168.65.1:32712: invalid WriteHeader code 0
goroutine 10 [running]:
net/http.(*conn).serve.func1()
	/usr/local/go/src/net/http/server.go:1943 +0xd3
panic({0x7eb760?, 0xc0003cb2b0?})
	/usr/local/go/src/runtime/panic.go:783 +0x132
net/http.checkWriteHeaderCode(...)
	/usr/local/go/src/net/http/server.go:1162
net/http.(*response).WriteHeader(0xc0003aa3c0, 0x0)
	/usr/local/go/src/net/http/server.go:1196 +0x839
main.errorHandler.func1({0x929188, 0xc0003aa3c0}, 0xc0003e6000)
	/go/src/k8s.io/ingress-nginx/images/custom-error-pages/main.go:152 +0xfeb
net/http.HandlerFunc.ServeHTTP(0xc18520?, {0x929188?, 0xc0003aa3c0?}, 0x62ad36?)
	/usr/local/go/src/net/http/server.go:2322 +0x29
net/http.(*ServeMux).ServeHTTP(0x474df9?, {0x929188, 0xc0003aa3c0}, 0xc0003e6000)
	/usr/local/go/src/net/http/server.go:2861 +0x1c7
net/http.serverHandler.ServeHTTP({0xc000025b40?}, {0x929188?, 0xc0003aa3c0?}, 0x1?)
	/usr/local/go/src/net/http/server.go:3340 +0x8e
net/http.(*conn).serve(0xc0000e4240, {0x929978, 0xc0003cf620})
	/usr/local/go/src/net/http/server.go:2109 +0x665
created by net/http.(*Server).Serve in goroutine 1
	/usr/local/go/src/net/http/server.go:3493 +0x485

After the change:

$ docker run --rm -p 8080:8080 custom-error-pages
2025/11/11 13:48:20 format not specified. Using text/html
2025/11/11 13:48:20 unexpected error opening file: open /www/0.html: no such file or directory
2025/11/11 13:48:20 unexpected error opening file: open /www/0xx.html: no such file or directory

If sending another code with curl -H "X-Code: 200" http://localhost:8080/.
Before:

$ docker run --rm -p 8080:8080 custom-error-pages
2025/11/11 13:57:25 format not specified. Using text/html
2025/11/11 13:57:25 unexpected error opening file: open /www/200.html: no such file or directory
2025/11/11 13:57:25 unexpected error opening file: open /www/2xx.html: no such file or directory
2025/11/11 13:57:25 http: superfluous response.WriteHeader call from main.errorHandler.func1 (main.go:170)

After the change:

$ docker run --rm -p 8080:8080 custom-error-pages
2025/11/11 13:58:45 format not specified. Using text/html
2025/11/11 13:58:45 unexpected error opening file: open /www/200.html: no such file or directory
2025/11/11 13:58:45 unexpected error opening file: open /www/2xx.html: no such file or directory

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added unit and/or e2e tests to cover my changes.
• All new and existing tests passed.

[netlify]

✅ Deploy Preview for kubernetes-ingress-nginx canceled.

Name	Link
🔨 Latest commit	3b235ed
🔍 Latest deploy log	https://app.netlify.com/projects/kubernetes-ingress-nginx/deploys/69134239e7630e000835eecb

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: jfyuen / name: Jean-François YUEN (363d0f9, 3b235ed, d81c055)

[k8s-ci-robot]

Welcome @jfyuen!

It looks like this is your first PR to kubernetes/ingress-nginx 🎉. Please refer to our pull request process documentation to help your PR have a smooth ride to approval.

You will be prompted by a bot to use commands during the review process. Do not be afraid to follow the prompts! It is okay to experiment. Here is the bot commands documentation.

You can also check if kubernetes/ingress-nginx has its own contribution guidelines.

You may want to refer to our testing guide if you run into trouble with your tests not passing.

If you are having difficulty getting your pull request seen, please follow the recommended escalation practices. Also, for tips and tricks in the contribution process you may want to read the Kubernetes contributor cheat sheet. We want to make sure your contribution gets all the attention it needs!

Thank you, and welcome to Kubernetes. 😃

[k8s-ci-robot]

Hi @jfyuen. Thanks for your PR.

I'm waiting for a github.com member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[Gacko]

/retitle Custom Error Pages: Do not write status code too soon.
/triage accepted
/kind bug
/priority backlog
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Gacko, jfyuen

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [Gacko]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[Gacko]

/cherry-pick release-1.14

[Gacko]

/cherry-pick release-1.13

[k8s-infra-cherrypick-robot]

@Gacko: once the present PR merges, I will cherry-pick it on top of release-1.14 in a new PR and assign it to you.

In response to this:

/cherry-pick release-1.14

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[k8s-infra-cherrypick-robot]

@Gacko: once the present PR merges, I will cherry-pick it on top of release-1.13 in a new PR and assign it to you.

In response to this:

/cherry-pick release-1.13

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[k8s-infra-cherrypick-robot]

@Gacko: new pull request created: #14162

In response to this:

/cherry-pick release-1.14

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[k8s-infra-cherrypick-robot]

@Gacko: new pull request created: #14163

In response to this:

/cherry-pick release-1.13

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.