Docker Tutorial Troubleshooting

My new job doesn’t start for several days. And, while I am spending some of my time teaching and relaxing, I don’t want to be completely idle. So, I’ve been spending part of my days either learning new technologies or reacquainting myself with technologies I haven’t used for a bit.

Recently, I’ve been exploring Docker. I got the application installed and running. However, after running the getting started application (docker run -d -p 80:80 docker/getting-started), I kept getting the following error:

400 Bad Request
Request Header Or Cookie Too Large
----------------------------------
nginx/1.21.0

Well, at least I was getting some response. Since this was a newly installed container, I expected it to just work. Also, since I was accessing the container through a standard web browser, I didn’t expect the request header or the cookies to be too large.

I went down a small rabbit hole trying to debug the application. While I learned a bunch of new docker commands and tried my hand at debugging in a new environment, it didn’t resolve my issue.

Eventually, I launched a copy of Fiddler so that I could see the raw request. While the request header looked normal, it sent a bunch more cookies than I had expected for something running on localhost.

Of course, as soon as I saw the first cookie, I realized what had happened: I currently teach an ASP.NET MVC Core class. What this means is I run a lot of student applications. But, since all those applications run on localhost, I had tons of extraneous ASP.NET cookies. Once I removed all the cookies, the Docker tutorial worked as expected. 😀