HTTP Diagnosis

Goal

You read an HTTP response along four axes, status code, headers, timing, and body, and tie it together into a health check script.

Why it matters

What is needed when you receive a report that "responses are slow" is not more dashboards but splitting the time of a single request by segment. The timing variables of curl -w tell you name resolution / TCP connection / TLS / server first byte / body transfer separately. If everything is normal up to time_connect and only time_starttransfer is large, the network is innocent and the application is the culprit - the debate ends right there. And a health check must always have a maximum wait time. Without it, the script stops forever when the other side does not respond, and if cron launches it every 5 minutes, processes pile up.

Steps

Use /root/www as the document root and /root/http as the working directory.

1. Create /root/www/index.html containing labhub-http-ok, start python3 -m http.server 8000 --bind 127.0.0.1 --directory /root/www in the background, and write its PID to /root/http/server_pid.txt.
2. Write only the number of the status code for a request to a nonexistent path to /root/http/status404.txt.
3. Receive only the response headers of /index.html and save them to /root/http/headers.txt. The body must not be mixed in.
4. Save the three values time_namelookup, time_connect, and time_total of the request on one line separated by spaces to /root/http/timing.txt.
5. Create /root/www/api.json. name must be labhub, and tags must be an array with two or more values. Then fetch that file from the server and write the second value of tags to /root/http/tag.txt.
6. Create the directory /root/www/sub and put one file in it. Write the status codes of requesting http://127.0.0.1:8000/sub without following redirects and with following them to /root/http/redirect.txt as the following two lines.
   • no_follow=<코드> (the code)
   • follow=<코드> (the code)
7. Write the status code for sending a POST request to /root/http/method.txt in the form post=<코드> (the code goes where the Korean word is).
8. Create /root/http/healthcheck.sh <URL>.
   • If 2xx, print ok code=<코드> time=<초> (the code and the seconds) and exit with code 0
   • If a response came but it is not 2xx, print down code=<코드> (the code) and exit with code 1
   • If the connection itself fails, print a message starting with down and exit with code 1. You must decide within 10 seconds.

Notes

• Extract only the status code with curl -s -o /dev/null -w '%{http_code}' URL. On connection failure, 000 comes out.
• curl -sI URL receives only the headers with a HEAD request.
• Set the maximum wait time with curl --max-time 5.
• Common mistake 1: if you get 200 even without -L in step 6, you requested it with a trailing slash. You must request it without the slash to get a 301.
• Common mistake 2: in step 8, if you compare 000 only as a number, you cannot tell a connection failure from normal.

Start a local HTTP server

Create /root/www/index.html containing labhub-http-ok, start python3 -m http.server 8000 --bind 127.0.0.1 --directory /root/www in the background, and write its PID to /root/http/server_pid.txt.

python3 -m http.server has an option to specify the document root. Bind it only to 127.0.0.1.

Extract only the status code

Write only the number of the status code for a request to a nonexistent path to /root/http/status404.txt.

There is a -w variable that discards the body and prints only the status code.

Receive only headers

Receive only the response headers of /index.html and save them to /root/http/headers.txt. The body must not be mixed in.

It is the option that sends a HEAD request. The body must not be mixed into the file.

Measure the time per segment

Save the three values time_namelookup, time_connect, and time_total of the request on one line separated by spaces to /root/http/timing.txt.

Print the three of -w's time_namelookup, time_connect, and time_total on one line separated by spaces.

Extract a value from a JSON response

Create /root/www/api.json. name must be labhub, and tags must be an array with two or more values. Then fetch that file from the server and write the second value of tags to /root/http/tag.txt.

Pass curl's output to jq. Array indexes start at 0.

Follow a redirect

Create the directory /root/www/sub and put one file in it. Write the status codes of requesting http://127.0.0.1:8000/sub without following redirects and with following them to /root/http/redirect.txt as the following two lines.

• no_follow=<코드> (the code)
• follow=<코드> (the code)

By default curl does not follow redirects. If a directory path has no trailing slash, the server returns a 301.

Response for an unimplemented method

Write the status code for sending a POST request to /root/http/method.txt in the form post=<코드> (the code goes where the Korean word is).

Specify the method with -X. This server implements only GET and HEAD.

Health check script

Create /root/http/healthcheck.sh <URL>.

• If 2xx, print ok code=<코드> time=<초> (the code and the seconds) and exit with code 0
• If a response came but it is not 2xx, print down code=<코드> (the code) and exit with code 1
• If the connection itself fails, print a message starting with down and exit with code 1. You must decide within 10 seconds.

You must distinguish three cases: healthy, non-2xx, and unreachable. Always set a maximum wait time.