Firewall and User-Agent

Why it is hard

Sometimes login works and TLS passes, yet requests fail oddly. The cause is the User-Agent request header. Two behaviors were found in the portal flow.

Situation	Result
No `User-Agent` sent	`classic.sejong.ac.kr` answers 404 after SSO
A library default (for example `Python-urllib`)	The firewall of `sjpt.sejong.ac.kr` blocks it and returns an “Alert!!!” HTML page

Your code is correct in both cases and only the response is strange, so it is easy to blame parsing or the login.

What the library does

It sends a non-empty, non-default, identifying User-Agent on every request. The format fixed in the contract is:

pro-sejongpeia-<language>/<version>

Python: pro-sejongpeia-python/<version>
Node.js: pro-sejongpeia-node/<version>
Java public data calls: pro-sejongpeia-java
Authentication requests follow the format above.

If you implement it yourself

Set a non-empty, distinctive User-Agent on every HTTP client.
Rather than imitating a browser, leave the library name and version visible. It makes finding the cause easier for both sides when something goes wrong.
When you get a 404 or an “Alert!!!” page, suspect the header before the parser.

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