Jira 5.2 Seraph SSO Login behind reverse proxy

Since a few days I'm trying to enable SSO for Jira 5.2 and figured out, that the help page from Jira is outdated.

Each example uses an old version of atlassian-seraph (Jira 5.2 uses 2.6.0).

Goal: I want to get automatically logged in into Jira if I'm logged in into Webseal (reverse proxy).

Background:

• Jira is behind a reverse proxy (see picture).
• This proxy authentificatates the user and holds the session.
• If I'm logged in I want to be logged in in Jira, too
• The only information provided is the user name

Question:

How to write a custom login module that reads the username from http_header and authentificates the user?

Links:

• https://confluence.atlassian.com/display/DEV/Single+Sign-on+Integration+with+JIRA+and+Confluence
• http://docs.atlassian.com/atlassian-seraph/latest/sso.html
• https://answers.atlassian.com/questions/23245/how-to-integrate-jira-with-my-company-s-sso