Can I get HaProxy to forward windows auth credentials onto IIS? I have searched for a day and cannot find much information out there.
What do you mean by "windows authentication" ? Is it NTLM auth over HTTP ? It's not very clear from your question. If it's NTLM auth, you need haproxy to work in tunnel mode because NTLM auth is not HTTP-compliant and authenticates the connection instead of the request. So you want haproxy to forward all traffic from the same connection to the same server. Using NTLM auth over proxies is dangerous BTW because you never know if proxies will multiplex connections or not, which could result in having multiple users browsing with the same account (haproxy does not do this right now).