[squid-users] proxy-connection question

I have a setup where I authenticate to squid with Negotiate and then I
authenticate to an apache web server with Negotiate.

The problem I have is that to a web server running on Solaris it works, but
to another on Linux it doesn't. The only difference I can see is that when I
go to the Solaris web server I see a Proxy-connection: keep-alive header
whereas with the Linux server I get Proxy-connection: close

What/Who determines the selection of close or keep-alive ?

Thank you
Markus