Yeps. This is KNOWN_BUG #10 unfortunately. There was previously some work on fixing this posted to curl-library, but those patches never got merged and I've not seen any updates recently. We'll happily accept patches that fix this situation.

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** [bugs:#1260] --proxy-negotiate ignored without --proxy-user**
**Status:** pending
**Labels:** negotiate proxy authentication 
**Created:** Wed Jul 31, 2013 07:35 AM UTC by Alan
**Last Updated:** Wed Jul 31, 2013 07:35 AM UTC
**Owner:** Daniel Stenberg
Curl doesn't try to use Kerberos (negotiate) authentication with the proxy unless I specify a bogus --proxy-user parameter.
curl -V output:
curl 7.31.0 (x86_64-unknown-linux-gnu) libcurl/7.31.0 OpenSSL/1.0.1e zlib/1.2.8                         
Protocols: dict file ftp ftps gopher http https imap imaps ldap ldaps pop3 pop3s rtsp smtp smtps telnet tftp 
Features: AsynchDNS GSS-Negotiate IPv6 Largefile NTLM NTLM_WB SSL libz TLS-SRP                                                               
This doesn't work:
curl -x http://myproxy:8080 --proxy-negotiate http://myurl/
This does:
curl -x http://myproxy:8080 --proxy-negotiate --proxy-user fakeuser:fakepass http://myurl/
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