Recently I was engaged on this TechNet Forum thread and I thought that it was an interesting question and concern. Let me explain the scenario on which I was able to repro the behavior on my own lab:
Contoso has a security policy where only users that belong to the Internet Users group can access Internet. To enforce that they created a rule on ISA Server on which only members of the Internet Group can access Internet. Contoso uses Internet Explorer 6 on all workstations of their network. The process today is that once the user receives authorization to access Internet, the AD Admin adds the user to Internet Users group and the user just need to open a new browser session and he will have access. Recently Contoso upgraded 50% of their workstations to use Internet Explorer 8, they notice the following behavior since this change: if an user that doesn’t belong to Internet Users group is already logged on the workstation and the administrator add this user to the Internet User’s group, the user receives an error when try to browse Internet, the error says that ISA denied the request. If the user logs off from the windows and logon again it works fine.
Why this is happening?
What it is happening on this case is simply the way that Internet Explorer 8 works for the authentication part of the conversation. Since the version 7, Internet Explorer was capable to use Kerberos for proxy authentication (while IE6 and lower only use NTLM and Basic). ISA Server will negotiate the authentication with the browser and will authenticate according to the method that is supported on both sides. The difference in this case is the following:
With IE7 (and higher)
1. Client sends the GET request to www.microsoft.com (for example). This request goes as anonymous.
2. ISA will send the 407 asking for authentication.
3. If client has already a Kerberos cached ticket it will not go to the DC to get a new one, if there is no ticket then it will go to the DC to get an updated ticket to send to ISA.
4. Client will send another GET Request now with the credentials and the ticket.
5. ISA will verify the request and allow (or deny according to the rule).
Note: in this case since ISA doesn't go to the DC, it relies on user's token to access the resource. Since the current ticket doesn’t include the update on user’s group membership, ISA will verify that the user doesn't below to the group and will deny the request.
3. Client sends another GET request with the credentials (NTLM).
4. ISA goes to the DC to authenticate the user.
Note: since ISA goes to the DC it will get an updated version of the user's group membership and will verify that the user now belongs to this group.
5. ISA will allow the user to pass through.
Under the Hood
Prior to get to this conclusion I decided to do some tests to validate the theory and the tests worked pretty fine. Here it is the traffic from the client workstation with the user “Yuri” logged in, right after the inclusion of the user to Internet User’s group:
1. Client sends the GET Request
10.20.20.201 10.20.20.1 HTTP HTTP:Request, GET http://www.microsoft.com/
2. ISA asks for authentication:
10.20.20.1 10.20.20.201 HTTP HTTP:Response, HTTP/1.1, Status Code = 407, URL: http://www.microsoft.com/
StatusCode: 407, Proxy authentication required
Reason: Proxy Authentication Required ( The ISA Server requires authorization to fulfill the request. Access to the Web Proxy filter is denied. )
Via: 1.1 ISACONTN1
+ ProxyAuthenticate: Negotiate
+ ProxyAuthenticate: Kerberos
+ ProxyAuthenticate: NTLM
+ ContentType: text/html
+ payload: HttpContentType = text/html
Note: Notice that ISA is negotiating the authentication method.
3. Client sends another GET request now with the credential.
10.20.20.201 10.20.20.1 HTTP HTTP:Request, GET http://www.microsoft.com/ , Using SPNEGO Authorization
+ URI: http://www.microsoft.com/
UserAgent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; .NET CLR 1.1.4322)
Accept-Encoding: gzip, deflate
Cookie: MUID=42623805587D4F6EB894604864D99E47; WT_FPC=id=220.127.116.11-401582496.30090674:lv=1279361835221:ss=1279361835221; MC1=GUID=ba325a1581e1aa4dbcd0988320053feb&HASH=155a&LV=20107&V=3; A=I&I=AxUFAAAAAABDBgAAgb+LoGxG8UwjOO2L0QId7Q!!
- ProxyAuthorization: Negotiate
- Authorization: Negotiate YIIE5QYGKwYBBQUCoIIE2TCCBNWgJDAiBgkqhkiC9xIBAgIGCSqGSIb3EgECAgYKKwYBBAGCNwICCqKCBKsEggSnYIIEowYJKoZIhvcSAQICAQBuggSSMIIEjqADAgEFoQMCAQ6iBwMFACAAAACjggO7YYIDtzCCA7OgAwIBBaEOGwxDT05UT1NPLk1TRlSiKTAnoAMCAQKhIDAeGwRIVFRQGxZpc2Fjb250bjEuY
- GssapiKrb5: 0x1
Note: Notice that client is using Kerberos.
4. This ticket doesn’t contain an updated information about user’s group membership, hence ISA Server sends a deny.
10.20.20.1 10.20.20.201 HTTP HTTP:Response, HTTP/1.1, Status Code = 502, URL: http://www.microsoft.com/
StatusCode: 502, Bad gateway
Reason: Proxy Error ( The ISA Server denied the specified Uniform Resource Locator (URL). )
5. At this point, I ran the command klist on the workstation and here are the tickets that I had at that point:
C:\Program Files\Windows Resource Kits\Tools>klist tickets
Cached Tickets: (3)
KerbTicket Encryption Type: RSADSI RC4-HMAC(NT)
End Time: 7/17/2010 18:34:46
Renew Time: 7/24/2010 8:34:46
6. I thought to myself: well, if I wipe all the tickets I will force the user to go to the DC and get an updated ticket, this way if I try again to access Internet, I will use the updated ticket. To do that I ran the command: klist purge. I deleted all tickets and the end result was:
C:\Program Files\Windows Resource Kits\Tools>klist purge
Cached Tickets: (0)
Now is time for the truth, tried to access Internet again and then it WORKED. Here it is the netmon trace for this second attempt with my Kerberos ticket cache clean:
3. Client goes to the DC to get ticket for the user “Yuri”:
10.20.20.201 10.20.20.20 KerberosV5 KerberosV5:AS Request Cname: yuri Realm: CONTOSO.MSFT Sname: krbtgt/CONTOSO.MSFT
- Kerberos: AS Request Cname: yuri Realm: CONTOSO.MSFT Sname: krbtgt/CONTOSO.MSFT
- AsReq: Kerberos AS Request
+ KdcReq: KRB_AS_REQ (10)
4. The DC responds:
10.20.20.20 10.20.20.201 KerberosV5 KerberosV5:AS Response Ticket[Realm: CONTOSO.MSFT, Sname: krbtgt/CONTOSO.MSFT]
- Kerberos: AS Response Ticket[Realm: CONTOSO.MSFT, Sname: krbtgt/CONTOSO.MSFT]
- AsRep: Kerberos AS Response
+ KdcRep: KRB_AS_REP (11)
5. Client goes again to the DC to get a ticket to the HTTP Service (ISA SPN):
10.20.20.201 10.20.20.20 KerberosV5 KerberosV5:TGS Request Realm: CONTOSO.MSFT Sname: HTTP/isacontn1.contoso.msft
- Kerberos: TGS Request Realm: CONTOSO.MSFT Sname: HTTP/isacontn1.contoso.msft
- TgsReq: Kerberos TGS Request
+ KdcReq: KRB_TGS_REQ (12)
6. The DC responds:
10.20.20.20 10.20.20.201 KerberosV5 KerberosV5:TGS Response Cname: Yuri
- Kerberos: TGS Response Cname: Yuri
- TgsRep: Kerberos TGS Response
- KdcRep: KRB_TGS_REP (13)
+ PvNo: 5
+ MsgType: KRB_TGS_REP (13)
+ Crealm: CONTOSO.MSFT
+ Cname: Yuri
+ Ticket: Realm: CONTOSO.MSFT, Sname: HTTP/isacontn1.contoso.msft
7. Client sends the GET request to ISA now with the new ticket:
- Authorization: Negotiate YIIFDQYGKwYBBQUCoIIFATCCBP2gJDAiBgkqhkiC9xIBAgIGCSqGSIb3EgECAgYKKwYBBAGCNwICCqKCBNMEggTPYIIEywYJKoZIhvcSAQICAQBuggS6MIIEtqADAgEFoQMCAQ6iBwMFACAAAACjggPjYYID3zCCA9ugAwIBBaEOGwxDT05UT1NPLk1TRlSiKTAnoAMCAQKhIDAeGwRIVFRQGxZpc2Fjb250bjEuY
8. ISA allows the traffic:
10.20.20.1 10.20.20.201 HTTP HTTP:Response, HTTP/1.1, Status Code = 302, URL: http://www.microsoft.com/
As you could see on this post this is not really an issue, neither on ISA nor on Internet Explorer, this is actually a behavioral change that Internet Explorer 7 and higher had. This is a good improvement, because Kerberos is much lighter and doesn’t put too much pressure on the DC. One classic problem with NTLM is that the authentication request against the DC is huge, read the article Improving Web Proxy Client Authentication Performance on ISA Server 2006 that I wrote couple of years ago and you will see the advantage that IE7 has over IE6.
But, if it is a requirement for your business that changes to group membership immediately reflect on user’s Internet browsing experience than you can use ISA’s IP address on the IE setting rather than the name, this way you are forcing to use NTLM rather than Kerberos. Just keep in mind that when too much pressure is added on the DC you can face the infamous “random prompt for authentication” on ISA due the 5783 and 5719 events.