Explicit proxy authentication

FortiGate supports multiple authentication methods. This topic explains using an external authentication server with Kerberos as the primary and NTLM as the fallback.

To configure Explicit Proxy with authentication:

1. Enable and configure the explicit proxy.
2. Configure the authentication server and create user groups.
3. Create an authentication scheme and rules.
4. Create an explicit proxy policy and assign a user group to the policy.
5. Verify the configuration.

Enable and configure the explicit proxy

To enable and configure explicit web proxy in the GUI:

1. Go to Network > Explicit Proxy.

2. Enable Explicit Web Proxy.

3. Select port2 as the Listen on Interfaces and set the HTTP Port to 8080.

4. Configure the remaining settings as needed.

5. Click Apply.

To enable and configure explicit web proxy in the CLI:

config web-proxy explicit
    set status enable
    set ftp-over-http enable
    set socks enable
    set http-incoming-port 8080
    set ipv6-status enable
    set unknown-http-version best-effort
end
config system interface
    edit "port2"
        set vdom "vdom1"
        set ip 10.1.100.1 255.255.255.0
        set allowaccess ping https ssh snmp http telnet
        set type physical
        set explicit-web-proxy enable
        set snmp-index 12
        end
    next
end

Configure the authentication server and create user groups

Since we are using an external authentication server with Kerberos authentication as the primary and NTLM as the fallback, Kerberos authentication is configured first and then FSSO NTLM authentication is configured.

For successful authorization, the FortiGate checks if user belongs to one of the groups that is permitted in the security policy.

When configuring an LDAP connection to an Active Directory server, an administrator must provide Active Directory user credentials. To secure this connection, use LDAPS on both the Active Directory server and FortiGate. See Configuring an LDAP server. Apply the principle of least privilege. For the LDAP regular bind operation, do not use credentials that provide full administrative access to the Windows server when using credentials. See Configuring least privileges for LDAP admin account authentication in Active Directory.

To configure an authentication server and create user groups in the GUI:

1. Configure Kerberos authentication:

   1. Go to User & Authentication > LDAP Servers.

   2. Click Create New.

   3. Set the following:

      Name	ldap-kerberos
      Server IP	172.18.62.220
      Server Port	389
      Common Name Identifier	cn
      Distinguished Name	dc=fortinetqa,dc=local

   4. Click OK

2. Define Kerberos as an authentication service. This option is only available in the CLI. For information on generating a keytab, see Generating a keytab on a Windows server.

3. Configure FSSO NTLM authentication:

   FSSO NTLM authentication is supported in a Windows AD network. FSSO can also provide NTLM authentication service to the FortiGate unit. When a user makes a request that requires authentication, the FortiGate initiates NTLM negotiation with the client browser, but does not process the NTLM packets itself. Instead, it forwards all the NTLM packets to the FSSO service for processing.

   1. Go to Security Fabric > External Connectors.

   2. Click Create New and select FSSO Agent on Windows AD from the Endpoint/Identity category.

   3. Set the Name to FSSO, Primary FSSO Agent to 172.16.200.220, and enter a password.

   4. Click OK.

4. Create a user group for Kerberos authentication:

   1. Go to User & Authentication > User Groups.

   2. Click Create New.

   3. Set the Name to Ldap-Group, and Type to Firewall.

   4. In the Remote Groups table, click Add, and set the Remote Server to the previously created ldap-kerberos server.

   5. Click OK.

5. Create a user group for NTLM authentication:

   1. Go to User & Authentication > User Groups.

   2. Click Create New.

   3. Set the Name to NTLM-FSSO-Group, Type to Fortinet Single Sign-On (FSSO), and add FORTINETQA/FSSO as a member.

   4. Click OK.

To configure an authentication server and create user groups in the CLI:

1. Configure Kerberos authentication:

   config user ldap
       edit "ldap-kerberos"
           set server "172.18.62.220"
           set cnid "cn"
           set dn "dc=fortinetqa,dc=local"
           set type regular
           set username "CN=root,CN=Users,DC=fortinetqa,DC=local"
           set password *********
       next
   end

2. Define Kerberos as an authentication service:

   config user krb-keytab
       edit "http_service"
           set pac-data disable
           set principal "HTTP/FGT.FORTINETQA.LOCAL@FORTINETQA.LOCAL"
           set ldap-server "ldap-kerberos"
           set keytab "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"
       next
   end

   For information on generating a keytab, see Generating a keytab on a Windows server.

3. Configure FSSO NTLM authentication:

   config user fsso
       edit "1"
           set server "172.18.62.220"
           set password *********
       next
   end

4. Create a user group for Kerberos authentication:

   config user group
       edit "Ldap-Group"
           set member "ldap" "ldap-kerberos"
       next
   end

5. Create a user group for NTLM authentication:

   config user group
       edit "NTLM-FSSO-Group"
           set group-type fsso-service
           set member "FORTINETQA/FSSO"
       next
   end

Create an authentication scheme and rules

Explicit proxy authentication is managed by authentication schemes and rules. An authentication scheme must be created first, and then the authentication rule.

To create an authentication scheme and rules in the GUI:

1. Create an authentication scheme:

   1. Go to Policy & Objects > Authentication Rules.

   2. Click Create New > Authentication Schemes.

   3. Set the Name to Auth-scheme-Negotiate and select Negotiate as the Method.

   4. Click OK.

2. Create an authentication rule:

   1. Go to Policy & Objects > Authentication Rules.

   2. Click Create New > Authentication Rules.

   3. Set the Name to Auth-Rule, Source Address to all, and Protocol to HTTP.

   4. Enable Authentication Scheme, and select the just created Auth-scheme-Negotiate scheme.

   5. Click OK.

To create an authentication scheme and rules in the CLI:

1. Create an authentication scheme:

   config authentication scheme
       edit "Auth-scheme-Negotiate"
           set method negotiate       <<< Accepts both Kerberos and NTLM as fallback
       next
   end

2. Create an authentication rule:

   config authentication rule
       edit "Auth-Rule"
           set status enable
           set protocol http
           set srcaddr "all"
           set ip-based enable
           set active-auth-method "Auth-scheme-Negotiate"
           set comments "Testing"
       next
   end

Create an explicit proxy policy and assign a user group to the policy

To create an explicit proxy policy and assign a user group to it in the GUI:

1. Go to Policy & Objects > Proxy Policy.

2. Click Create New.

3. Set Proxy Type to Explicit Web and Outgoing Interface to port1.

4. Set Source to all, and the just created user groups NTLM-FSSO-Group and Ldap-Group.

5. Also set Destination to all, Schedule to always, Service to webproxy, and Action to ACCEPT.

6. Click OK.

To create an explicit proxy policy and assign a user group to it in the CLI:

config firewall proxy-policy
    edit 1
        set proxy explicit-web
        set dstintf "port1"
        set srcaddr "all"
        set dstaddr "all"
        set service "web"
        set action accept
        set schedule "always"
        set logtraffic all
        set groups "NTLM-FSSO-Group" "Ldap-Group"
        set av-profile "av"
        set ssl-ssh-profile "deep-custom"
    next
end

Verify the configuration

Log in using a domain and system that would be authenticated using the Kerberos server, then enter the diagnose wad user list CLI command to verify:

# diagnose wad user list
ID: 8, IP: 10.1.100.71, VDOM: vdom1
  user name   : test1@FORTINETQA.LOCAL
  duration    : 389
  auth_type   : IP
  auth_method : Negotiate
  pol_id      : 1
  g_id        : 1
  user_based  : 0
  expire      : no
  LAN:
    bytes_in=4862 bytes_out=11893
  WAN:
    bytes_in=7844 bytes_out=1023

Log in using a system that is not part of the domain. The NTLM fallback server should be used:

# diagnose wad user list
ID: 2, IP: 10.1.100.202, VDOM: vdom1
  user name   : TEST31@FORTINETQA
  duration    : 7
  auth_type   : IP
  auth_method : NTLM
  pol_id      : 1
  g_id        : 5
  user_based  : 0
  expire      : no
  LAN:
    bytes_in=6156 bytes_out=16149
  WAN:
    bytes_in=7618 bytes_out=1917

Generating a keytab on a Windows server

A keytab is used to allow services that are not running Windows to be configured with service instance accounts in the Active Directory Domain Service (AD DS). This allows Kerberos clients to authenticate to the service through Windows Key Distribution Centers (KDCs).

For an explanation of the process, see https://docs.microsoft.com/en-us/windows-server/administration/windows-commands/ktpass.

To generate a keytab on a Windows server:

1. On the server, create a user for the FortiGate:

   • The service name is the FQDN for the explicit proxy interface, such as the hostname in the client browser proxy configuration. In this example, the service name is FGT.

   • The account only requires domain users membership.

   • The password must be very strong.

   • The password is set to never expire.

2. Add the FortiGate FQDN in to the Windows DNS domain, as well as in-addr.arpa.

3. Generate the Kerberos keytab using the ktpass command on Windows servers and many domain workstations:

   # ktpass -princ HTTP/<domain name of test fgt>@realm -mapuser <user> -pass <password> -crypto all -ptype KRB5_NT_PRINCIPAL -out fgt.keytab

   For example:

   ktpass -princ HTTP/FGT.FORTINETQA.LOCAL@FORTINETQA.LOCAL -mapuser FGT -pass *********** -crypto all -ptype KRB5_NT_PRINCIPAL -out fgt.keytab

   If the FortiGate is handling multiple keytabs in Kerberos authentication, use different passwords when generating each keytab.

4. Encode the keytab to base64 in a text file:

   • On Windows: certutil -encode fgt.keytab tmp.b64 && findstr /v /c:- tmp.b64 > fgt.txt

   • On Linux: base64 fgt.keytab > fgt.txt

   • On MacOS: base64 -i fgt.keytab -o fgt.txt

5. Use the code in fgt.txt as the keytab parameter when configuring the FortiGate.