How to implement GSS-TSIG on ISC BIND Written by Patrick H. Piper - Last Updated Thursday, 09 December 2010 07:48 The purpose of this article demonstrate how to get GSS-TSIG or secure dynamic updates working using ISC Bind DNS on a *NIX server. After several hours of trying to get this to work, perhaps this article would have been better named "GSS-TSIG on ISC Bind -- The Missing Manual". I know in working with others, we experienced many trials and tribulations in getting it all to work. GSS-TSIG DNS Updates or secure dynamic updates is an extension to TSIG based updates which implements secure key exchange. GSS API calls for the use of Kerberos for authentication, integrity and confidentiality by establishing a limited lifetime security context. Once the security context is established, special TKEY resource records are used to securely exchange key material between the DNS Server and DNS Client. GSS-TSIG support has been present in the ISC Bind code since version 9.5.0, circa mid-summer of 2008. In this HOW-TO, we compiled ISC Bind 9.7.1-P2 on Fedora 13 (32-bit) and used a single Microsoft Windows 2008 Server running as an Active Directory Domain Controller for example.com. Before we demonstrate secure dynamic updates, we must first address a "chicken-and-egg" issue. We need our Active Directory Domain Controller up and running prior so we can configure our AD user and Kerberos Service Principal. Prior to running dcpromo to promote our first AD Domain Controller or DC, we must have DNS up and running with dynamic DNS support. So, It is recommended that ISC Bind is built and configured to be authoritative for the AD Domain and support dynamic DNS updates using the allow-update directive by supplying the IP address of the AD DC. Building ISC Bind on Fedora, CentOS, or Red Hat Linux In this exercise, a minimal install of the Fedora 13 and/or CentOS 5.5 (32-bit) operating system was installed. Ensure that the following RPM dependencies openssl-devel, gcc, make, perl, krb5-workstation, and krb5-devel. This can be done using the yum pkg manager as follows: yum install openssl-devel yum install gcc make perl yum install krb5-workstation krb5-devel You will note that we did not specify libgssapi* modules. The reason for this is that the 1/6 How to implement GSS-TSIG on ISC BIND Written by Patrick H. Piper - Last Updated Thursday, 09 December 2010 07:48 GSS-API code is present and embedded in the krb5-workstation and krb5-devel library and development package. In our test, we used the latest version of ISC Bind (9.7.1-P2 was the current version at the time of this article). Unpack this version of bind and build as follows: ./configure --with-openssl --with-gssapi make && make install Some key assumptions: - Configure our DNS server, dns1.example.com to be authoritative for the zone example.com - Configure our DNS server, dns1.example.com to be authoritative for the reverse zone 168.192.in-addr.arpa - Configure the zone(s) example.com and 168.192.in-addr.arpa to be dynamically updatable via the allow-update directive using the IP address of the AD DC(s) so they can inject any/all AD-related DNS records. Once that is complete, our ISC Bind name server should accept insecure dynamic DNS updates from the Domain Controllers for the forward zone example.com and the reverse zone 168.192.in-addr.arpa. At this point it is time to bring up the AD Domain Controller. Active Directory Domain Controller Promotion With our Unix DNS server configured to support dynamic updates, we change our focus to the configuration of the AD Domain and DC itself. At this point, we can now run dcpromo.exe on our Windows Server 2008 to promote it to a Domain Controller or DC for example.com. Follow the wizard and at the conclusion of dcpromo.exe, make sure that all the AD related records were properly "injected" into our ISC Bind Server. Our DC will populate the BIND DNS server with the records that are contained in the %systemroot%system32confignetlogon.dns file. You can verify and validate that this occurred by either querying the name server for specific SRV, A, and CNAME records or by displaying the contents of the db.example.com.jnl file using the Bind-provided named-journalprint command. Create AD user & Service Principal Using the Microsoft Active Directory Users and Computers Microsoft Management Console or MMC, create an AD user called "dns1" in a zone such as example.com as shown in Fig. 1 - New User 2/6 How to implement GSS-TSIG on ISC BIND Written by Patrick H. Piper - Last Updated Thursday, 09 December 2010 07:48 below. NOTE: we chose the name "dns1" because it will represent the "instance" or hostname of the DNS server we plan to run ISC Bind on, i.e. dns1.example.com. You should plan to add an AD User for each DNS server that will perform GSS-TSIG secure dynamic updates. Click Next, and set & confirm a strong password. In this example, we chose to set a one-time fixed password so that we would not have to rebuild Kerberos keytab files with the new credentials. If you permit the password to change or require it to be rotated for security reasons, you will need to update the Kerberos keytab files that get configured on the remote DNS Servers. When done, click next to view the final user creation screen shown in Fig. 3. Click Finish to complete the task of creating our user. Next, we must create a Kerberos Service Principal Name or SPN, that is mapped to our Active Directory User dns1. This is accomplished using the ktpass.exe utility that is installed on modern Microsoft Windows 2008 Server Installations. If you are operating on Microsoft Windows 2003, you will need to install the Microsoft "Support Tools". You MUST make sure to install the same release of the Support Tools as the version of Windows Server you are operating. Run the ktpass.exe utility with the following flags and/or options: Parameter/Value Description -princ DNS/[email protected] This represents the Kerberos Service Principal Account for the <instance -mapuser [email protected] The Kerberos SPN is mapped to this AD User Account that was created -mapOp set Specifies how the mapping attribute is set +DesOnly Sets the encryption type to Des only -ptype KRB5_NT_PRINCIPAL This is the type of Kerberos Principal -pass Pa$$w0rd The password of the AD user [email protected] must be correctly pas -out dns1.keytab This option names the output file for ktpass.exe. The file that is outputted is 3/6 How to implement GSS-TSIG on ISC BIND Written by Patrick H. Piper - Last Updated Thursday, 09 December 2010 07:48 Fig. plan 1ktpass.exe command - New User file Fig. 2a -temporary Password generated Fig. 3 -on Finish as SPN many Fig. instances 4Bind - ktpass.exe usage The dns1.keytab you Linux Copy each hosts to keytab operate. that should were be In running tocopied our above testing, ISC to /tmp should Bind wedirectory on 9.7.2-P2, be dns1.example.com. executed dns1.example.com the two corresponding for such instance and DNS dns2.example.com. forserver or two Linux (2) instances different host, e.g. Configuring The Kerberos Client on Linux The first step is to ensure that the krb5-workstation utilities are properly installed, and that the location of the utilities are in your current PATH environment variable. In our case, we added /usr/kerberos/bin to our default path. Set up Kerberos with a config similar to that shown below: [logging] default = FILE:/var/log/krb5libs.log kdc = FILE:/var/log/krb5kdc.log admin_server = FILE:/var/log/kadmind.log [libdefaults] default_realm = EXAMPLE.COM default_tkt_enctypes = des-cbc-md5 default_tgs_enctypes = des-cbc-md5 dns_lookup_realm = true dns_lookup_kdc = true ticket_lifetime = 30d forwardable = yes default_keytab_name = FILE:/etc/krb5.keytab [realms] EXAMPLE.COM = { kdc = dc1.example.com:88 admin_server = dc1.example.com:749 default_domain = example.com } [domain_realm] .example.com = EXAMPLE.COM example.com = EXAMPLE.COM [appdefaults] pam = { debug = false ticket_lifetime = 36000 renew_lifetime = 36000 forwardable = true krb4_convert = false } Run the ktutil command to read in the freshly copied keytab file, and write out the full contents to our target keytab file as follows: [root@dns1 ~]# ktutil ktutil: rkt /tmp/dns1.keytab ktutil: list slot KVNO Principal ---- ------------------------------------------------------------------------ 1 3 DNS/[email protected] ktutil: wkt /etc/krb5.keytab ktutil: quit [root@dns1 4/6 How to implement GSS-TSIG on ISC BIND Written by Patrick H. Piper - Last Updated Thursday, 09 December 2010 07:48 ~]# NOTE: If you are running ISC Bind as a lesser privileged user than the root user, you MUST set new ownership on the /etc/krb5.keytab file to the user that is running Bind. In our testing, we set the owner to 'named'.Now, that our keytab file is in place, and our krb5.conf file is set up, you can initiate Kerberos authentication in obtaining Kerberos Tickets from the AD Server. This is done via the following command: /usr/kerberos/bin/kinit -k -t /etc/krb5.keytab DNS/dns1.example.com If there is no output and/or the return value $? is 0, then the command completed without any errors. You can now view the Kerberos tickets that have been granted by: [root@dns1 ~]# klist Ticket cache: FILE:/tmp/krb5cc_0 Default principal: DNS/[email protected] Valid starting Expires Service principal 10/02/10 11:12:58 10/02/10 21:12:54 krbtgt/[email protected] renew until 10/09/10 11:12:58 Kerberos 4 ticket cache: /tmp/tkt0 klist: You have no tickets cached At this point, you should now have a working Kerberos Workstation or client that can authenticate and obtain Kerberos Tickets via the AD Domain Controller. If you run kdestroy, this command will remove any cached tickets from the Kerberos Client. Remove these cached tickets so that you don't have any cached tickets that will expire. Once that is done, the named process should renew tickets as needed to remain authenticated and capable of performing secure dynamic updates. Configuring ISC Bind to Perform GSS-TSIG Secure Dynamic Updates Previously, we had configured our DNS server to support dynamic DNS updates in the clear (insecure) by using the allow-update directive. Now that our server has been "Kerberized", you can change the configuration of the name server to accept only secure dynamic updates by tweaking the server's global and zone configuration(s). In the global options block of the named.conf file, you will need to add the tkey-gssapi-cred ential and tkey-d omain directives. The following was used for the example.com domain name server dns1.example.com: tkey-gssapi-credential "DNS/[email protected]"; tkey-domain "EXAMPLE.COM"; It was during the next part of the configuration, that we experienced difficulty in getting our configuration to work. Since we are configuring secure dynamic updates on two zone(s), example.com, and 168.192.in-addr.arpa, we first needed to remove the allow-update { 5/6 How to implement GSS-TSIG on ISC BIND Written by Patrick H. Piper - Last Updated Thursday, 09 December 2010 07:48 update_acl; }; because that directive is based upon IP access lists and is associated with insecure updates. The newer update-policy directive is what should be used to configure zones to support secure dynamic updates. We struggled with the documentation and exact syntax of this policy while trying to get GSS-TSIG working. The following syntax was used to get things working for our two (2) zones: zone "example.com" { type master; file "db.example.com"; check-names ignore; allow-transfer { localhost; 192.168.0.0/16; }; update-policy { grant * subdomain example.com. ANY; }; }; zone "168.192.in-addr.arpa" { type master; file "db.168.192.in-addr.arpa"; check-names ignore; allow-transfer { localhost; 192.168.0.0/16; }; update-policy { grant * subdomain 168.192.in-addr.arpa. PTR TXT; }; }; While the above configuration allowed our ISC Bind Server to handle GSS-TSIG secure dynamic updates, it is far less than optimal from a security perspective. With all that said, how do we really know it's working? During testing, it's best to start named at a low debugging level so that log output shows details and evidence of the GSS-TSIG signed updates. Running ISC Bind in Debug level 6 will output logged messages as shown in gss-tsig.log . This is the first part of a two (2) article series on GSS-TSIG Updates using ISC Bind. Part 2 - will cover some of the "gotchas", exceptions, and provide some additional recipes for a more secure update-policy usage. 6/6
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