Configuring Kerberos Network Application Servers

Network application servers are hosts that provide access using one or more of the following network applications: ftp, rcp, rlogin, rsh, ssh, and telnet. Only a few steps are required to enable the Kerberos version of these commands on a server.

How to Configure a Kerberos Network Application Server

This procedure uses the following configuration parameters:

Application server = boston
admin principal = kws/admin
DNS domain name = example.com
Realm name = EXAMPLE.COM

Before You Begin

This procedure requires that the master KDC has been configured. To fully test the process, several Kerberos clients must be configured.

Become superuser on the server.
(Optional) Install the NTP client or another clock synchronization mechanism.
See Synchronizing Clocks Between KDCs and Kerberos Clients for information about NTP.
Add principals for the new server and update the server's keytab file.
The following command reports the existence of the host principal:
```
boston # klist -k |grep host
4 host/boston.example.com@EXAMPLE.COM
4 host/boston.example.com@EXAMPLE.COM
4 host/boston.example.com@EXAMPLE.COM
4 host/boston.example.com@EXAMPLE.COM
```
If the command does not return a principal, then create new principals using the following steps.
How to use the GUI Kerberos Administration Tool to add a principal is explained in How to Create a New Kerberos Principal. The example in the following steps shows how to add the required principals using the command line. You must log in with one of the admin principal names that you created when configuring the master KDC.
```
boston # /usr/sbin/kadmin -p kws/admin
Enter password: <Type kws/admin password>
kadmin: 
```
1. Create the server's host principal.
  The host principal is used in the following ways:
  - To authenticate traffic when using the remote commands, such as rsh and ssh.
  - By pam_krb5 to prevent KDC spoofing attacks by using the host principal to verify that a user's Kerberos credential was obtained from a trusted KDC.
  - To allow the root user to automatically acquire a Kerberos credential without requiring that a root principal exist. This can be useful when doing a manual NFS mount where the share requires a Kerberos credential.
  This principal is required if traffic using the remote application is to be authenticated using the Kerberos service. If the server has multiple hostnames associated with it, then create a principal for each hostname using the FQDN form of the hostname.
```
kadmin: addprinc -randkey host/boston.example.com
Principal "host/boston.example.com" created.
kadmin: 
```
2. Add the server's host principal to the server's keytab file.
  If the kadmin command is not running, restart it with a command similar to the following: /usr/sbin/kadmin -p kws/admin
  If the server has multiple host names associated with it, then add a principal to the keytab for each hostname.
```
kadmin: ktadd host/boston.example.com
Entry for principal host/boston.example.com with kvno 3, encryption type AES-256 CTS mode
          with 96-bit SHA-1 HMAC added to keytab WRFILE:/etc/krb5/krb5.keytab.
Entry for principal host/boston.example.com with kvno 3, encryption type AES-128 CTS mode
          with 96-bit SHA-1 HMAC added to keytab WRFILE:/etc/krb5/krb5.keytab.
Entry for principal host/boston.example.com with kvno 3, encryption type Triple DES cbc
          mode with HMAC/sha1 added to keytab WRFILE:/etc/krb5/krb5.keytab.
Entry for principal host/boston.example.com with kvno 3, encryption type ArcFour
          with HMAC/md5 added to keytab WRFILE:/etc/krb5/krb5.keytab.
Entry for principal host/boston.example.com with kvno 3, encryption type DES cbc mode
          with RSA-MD5 added to keytab WRFILE:/etc/krb5/krb5.keytab.
kadmin:
```
3. Quit kadmin.
```
kadmin: quit
```

How to Use the Generic Security Service With Kerberos When Running FTP

The generic security service (GSS) can be used to applications to easily use Kerberos for authentication, integrity, and privacy. The following steps show how to enable the GSS service for ProFTPD.

Become superuser on the FTP server.
Add principals for the FTP server and update the server's keytab file.
These steps might not be needed if the changes were made earlier.
1. Start the kadmin command.
```
ftpserver1 # /usr/sbin/kadmin -p kws/admin
Enter password: <Type kws/admin password>
kadmin: 
```
2. Add the host service principal for the FTP server.
```
kadmin: addprinc -randkey host/ftpserver1.example.com 
```
3. Add the host service principal to the server's keytab file..
```
kadmin: ktadd host/ftpserver1.example.com 
```

Enable GSS for the FTP server.

Make the following changes to the /etc/proftpd.conf file.

# cat /etc/proftpd.conf
#User          ftp
#Group         ftp

User root
Group root

UseIPv6 off

LoadModule mod_gss.c

GSSEngine on
GSSKeytab /etc/krb5/krb5.keytab

Restart the FTP server.
```
# svcadm restart network/ftp
```

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