Kerberos
Kerberos is a network authentication system. See krb5 documentation.
Installation
Install the krb5 package on your clients and server.
It is highly recommended to use a time synchronization daemon to keep client/server clocks in sync.
If hostname resolution has not been configured, you can manually add your clients and server to the hosts(5) file of each machine. Note that the FQDN (myclient.example.com) must be the first hostname after the IP address in the hosts file.
Server configuration
Domain creation
Edit /etc/krb5.conf to configure your domain:
/etc/krb5.conf
[libdefaults]
default_realm = EXAMPLE.COM
[realms]
EXAMPLE.COM = {
admin_server = kerberos.example.com
# use "kdc = ..." if the kerberos SRV records aren't in DNS (see Advanced section)
kdc = kerberos.example.com
# This breaks krb4 compatibility but increases security
default_principal_flags = +preauth
}
[domain_realm]
example.com = EXAMPLE.COM
.example.com = EXAMPLE.COM
[logging]
kdc = SYSLOG:NOTICE
admin_server = SYSLOG:NOTICE
default = SYSLOG:NOTICE
This file's format is described in the MIT Kerberos documentation
Create the database:
# kdb5_util -r EXAMPLE.COM create -s
Loading random data Initializing database '/var/lib/krb5kdc/principal' for realm 'EXAMPLE.COM', master key name 'K/M@EXAMPLE.COM' You will be prompted for the database Master Password. It is important that you NOT FORGET this password. Enter KDC database master key: *** Re-enter KDC database master key to verify: ***
Finally, enable and start the Kerberos services:
# systemctl enable --now krb5-kdc krb5-kadmind
Add principals
Start the Kerberos administration tool, using local authentication
# kadmin.local
Authenticating as principal root/admin@EXAMPLE.COM with password. kadmin.local:
Add a user principal to the Kerberos database:
kadmin.local: addprinc myuser@EXAMPLE.COM
WARNING: no policy specified for myuser@EXAMPLE.COM; defaulting to no policy Enter password for principal "myuser@EXAMPLE.COM": *** Re-enter password for principal "myuser@EXAMPLE.COM": *** Principal "myuser@EXAMPLE.COM" created.
Add the KDC principal to the Kerberos database:
kadmin.local: addprinc -randkey host/kerberos.example.com
WARNING: no policy specified for host/kerberos.example.com@EXAMPLE.COM; defaulting to no policy Principal "host/kerberos.example.com@EXAMPLE.COM" created.
Finally, Add the KDC principal to the server's keytab:
kadmin.local: ktadd host/kerberos.example.com
Entry for principal host/kerberos.example.com with kvno 2, encryption type aes256-cts-hmac-sha1-96 added to keytab FILE:/etc/krb5.keytab. Entry for principal host/kerberos.example.com with kvno 2, encryption type aes128-cts-hmac-sha1-96 added to keytab FILE:/etc/krb5.keytab.
Quit the Kerberos administration tool:
kadmin.local: quit
You should now be able to get a Kerberos ticket:
$ kinit
Password for myuser@EXAMPLE.COM: ***
$ klist
Ticket cache: FILE:/tmp/krb5cc_1000 Default principal: myuser@EXAMPLE.COM Valid starting Expires Service principal 08/30/2017 14:26:09 08/31/2017 14:26:09 krbtgt/EXAMPLE.COM@EXAMPLE.COM
Firewall
Add ALLOW rules to your firewall for any applicable ports/protocols:
- 88, TCP and UDP for Kerberos v5
- 749, TCP and UDP for kadmin if you plan to configure it
- 750, TCP and UDP for Kerberos v4 if you need backwards compatibility
DNS records
This is not necessary if you specify the kerberos and kadmin server in each machine's krb5.conf
db.example.com
kerberos.example.com. A 1.2.3.4 _kerberos.example.com. TXT "EXAMPLE.COM" _kerberos._udp.example.com. SRV 0 0 88 kerberos.example.com. _kerberos-adm._udp.example.com. SRV 0 0 749 kerberos.example.com.
Do not forget reverse DNS.
Client configuration
Edit the client's /etc/krb5.conf to match your server's configuration. You can copy this file from the server, or just set the required realm information.
Testing
You should now be able to get a Kerberos ticket on the client:
$ kinit
Password for myuser@EXAMPLE.COM: ***
$ klist
Ticket cache: FILE:/tmp/krb5cc_1000 Default principal: myuser@EXAMPLE.COM Valid starting Expires Service principal 08/30/2017 15:36:10 08/31/2017 15:36:10 krbtgt/EXAMPLE.COM@EXAMPLE.COM
Configuring kadmin
You will need /etc/krb5.conf configured on the kadmin client, and the server's firewall configured for kadmin.
Configuring kadmin ACL
Create a principal for administration:
kadmin.local: add_principal myuser/admin@EXAMPLE.COM
WARNING: no policy specified for myuser/admin@EXAMPLE.COM; defaulting to no policy Enter password for principal "myuser/admin@EXAMPLE.COM": *** Re-enter password for principal "myuser/admin@EXAMPLE.COM": *** Principal "myuser/admin@EXAMPLE.COM" created.
Add the user to the kadmin ACL file:
/var/lib/krb5kdc/kadm5.acl
myuser/admin@EXAMPLE.COM *
This file's format is described in the MIT Kerberos documentation
Configure kdc.conf:
/var/lib/krb5kdc/kdc.conf
[kdcdefaults]
kdc_ports = 750,88
[realms]
EXAMPLE.COM = {
database_name = /var/lib/krb5kdc/principal
acl_file = /var/lib/krb5kdc/kadm5.acl
key_stash_file = /var/lib/krb5kdc/.k5.EXAMPLE.COM
kdc_ports = 750,88
max_life = 10h 0m 0s
max_renewable_life = 7d 0h 0m 0s
}
This file's format is described in the MIT Kerberos documentation
Restart krb5-kdc.servce and krb5-kadmind.
You can now use kadmin as your own user, authenticating with kerberos:
$ kadmin
Authenticating as principal myuser/admin@EXAMPLE.COM with password. Password for myuser/admin@EXAMPLE.COM: *** kadmin:
Service principals and keytabs
First, ensure you have configured krb5.conf on all involved machines.
A kerberos principal has three components, formatted as `primary/instance@REALM`. For user principals, the primary is your username and the instance is omitted or is a role (eg. "admin"): `myuser@EXAMPLE.COM` or `myuser/admin@EXAMPLE.COM`. For hosts, the primary is "host" and the instance is the server FQDN: `host/myserver.example.com@EXAMPLE.COM`. For services, the primary is the service abbreviation and the instance is the FQDN: `nfs/myserver.example.com@EXAMPLE.COM`. The realm can often be omitted, the local computer's default realm is usually assumed.
With remote kadmin
This is the easier method, but requires you to have configured kadmin.
Open kadmin as root (so we can write the keytab) on the client, authenticating with your admin principal:
client# kadmin -p myuser/admin
Authenticating as principal myuser/admin with password. Password for myuser/admin@EXAMPLE.COM: kadmin:
Add a principal for any services you will be using, eg. "host" for SSH authentication or "nfs" for NFS:
kadmin: addprinc -randkey host/kbclient.example.com
WARNING: no policy specified for host/kbclient.example.com@EXAMPLE.COM; defaulting to no policy Principal "host/kbclient.example.com@EXAMPLE.COM" created.
Save each key to the local keytab:
kadmin: ktadd host/kbclient.example.com
Entry for principal host/kbclient.example.com with kvno 2, encryption type aes256-cts-hmac-sha1-96 added to keytab FILE:/etc/krb5.keytab. Entry for principal host/kbclient.example.com with kvno 2, encryption type aes128-cts-hmac-sha1-96 added to keytab FILE:/etc/krb5.keytab.
Without remote kadmin
Start kadmin on the Kerberos server, using either unix or kerberos authentication:
# kadmin.local
Authenticating as principal root/admin@EXAMPLE.COM with password. kadmin.local:
Add a principal for any services you will be using, eg. "host" for SSH authentication or "nfs" for NFS:
kadmin.local: addprinc -randkey host/kbclient.example.com
WARNING: no policy specified for host/kbclient.example.com@EXAMPLE.COM; defaulting to no policy Principal "host/kbclient.example.com@EXAMPLE.COM" created.
Save each key to a new keytab to be transferred to the client:
kadmin.local: ktadd -k kbclient.keytab host/kbclient.example.com
Entry for principal host/kbclient.example.com with kvno 2, encryption type aes256-cts-hmac-sha1-96 added to keytab FILE:/etc/krb5.keytab. Entry for principal host/kbclient.example.com with kvno 2, encryption type aes128-cts-hmac-sha1-96 added to keytab FILE:/etc/krb5.keytab.
Finally, copy kbclient.keytab from the server to the client using SCP or similar, then put it in place with correct permissions:
# install -b -o root -g root -m 600 kbclient.keytab /etc/krb5.keytab
Finally, delete kbclient.keytab from the server and client.
Cross-Realm Trust
Set up a second server as shown above, then create the cross-realm principal on both KDCs. Cross-realm principals must be created with strong passwords, not -randkey, and the same password must be used on both KDCs. The principal must have the same key version number (kvno) in both KDCs.
To grant EXAMPLE.COM principals access to EXAMPLE.ORG resources, you would use the following principal:
kadmin# addprinc krbtgt/EXAMPLE.ORG@EXAMPLE.COM
The [capaths] section of krb5.conf can be used to further control cross-realm trust relationships.
SSH authentication
Use the instructions in Service principals and keytabs to create a principal for the "host" service for both client and server, then put the client's keys in the client's keytab and the server's keys in the server's keytab.
Modify your SSH server configuration to enable GSSAPI authentication:
/etc/ssh/sshd_config
# GSSAPI Options GSSAPIAuthentication yes GSSAPICleanupCredentials yes
And modify your client configuration to send GSSAPI requests:
/etc/ssh/ssh_config
Host * GSSAPIAuthentication yes GSSAPIDelegateCredentials yes
Get a ticket-granting ticket on the client before using ssh:
$ kinit myuser@EXAMPLE.COM
Password for myuser@EXAMPLE.COM: ***
Pass the -v option to ssh to watch what's happening:
$ ssh sshserver.example.com -v
debug1: Authentications that can continue: publickey,gssapi-with-mic,password debug1: Next authentication method: gssapi-with-mic debug1: Delegating credentials debug1: Delegating credentials debug1: Authentication succeeded (gssapi-with-mic). Authenticated to sshserver.example.com ([192.168.100.136]:22). debug1: channel 0: new [client-session] debug1: Requesting no-more-sessions@openssh.com debug1: Entering interactive session. debug1: pledge: network debug1: client_input_global_request: rtype hostkeys-00@openssh.com want_reply 0 Last login: Wed Aug 30 15:52:41 2017 from 192.168.100.1
And you should now see a host ticket on the client:
client$ klist
Ticket cache: FILE:/tmp/krb5cc_1000 Default principal: myuser@EXAMPLE.COM Valid starting Expires Service principal 08/30/2017 15:37:40 08/31/2017 15:37:40 krbtgt/EXAMPLE.COM@EXAMPLE.COM 08/30/2017 15:53:04 08/31/2017 15:37:40 host/sshserver.example.com@EXAMPLE.COM
Authorize other principals
To allow a different kerberos principal to authenticate to a user account, add the principal name to the target account's .k5login file. For example, to allow robert@EXAMPLE.COM to SSH to alice's account:
/home/alice/.k5login
robert@EXAMPLE.COM
NFS security
First, configure your NFS server server. Also see NFS Troubleshooting. Configuring a time synchronization daemon on both the clients and the server is strongly recommended. Clock drift will cause this to break, and the error message will not be helpful.
Use the instructions in Service principals and keytabs to create a principal for the "nfs" service for both client and server, then put the client's keys in the client's keytab and the server's keys in the server's keytab.
NFS server
Add a Kerberos export option. Multiple options can be specified using a colon as a delimiter:
- sec=krb5 uses kerberos for authentication only, and transmits the data unauthenticated and unencrypted.
- sec=krb5i uses kerberos for authentication and integrity checking, but still transmits data unencrypted.
- sec=krb5p uses kerberos for authentication and encryption.
- sec=sys does not use kerberos
/etc/exports
/srv/export *(rw,async,no_subtree_check,no_root_squash,sec=krb5p:krb5)
And reload the exports:
# exportfs -arv
NFS client
Mount the exported directory:
# mount nfsserver:/srv/export /mnt/
You can add -vv for verbose information, and may need -t nfs4 and -o sec=krb5p or your chosen security option.
Check that it worked with the mount command:
mount | grep krb5
nfsserver:/srv/export on /mnt type nfs4 (rw,relatime,vers=4.1,rsize=131072,wsize=131072,namlen=255,hard,proto=tcp,port=0,timeo=600,retrans=2,sec=krb5,clientaddr=192.168.100.139,local_lock=none,addr=192.168.100.136)
Browsers
Some browsers have support for Kerberos protocol but disable it by default. Here are the instructions how to enable it:
Chromium
Chromium needs to be run with a command line parameter that specifies a list of sites where Kerberos authentication is allowed. The easiest way is to add persistent flag to the config file:
/etc/chromium/policies/managed/test_policy.json
{
"AuthServerWhitelist": "*.mycompany.com",
"DisableAuthNegotiateCnameLookup": true
}
Firefox
To configure Firefox with trusted sites visit about:config and set network.negotiate-auth.trusted-uris property to FOO.COM (Note: for Firefox there is no "*."; for Chrome, there is).
Troubleshooting
Cannot set GSSAPI authentication names
Cannot set GSSAPI authentication names, aborting
Your realm is missing either the kadmin/admin or kadmin/changepw principal.
For clients, invalid arguments/options may happen on first setup if rpc-gssd is not loaded. Loading it is usually acomplished by enabling and starting nfs-client.target, but after first setup this target will need a restart.
SSH authentication fails while connecting to a server requiring GSSAPI with KeyExchange
If any of the following errors are encountered:
$ ssh -v -o GSSAPIDelegateCredentials=yes -o GSSAPIAuthentication=yes <user>@<IP address> Unable to negotiate with <IP address> port 22: no matching key exchange method found. Their offer: gss-group14-sha1-...
$ ssh -v -o GSSAPIDelegateCredentials=yes -o GSSAPIKeyExchange=yes -o GSSAPIAuthentication=yes <user>@<IP address> command-line: line 0: Bad configuration option: gssapikeyexchange
it means that package openssh is not configured with GSSAPI patch for OpenSSH. You can install openssh-gssapiAUR or follow this method.