Fix grub boot loader - Debian 7

Boot from CDROM, choose Rescue Mode and Advanced options. When prompted, request a shell on the root partition With the grub tools (apt-get install grub if tools are missing):

grub-mkdevicemap
grub-install /dev/sda
update-grub

Reboot

Start service at boot (autostart) - CentOS 6

chkconf is the command, for example auto start Apache at boot:

chkconfig httpd on

Fixing deleted /var/run link and restoring SELinux permissions - CentOS 7

So some idiot with sudo permissions deleted the symlink /var/run. To restore it, log on to another system and find out the permissions of this link:

# ls -ld /var/run
lrwxrwxrwx. 1 root root 6 Feb 25  2015 /var/run -> ../run
# ls -Z /var/run
lrwxrwxrwx. root root system_u:object_r:var_run_t:s0   /var/run -> ../run
#

Log on to broken system as root and do the following:

# cd /var
# ln -s ../run run
# chcon -h system_u:object_r:var_run_t:s0 run
# ls -Z run
lrwxrwxrwx. root root system_u:object_r:var_run_t:s0   /var/run -> ../run

Kerberos Keytab Error - CentOS 6

Oct 22 21:10:38 server sshd[52210]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=server.domain.com  user=st14988
Oct 22 21:10:38 server sshd[52210]: pam_krb5[52210]: error reading keytab 'FILE:/etc/krb5.keytab'
Oct 22 21:10:38 server sshd[52210]: pam_krb5[52210]: TGT verified
Oct 22 21:10:38 server sshd[52210]: pam_krb5[52210]: authentication succeeds for 'abc' (abc@server.domain.com)
Oct 22 21:10:38 server sshd[52210]: pam_unix(sshd:session): session opened for user abc by (uid=0)

To disable keytab validation and hence suppress these log messages, add the no_validate option to your PAM settings.

auth        sufficient    pam_krb5.so use_first_pass no_validate

On my CentOS 6 servers, these are in the following files:

/etc/pam.d/password-auth-ac
/etc/pam.d/system-auth-ac

RedHat Channel Subscription

To list all available channels:

[root@ ~]# rhn-channel -L  --user=rhn_user --password=rhn_password

To subscribe to a channel (e.g. options):

[root@ ~]# rhn-channel --add --channel=rhel-x86_64-server-optional-6 --user=rhn_user --password=rhn_password

To list channel subscribed:

[root@ ~]# rhn-channel -l
rhel-x86_64-server-6
rhel-x86_64-server-optional-6

To register to RHN (do this first!): To register and subscribe in one step:

[root@ ~]# subscription-manager register --username  --password  --auto-attach

This is the old way or using a satellite server:

[root@ ~]# rhn_register

Remove node from Puppet Enterprise system

On the PE master:

[root@pe-master ~]# puppet cert clean node.domain.com
Notice: Revoked certificate with serial 34
Notice: Removing file Puppet::SSL::Certificate node.domain.com at '/etc/puppetlabs/puppet/ssl/ca/signed/node.domain.com
Notice: Removing file Puppet::SSL::Certificate node.domain.com at '/etc/puppetlabs/puppet/ssl/certs/node.domain.com

[root@pe-master ~]# puppet node deactivate node.domain.com
Submitted 'deactivate node' for node.domain.com with UUID 1d6c7992-d4cf-4659-91e2-23c751afacc2

[root@pe-master ~]# puppet cert revoke node.domain.com
Notice: Revoked certificate with serial 34

Restart pe: service pe-puppet restart On the client:

[root@pe-client ~]# find /etc/puppetlabs/puppet/ssl -name node.domain.com -delete

Firewall Commands on CentOS 7

Open a firewall port

   30  firewall-cmd --zone=public --add-port=4407/tcp --permanent
   31  firewall-cmd --reload

Show opened ports

  568  firewall-cmd --zone=public --list-ports

List connected servies

  567  firewall-cmd --zone=public --list-services