1. Linux Administration Odds and Ends
Cron, YUM repo, and DHCP

2. Cron Cron daemon – controls periodic processes in the Linux system
Reads one or more configuration files containing lists of command lines and times they are to run.
crontab aka “cron table” – cron configuration file
Cron wakes and sleeps every minute to check all configuration files, reloads any files that have changed, and executes any that are scheduled.

3. Cron locations
Each user in the system can store their own cron file in /var/spool/cron
System maintenance files located /etc/cron.d and /etc/crontab
Generally /etc/crontab is the file sys admins change by hand
/etc/cron.d is the location software packages can install crontab entires

4. Cron File Format # designates comments
minute hour day month weekday [username] command
First 6 fields separated by whitespace
Username found only in /etc/crontab and /etc/cron.d

5. Cron details Each of the time-related fields may contain:
A star, which matches everything
A single integer, which matches exactly
Two integers separated by a dash, matching a range of values
A range followed by a slash and a step value, e.g., 1-10/2 (Linux only)
A comma-separated list of integers or ranges, matching any value

6. Cron Examples
* * 1-5
Means to run every Monday through Friday at 10:45am
* * 0-3, 6 /staff/trent/bin/checkservers
Means to run checkservers command everday at 11:55pm except Thursdays and Fridays

7. Crontab Management
crontab filename installs filename as your crontab replacing any previous version.
crontab arguments:
e checks out a copy of your crontab for editing
l lists crontab contents
r removes crontab
u username (used by root to edit or view a users crontab file)
/etc/cron.allow and /etc/cron.deny
If netiher file exists only root can submit crontabs

8. Common Uses for Cron Why would this be useful?
Automated system backups
Automated cleaning of tmp files or log files
Automated system usage reports
Automated reports of failed logins

9. Setup YUM Repo
Mount the local media like CD, DVD, USB stick or ISO image that contains CentOS 7 / RHEL 7 / SL 7 / OL 7 to your PC.
~]# cd /
/]# mkdir localrepo
/]# cp -rv /media/* /localrepo/ (copy repos from disk image)
~]# rm -rf /etc/yum.repos.d/* (this removes online repo)

10. Setup YUM Repo cont’d
~]# vim /etc/yum.repos.d/local.repo
(add the following below to the local.repo file)
[centos7]
name=centos7
baseurl=file:///localrepo/
enabled=1
gpgcheck=0

11. Setup YUM Repo cont’d [root@localhost ~]# createrepo /localrepo/
~]# yum clean all
~]# yum repolist all
~]# yum update

12. The Network File System
Chapter 18
The Network File System

13. NFS NFS – not a video game!
The Network File System (NFS) is a network protocol which allows one machine to access files stored on a second system.

14. NFS
An NFS server system exports certain directory hierarchies from its local disc partitions.
An NFS client system can then mount directories from that hierarchy as if they were filesystems stored on local disc. This requires a certain amount of cooperation between the two systems which are involved.
An NFS client system inherits file ownerships and permissions from its server. If a particular user needs to access files on an NFS server, the UID and GID information for that user needs to be consistent on the two machines.

15. NFS NFS is designed to work across many different types of platforms.
It is possible to export a directory from one type of Linux system and mount it on a second machine which is running an entirely different kind of Linux.
Network filesystems should provide users with a seamless experience.
Intended to be more or less transparent across networks.

16. NFS Daemons
NFS client and server systems need to be running various system daemons before NFS will work reliably.
NFS is an RPC based service, consequently the RPC portmapper must be running. They should also run the lockd and statd daemons for file locking to work reliably.
File locking is used to guarantee exclusive access to a file.

18. Server Side NFS Running NFS sever on CentOS
sudo service nfs start
sudo service nfs status
To add directories that the NFS server should allow others to mount over the network modify /etc/exports and then run /usr/sbin/exportfs -a

19. More on the exports file

20. And More

21. Server side NFS For example, the /etc/exports configuration
/directory1 server.example.com(options)
/directory /24(options)

22. Client side NFS
Before an NFS file system can be mounted, it must be properly exported on the server side.
To verify that a server has properly exported its filesystems from the client’s perspective, use the client’s showmount command
E.g. showmount –e <NFS server>
Dash “e” tells the server to display its export list

23. Client Side NFS cont’ To mount filesystem:
$ sudo mount -t nfs4 -o rw,hard,intr,bg host_name:mount location/ /(local mount point)

24. Client Side NFS cont’ You can check NFS mount with the df command
You can also remove the mounted file system with the unmount –f command
Add mounts that are part of a system’s permanent configuration in /etc/fstab automatically at boot time.

25. /etc/fstab
The device name or other means of locating the partition or data source.
The mount point, where the data is to be attached to the filesystem.
The filesystem type, or the algorithm used to interpret the filesystem.
Options, including if the filesystem should be mounted at boot.
dump-freq adjusts the archiving schedule for the partition (used by dump).
pass-num Controls the order in which fsck checks the device/partition for errors at boot time

26. nfsstat Dumps the output of statistics maintained by the nfs system
Usage: nfsstat –[s/c]
(s option is for server operations)
(c is for client operations)

27. Dedicated NFS servers
Stand alone hw that tends to be $$$ but has the following advantages:
As storage requirements grow, they can scale smoothly to support terabytes of storage and hundreds of users.
They are more reliable than stand-alone boxes thanks to their simplified software, redundant hardware, and use of disk mirroring.
They usually provide file service for both Linux and Windows clients.
Most even contain integrated web, FTP, and SFTP servers.
They are often easier to administer than Linux file servers.
They often include backup and checkpoint facilities that are superior to those found on vanilla Linux systems.

28. Automatic Mounting
Configuring /etc/fstab for many hosts in a large system is tedious
Many clients trying to access a NFS server can cause problems if the server crashes
Automount daemon to mount filesystems when they are referenced and to unmount them when they are no longer being used.

29. Automatic Mounting cont’
Also limit the number of mount points
Transparent to the user
Automount understands three different kinds of
configuration files. (referred to as “maps”):
direct maps, indirect maps, and master maps
Direct and indirect maps provide information about the filesystems to be automounted.
A master map lists the direct and indirect maps that automount should pay attention to.

30. Automount cont’ The default master map file is /etc/auto.master
(example below)
# Directory Map
/harp /etc/auto.harp
/ /etc/auto.direct

31. Important things we did not cover
postfix
sendmail
procmail
Sharing Network Files
Kerberos
LDAP
NIS
Hardening/Security