IP Forwarding on RHEL

We can modify IP_forwarding temporally or permanently

For temporally

There are two methods for temporally change,

Method 1

Check if IP Forwarding was already enabled or not

[root@Ram ~]# cat /proc/sys/net/ipv4/ip_forward
0

if it is 0 it is disabled, if 1 then it is enabled

Modify the value

[root@Ram ~]# echo 1 > /proc/sys/net/ipv4/ip_forward
[root@Ram ~]# cat /proc/sys/net/ipv4/ip_forward
1

Method 2

[root@Ram ~]# sysctl -a|grep ip_forward
net.ipv4.ip_forward = 0

[root@Ram ~]# sysctl -w net.ipv4.ip_forward=1
net.ipv4.ip_forward = 1

[root@Ram ~]# sysctl -a|grep ip_forward
net.ipv4.ip_forward = 1




For permanent change,

Modify following line in /etc/sysctl.conf to 1 from 0

# Controls IP packet forwarding
net.ipv4.ip_forward = 1


Execute following command to load the configuration

[root@Ram ~]# sysctl -p

LVM creationg, Extend and reduce in RHEL

fdisk -l

Disk /dev/sda: 8589 MB, 8589934592 bytes

Added new hard disk(2 GB) using Vmware settings


From the host, used following command to scan new disk

echo "- - -" > /sys/class/scsi_host/host2/scan

Disk /dev/sda: 8589 MB, 8589934592 bytes
Disk /dev/sdb: 2147 MB, 2147483648 bytes


Noticed new disk /dev/sdb after scan


Now Creating new LVM using new disk

1) Creating Physical volume

[root@Ram ~]# pvcreate /dev/sdb
Physical volume "/dev/sdb" successfully created

2) Assign physical volume to volume group

[root@Ram ~]# vgcreate vg1 /dev/sdb
Volume group "vg1" successfully created

3) Check your work

[root@Ram ~]# vgs
VG #PV #LV #SN Attr VSize VFree
vg1 1 0 0 wz--n- 2.00g 2.00g

[root@Ram ~]# pvs
PV VG Fmt Attr PSize PFree
/dev/sdb vg1 lvm2 a- 2.00g 2.00g

Now Create Logical Volumes of 1 GB initially. I will explain how to extend and reduce after this

[root@Ram ~]# lvcreate -L 1GB -n lg1 vg1
Logical volume "lg1" created

[root@Ram ~]# mke2fs -j /dev/vg1/lg1
mke2fs 1.41.12 (17-May-2010)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
65536 inodes, 262144 blocks
13107 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=268435456
8 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks:
32768, 98304, 163840, 229376

Writing inode tables: done
Creating journal (8192 blocks): done
Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 26 mounts or
180 days, whichever comes first. Use tune2fs -c or -i to override.


Now Create new filesystem

[root@Ram ~]# mkdir /test
[root@Ram ~]# mount /dev/vg1/lg1 /test
[root@Ram ~]# df -h
/dev/mapper/vg1-lg1 1008M 34M 924M 4% /test

Now filesystem is mounted but when you restart the system, it will not mount it again. For that, we need to update /etc/fstab file

/dev/mapper/vg1-lg1 /test ext4 defaults 0 0

After updating, we can make sure it is correct by unmounting and mounting it again

[root@Ram ~]# umount /test

[root@Ram ~]# mount /test

Cool. Now we will extend the filesystem from 1 GB to 1.5 GB.

First we will check if we have enough space for this

[root@Ram ~]# vgs
VG #PV #LV #SN Attr VSize VFree
vg1 1 1 0 wz--n- 2.00g 1020.00m


Yep, we have space.

[root@Ram ~]# lvextend -L +512MB /dev/vg1/lg1
Extending logical volume lg1 to 1.50 GiB
Logical volume lg1 successfully resized

It will still show 1 GB

/dev/mapper/vg1-lg1 1008M 34M 924M 4% /test

Now we have to execute following command to extend

root@Ram ~]# resize2fs /dev/vg1/lg1
resize2fs 1.41.12 (17-May-2010)
Filesystem at /dev/vg1/lg1 is mounted on /test; on-line resizing required
old desc_blocks = 1, new_desc_blocks = 1
Performing an on-line resize of /dev/vg1/lg1 to 393216 (4k) blocks.
The filesystem on /dev/vg1/lg1 is now 393216 blocks long.

[root@Ram ~]# df -h

/dev/mapper/vg1-lg1 1.5G 34M 1.4G 3% /test


We got email from developer to reduce the filesystem to 1 GB since he accidently sent wrong filesystem

Ok, the problem for reducing filesystem, we have to unmount the filesystem

[root@Ram ~]# umount /test

Check the filesytem

[root@Ram ~]# e2fsck -f /dev/vg1/lg1
e2fsck 1.41.12 (17-May-2010)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
/dev/vg1/lg1: 11/98304 files (0.0% non-contiguous), 14765/393216 blocks

[root@Ram ~]# resize2fs /dev/vg1/lg1 1024M
resize2fs 1.41.12 (17-May-2010)
Resizing the filesystem on /dev/vg1/lg1 to 262144 (4k) blocks.
The filesystem on /dev/vg1/lg1 is now 262144 blocks long.


[root@Ram ~]# lvreduce -L 1024M /dev/vg1/lg1
WARNING: Reducing active logical volume to 1.00 GiB
THIS MAY DESTROY YOUR DATA (filesystem etc.)
Do you really want to reduce lg1? [y/n]: y
Reducing logical volume lg1 to 1.00 GiB
Logical volume lg1 successfully resized

Mount the filesystem

[root@Ram ~]# mount /test

[root@Ram ~]# df -h
/dev/mapper/vg1-lg1 1008M 34M 924M 4% /test

ACL setup on RHEL

In the following setting up ACL under home directory which is mounted under /(root)

# mount -o remount,acl /
# mount |grep /
/dev/sda2 on / type ext4 (rw,usrquota,grpquota,acl)

# touch testacl.txt
# getfacl /home/testacl.txt
getfacl: Removing leading '/' from absolute path names
# file: home/testacl.txt
# owner: root
# group: root
user::rw-
group::r--
other::r--

[root@Ram home]# setfacl -m u:ram:rwx /home/testacl.txt
[root@Ram home]# getfacl /home/testacl.txt
getfacl: Removing leading '/' from absolute path names
# file: home/testacl.txt
# owner: root
# group: root
user::rw-
user:ram:rwx
group::r--
mask::rwx
other::r--

Blocking port 80 using IPTABLES in RHEL

This is just basic example using iptables, it needs more practice since it is complicated task.

Make sure port 80 is accessible on the server.

# telnet 80

to check iptables contents:

# less /etc/sysconfig/iptables

For blocking port 80 using command line

# iptables -I INPUT -p tcp --dport 80 -j REJECT

Save the configuration

# service iptables save

Saving firewall rules to /etc/sysconfig/iptables: [ OK ]

Restart the service

# service iptables restart

Flushing firewall rules: [ OK ]

Setting chains to policy ACCEPT: filter [ OK ]

Unloading iptables modules: [ OK ]

Applying iptables firewall rules: [ OK ]

Loading additional iptables modules: ip_conntrack_netbios_n[ OK ]

Make sure port 80 is not accessible now.

# telnet 80

To remove the configuration enter the following command, save and restart the service:

# iptables -D INPUT -p tcp --dport 80 -j REJECT

Now, you should access port 80 again.

Resetting ROOT password on RHEL 5 and RHEL 6

If you don't know root password on RHEL 5. it is straight forward. Reboot to the single user mode and change the password using passwd.

How to boot to the single user mode.

When system reboot, Press Enter and Highlight Kernel line and press E. Enter single or s at the end of the line and enter.
Enter b to boot to the single user mode.

You will get # and type passwd to change the password.

Reboot the system

The only difference in RHEL 6 is, we have to disable selinux to be able to change the root password in the single user mode.

How to disable seLinux

Use following command to verify, whether is enabled or not.

# /usr/sbin/getenforce

Edit /etc/selinux/config file and change following line

SELINUX=enforcing to disabled and reboot the system to the single user mode using same steps as RHEL 5. Change the password using passwd

Using KSAR for Graphs from SAR

As UNIX administrator, we use sar to see the server stats. KSAR is free gui tool, where it will show sar output in graphs and also we can save the output in pdf file.

--> Download GUI tool from

http://sourceforge.net/projects/ksar/files/latest/download

--> unzip the gip file

--> under Ksar-5.N.N folder, click on ksat executable jar file (It will open gui window)

Either you can see full day output using data -> Lauch SSH Command -> Connect to remote server

or

Copy SAR output to local desktop (If you have sar output from previous days)

Enter Data -> Load from text file

To convert to text file

LC_ALL=C sar -A -f /var/adm/sa/sa11111 > sar111111.txt

LVM commands

pvs Displays physical volumes

vgs Displays volume groups

lvs Displays logical volumes

pvdisplay Displays detailed informationon physical volumes

vgdisplay Displays detailed informationon volume groups

lvdisplay Displays detailed informationon logical volumes

pvcreate Creates a new physical volume

vgcreate Creates a new volume group

lvcreate Creates a new logical volume

vgextend Extends an existing volume group

lvextend Extends a logical volume

lvresize Resizes a logical volume

lvreduce Reduces a logical volume

lvrename Renames a logical volume

pvmove Moves/migrates data from one physicalvolume to another

vgreduce Reduces a volume group

pvremove Removes a physical volume

vgremove Removes a volume group

lvremove Removes a logical volume

Some of the PRCTL commands

Most of the times, we get email from oracle team to modify some of the project setting. Following are the some of the commands:

Command to check value for project.max-shm-memory

# prctl -n project.max-shm-memory -i projectuser.oracle

Command to increase memory to 96 GB memory for running Process

# prctl -n project.max-shm-memory -r -v 96GB-i project user.oracle

Command to Make it permanentby updating /etc/project file

# projmod -sK "project.max-shm-memory=(privileged,103079215104,deny)"user.oracle

Command to check all parametersfor current user

# prctl $$

Command to check all parametersused for Oracle user

# prctl -i project user.oracle