SELinux Survival Guide 
Wednesday, 10 October 2018, 12:56 - Linux Stuff, RedHat Stuff
Posted by Administrator
On SELinux enabled systems (default on CentosOS/RHEL 6.x and higher), it may deny access when system utilities are called from a daemon's context used for automation or monitoring purposes.

You will see some deny messages within /var/log/audit.log that indicate SELinux is blocking access.

So follow this procedure for simply allow things denied by SELinux policies:

Build SELinux Policy


1. Set concerning context to permissive (will still log whitn audit.log:
semanage permissive -a zabbix_agent_t
2. Allow logging even rules are set to dontaudit:
semodule -DB
3. Now let the programme or script do its intended job.

Important: If the programme is doing things that wouldn't be done at every run, like caching (e.g. yum), try to clean programme's cache before running so you catch everything it may do!

4. Search for log entries and build a policy module & package out of it, analysis beginning from date today" (and optionally a time spec):
ausearch -r -m avc -ts today [HH:MM] | audit2allow -M zabbix_megacli

5. Import policy package:
semodule -i zabbix_megacli.pp

6. Disable permissive mode for context again:
semanage permissive -d zabbix_agent_t

7. Disable logging of rules defined as dontaudit:
semodule -B

8. Test if intended stuff works now!

Adjust policy


When you still see some single denials within audit.log, and quickly what to complete the policy with the rules seen, you may:

1. Edit zabbix_megacli.te and add missing operations like write, lock, etc. to the allow rules - don't forget to also specify those ops within concerning class!

2. Compile module file:
checkmodule -M -m -o zabbix_megacli.mod zabbix_megacli.te
3. (Re-)create the module package from module file:
semodule_package -o zabbix_megacli.pp -m zabbix_megacli.mod

For more info, see here:
https://www.centos.org/docs/5/html/Deployment_Guide-en-US/sec-sel-building-policy-module.html
3. Import policy package:
semodule -i zabbix_megacli.pp


Apply Policy to other hosts


1. Copy the policy package (<policy>.pp) to the host you want to apply policy

2. Run the following command on every machine to load the package:
semodule -i zabbix_megacli.pp

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Windows always fails on installing monthly security rollup update 
Monday, 14 May 2018, 02:00 - Windows Stuff, Linux Stuff
Posted by Administrator
When trying to install Windows security rollup update on computer with dual boot, Windows Update always fails.

Symptom


During shutdown, Windows starts preparing the update and during the next boot, it continues until around 80-100%. Then it fails, rolls back the upgrade and reboots again. After that, the update is still listed for installation and shows as failed attempt in the update log with error code 80004005.

Solution


1. Get Windows to boot using its native boot loader


Windows will then boot the active partition from MBR, temporarily remove the boot loader!. So make sure that partition where Windows is installed is the active partition (e.g. using Disk Management)!

Start Ubuntu (either installed one or from a USB Stick) and run following commands:
sudo apt-get install mbr
sudo install-mbr -i n -p D -t 0 /dev/sdX
(replace sdX with the disk where Windows is installed!)

Attention: This makes your linux installation unbootable if you run mbr command on the disk you normally boot Linux from, so ensure:
- you have a current backup of your value data
- have a USB stick at hand with Ubuntu ISO Image

If you have installed Windows on an other (second) harddisk, also go to BIOS Setup and change boot order so the disk containing Windows is in first order (before the one containing Linux).

2. Install Security Rollup Update


On subsequent reboots, your computer will now boot directly into Windows (without showing GRUB menu anymore).

Start Security Rollup Update again:
* Go to Windows Update (Control Panel -> Sytem and Security -> Windows Update)
* Choose "Check for updates"
* Make sure Security Rollup is selected
* Choose "Install Updates"

This time, after 2-3 reboots, update should succeed.

3. Make Linux bootable again


a) When changed boot order to start Windows directly from another disk, go to BIOS Setup again and switch order back, so the harddisk with GRUB installed will be ordered before the HD containing Windows installation.

b) When installed "original" MBR to the disk where GRUB was installed, you have to repair the Linux Bootloader:
* boot using a USB Stick containing e.g. Ubuntu Linux ISO image
* mount your root and boot Linux partitions, e.g.
mkdir -p /mnt/root && mount /dev/sda6 /mnt/root && mount /dev/sda5 /mnt/root/boot
(replace with device where your linux partitions resides, if in doubt, first run fdisk -l /dev/sda)

* chroot your Linux installation:
chroot /mnt/root /bin/bash
* install grub again to Master Boot Record, e.g.
grub-install /dev/sda
(grub configuration should be available on /boot)

* Exit chroot environment
exit
* Unmount Linux partitions:
umount /mnt/root/boot /mnt/root
* reboot your system

You now should see GRUB boot menu again, where you can boot either Linux or Windows...

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Reverse Proxy mit HTTP Auth im Backend 
Friday, 5 May 2017, 15:27 - General, Knowledge, Apache Stuff, Linux Stuff, Nginx
Posted by Administrator
Damit man über einen Reverse-Proxy auf einen Web-Server zugreifen kann, welcher seinerseits wieder mit HTTP Basic Authentifizierung geschützt ist (und im Backend andere Login-Informationen als für die Anmeldung am Reverse Proxy erforderlich sind), muss die HTTP-Authentifizierung für den Backend-Server im Proxy-Abschnitt mitgegeben werden.

Dazu muss zuerst Benutzername und Passwort in eine Base64-Zeichenkette encodiert werden:
echo -n "User:Pass" | base64
VXNlcjpQYXNz
(auch wenn kein Benutzername benutzt wird, muss das Doppelpunkt im zu encodierenden String enthalten sein!)

Danach in der Konfiguration des als Reverse-Proxy verwendeten Frontend-Servers folgendes z.B. in einen Location-Abschnitt hinzufügen.

Apache:
RequestHeader set Authorization "Basic VXNlcjpQYXNz"

Nginx:
proxy_set_header Authorization "Basic VXNlcjpQYXNz";


Technischer Hintergrund:

Sofern dieselben Anmelde-Informationen im Backend verwendet werden wie im Frontend (Reverse-Proxy), sollte dieses bei der nachfolgenden HTTP-Auth Anfrage transparent vom Client Web-Browser weitergereicht werden, und obiger Parameter ist nicht notwendig.

Wird hingegen versucht, sich mit unterschiedlichen HTTP-Auth Passwörter anzumelden (zuerst dasjenige für den Reverse-Proxy, dann dasjenige, welches der Backend-Webserver verlangt), ist darauf sofort die Anmeldung am Proxy nicht mehr gültig -> Ein Zugriff würde so also nie funktionieren!
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Integrate new drivers into a RedHat Network Boot Disk  
Tuesday, 23 September 2003, 00:00 - Technology, Linux Stuff, RedHat Stuff
Posted by Administrator
It is possible that the hardware changes with new server models and you cannot boot any more from the RedHat Bootdisk. Then you need to inspect your new hardware and see what new devices are built in and get a driver for it.

For more convenience, I have written some simple scripts, which do the most annoying tasks.

The drivers need to be changed on different locations:

Network drivers must be put on the boot-disk, because all other packages, drivers, etc. are loaded from a network location

Ungzip bootnet.img.gz with:
        gunzip bootnet.img.gz 

Mount the Image as a loop filesystem on /mnt/bootimage:
        unpack_bootimage.sh bootnet.img 

Unpack and mount the inital ramdisk with the following script, give the initrd-file under /mnt/bootimage/ as argument. The inital ramdisk will be mounted under /mnt/initrd:
        unpack_initrd.sh initrd.img 

Unpack the modules with the following script:
       unpack_modules.sh /mnt/initrd/modules/modules.cgz 

Now copy the new driver module(s) to /var/tmp/modules/<kernel-version> directory
Change /mnt/initrd/modules/pcitable and add a new line with the vendor ID, product ID, driver name and description to the file (see example below):
        0x8086  0x1010  "e1000"         "Intel Corporation|PRO/1000"

Change the file /mnt/initrd/modules/module-info and add a line for each the driver name, type and description (see the following example):
        e1000
eth
"Intel EtherExpress Pro 1000"

Add a line for the new driver to the file /mnt/initrd/modules/modules.dep if the new driver module is dependent on othe kernel modules. Mostly not necessary for ethernet adapters.

Re-pack the modules to the archive, kernel-version-directory under /var/tmp/modules as 1st argument, cpio archive file as 2nd argument:
        pack_modules.sh 2.4.20-18.7BOOT /mnt/initrd/modules/modules.cgz 

"exit" from the initrd-mount, if you are there, /mnt/initrd or subdirectories of it musn't be your current dir!

If you also need to update the boot-kernel (when used some modules not for acutal kernel version), copy the new vmlinuz kernel image to /mnt/bootimage
Unmount and pack the initial ramdisk, give the initrd-file under /mnt/bootimage to be updated as argument:
        pack_initrd.sh initrd.img 

Now unmount the bootimage and write it to a boot floppy with this script:
        pack_bootimage.sh bootnet.img 

Most other drivers, like for SCSI-Controllers are in the stage2 Image
Mount the Stage 2 image (network-connected part of the installation):
        mount -o loop /install/cdrom/RedHat/base/stage2.img /mnt/image 

Unpack the modules with the following script:
        unpack_modules.sh /mnt/image/modules/modules.cgz 

Now copy the new driver module(s) to /var/tmp/<kernel-version> directory

Important: If the new modules are built for another kernel version as the one on the boodisk, the bootdisk image must be updated with the corresponding kernel image. Further, all modules need to be replaced by one's of the same kernel version as the kernel image!

Change /mnt/image/modules/pcitable and add a new line with the vendor ID, product ID, driver name and description to the file (see example below):
        0x9005  0x801f  "aic79xx"       "Adaptec|AIC7902 Ultra 320 SCSI Adapter" 

Change the file /mnt/initrd/modules/module-info and add a line for each the driver name, type and description (see the following example):
        aic79xx
scsi
"Adaptec AIC79xx Ultra 320 SCSI Host Adapter"

Add a line for the new driver to the file /mnt/image/modules/modules.dep if the new driver module is dependent on other kernel modules. Example:

        aic79xx: scsi_mod 


Re-pack the modules to the archive, kernel-version-directory under /var/tmp/ as 1st argument (see /var/tmp/modules), cpio archive file as 2nd argument:
        pack_modules.sh <kernel-version> /mnt/image/modules/modules.cgz 

Leave the mountpoint of the image (/mnt/image or subdirectories of it musn't be your current dir!)

Unmount the image:
        umount /mnt/image 


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