How do I verify that a private key matches a certificate? 
Friday, 11 February 2022, 12:47 - Technology, OpenSSL
Posted by Administrator
To verify that an RSA private key matches the RSA public key in a certificate you need to i) verify the consistency of the private key and ii) compare the modulus of the public key in the certificate against the modulus of the private key.

To verify the consistency of the RSA private key:
openssl rsa -check -noout -in myserver.key
RSA Key is ok

If it doesn't say 'RSA key ok', it isn't OK!"

To view its modulus:
openssl rsa -modulus -noout -in myserver.key | openssl md5

To view the modulus of the RSA public key in a certificate:
openssl x509 -modulus -noout -in myserver.crt | openssl md5

If the first commands shows any errors, or if the modulus of the public key in the certificate and the modulus of the private key do not exactly match, then you're not using the correct private key.
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Check CRL for revoked certificates and valitity of CRL itself 
Saturday, 15 January 2022, 14:23 - OpenSSL
Posted by Administrator
To find out if a client certificate was rejected or if the Certificate Revocation List itself is still valid (not older than "Next Update" attribute defined):
openssl crl -inform DER -text -noout -in mycrl.crl

Most CRLs are DER encoded, but you can use -inform PEM if your CRL is not binary. If you’re unsure if it is DER or PEM open it with a text editor. If you see —–BEGIN X509 CRL—– then it’s PEM and if you see strange binary-looking garbage characters it’s DER.
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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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