*nix notes

[ikitaez]

How to add swap space on linux

fallocate -l 2G /swapfile

chmod 600 /swapfile

mkswap /swapfile

swapon /swapfile

After this edit /etc/fstab and add swap file to allow it on booting.

add:

/swapfile none swap sw 0 0

[ikitaez]

How to replace dead hdd in software raid with mdadm

Lets say you have RAID1 array created with mdadm and you want to replace disk sdb with new one.

To view raid devices:

cat /prod/mdstat

Then remove associated partitions with md devices:

mdadm /dev/md0 --remove /dev/sdb2 mdadm /dev/md1 --remove /dev/sdb3

If it gives error that it cant be removed, then before removing make them failed:

mdadm /dev/md0 -f /dev/sdb2 mdadm /dev/md1 -f /dev/sdb3

Then insert new disk and copy partitions from working disk sda to new disk sdb:

sfdisk -d /dev/sda | sfdisk /dev/sdb

Then add new drive to arrays:

mdadm /dev/md0 --add /dev/sdb2 mdadm /dev/md1 --add /dev/sdb3

And finally install grub to new disk:

grub-install /dev/sdb

[ikitaez]

How to check ssl certificate matches private key

To check certificate type:

openssl x509 -noout -modulus -in cert.crt | openssl md5

To check key:

openssl rsa -noout -modulus -in cert.key | openssl md5

If md5 sum of both commands will be same, then cert and key matches.

[ikitaez]

How to login to remote server with ssh key

On your server run:

ssh-keygen -t rsa

You will be asked for key location and password, just leave them empty and then keys will be created in home directory of your user, for exmaple:

/root/.ssh

Then we need to copy public key to remote server:

ssh-copy-id -i ~/.ssh/id_rsa.pub user@server

Replace user with user on remote side and server with server address. You will be asked for password of remote server, fill it.

After this you can connect to remote server with command:

ssh user@server

[ikitaez]

Plesk mysql governor error

If you see in /var/log/messages such errors:

Apr 14 01:53:37 service-1 systemd: Starting MySQL watcher service...

Apr 14 01:53:37 service-1 systemd: Started MySQL watcher service.

Apr 14 01:53:37 service-1 mysqld: 2022-04-14 1:53:37 140260687537920 [Warning] Access denied for user 'admin'@'localhost' (using password: YES)

Apr 14 01:53:37 service-1 mysqld: 2022-04-14 1:53:37 140260688766720 [Warning] Access denied for user 'root'@'localhost' (using password: NO)

Apr 14 01:53:37 service-1 mysqld: 2022-04-14 1:53:37 140260685080320 [Warning] Access denied for user 'root'@'localhost' (using password: NO)

Apr 14 01:53:37 service-1 systemd: db_governor.service: main process exited, code=exited, status=1/FAILURE

Apr 14 01:53:37 service-1 systemd: Unit db_governor.service entered failed state.

Apr 14 01:53:37 service-1 systemd: db_governor.service failed.

Apr 14 01:53:40 service-1 systemd: db_governor.service holdoff time over, scheduling restart.

Apr 14 01:53:40 service-1 systemd: Stopped MySQL watcher service.

Then you have incorrect password of mysql admin user in db governer config. To fix it:

nano /etc/container/mysql-governor.xml

Look for section:

<connector login="admin" password="

There will be encoded password started with $AES

Now we need to get actual password, to do it run:

cat /etc/psa/.psa.shadow

You will see working password in same encoded format, now replace old password in config with working one from .psa.shadow . Save file and exit. DB governor will be auto reconnected.

[ikitaez]

How to restart VMware services

To restart VMware services on host, run this command:

/sbin/services.sh restart

It can be helpful to apply changes after changing SSL certificate on host.

[ikitaez]

How to get Lets Encrypt SSL certificate

First of all you need to install certbot utility. If you have any webserver runned you must stop it.

Then use command:

certbot certonly -d yourdommain.net

Answer the questions, on How would you like to authenticate with the ACME CA? step choose number 1

If everything goes well you will find cert and key in following dirs:

Cert: /etc/letsencrypt/live/yourdomain.net/fullchain.pem Key: /etc/letsencrypt/live/yourdomain.net/privkey.pem

Now you can configure your webserver with it.

[ikitaez]

How to find and replace text in all files

If you want to replace text in current directory and all it's subdirectories then use this:

find . -type f -name "*.txt" -exec sed -i 's/foo/bar/g' {} \;

This will change foo to bar in all txt files.

[ikitaez]

How to find files changed between two dates

If you need to find all files that was changed in system between two dates, for example 28 March 2020 and 30 March 2020 then use command below:

find / -type f -newermt "2020-03-28" ! -newermt "2020-03-30" -ls

[ikitaez]

How to change default editor in Linux

For example, you want to edit cron jobs, and after typing crontab -e you see that vi was opened. You can change it to nano. To do it, type:

export EDITOR='nano'

[ikitaez]

mysql table import from dump

Imagine that you have mysql dump of database (testdb.sql) and you want to restore only one table (mytable). Then do this:

sed -n -e '/DROP TABLE.*mytable/,/UNLOCK TABLES/p' testdb.sql > testdb_mytable.sql

This command will create file testdb_mytable.sql with data only for this table. Then you can import it:

mysql -uUser -p testdb < testdb_mytable.sql

[ikitaez]

Cisco Catalyst port mirror

To configure port mirror do this:

conf t

monitor session 1 source interface fa0/1

monitor session 1 destination interface fa0/10

Where fa0/1 is port we want to mirror and fa0/10 is destination port. This will work for Catalyst 2940, 2955, 2960, 2970, 3550, 3560, 3560-E, 3750 and 3750-E.

For Catalyst 2900XL/3500XL series use this:

conf t

int fa0/10

port monitor fa0/1

Where fa0/10 is destination port we will mirror traffic for, and fa0/1 is source port.

[ikitaez]

How to create partition in Linux for 2TB drive and bigger

parted /dev/sdb

mkpart primary 2048s 100%

q

mkfs.ext4 /dev/sdb1

[ikitaez]

How to install iptables service in CentOS

By default CentOS uses firewalld service to control firewall rules. If you want to configure firewall with just iptables rules, you need to do following

Disable everything of firewalld

systemctl stop firewalld.service

systemctl disable firewalld.service

Install iptables-service

yum install iptables-services

Enable and start service

systemctl enable iptables.service

systemctl start iptables.service

Now you can write rules in console and after you done type:

iptables-save

Rules will be saved and loaded after reboot.

[ikitaez]

How to allow SSH access only to your IP

iptables -A INPUT -p tcp -s your_ip_here --dport 22 -j ACCEPT

iptables -A INPUT -p tcp -s another_your_ip_here --dport 22 -j ACCEPT

iptables -A INPUT -p tcp --dport 22 -j DROP

[ikitaez]

How to remove disk in CentOS

Make device offline

echo "offline" > /sys/block/sda/device/state

Delete disk

echo "1" > /sys/block/sda/device/delete

Where sda is your drive device.

[ikitaez]

How to update FreeBSD 10.2 to 10.3

freebsd-update -r 10.3-RELEASE upgrade

And follow instructions on console.