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 VirusTotal https://www.virustotal.com/gui/home/upload Whatismyip https://whatismyipaddress.com/blacklist-check Barracuda https://www.barracudacentral.org/lookups mxtoolbox https://mxtoolbox.com/blacklists.aspx haveibeenpwned https://haveibeenpwned.com/ dnschecker https://dnschecker.org/ip-blacklist-checker.php shodan IoT https://www.shodan.io/ references https://www.spamhaus.org/drop/

Error message 1. ssh by option -o KexAlgorithms when you meet error message %   ssh -oKexAlgorithms=+diffie-hellman-group1-sha1,diffie-hellman-group14-sha1 account@domain-name or 2. edit /etc/ssh/ssh_config unsharp middle two options and add two options at latest 3 reboot or  configure encryption strength to 2048 bits when you generate keys   references https://www.petenetlive.com/kb/article/0001245 https://www.volico.com/wiki/pages/viewpage.action?pageId=15728687

 1.  $ sudo systemctl stop mysql $ sudo mkdir  /var/run/mysqld; sudo chown mysql /var/run/mysqld 2. $ mysqld_safe --skip-grant-tables & $ mysql --user=root mysql 3. -> update mysql.user set authentication_string=null where user='root';

After minial installation or normal installation of Ubuntu  1. update repository and upgrade packages from apt command $ sudo apt update $ sudo apt upgrade $ sudo reboot 2.  install build-essential for library vmware needed to compile $ sudo apt install build-essential $ sudo apt install dkms 3 . download vmware workstation from website as https://www.vmware.com/go/getworkstation-linux 4. $  chmod 755 VMware-Workstation-Full-16.2.3-19376536.x86_64.bundle   # latest version $ cd ~/Download; sudo ./VMware-Workstation-Full-16.2.3-19376536.x86_64.bundle $ vmware &   # or search vmware from gnome and click app to execute

 1. change to root $ su - 2. install epel, mod_ssl, httpd, snapd $ yum -y install epel-* mod_ssl httpd $ yum -y install snapd $ systemctl enable snapd; systemctl start snapd 3. create a soft link for /snap $ ln -s /var/lib/snapd/snap /snap 4. test snapd for latest version and install classic certbot $ snap install core; snap refresh core $ snap install --classic certbot 5.  $ ln -s  /snap/bin/certbot /usr/bin/certbot $ certbot --apache 6. renew cert with certbot periodically or add in corn table   certbot renew --dry-run certbot renew   References https://snapcraft.io/docs/installing-snap-on-centos https://certbot.eff.org/instructions?ws=apache&os=centosrhel7 https://letsencrypt.org/zh-tw/donate/

for CentOS  1.  enable selinux to permit httpd_unified $ setsebool -P httpd_unified 1   2. go to parent folder of cache_tmp if it could not be written   $ semanage fcontext -a -t httpd_sys_rw_content_t 'cache_tmp'  $ restorecon -v 'cache_tmp'   Reference  https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/selinux_users_and_administrators_guide/sect-managing_confined_services-the_apache_http_server-booleans SElinux Alert Browser GUI