Linux

Fixing Disk Space Not Freed on Linux

Fixing Disk Space Not Freed on Linux Use df -ah and du -h --max-depth=1 The total from du is far smaller than the total reported by df. When a process deletes files but keeps running, the files are not actually removed, so disk space is not freed, and those files are not counted. lsof |grep delete

Wednesday, July 10, 2019 Read

Install Font Libraries and Chinese Fonts on Linux CentOS 7

Install Font Libraries and Chinese Fonts on Linux CentOS 7 yum -y install fontconfig Now you can see the fonts and fontconfig directories under /usr/shared (they did not exist before). Before that we need to create a directory. First create /usr/shared/fonts/chinese. mkdir /usr/shared/fonts/chinese Copy the fonts you need and upload them to /usr/shared/fonts/chinese. Here I use SimSun and HeiTi (used in reports). You will see files with ttf and ttc extensions.

Tuesday, December 18, 2018 Read

What is the SHA256 that comes on the sshd entry in auth.log?

What is the SHA256 that comes on the sshd entry in auth.log? ssh-keygen -lf .ssh/id_rsa.pub cat .ssh/id_rsa.pub | awk '{ print $2 }' | # Only the actual key data without prefix or comments base64 -d | # decode as base64 sha256sum | # SHA256 hash (returns hex) awk '{ print $1 }' | # only the hex data xxd -r -p | # hex to bytes base64 # encode as base64

Monday, December 17, 2018 Read

Netcat (Linux nc) Practical Examples for Network Admins

Netcat (Linux nc) Practical Examples for Network Admins Send a test UDP packet to a remote server This command sends a UDP test packet to the specified host and port. The -w1 option sets the timeout to 1 second. echo -n "foo" | nc -u -w1 192.168.1.8 5000 Open a UDP port to receive data nc -lu localhost 5000 Port scanning on a remote host This command scans TCP ports in the ranges 1-1000 and 2000-3000 on the specified host to see which ports are open.

Friday, November 9, 2018 Read

Simulate Network Anomalies with TC and Netem

Simulate Network Anomalies with TC and Netem Netem and TC brief overview Netem is a network emulation module provided by Linux 2.6 and later kernels. It can be used on a good LAN to simulate complex Internet transmission performance, such as low bandwidth, latency, packet loss, and so on. Many Linux distributions with kernel 2.6+ enable this module by default, such as Fedora, Ubuntu, Redhat, OpenSuse, CentOS, Debian, etc. TC is a user-space tool in Linux, short for Traffic Control. TC controls the operating mode of the Netem module. In other words, to use Netem you need at least two conditions: the Netem module must be enabled in the kernel, and the corresponding user-space tool TC must be available.

Saturday, September 15, 2018 Read

Quagga Routing - Install, Configure and setup BGP

Tuesday, August 14, 2018 Read

Systemd Tutorial: Practical Part

Systemd Tutorial: Practical Part $ systemctl cat sshd.service [Unit] Description=OpenSSH server daemon Documentation=man:sshd(8) man:sshd_config(5) After=network.target sshd-keygen.service Wants=sshd-keygen.service [Service] EnvironmentFile=/etc/sysconfig/sshd ExecStart=/usr/sbin/sshd -D $OPTIONS ExecReload=/bin/kill -HUP $MAINPID Type=simple KillMode=process Restart=on-failure RestartSec=42s [Install] WantedBy=multi-user.target [Unit] Section: Startup Order and Dependencies After field: if network.target or sshd-keygen.service needs to start, then sshd.service should start after them. Correspondingly, the Before field defines which services sshd.service should start before. Note that After and Before only involve startup order, not dependency relationships.

Thursday, August 9, 2018 Read