How to fix tcpdump error with file permission denied

Question: When I run tcpdump with “-r” option to read from a packet dump file, I am getting an error from tcpdump saying that “file permission denied”. I am getting this error even when I run tcpdump with root privilege. How can I fix this error? If this problem happens on Ubuntu, it is possible […]
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How to configure static DNS on CentOS or Fedora

Question: On CentOS, I am getting an IP address assigned by DHCP. However, I want to use public DNS servers (e.g., Google DNS), not those assigned by a DHCP server. In general, how can I configure DNS servers statically on CentOS or Fedora? If you want to hard-code DNS servers to use on CentOS or […]
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How to create a GRE tunnel on Linux

Question: I want to connect to remote networks by using a GRE tunnel. How can I create a GRE tunnel between two end points on Linux? GRE tunnels are IP-over-IP tunnels which can encapsulate IPv4/IPv6 and unicast/multicast traffic. To create a GRE tunnel on Linux, you need ip_gre kernel module, which is GRE over IPv4 […]
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How to check which ports are open on Linux

Question: How can I check which ports are currently open on my Linux system? If a port is open, it means that there is an actively running program (e.g., daemon) which uses the port number to communicate with other programs. If you want to check which ports are open on local Linux system, you can […]
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