Linux Ask! is a Q & A web site specific for Linux related questions. Questions are collected, answered and audited by experienced Linux users.
How to kill a TCP connection using tcpkill?
Answer:
You can use the tcpkill program.
tcpkill came with the dsniff in Debian/Ubuntu, you need to install it manually.
# sudo apt-get install dsniff
Usages:
1. Kill all outgoing web (port 80) connection:
# tcpkill -i eth0 port 80
2. Kill all connection by IP
# tcpkill -i eth0 host www.google.com
What is the difference between ip address 0.0.0.0 and 127.0.0.1?
Answer:
Sometimes when using the netstat command, which print out something like
# netstat -ntpl
(No info could be read for "-p": geteuid()=1001 but you should be root.)
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN -
...
What is the real meaning of 0.0.0.0?
In fact, this special IP means "all the IP addresses on the local machine". So if you have seen a service listening on 0.0.0.0, it means you can connect to the service using any IP addresses of the machine.
How to take down/up a network interface in Linux?
Answer:
To take the eth0 interface (the first network card) down:
# sudo ifconfig eth0 down
To take the eth0 interface up:
# sudo ifconfig eth0 up
Find out which programs are listening TCP port
Answer:
Use the command: netstat -tlnp
# netstat -tlnp
(Not all processes could be identified, non-owned process info
will not be shown, you would have to be root to see it all.)
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State
tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN
tcp 0 0 0.0.0.0:443 0.0.0.0:* LISTEN
tcp 0 0 0.0.0.0:80 0.0.0.0:* LISTEN
Disable reply to ping
Answer:
To stop other machine from pinging your machine, you the following command
# sysctl -w net.ipv4.icmp_echo_ignore_all=1
To turn it on again:
# sysctl -w net.ipv4.icmp_echo_ignore_all=0
