How do I look up which package brought a particular command?

For example, there is a command "send_arp", I'm not sure which package should I uninstall if I want to remove it.

You can determine the file name for a command line utility using the which command. For example, we can look up the path of the ls command:

$ which ls
/bin/ls

With that information, you can find out which package owns the file using dpkg:

$ dpkg -S /bin/ls
coreutils: /bin/ls

That tells us that ls is provided by the coreutils package.

In addition to James's post you can also do this on-line: send-arp. That way you can find not only the package but also all the dependencies (if there are any): cluster-agents