I searched the web alot but cannot find an answer. When I pipe something big over nc, does it makes sure everything is went through? I mean is there any checksum/verification mechanism against missing packages?

This is not nc's business. that's TCP's responsibility as a network stack level.

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Exactly what acid_kewpie said. (Except it's transport layer. )

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edit: Ah, I think you meant "network stack level" to describe OSI. Never mind me.

Ahh I wasn't talking models, just "not the programs problem"

In some ways, netcat can be thought of as a cable. It just moves data. Doesn't care about it at all.

So TCP takes care of it? So I can dump a huge hdd over nc using dd and I can be sure that everything will go through?

If the TCP conversation successfully completes (and nc(1) appears to be good about warning you if it does not), you should be all set. If you really, really want to be sure, then use a cryptographic digest to confirm.

BTW, why use nc(1) for this? What about scp(1)?

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edit: Or if you don't want to create a large file locally, then pipe your dd(1) to ssh(1) a la:

For some reason the link I got this from is down so I can't quote it.

"
Using DD over Netcat
Netcat opens an encryption-less connection from one host to another, which is why it outperforms SSH. If using the netcat method, take a moment to consider the implications of sending raw, unecrypted data over your network. We strongly recommend against using this method for WAN data transfers, unless you are doing so over an encrypted tunnel (e.g. VPN).
We will assume for the purposes of this tutorial that you have the nc version of netcat. If you have the the other, then the command line options will be slightly different but the idea is the same. You set up a listening server on the destination, and then you send data to the port you've specified form the source. Let's assume we are transferring a full disk image from serverA (/dev/sda) to serverB (dev/sdb). We are going to assume block size of incoming data for dd will be 16MB and that it will be bzip compressed. On serverB we would run the following:
nc -l 19000|bzip2 -d|dd bs=16M of=/dev/sdb
This tells netcat to listen on port 19000 for incoming data, then pipe that data to bzip for decompression, and then finally pipe the decompressed data to dd to be written to /dev/sdb.
Once we have this listening (you won't see any output after you hit Enter), we can move on to starting the data transfer on serverA:
dd bs=16M if=/dev/sda|bzip2 -c|nc serverB.example.net 19000
You again will not see any output after you've hit Enter, but do not fret! You can start another session (or launch the netcat in a screen session and back out), and run a tcpdump on port 19000 on serverB to ensure that traffic is indeed flowing. You'll get a DD output on both ends summarizing the read/write time and bytes transferred when the process is complete. In this case no additional configuration is needed. /dev/sdb is a mountable and readable block device that's ready for use!
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