This paper walks through various scenarios where dd and netcat can be used to backup files for administrative and forensic purposes. This paper is expected to grow over time and will be updated as time permits.

The first example uses OpenBSD 3.9 with nc that comes with base as the server host, and NetBSD 3.99.14 with netcat-1.10nb2 from pkgsrc as the client.

We will setup OpenBSD to listen on port 9999, and redirect output to “backup.file”

  server# nc -l -p 9999 > backup.file

 
Once nc has received the file, it will automatically terminate the process.

Now, let’s backup a file. (Assume 10.0.0.2 is the IP address of the OpenBSD server).

  client# cat file-to-backup | nc 10.0.0.2 9999

 
It’s as simple as that. And you can just verify the md5 checksum to be sure.

  client# md5 file-to-backup
  MD5 (file-to-backup) = 8320752ece7caf11f7212d790b48cb92

  server# md5 backup.file
  MD5 (backup.file) = 8320752ece7caf11f7212d790b48cb92

 
UPDATE: If you’re interested in encrypting these transfers, please refer to our article here