package manager and unrar for readynas pro 4?

nas-02:~# apt-get install unrar
Reading package lists... Done
Building dependency tree... Done
E: Couldn't find package unrar
nas-02:~# apt-get update
Get:1 http://archive.debian.org etch Release.gpg [1033B]
Get:2 http://www.readynas.com 4.2.19/ Release.gpg [490B]                      
Get:3 http://archive.debian.org etch Release [67.8kB]  
Get:4 http://www.readynas.com 4.2.19/ Release [1181B]
Ign http://www.readynas.com 4.2.19/ Release              
Get:5 http://www.readynas.com 4.2.19/ Packages [18.9kB]
Get:6 http://archive.debian.org etch/main Packages [5477kB]
Fetched 5567kB in 5s (1063kB/s)    
Reading package lists... Done
W: GPG error: http://www.readynas.com 4.2.19/ Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B7F02108D553259A
W: You may want to run apt-get update to correct these problems
nas-02:~# apt-get install unrar
Reading package lists... Done
Building dependency tree... Done
Package unrar is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package unrar has no installation candidate