1) In the terminal, install netatalk so that your Ubuntu machine will communicate via AppleTalk. While you're there, make sure that avahi-daemon is installed (it is by default on 11.10):
sudo apt-get install netatalk avahi-daemon
2) Now edit the afpd config file (AppleTalk Filing Protocol) so we can communicate with the directory properly over the network. To see a description of each option, view the afpd website or "man afpd" in terminal:
sudo gedit /etc/netatalk/afpd.confAdd this to the bottom of the file:- -tcp -noddp -uamlist uams_guest.so,uams_dhx.so,uams_dhx2.so -n3) Create the directory for you TimeMachine backups:mkdir /dir/dir/YourBackup/FolderI chose /media/Backups/macbook/TimeMachine4) Edit the permissions for that folder so that everyone can have read/write access.5) Edit the netatalk AppleVolumes.default file to point to your directory:/media/Backups/macbook/TimeMachine "TimeMachine" cnidscheme:dbd options:usedots,tm,uprivThe "tm" enables for TimeMachine use. This page details all the options for AppleVolumes.default6) Tell the avahi daemon to broadcast AppleTalk over the network:sudo gedit /etc/avahi/services/afpd.serviceThis will open an empty (hopefully) file.Enter the following, save, and close:<?xml version="1.0" standalone='no'?><!--*-nxml-*--> <!DOCTYPE service-group SYSTEM "avahi-service.dtd"> <service-group> <name replace-wildcards="yes">%h</name> <service> <type>_afpovertcp._tcp</type> <port>548</port> </service> <service> <type>_device-info._tcp</type> <port>0</port> <txt-record>model=Xserve</txt-record> </service> </service-group>7) Restart the services to read in the new config files:service netatalk restart service avahi-deamon restart8) In order to not fill all of the hard drive space on the Ubuntu server with MacBook backups, create a disk image from OS X:Disk Utility > New ImageCreate whatever size image you'd like. This will look like an external hard drive to TimeMachine, which it will fill until it is out of space, and then start replacing the oldest data first.Save this to your networked folder on the Ubuntu server (/media/Backups/macbook/TimeMachine/backup.dmg)9) On the Mac, select the Go menu > Connect to Folder > Browse > TimeMachine10) Enable TimeMachine and see if that drive is available. If not, I found this small hack for OS X on engadget.com:defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1Enter that into your terminal and try again.