Thanks once again Cas,

The installed versions I have are:
qbittorrent: 2.9.7-1
libtorrent-rasterbar6: 0.15.10-1

For deluged I'm using v 1.3.5 with
python-libtorrent: 0.15.10
libtorrent-rasterbar6: 0.15.10-1

If it's of interest:
libboost: 1.46.1

You're right Cas, it doesn't seem to be due to libtorrent since everything works fine with qbittorrent and rtorrent (which use libtorrent-rasterbar6 and libtorrent14 respectively). Do you know what library is involved with the DNS lookup? If I can narrow it down to a particular library I can certain...

Right - so I removed (purged) all the deluge packages, and any packages with "libtorrent" in the name. Then rebooted and then reinstalled deluge. Still the same problem.

Any more debugging ideas?

Thanks again for the reply Cas. So yes, I checked the resolv.conf (before I upgraded Ubuntu) and it pointed to my router as expected. After I upgraded to Ubuntu 12.04 resolv.conf points to localhost since 12.04 uses dnsmasq for DNS. Since the error occurred before dnsmasq was installed, it seems unl...

Thanks for the reply Cas. Indeed, the underlying host tracker is actually torrent.ubuntu.com, but I can resolve that (and ping it) from the command-line. The deluge error just mentions ubuntu.com (and similar for other torrents). I appreciate that the error is probably generated by a (third-party) l...

Deluge was working fine on Ubuntu, and then I started getting the following for all my torrents: Tracker Status: ubuntu.com: Error: Host not found (authoritative) I can nslookup and ping ubuntu.com on the machine so the DNS is working. (Obviously the "ubuntu.com" bit varies according to th...