On Wed, 2011-08-03 at 12:04 +0200, Mike Hearn wrote:
> This is expected to happen from time to time of course as it's
> inherently racy, but there are a lot of bad nodes appearing in the DNS
> seeds.

> $ nmap -oG /tmp/x -p 8333 `dig +short bitseed.bitcoin.org.uk
> dnsseed.bluematt.me bitseed.xf2.org`
> ...
> Nmap done: 48 IP addresses (25 hosts up) scanned in 9.80 seconds

> $ grep -c 'closed' /tmp/x
> 6

> So of 48 IPs returned only 19 are actually usable. This is slowing
> down peer bringup for the Android apps, which don't currently save the
> addresses of last-used peers (yes, I know we should fix this).
Its actually much, much less.  You forgot to grep for filtered, which
are also worthless and you didn't make an actual connection to the node,
meaning there is no way to tell if the node has its connection slots
full (a node which has the maximum connection count will ack a syn, but
will drop the connection after the first message, so nmap thinks the
port is open).
I just tested and I show 0 accepting from bitseed.xf2.org and 0 from
bitcoin.bitcoin.co.uk.  dnsseed.bluematt.me rotates every 2 minutes to
the most recently checked so it tends to be pretty good if you get it
right after a rotate, if you wait to long, those slots fill up quick.
> 
> I was talking to a friend a few days ago about Bitcoin, he seemed
> interested. I'm hoping he might take on DNS seeding as a project. A
> custom DNS server that watches the network to find long-lived peers
> that run the latest version would be helpful for resolving this kind
> of thing.
Point him to https://github.com/TheBlueMatt/dnsseed it could use a bit
of cleanup, but it works.
If a different DNS Server were used to could pull directly from the
database in a more dynamic way it would probably work better too (it was
originally set up on MySQL and PowerDNS, but that is quite a resource
hog compared to SQLite and BIND, but the original backend is still there
and could work if you have a beefy enough server).

Matt