Martin Klier


Oracle 11g JDBC driver hangs blocked by /dev/random – entropy pool empty

On a headless (=without console) network server, the 11g JDBC driver used for (java) application connect may cause trouble. In my case, it refused to connect to the DB without any error, trace or log entry. It simply hung. After several hours, it connected one time, and freezed again. Remote debugging done by the development clarified that it locks after calling SeedGenerator() and SecureRandom().

Reason: The JDBC 11g needs about 40 bytes of secure random numbers, gathered from /dev/random, to encrypt its connect string.
But public-available “man 4 random” says:

When read, the /dev/random device will only return random bytes within the estimated number of bits of noise in the entropy pool. /dev/random should be suitable for uses that need very high quality randomness such as one-time pad or key generation. When the entropy pool is empty, reads from /dev/random will block until additional environmental noise is gathered.

So far so good, now the question arises: Why does this mystic “entropy pool” runs out of gas?
The answer is as simple as unsatisfying: because too less entropy “noise” was generated by the system. You can check the “filling level” (maybe zero?) of your pool and the overall size of the pool (usually 4096) by issuing

cat /proc/sys/kernel/random/entropy_avail
cat /proc/sys/kernel/random/poolsize

Hint: /dev/random will deliver one new random number as soon as the pool has reached more than 64 entropy units.

So why does my box not generate more entropy noise?
Because only few drivers will fill the entropy pool, first of all keyboard and mouse. Sounds very useful on a server in a datacenter, isn’t it? Some block device and network drivers seem to do so as well, and I have read from guys on the net changing their network card and driver to enjoy this “feature”! But let’s stop ranting, /dev/random is simply made for high security randomness, and if it can’t make sure that randomness is as good as possible in this deterministic world, it stops. Intelligent people have created /dev/urandom for that, like “man 4 random” clearly states:

A read from the /dev/urandom device will not block waiting for more entropy. As a result, if there is not sufficient entropy in the entropy pool, the returned values are theoretically vulnerable to a cryptographic attack on the algorithms used by the driver. Knowledge of how to do this is not available in the current non-classified literature, but it is theoretically possible that such an attack may exist. If this is a concern in your application, use /dev/random instead.

Now let’s get back on our JDBC problem. Oracle JDBC 11g seems to use /dev/random by default, which causes usually no trouble on clients running with console access by a user, because his/her unpredictable 🙂 actions will keep the entropy pool well-fed. But to make it usable on a headless server with a latently empty entropy pool, you should do several things, in descending security order (without warranty):

  1. Involve an audio entropy daemon like AED to gather noise from your datacenter with an open microphone, maybe combine it with a webcam noise collector like VED. Other sources are talking about “Cryptographic Randomness from Air Turbulence in Disk devices“. 🙂
  2. Use the Entropy Gathering Daemon to collect weaker entropy from randomness of userspace programs.
  3. Talk your JDBC into using /dev/urandom instead:

Enjoy Oracle on Linux/Unix!
Yours, Usn

EDIT: Corrected Air Turbulence link

EDIT: Have a look at haveged (collecting good entropy on basis of CPU clock flutter)

Creating Oracle AWR reports quicksheet
Get Oracle User DDL with dbms_metadata

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