Difference between /dev/urandom and /dev/./urandom (Was: swingbench connection issue)

From: Yong Huang <yong321_at_yahoo.com>
Date: Wed, 14 Nov 2012 09:19:22 -0800 (PST)
Message-ID: <1352913562.27736.YahooMailClassic_at_web184806.mail.gq1.yahoo.com>

> This seems to have solved it
> https://kr.forums.oracle.com/forums/thread.jspa?threadID=941911
> *rm /dev/random*
> *ln -s /dev/urandom /dev/random
> *strange that the problem arises with one database on the box but not
> another on the same box

I have another puzzle related to /dev/(u)random. I posted somewhere else without an answer.

On RHEL 6, a very simple JDBC java program returns instantly if I pass /dev/./urandom to -Djava.security.egd (entropy gathering device). But if I pass /dev/urandom to it, or omit this option altogether (I assume it uses /dev/random), it takes a long time (unless entropy happens to have accumulated enough in the past, which doesn't happen often on this headless server).

$ time java -Djava.security.egd=file:/dev/./urandom -cp .:/u01/app/oracle/product/11.2.0/db/jdbc/lib/ojdbc6.jar:/u01/app/oracle/product/11.2.0/db/jdk/jre/lib TestClient <-- ojdbc5.jar behaves the same as ojdbc6 Got it. <-- this is just the output of TestClient

real    0m0.848s  <-- always subsecond
user    0m1.064s
sys     0m0.083s

$ time java -Djava.security.egd=file:/dev/urandom -cp .:/u01/app/oracle/product/11.2.0/db/jdbc/lib/ojdbc6.jar:/u01/app/oracle/product/11.2.0/db/jdk/jre/lib TestClient Got it.
real    0m7.999s  <-- anywhere from a few to tens of seconds
user    0m1.041s
sys     0m0.100s

This is very reproducible. I can't think of any difference whether there's one more "directory" layer "." between /dev/ and urandom. Literally, it just tells the process to go to *the current* directory which is already /dev/, before it goes on to read urandom. Any thought?

Received on Wed Nov 14 2012 - 18:19:22 CET

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