Messages in this thread |  | | | Date | Tue, 04 Dec 2007 15:16:53 +0100 | | From | Eric Dumazet <> | | Subject | Re: Why does reading from /dev/urandom deplete entropy so much? |
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Marc Haber a écrit : > While debugging Exim4's GnuTLS interface, I recently found out that > reading from /dev/urandom depletes entropy as much as reading from > /dev/random would. This has somehow surprised me since I have always > believed that /dev/urandom has lower quality entropy than /dev/random, > but lots of it. > > This also means that I can "sabotage" applications reading from > /dev/random just by continuously reading from /dev/urandom, even not > meaning to do any harm. > > Before I file a bug on bugzilla, can I ask why /dev/urandom wasn't > implemented as a PRNG which is periodically (say, every 1024 bytes or > even more) seeded from /dev/random? That way, /dev/random has a much > higher chance of holding enough entropy for applications that really > need "good" entropy. > > Entropy depletion has become an issue on Linux systems since the > network was removed from the group of entropy sources a few years ago. > Of the around 20 Linux systems that I currently have root privileges > on, only two have a hardware RNG. > Some network drivers still feed entropy pools :
$ find drivers/net | xargs grep -n IRQF_SAMPLE_RANDOM drivers/net/mv643xx_eth.c:1336: IRQF_SHARED | IRQF_SAMPLE_RANDOM, dev->name, dev); drivers/net/3c527.c:437: err = request_irq(dev->irq, &mc32_interrupt, IRQF_SHARED | IRQF_SAMPLE_RANDOM, DRV_NAME, dev); drivers/net/xen-netfront.c:1371: IRQF_SAMPLE_RANDOM, netdev->name, drivers/net/macb.c:1131: err = request_irq(dev->irq, macb_interrupt, IRQF_SAMPLE_RANDOM, drivers/net/tg3.c:7268: flags = IRQF_SAMPLE_RANDOM; drivers/net/tg3.c:7273: flags = IRQF_SHARED | IRQF_SAMPLE_RANDOM; drivers/net/tg3.c:7291: IRQF_SHARED | IRQF_SAMPLE_RANDOM, dev->name, dev); drivers/net/qla3xxx.c:3648: unsigned long irq_flags = IRQF_SAMPLE_RANDOM | IRQF_SHARED; drivers/net/3c523.c:292: ret = request_irq(dev->irq, &elmc_interrupt, IRQF_SHARED | IRQF_SAMPLE_RANDOM, drivers/net/cris/eth_v10.c:704: IRQF_SAMPLE_RANDOM, cardname, (void *)dev)) { drivers/net/atl1/atl1_main.c:1967: int irq_flags = IRQF_SAMPLE_RANDOM; drivers/net/niu.c:4647: IRQF_SHARED | IRQF_SAMPLE_RANDOM, drivers/net/netxen/netxen_nic_main.c:866: IRQF_SHARED|IRQF_SAMPLE_RANDOM, netdev->name, drivers/net/ibmlana.c:784: result = request_irq(priv->realirq, irq_handler, IRQF_SHARED | IRQF_SAMPLE_RANDOM, dev->name, dev); So I guess you could patch your kernel to get more entropy sources (even if they are not *perfect* for some purists)
> What can I do to keep sufficiently high entropy pools even if I read > from /dev/urandom more than once in a while? > > Greetings > Marc > >
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