urandom(4) -- Linux man page



random, urandom - kernel random number source devices  


The character special files /dev/random and /dev/urandom (present since Linux 1.3.30) provide an interface to the kernel's random number generator. File /dev/random has major device number 1 and minor device number 8. File /dev/urandom has major device number 1 and minor device number 9.

The random number generator gathers environmental noise from device drivers and other sources into an entropy pool. The generator also keeps an estimate of the number of bits of noise in the entropy pool. From this entropy pool random numbers are created.

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.

When read, /dev/urandom device will return as many bytes as are requested. 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.  


If your system does not have /dev/random and /dev/urandom created already, they can be created with the following commands:

        mknod -m 644 /dev/random c 1 8
        mknod -m 644 /dev/urandom c 1 9
        chown root:root /dev/random /dev/urandom

  When a Linux system starts up without much operator interaction, the entropy pool may be in a fairly predictable state. This reduces the actual amount of noise in the entropy pool below the estimate. In order to counteract this effect, it helps to carry entropy pool information across shut-downs and start-ups. To do this, add the following lines to an appropriate script which is run during the Linux system start-up sequence:

        echo "Initializing kernel random number generator..."
        # Initialize kernel random number generator with random seed 
        # from last shut-down (or start-up) to this start-up.  Load and 
        # then save 512 bytes, which is the size of the entropy pool.
        if [ -f /var/random-seed ]; then
                cat /var/random-seed >/dev/urandom
        dd if=/dev/urandom of=/var/random-seed count=1

Also, add the following lines in an appropriate script which is run during the Linux system shutdown:

        # Carry a random seed from shut-down to start-up for the random 
        # number generator.  Save 512 bytes, which is the size of the 
        # random number generator's entropy pool.
        echo "Saving random seed..."
        dd if=/dev/urandom of=/var/random-seed count=1


The files in the directory /proc/sys/kernel/random (present since 2.3.16) provide an additional interface to the /dev/random device.

The read-only file entropy_avail gives the available entropy. Normally, this will be 4096 (bits), a full entropy pool.

The file poolsize gives the size of the entropy pool. Normally, this will be 512 (bytes). It can be changed to any value for which an algorithm is available. Currently the choices are 32, 64, 128, 256, 512, 1024, 2048.

The file read_wakeup_threshold contains the number of bits of entropy required for waking up processes that sleep waiting for entropy from /dev/random. The default is 64. The file write_wakeup_threshold contains the number of bits of entropy below which we wake up processes that do a select() or poll() for write access to /dev/random. These values can be changed by writing to the files.

The read-only files uuid and boot_id contain random strings like 6fd5a44b-35f4-4ad4-a9b9-6b9be13e1fe9. The former is generated afresh for each read, the latter was generated once.  




The kernel's random number generator was written by Theodore Ts'o (tytso@athena.mit.edu).  


mknod (1)
RFC 1750, "Randomness Recommendations for Security"