Can't open /dev/random的解决方法和一点/dev/random的资料【by joyride】

一. 解决“Can't open /dev/random”的问题

今天在移植appweb时遇到一个错误：Can't open /dev/random。搞了半天终于解决了。

如果我们使用的linux版本（1.3.30以上）支持random，那么运行下面的命令（必需以root身份运行）

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

即可在/dev下创建出两个设备文件random和urandom，上面的问题“Can't open /dev/random”解决。

经试验验证，设备random和urandom是必需成对创建（虽然只报告了Can't open /dev/random错误）。若没有创建urandom而只创建了random，仍然还是报告Can't open /dev/random错误。

二. 以下是关于/dev/random的一点资料

英文部分转自http://blog.csdn.net/wonder4/archive/2008/03/18/2194411.aspx，中文部分是我自己翻译的，水平有限，尽力而译（紫色文字是我自己加的，英文原文中没有；红色部分表示根据理解翻译的，不是直译过来的）。

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.

特殊字符文件/dev/random和/dev/urandom（linux从1.3.30内核起支持）提供了一个与linux内核中的随机数发生器的接口。文件/dev/random的主设备号是1，次设备号是8；文件/dev/urandom的主设备号是1，次设备号是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.

（太专业了，看不懂。大概的意思是：随机数发生器搜集一些环境噪声（从网上搜到的：例如，键盘驱动程序收集两个按键之间时间的信息）放到熵池中，同时熵池对噪声的位数进行估计，然后熵池用这些随机信息来生成随机数。）

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.

当读取/dev/random设备时将从熵池中返回一个随机数（return random bytes 我理解为返回的随机数由多个随机字节组成）。/dev/random应该适用于高性能的应用，如一次性密码或用于生成密匙等。一旦熵池为空时，读取/dev/random的操作将被阻塞，直到有新的环境噪声被（随机数发生器）捕获（，然后被放入熵池以产生出新的随机数）。

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.

（还是太专业，不太懂。大概意思是：/dev/urandom是/dev/random的非阻塞版本，即当熵池中没有可取的随机数时，读取/dev/urandom设备仍然会返回一个随机数，只是该随机数的安全性理论上不高。如果应用对安全性要求很高，那么应该使用/dev/random。）

Configuring

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

如果你的系统没有/dev/random和/dev/urandom，那么可以通过下面的命令来创建：

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:

在linux系统启动过程中，如果没有足够多的交互处理，那么熵池中的噪声就很少（可能低于估计随机数需要的数量），于是熵池产生随机数也容易被言中。为了处理这个问题，熵池可以从关机和启动过程来取得噪声。实现的方法是加入下面的行到一个合适的linux启动脚本中：

echo "Initializing random number generator..."
    random_seed=/var/run/random-seed
    # Carry a random seed from start-up to start-up
    # Load and then save the whole entropy pool
    if [ -f $random_seed ]; then
        cat $random_seed >/dev/urandom
    else
        touch $random_seed
    fi
    chmod 600 $random_seed
    poolfile=/proc/sys/kernel/random/poolsize
    [ -r $poolfile ] && bytes='cat $poolfile' || bytes=512
    dd if=/dev/urandom of=$random_seed count=1 bs=$bytes

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

加入系列行到一个合适的linux系统的关机执行脚本中：

# Carry a random seed from shut-down to start-up
    # Save the whole entropy pool
    echo "Saving random seed..."
    random_seed=/var/run/random-seed
    touch $random_seed
    chmod 600 $random_seed
    poolfile=/proc/sys/kernel/random/poolsize
    [ -r $poolfile ] && bytes='cat $poolfile' || bytes=512
    dd if=/dev/urandom of=$random_seed count=1 bs=$bytes

下面是关于/dev/random在proc目录下的一个接口。目录/proc/sys/kernel/random（linux 2.3.16版开始）下的文件提供了另外一个与/dev/random设备的接口。水平有限，就不再翻译了。

Proc Interface

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.

Files

/dev/random
/dev/urandom

Author

The kernel's random number generator was written by Theodore Ts'o ().