关于sudo的一些问题

今天在研究dsniff的时候，需要打开ip转发，执行命令如下：
 sudo echo 1> /proc/sys/net/ipv4/ip_forward

本意以为一切顺利，但是却出现这样的提示：
bash: /proc/sys/net/ipv4/ip_forward: Permission denied

百思不得其解，在Google大哥的帮助下找到了如下文章：

(来自：)

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May 17th 2006 Posted to ,

The other day I wanted to enable IP forwarding on my Linux box (so that it could forward packets from a tun virtual interface being used by to the physical interface connected to my home network).

I looked up it up and it turns out that it’s a simple setting in a file in the /proc filesystem, so I did what seemed obvious and logical at the time:

marca:~$ sudo echo "1" > /proc/sys/net/ipv4/ip_forward
-bash: /proc/sys/net/ipv4/ip_forward: Permission denied

I took this to mean that my kernel was not compiled with ip_forward support and then wasted a bunch of time building a new kernel.

Finally, it dawned on me. Duh. The echo command is a shell built-in so sudo has no effect.

I didn’t need a new kernel. All I had to do was:

marc:~$ sudo bash
root:~# sudo echo "1" > /proc/sys/net/ipv4/ip_forward 
root:~# cat /proc/sys/net/ipv4/ip_forward
1

or even:

marc:~$ sudo sh -c 'echo "1" > /proc/sys/net/ipv4/ip_forward'

Sigh.

I thought of the idea of preventing this in the future by defining a bash function that detects builtins:

function sudo()
{
    if [ $(type -t "$1") == "builtin" ]; then
        echo "sudo bash function: ERROR - \"$1\" is a shell builtin" 1>&2
        return 1
    fi

    command sudo "$@"
}

which works for some cases but unfortunately doesn’t help for the case above, because the redirection permissions are checked before the function is executed. Sigh.

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大概意思是说echo是一个shell-builtin的命令，所以sudo不起作用，因此可以换做如下任一命令：

$ sudo sh -c 'echo "1" > /proc/sys/net/ipv4/ip_forward'

或

$ sudo bash
# sudo echo "1" > /proc/sys/net/ipv4/ip_forward 

我经过试验是可以成功的，但是还是有些疑惑，为什么shell-builtin就不起作用了呢？sudo它本质上是个什么机制？
渴望各位的解答:-)