还有一种“djb方式”。您可以使用此方法以运行在tcpserver下任何端口上的根用户身份启动您的进程，然后在进程启动后，它将把进程的控制权立即交给您指定的用户。

#!/bin/sh

UID=$(id -u username)
GID=$(id -g username)
exec tcpserver -u "${UID}" -g "${GID}" -RHl0 0 port /path/to/binary &

更多信息，请参见:http://thedjbway.b0llix.net/daemontools/uidgid.html

标准方法是将它们设置为“setuid”，以便它们以根用户身份启动，然后在它们绑定到端口但开始接受到该端口的连接之前丢弃根用户特权。您可以在Apache和INN的源代码中看到这样的好例子。我听说莱特特警局是另一个很好的例子。

另一个例子是Postfix，它使用多个通过管道通信的守护进程，其中只有一两个守护进程(除了接受或发出字节外几乎不做任何事情)以根身份运行，其余的守护进程以较低的权限运行。

或者修补内核并删除检查。

(最后的选择，不推荐)。

在net/ipv4/af_inet.c中，删除读取的两行

      if (snum && snum < PROT_SOCK && !capable(CAP_NET_BIND_SERVICE))
              goto out;

内核将不再检查特权端口。

我尝试了iptables PREROUTING REDIRECT方法。在旧的内核中，IPv6似乎不支持这种类型的规则。但显然，现在ip6tables v1.4.18和Linux内核v3.8支持它。

我还发现PREROUTING REDIRECT对机器内启动的连接不起作用。要处理来自本地机器的连接，还需要添加一个OUTPUT规则-参见iptables端口重定向不适用于本地主机。例如:

iptables -t nat -I OUTPUT -o lo -p tcp --dport 80 -j REDIRECT --to-port 8080

I also found that PREROUTING REDIRECT also affects forwarded packets. That is, if the machine is also forwarding packets between interfaces (e.g. if it's acting as a Wi-Fi access point connected to an Ethernet network), then the iptables rule will also catch connected clients' connections to Internet destinations, and redirect them to the machine. That's not what I wanted—I only wanted to redirect connections that were directed to the machine itself. I found I can make it only affect packets addressed to the box, by adding -m addrtype --dst-type LOCAL. E.g. something like:

iptables -A PREROUTING -t nat -p tcp --dport 80 -m addrtype --dst-type LOCAL -j REDIRECT --to-port 8080

另一种可能是使用TCP端口转发。例如使用socat:

socat TCP4-LISTEN:www,reuseaddr,fork TCP4:localhost:8080

然而，这种方法的一个缺点是，在端口8080上侦听的应用程序不知道传入连接的源地址(例如用于日志记录或其他识别目的)。

我知道这是一个老问题，但是现在有了最新的(>= 4.3)内核，这个问题终于有了一个很好的答案——环境能力。

快速的答案是从git中获取libcap的最新(尚未发布的)版本的副本并编译它。将生成的progs/capsh二进制文件复制到某个地方(/usr/local/bin是一个不错的选择)。然后，作为根用户，使用

/usr/local/bin/capsh --keep=1 --user='your-service-user-name' \
    --inh='cap_net_bind_service' --addamb='cap_net_bind_service' \ 
    -- -c 'your-program'

按顺序，我们是

声明当切换用户时，我们希望保持当前的功能集 切换用户和组到“your-service-user-name” 将cap_net_bind_service功能添加到继承的&环境集 派生bash -c 'your-command'(因为capsh会自动用——后面的参数启动bash)

这里隐藏着很多秘密。

Firstly, we are running as root, so by default, we get a full set of capabilities. Included in this is the ability to switch uid & gid with the setuid and setgid syscalls. However, ordinarily when a program does this, it loses its set of capabilities - this is so that the old way of dropping root with setuid still works. The --keep=1 flag tells capsh to issue the prctl(PR_SET_KEEPCAPS) syscall, which disables the dropping of capabilities when changing user. The actual changing of users by capsh happens with the --user flag, which runs setuid and setgid.

The next problem we need to solve is how to set capabilities in a way that carries on after we exec our children. The capabilities system has always had an 'inherited' set of capabilities, which is " a set of capabilities preserved across an execve(2)" [capabilities(7)]. Whilst this sounds like it solves our problem (just set the cap_net_bind_service capability to inherited, right?), this actually only applies for privileged processes - and our process is not privileged anymore, because we already changed user (with the --user flag).

新的环境能力集解决了这个问题——它是“在一个没有特权的程序的执行中保存的一组能力。”通过将cap_net_bind_service放在环境集中，当capsh exec执行我们的服务器程序时，我们的程序将继承此功能，并能够将侦听器绑定到低端口。

如果您有兴趣了解更多信息，功能手册页详细解释了这一点。通过strace运行capsh也是非常有用的!

还有一种“djb方式”。您可以使用此方法以运行在tcpserver下任何端口上的根用户身份启动您的进程，然后在进程启动后，它将把进程的控制权立即交给您指定的用户。

#!/bin/sh

UID=$(id -u username)
GID=$(id -g username)
exec tcpserver -u "${UID}" -g "${GID}" -RHl0 0 port /path/to/binary &

更多信息，请参见:http://thedjbway.b0llix.net/daemontools/uidgid.html