TLDR:对于“答案”(如我所见)，跳到这个答案中的>>TLDR<<部分。

好吧，我已经想明白了(这次是真的)，这个问题的答案，我的这个答案也是一种道歉的方式，因为我推广了另一个我认为是“最好的”的答案(在这里和推特上)，但在尝试之后，发现我错了。孩子们，从我的错误中吸取教训吧:在你自己真正尝试过之前，不要推销任何东西!

我在这里复习了所有的答案。我已经尝试了其中的一些(而选择不尝试其他的，因为我不喜欢这些解决方案)。我认为解决方案是使用systemd及其Capabilities=和capactiesbindingset = settings。在思考了一段时间后，我发现这不是解决方案，因为:

功能旨在限制根进程!

正如OP所明智地指出的那样，最好避免这种情况(如果可能的话，对于所有守护进程!)。

You cannot use the Capabilities related options with User= and Group= in systemd unit files, because capabilities are ALWAYS reset when execev (or whatever the function is) is called. In other words, when systemd forks and drops its perms, the capabilities are reset. There is no way around this, and all that binding logic in the kernel is basic around uid=0, not capabilities. This means that it is unlikely that Capabilities will ever be the right answer to this question (at least any time soon). Incidentally, setcap, as others have mentioned, is not a solution. It didn't work for me, it doesn't work nicely with scripts, and those are reset anyways whenever the file changes.

在我微薄的辩护中，我确实说过(在我现在删除的评论中)，詹姆斯的iptables建议(OP也提到了)是“第二优解决方案”。: - p

> > TLDR < <

解决方案是将systemd与动态iptables命令结合起来，就像这样(取自DNSChain):

[Unit]
Description=dnschain
After=network.target
Wants=namecoin.service

[Service]
ExecStart=/usr/local/bin/dnschain
Environment=DNSCHAIN_SYSD_VER=0.0.1
PermissionsStartOnly=true
ExecStartPre=/sbin/sysctl -w net.ipv4.ip_forward=1
ExecStartPre=-/sbin/iptables -D INPUT -p udp --dport 5333 -j ACCEPT
ExecStartPre=-/sbin/iptables -t nat -D PREROUTING -p udp --dport 53 -j REDIRECT --to-ports 5333
ExecStartPre=/sbin/iptables -A INPUT -p udp --dport 5333 -j ACCEPT
ExecStartPre=/sbin/iptables -t nat -A PREROUTING -p udp --dport 53 -j REDIRECT --to-ports 5333
ExecStopPost=/sbin/iptables -D INPUT -p udp --dport 5333 -j ACCEPT
ExecStopPost=/sbin/iptables -t nat -D PREROUTING -p udp --dport 53 -j REDIRECT --to-ports 5333
User=dns
Group=dns
Restart=always
RestartSec=5
WorkingDirectory=/home/dns
PrivateTmp=true
NoNewPrivileges=true
ReadOnlyDirectories=/etc

# Unfortunately, capabilities are basically worthless because they're designed to restrict root daemons. Instead, we use iptables to listen on privileged ports.
# Capabilities=cap_net_bind_service+pei
# SecureBits=keep-caps

[Install]
WantedBy=multi-user.target

在这里，我们完成了以下工作:

守护进程侦听5333，但是由于iptables，连接成功地在53上被接受 我们可以将命令包含在单元文件本身中，从而为人们省去了麻烦。Systemd为我们清理防火墙规则，确保在守护进程未运行时删除它们。 我们从不以根用户身份运行，并且使特权升级成为不可能(至少systemd声称是这样)，即使守护进程被破坏并设置uid=0。

不幸的是，Iptables仍然是一个相当丑陋且难以使用的实用工具。例如，如果守护进程正在侦听eth0:0而不是eth0，则命令略有不同。

端口重定向对我们来说是最有意义的，但我们遇到了一个问题，我们的应用程序将在本地解决一个url，也需要重新路由;(意思是你在狂欢)。

这也将允许您在访问本地机器上的url时被重定向。

iptables -A PREROUTING -t nat -p tcp --dport 80 -j REDIRECT --to-port 8080
iptables -A OUTPUT -t nat -p tcp --dport 80 -j REDIRECT --to-port 8080

2015年9月:

ip6tables现在支持IPV6 NAT: http://www.netfilter.org/projects/iptables/files/changes-iptables-1.4.17.txt

您将需要内核3.7+

证明:

[09:09:23] root@X:~ ip6tables -t nat -vnL
Chain PREROUTING (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
    0     0 REDIRECT   tcp      eth0   *       ::/0                 ::/0                 tcp dpt:80 redir ports 8080
    0     0 REDIRECT   tcp      eth0   *       ::/0                 ::/0                 tcp dpt:443 redir ports 1443

Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain OUTPUT (policy ACCEPT 6148 packets, 534K bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain POSTROUTING (policy ACCEPT 6148 packets, 534K bytes)
 pkts bytes target     prot opt in     out     source               destination

出于某种原因，没有人提到降低sysctl net.ipv4。Ip_unprivileged_port_start到您需要的值。 示例:我们需要将应用程序绑定到443端口。

sysctl net.ipv4.ip_unprivileged_port_start=443

有些人可能会说，存在潜在的安全问题:非特权用户现在可能绑定到其他特权端口(444-1024)。 但是你可以用iptables通过阻塞其他端口轻松解决这个问题:

iptables -I INPUT -p tcp --dport 444:1024 -j DROP
iptables -I INPUT -p udp --dport 444:1024 -j DROP

与其他方法的比较。这个方法:

from some point is (IMO) even more secure than setting CAP_NET_BIND_SERVICE/setuid, since an application doesn't setuid at all, even partly (capabilities actually are). For example, to catch a coredump of capability-enabled application you will need to change sysctl fs.suid_dumpable (which leads to another potential security problems) Also, when CAP/suid is set, /proc/PID directory is owned by root, so your non-root user will not have full information/control of running process, for example, user will not be able (in common case) to determine which connections belong to application via /proc/PID/fd/ (netstat -aptn | grep PID). has security disadvantage: while your app (or any app that uses ports 443-1024) is down for some reason, another app could take the port. But this problem could also be applied to CAP/suid (in case you set it on interpreter, e.g. java/nodejs) and iptables-redirect. Use systemd-socket method to exclude this problem. Use authbind method to only allow special user binding. doesn't require setting CAP/suid every time you deploy new version of application. doesn't require application support/modification, like systemd-socket method. doesn't require kernel rebuild (if running version supports this sysctl setting) doesn't do LD_PRELOAD like authbind/privbind method, this could potentially affect performance, security, behavior (does it? haven't tested). In the rest authbind is really flexible and secure method. over-performs iptables REDIRECT/DNAT method, since it doesn't require address translation, connection state tracking, etc. This only noticeable on high-load systems.

根据具体情况，我将在sysctl、CAP、authbind和iptables-redirect之间进行选择。我们有这么多选择真是太好了。

使用systemd，您只需要稍微修改您的服务以接受预激活的套接字。

稍后可以使用systemd套接字激活。

不需要任何功能、iptables或其他技巧。

这是这个简单的python http服务器示例中的相关systemd文件的内容

文件httpd-true.service

[Unit]
Description=Httpd true 

[Service]
ExecStart=/usr/local/bin/httpd-true
User=subsonic

PrivateTmp=yes

文件httpd-true.socket

[Unit]
Description=HTTPD true

[Socket]
ListenStream=80

[Install]
WantedBy=default.target

我的“标准解决方案”使用socat作为用户空间重定向器:

socat tcp6-listen:80,fork tcp6:8080

注意，这不会扩展，分叉是昂贵的，但这是socat工作的方式。