最近在学习 iptables NFQUEUE 的时候，顺手使用 Go 语言写了一个例子。

源代码：github.com/Asphaltt/go-nfnetlink-example

例子的效果

使用 iptables NFQUEUE 监听新建 tcp 连接：

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./nfnetlink-example
A new tcp connection will be established: 192.168.x.yyy:61883 -> xxx.yyy.219.56:80
A new tcp connection will be established: 192.168.x.yyy:61886 -> xxx.yyy.219.56:80
A new tcp connection will be established: 192.168.x.yyy:61887 -> xxx.yyy.235.132:80
A new tcp connection will be established: 192.168.x.yyy:61888 -> xxx.yyy.235.132:80
A new tcp connection will be established: 192.168.x.yyy:61889 -> xxx.yyy.137.56:443
A new tcp connection will be established: 192.168.x.yyy:61890 -> xxx.yyy.137.56:443

简洁的代码

使用了 go-nfnetlink 纯 Go 实现的 nfnetlink 库，不依赖 libnetfilter_queue Linux 系统库，编译后即可使用。

从 iptables NFQUEUE 中接收 tcp 连接的 SYN 包，并从 SYN 包中解析得到源 IP 地址、源 tcp 端口、目的 IP 地址、目的 tcp 端口等信息。

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	q := nfqueue.NewNFQueue(1)

	ps, err := q.Open()
	if err != nil {
		fmt.Printf("Error opening NFQueue: %v\n", err)
		os.Exit(1)
	}
	defer q.Close()

	for p := range ps {
		networkLayer := p.Packet.NetworkLayer()
		ipsrc, ipdst := networkLayer.NetworkFlow().Endpoints()

		transportLayer := p.Packet.TransportLayer()
		tcpsrc, tcpdst := transportLayer.TransportFlow().Endpoints()

		fmt.Printf("A new tcp connection will be established: %s:%s -> %s:%s\n",
			ipsrc, tcpsrc, ipdst, tcpdst)
		p.Accept()
	}

使用的 iptables 规则如下：

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iptables -t raw -I PREROUTING -p tcp --syn -j NFQUEUE --queue-num=1 --queue-bypass

在 raw 表 PREROUTING 链上匹配 tcp 连接的 SYN 包。

iptables NFQUEUE

NFQUEUE

This target passes the packet to userspace using the nfnetlink_queue handler. The packet is put into the queue identified by its 16-bit queue number. Userspace can inspect and modify the packet if desired. Userspace must then drop or reinject the packet into the kernel. Please see libnetfilter_queue for details. nfnetlink_queue was added in Linux 2.6.14. The queue-balance option was added in Linux 2.6.31, queue-bypass in 2.6.39.

• –queue-num value

  This specifies the QUEUE number to use. Valid queue numbers are 0 to 65535. The default value is 0.

• –queue-balance value**:**value

  This specifies a range of queues to use. Packets are then balanced across the given queues. This is useful for multicore systems: start multiple instances of the userspace program on queues x, x+1, .. x+n and use “–queue-balance x**:**x+n”. Packets belonging to the same connection are put into the same nfqueue.

• –queue-bypass

  By default, if no userspace program is listening on an NFQUEUE, then all packets that are to be queued are dropped. When this option is used, the NFQUEUE rule behaves like ACCEPT instead, and the packet will move on to the next table.

• –queue-cpu-fanout

  Available starting Linux kernel 3.10. When used together with –queue-balance this will use the CPU ID as an index to map packets to the queues. The idea is that you can improve performance if there’s a queue per CPU. This requires –queue-balance to be specified.

Doc: iptables-extensions