sejong.jeonjo@gmail.com

C language, Go(Golang), Netfilter, Netlink를 이용하여 Linux network을 제어하고 모니터링하는 방법을 알아보자~

개념 이해하기: Netfilter hooks into Linux networking packet flows

The following schematic shows packet flows through Linux networking:

Linux Netfilter + C example code

참고 문서:   https://pr0gr4m.github.io/linux/kernel/netfilter/
  이론 설명과 함께 잘 동작하는 예시가 있어서 쉽게 이해할 수 있다.
  아래는 위 블로그의 끝 부분에 있는 HTTP Traffic만 선별하여 Drop하는 예제 코드인데,
  그냥 이 예제 코드만 봐도 이해할 수 있을 것이다.

http_netfilter.c

#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/netfilter.h>
#include <linux/netfilter_ipv4.h>
#include <linux/ip.h>
#include <linux/tcp.h>
#include <linux/netdevice.h>

static unsigned int hook_http(void *priv,
		struct sk_buff *skb, const struct nf_hook_state *state)
{
	struct iphdr *iph;
	struct tcphdr *th;
	char *data = NULL;
	int length = 0;
	
	if (!skb)
		return NF_ACCEPT;

	iph = ip_hdr(skb);

	if (iph->protocol == IPPROTO_TCP) {
		th = tcp_hdr(skb);

		length = skb->len - iph->ihl * 4 - th->doff * 4;
		if (length <= 4)
			return NF_ACCEPT;

		data = kzalloc(length, GFP_KERNEL);
		memcpy(data, (unsigned char *)th + th->doff * 4, length);

		if (strstr(data, "HTTP") != NULL) {
			printk("[Kernel:%s] HTTP Detected\n", __func__);
			kfree(data);
			return NF_DROP;
		}

		kfree(data);
	}

	return NF_ACCEPT;
}

static struct nf_hook_ops *nf_ops = NULL;

static int __init nfilter_init(void)
{
	nf_ops = (struct nf_hook_ops *)kzalloc(sizeof(struct nf_hook_ops), GFP_KERNEL);
	nf_ops->hook = (nf_hookfn *)hook_http;
	nf_ops->pf = PF_INET;
	nf_ops->hooknum = NF_INET_LOCAL_IN;
	nf_ops->priority = NF_IP_PRI_FIRST;

	nf_register_net_hook(&init_net, nf_ops);
	printk("[Kernel:%s] NFilter Init\n", __func__);
	return 0;
}

static void __exit nfilter_exit(void)
{
	nf_unregister_net_hook(&init_net, nf_ops);
	kfree(nf_ops);
	printk("[Kernel:%s] NFilter Exit\n", __func__);
}

module_init(nfilter_init);
module_exit(nfilter_exit);
MODULE_LICENSE("GPL");

Makefile

obj-m += http_netfilter.o

KDIR := /lib/modules/$(shell uname -r)/build

default:
	$(MAKE) -C $(KDIR) M=$(PWD) modules

CC := gcc

%.c%:
	${CC} -o $@ $^

clean:
	$(MAKE) -C $(KDIR) M=$(PWD) clean
	rm -f ${TARGETS}

Build & Test

##
## HTTP Server 장비에서 아래 명령을 수행.
##

$ make

$ sudo insmod http_netfilter.ko

$


---

##
## 다른 PC에서 아래와 같이 HTTP Traffic을 발생시켜본다.
##
$ curl -v http://my-test.server.domain/

*   Trying my-test.server.domain:80...
* Connected to my-test.server.domain port 80 (#0)
> GET / HTTP/1.1
> Host: my-test.server.domain
> User-Agent: curl/7.77.0
> Accept: */*
> ##
## TCP Session만 수립될 뿐, 
## 실제 HTTP Response 패킷을 받지 못해서 이 상태로 계속 남아있다가 Timed out 처리된다.
##
* Recv failure: Operation timed out
* Closing connection 0
curl: (56) Recv failure: Operation timed out
$


---

##
## HTTP Server 장비에서 아래 명령을 수행.
##

$ dmesg --color --follow
... 중간 생략 ..
[264707.035282] [Kernel:hook_http] HTTP Detected
[264711.387549] [Kernel:hook_http] HTTP Detected
... 중간 생략 ..

Netlink for C language

Wikipedia

• https://en.wikipedia.org/wiki/Netlink

Netlink Protocol Library Suite (libnl)

• https://www.infradead.org/~tgr/libnl/

Core Library Developer's Guide (libnl)

• https://www.infradead.org/~tgr/libnl/doc/core.html

Routing Library Developer's Guide (libnl-route)

• https://www.infradead.org/~tgr/libnl/doc/route.html

Example Collection

• https://medium.com/thg-tech-blog/on-linux-netlink-d7af1987f89d
• https://olegkutkov.me/2018/02/14/monitoring-linux-networking-state-using-netlink/

위 문서를 읽고 나서, 아래 예제를 테스트하면서 이해하기.

$  cat detect_chg_event.c


/**
 * How to build
 *   $  gcc -o detect_chg_event detect_chg_event.c
 * How to run
 *   $  ./detect_chg_event
 */

#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <ifaddrs.h>
#include <net/if.h>
#include <netdb.h>
#include <netinet/in.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>


static int
create_sock (const char *nic)
{
    struct sockaddr_nl addr;
    int                sock;

    memset (&addr, 0, sizeof (addr));
    addr.nl_family = AF_NETLINK;
    addr.nl_groups = RTMGRP_LINK | RTMGRP_IPV4_IFADDR | RTMGRP_IPV6_IFADDR;

    sock = socket (PF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
    if (sock < 0)
    {
        fprintf (stderr, "failed to open NETLINK_ROUTE socket for %s - %s(%d)",
                nic, strerror (errno), errno);
        return -1;
    }

    if (bind (sock, (struct sockaddr *)&addr, sizeof(addr)) < 0)
    {
        fprintf (stderr, "failed to bind NETLINK_ROUTE socket for %s - %s(%d)",
                nic, strerror (errno), errno);
        close (sock);
        return -1;
    }

    return sock;
}


/**
 * NOTE: Cheeck if NIC status is changed
 */
static int
ip_changed (int         sock,
			const char *nic)
{
    struct nlmsghdr   *nlh;
    char               buffer[4096];
    int                len;
    int                idx;
    int                found;

    len = recv (sock, buffer, sizeof (buffer), 0);
    if (len <= 0)
    {
        fprintf (stderr, "NETLINK_ROUTE socket recv() failedn");
        return -1;
    }

    printf("\n %s %s(%d) Receive message from raw socket \n",
            __func__, __FILE__, __LINE__);

    found = 0;
    idx = if_nametoindex (nic);

    printf("\n %s %s(%d) Index of %s: %d \n",
            __func__, __FILE__, __LINE__, nic, idx);

    for (	nlh = (struct nlmsghdr *) buffer;
            NLMSG_OK (nlh, len);
            nlh = NLMSG_NEXT (nlh, len))
    {
        if (nlh->nlmsg_type == NLMSG_DONE)
            break;
        if (nlh->nlmsg_type == NLMSG_ERROR)
            continue;
        if (!(NLMSG_OK (nlh, len)))
            continue;

        printf("\n %s %s(%d) Netlink MSG Type: %d\n",
                __func__, __FILE__, __LINE__, nlh->nlmsg_type);

        /*
         * NOTE:
         *   RTM_NEWADDR, RTM_NEWLINK 에 관한 정의는 rtnetlink.h 파일에서 확인할 수 있다.
         *     - /usr/include/linux/rtnetlink.h
         */
        switch (nlh->nlmsg_type)
        {
            case RTM_NEWADDR:
                {
                    struct ifaddrmsg *ifa = (struct ifaddrmsg *)NLMSG_DATA (nlh);

                    if (ifa->ifa_index == idx)
                        found = 1;
                }
                break;
            case RTM_NEWLINK:
                {
                    struct ifinfomsg *ifi = (struct ifinfomsg *)NLMSG_DATA (nlh);

                    if (ifi->ifi_index == idx)
                        found = 1;
                }
                break;
            default:
                break;
        }
    }

    return found;
}


static int
get_nic_addr (  const char     *nic,
				struct ifaddrs *ifaddr,
				int             wanted_family,
				char           *host,
				int             host_len,
				int            *active)
{
    struct ifaddrs *ifa;

    for (ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next)
    {
        int family;
        int s;

        if (ifa->ifa_addr == NULL)
            continue;

        if (strcmp (ifa->ifa_name, nic))
            continue;

        /* Skip unwanted families. */
        family = ifa->ifa_addr->sa_family;
        if (family != wanted_family)
            continue;

        *active = (ifa->ifa_flags & IFF_RUNNING) ? 1 : 0;

        s = getnameinfo (   ifa->ifa_addr,
                            family == AF_INET ? sizeof (struct sockaddr_in) :
                            sizeof (struct sockaddr_in6),
                            host,
                            host_len,
                            NULL,
                            0,
                            NI_NUMERICHOST);
        if (s != 0)
        {
            fprintf (stderr, "failed to getnameinfo() for '%s - %s(%d)",
                    ifa->ifa_name, strerror (errno), errno);
            continue;
        }

        /* Get the address of only the first network interface card. */
        return 1;
    }

    return 0;
}


static void
print_ip (const char *nic)
{
    struct ifaddrs *ifaddr;
    char            addr[NI_MAXHOST];
    int             active;

    printf("\n %s(%d) nic: %s \n",
            __FILE__, __LINE__, nic);

    if (getifaddrs (&ifaddr) == -1)
    {
        fprintf (stderr, "failed to getifaddrs() - %s(%d)", strerror (errno), errno);
        return;
    }

    // NOTE: IPv4
    if (!get_nic_addr (nic, ifaddr, AF_INET, addr, sizeof (addr), &active))
    {
        // If IPv4 configuration is not available,
        // then try to get the Ipv6 configuration.
        printf("\n %s(%d) nic: %s  addr: %s  active: %d \n",
                __FILE__, __LINE__, nic, addr, active);
        // NOTE: IPv6
        if (!get_nic_addr (nic, ifaddr, AF_INET6, addr, sizeof (addr), &active))
        {
            // Nothing to do.
            strcpy (addr, "127.0.0.1");
            active = 0;
        }
    } else {
        printf("\n %s(%d) nic: %s  addr: %s  active: %d \n",
                __FILE__, __LINE__, nic, addr, active);
    }

    freeifaddrs (ifaddr);

    printf("\n %s %s(%d) %s is %s (link %s) \n",
            __func__, __FILE__, __LINE__,
            nic, addr, active ? "active" : "inactive");
}


int
main (void)
{
    // FIXME: enp7s0 --> my machine's network interface name
    char *nic = "enp7s0";
    int   sock;

    print_ip (nic);

    sock = create_sock (nic);
    if (sock < 0)
        return -1;

    while (1)
    {
        int ret;

        ret = ip_changed (sock, nic);
        if (ret < 0)
            return -1;

        if (ret)
            print_ip (nic);

        printf("\n\n %s %s(%d) END OF LOOP \n\n\n",
                __func__, __FILE__, __LINE__);
    }

    close (sock);

    return 0;
}


$

$  gcc -o detect_chg_event detect_chg_event.c

$  ./detect_chg_event

ip_changed detect_chg_event.c(73) Receive message from raw socket

 ip_changed detect_chg_event.c(79) Index of enp7s0: 2

 ip_changed detect_chg_event.c(93) Netlink MSG Type: 16

 detect_chg_event.c(181) nic: enp7s0

 detect_chg_event.c(205) nic: enp7s0  addr: 10.1.4.51  active: 1

 print_ip detect_chg_event.c(211) enp7s0 is 10.1.4.51 (link active)


 main detect_chg_event.c(239) END OF LOOP
 
 ...
 ...

위 예제에서 detect_chg_event 명령을 실행시켜 놓고, 아래와 같이 명령을 실행해본다.

$  ifconfig  enp7s0  mtu 1501
$  ifconfig  enp7s0  mtu 1500
$  ifconfig  enp7s0  down
$  ifconfig  enp7s0  up

detect_chg_event 예제 앱이 enp7s0 장치의 상태 변화를 감지해서 터미널에 감지한 내용을 출력해줄 것이다.

Netlink library for go

https://github.com/vishvananda/netlink

https://tomdnetto.net/post/linux_networking_from_go_nftables

https://tomdnetto.net/post/advanced_nftables_with_go