浅谈原始套接字 SOCK_RAW 的内幕及其应用（port scan, packet sniffer, syn flood, icmp flood）
一、SOCK_RAW 内幕
首先在讲SOCK_RAW 之前，先来看创建socket 的函数：
int socket(int domain, int type, int protocol);
domain ：指定通信协议族（protocol family/address）
/usr/include/i386-linux-gnu/bits/socket.h
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/* Supported address families. */
#define AF_UNSPEC 0 #define AF_UNIX 1 /* Unix domain sockets */ #define AF_LOCAL 1 /* POSIX name for AF_UNIX */ #define AF_INET 2 /* Internet IP Protocol */ #define PF_PACKET 17 /* Packet family. */
/* ... */ /* Protocol families, same as address families. */ #define PF_UNSPEC AF_UNSPEC #define PF_UNIX AF_UNIX #define PF_LOCAL AF_LOCAL #define PF_INET AF_INET #define AF_PACKET PF_PACKET
/* ... */ |
type：指定socket类型（type）
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enum sock_type
{ SOCK_STREAM = 1, SOCK_DGRAM = 2, SOCK_RAW = 3, SOCK_RDM = 4, SOCK_SEQPACKET = 5, SOCK_DCCP = 6, SOCK_PACKET = 10, }; |
protocol ：协议类型（protocol）
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/* Standard well-defined IP protocols. */
enum { IPPROTO_IP = 0, /* Dummy protocol for TCP */ IPPROTO_ICMP = 1, /* Internet Control Message Protocol */ IPPROTO_IGMP = 2, /* Internet Group Management Protocol */ IPPROTO_IPIP = 4, /* IPIP tunnels (older KA9Q tunnels use 94) */ IPPROTO_TCP = 6, /* Transmission Control Protocol */ IPPROTO_EGP = 8, /* Exterior Gateway Protocol */ IPPROTO_PUP = 12, /* PUP protocol */ IPPROTO_UDP = 17, /* User Datagram Protocol */ IPPROTO_IDP = 22, /* XNS IDP protocol */ IPPROTO_DCCP = 33, /* Datagram Congestion Control Protocol */ IPPROTO_RSVP = 46, /* RSVP protocol */ IPPROTO_GRE = 47, /* Cisco GRE tunnels (rfc 1701,1702) */ IPPROTO_IPV6 = 41, /* IPv6-in-IPv4 tunnelling */ IPPROTO_ESP = 50, /* Encapsulation Security Payload protocol */ IPPROTO_AH = 51, /* Authentication Header protocol */ IPPROTO_BEETPH = 94, /* IP option pseudo header for BEET */ IPPROTO_PIM = 103, /* Protocol Independent Multicast */ IPPROTO_COMP = 108, /* Compression Header protocol */ IPPROTO_SCTP = 132, /* Stream Control Transport Protocol */ IPPROTO_UDPLITE = 136, /* UDP-Lite (RFC 3828) */ IPPROTO_RAW = 255, /* Raw IP packets */ IPPROTO_MAX }; |
你是否曾经有过这样的疑惑，当我们在Linux下这样调用 socket(AF_INET, SOCK_STREAM, 0); 时，第三个参数为0，内核是如何找到合适的协议如IPPROTO_TCP 的？实际上是调用 pffindtype 函数实现的。下面来看看FreeBSD的源码，linux 的实现差不多，有个小区别等会指出。
在freeBSD 上创建一个socket 会调用socreate() 函数：
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/*
* socreate returns a socket with a ref count of 1. The socket should be * closed with soclose(). */ int socreate(int dom, struct socket **aso, int type, int proto, struct ucred *cred, struct thread *td) { struct protosw *prp; struct socket *so; int error; if (proto) prp = pffindproto(dom, proto, type); else prp = pffindtype(dom, type); /* .... */ } |
从函数可以看出当proto 为0 则调用pffindtype() 函数，否则调用pffindproto() 函数，两个函数如下：
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struct protosw *
pffindtype(int family, int type) { struct domain *dp; struct protosw *pr; for (dp = domains; dp; dp = dp->dom_next) if (dp->dom_family == family) goto found; return (0); found: for (pr = dp->dom_protosw; pr < dp->dom_protoswNPROTOSW; pr++) if (pr->pr_type && pr->pr_type == type) return (pr); return (0); } struct protosw * pffindproto(int family, int protocol, int type) { struct domain *dp; struct protosw *pr; struct protosw *maybe = 0; if (family == 0) return (0); for (dp = domains; dp; dp = dp->dom_next) if (dp->dom_family == family) goto found; return (0); found: for (pr = dp->dom_protosw; pr < dp->dom_protoswNPROTOSW; pr++) { if ((pr->pr_protocol == protocol) && (pr->pr_type == type)) return (pr); if (type == SOCK_RAW && pr->pr_type == SOCK_RAW && pr->pr_protocol == 0 && maybe == (struct protosw *)0) maybe = pr; } return (maybe); } |
不要被它吓到了，其实不难理解，但理解之前需要知道的是struct protosw 是个结构体，里面有.pr_type（SOCK_XXX） 和.pr_protocol（ IPPROTO_XXX ）等成员，所有的struct protosw 结构体存储于一个 inetsw[] 数组中，此外有一个全局的domain 链表，其中一个节点inetdomain 的成员指针指向了inetsw[] 数组，大致图形如下（不是很准确）：
注意最后一个wildcare entry，它的.pr_protocol 没有赋值故为0，如下
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/* raw wildcard */
{ .pr_type = SOCK_RAW, .pr_domain = &inetdomain,
.pr_flags = PR_ATOMIC | PR_ADDR, .pr_input = rip_input, .pr_ctloutput = rip_ctloutput, .pr_init = rip_init, .pr_usrreqs = &rip_usrreqs }, }; /* end of inetsw[] */ |
回过头来看pffindtype 和 pffindproto：
pffindtype: 1. 通过"family" 参数找到对应的domain 节点
2. 返回inetsw [] 数组中匹配“type" 参数的第一个struct protosw 结构体指针
pffindproto: 1. 通过"family" 参数找到对应的domain 节点
2.
返回inetsw [] 数组中匹配“type" --”protocol“ 参数对的第一个struct protosw 结构体指针
3. 如果参数对不匹配而且”type" 为 SOCK_RAW，则返回wildcard entry 指针
假设现在这样调用 socket(AF_INET, SOCK_RAW, 30); 则使用pffindproto() 函数查找，但因为协议值30未在内核中定义，故返回wildcard_RAW entry。同理，你可能看见过别人这样写：socket(AF_INET, SOCK_RAW, IPPROTO_TCP); 实际上在FreeBSD 下 用pffindproto 找，SOCK_RAW 与 IPPROTO_TCP 也是不匹配的，返回wildcard_RAW entry 。
再者，在FreeBSD 上这样调用 socket(AF_INET, SOCK_RAW, 0/* IPPRORO_IP*/); 是可以的，使用pffindtype() 函数查找，返回的第一个是default entry；但在linux 上这样调用会出错，errno = EPROTONOSUPPORT，这就是前面提到的两个系统中不同点。为什么会出错，看linux 源码：
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/* Upon startup we insert all the elements in inetsw_array[] into
* the linked list inetsw. */ static struct inet_protosw inetsw_array[] = { { .type = SOCK_STREAM, .protocol = IPPROTO_TCP, .prot = &tcp_prot, .ops = &inet_stream_ops, .capability = -1, .no_check = 0, .flags = INET_PROTOSW_PERMANENT | INET_PROTOSW_ICSK, }, { .type = SOCK_DGRAM, .protocol = IPPROTO_UDP, .prot = &udp_prot, .ops = &inet_dgram_ops, .capability = -1, .no_check = UDP_CSUM_DEFAULT, .flags = INET_PROTOSW_PERMANENT, }, { .type = SOCK_RAW, .protocol = IPPROTO_IP, /* wild card */ .prot = &raw_prot, .ops = &inet_sockraw_ops, .capability = CAP_NET_RAW, .no_check = UDP_CSUM_DEFAULT, .flags = INET_PROTOSW_REUSE, } }; |
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static int inet_create(struct net *net, struct socket *sock, int protocol)
{ /* ... */ /* Look for the requested type/protocol pair. */ answer = NULL; lookup_protocol: err = -ESOCKTNOSUPPORT; rcu_read_lock(); list_for_each_rcu(p, &inetsw[sock->type]) { answer = list_entry(p, struct inet_protosw, list); /* Check the non-wild match. */ if (protocol == answer->protocol) { if (protocol != IPPROTO_IP) break; } else { /* Check for the two wild cases. */ if (IPPROTO_IP == protocol) { protocol = answer->protocol; break; } if (IPPROTO_IP == answer->protocol) break; } err = -EPROTONOSUPPORT; answer = NULL; } /* ... */ } |
在这里提醒一下IPPROTO_IP = 0， 在inet_create()函数中，我们根据type的值，在全局数组struct inet_protosw inetsw[]里找到我们对应的协议转换开关。下面通过来分析几个调用来走一下上面的inet_create 函数（linux 下）：
1) socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
protocol = 6
*answer =
inetsw_array[0]
protocol == answer->protocol && protocol != IPPROTO_IP : TRUE
OK
2) socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
protocol = 17
*answer = inetsw_array[1]
protocol == answer->protocol && protocol != IPPROTO_IP : TRUE
OK
3) socket(AF_INET, SOCK_STREAM, 0);
protocol = 0
*answer = inetsw_array[0]
if (protocol == answer->protocol) : FALSE
check else :
/* Check for the two wild cases. */
if (IPPROTO_IP == protocol) {
protocol = answer->protocol;
break;
}
: TRUE
note that protocol value 0 is substituted with the real value of IPPROTO_TCP in line:
protocol = answer->protocol;
OK
/* 上面例子（3）解释了文章最开始提出的疑问，现在protocol 已经被替换成了6 */
4) socket(AF_INET, SOCK_DGRAM, 0);
protocol = 0
*answer = inetsw_array[1]
if (protocol == answer->protocol) : FALSE
check else :
/* Check for the two wild cases. */
if (IPPROTO_IP == protocol) {
protocol = answer->protocol;
break;
}
: TRUE
note that protocol value 0 is substituted with the real value of IPPROTO_UDP in line:
protocol = answer->protocol;
OK
5) socket(AF_INET, SOCK_RAW, 0);
protocol = 0
*answer = inetsw_array[2]
protocol == answer->protocol && protocol == IPPROTO_IP so : if (protocol != IPPROTO_IP) is FALSE
not OK -> EPROTONOSUPPORT
6) socket(AF_INET, SOCK_STREAM, 9); (where 9 can be any protocol except IPPROTO_TCP)
protocol = 9
*answer = inetsw_array[0]
if (protocol == answer->protocol) : FALSE
check else :
/* Check for the two wild cases. */
if (IPPROTO_IP == protocol) {
protocol = answer->protocol;
break;
}
if (IPPROTO_IP == answer->protocol)
break;
both are a FALSE
not OK -> EPROTONOSUPPORT
7) socket(AF_INET, SOCK_DGRAM, 9); (where 9 can be any protocol except IPPROTO_UDP)
same as above
not OK -> EPROTONOSUPPORT
8) socket(AF_INET, SOCK_RAW, 9); (where 9 can be *any* protocol except 0)
protocol = 9
*answer = inetsw_array[2]
if (protocol == answer->protocol) : FALSE
check else :
/* Check for the two wild cases. */
if (IPPROTO_IP == protocol) {
protocol = answer->protocol;
break;
}
: FALSE
if (IPPROTO_IP == answer->protocol)
break;
: TRUE
OK
那raw socket 接收缓冲区的数据是什么呢？看下面这个图：
真正从网卡进来的数据是完整的以太网帧，底层用sk_buff 数据结构描述，最终进入接收缓冲区recv buffer，而我们应用层调用read / recv /recvfrom 从接收缓冲区拷贝数据到应用层提供的buffer，对一般的套接字，如SOCK_STREAM， SOCK_DGRAM 来说，此时缓冲区只有user data，其他各层的头部已经被去除，而对于SOCK_RAW 来说是IP head + IP payload，当然也可以是arp/rarp 包，甚至是完整的帧（加上MAC头）。
假设现在我们要通过SOCK_RAW 发送数据，则需要调用setsockopt 设置IP_HDRINCL 选项（如果protocol 设为IPPROTO_RAW 则默认设置了IP_HDRINCL），即告诉内核我们自己来封装IP头部，其实头部中某些元素是可以偷懒让内核填充的：
需要注意的是，如果我们自己来封装IP头部，那么数据包传递出去的时候IP 层就不会参与运作，即如果数据包大于接口的MTU，那么不会进行分片而直接丢弃。
二、SOCK_RAW 应用
1、packet sniffer
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sock_raw = socket(AF_INET , SOCK_RAW , IPPROTO_TCP);
while(1) { data_size = recvfrom(sock_raw , buffer , 65535 , 0 , &saddr , &saddr_size); //Now process the packet ProcessPacket(buffer , data_size); } |
即创建原始套接字，调用recvfrom 接收数据，再调用processpacket 处理IP包，可以读出ip head 和 tcp head 各字段。
上述程序只可以接收tcp 包，当然udp 和 icmp 可以这样写：
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sock_raw = socket(AF_INET , SOCK_RAW , IPPROTO_UDP);
sock_raw = socket(AF_INET , SOCK_RAW , IPPROTO_ICMP); |
但是不能以为 sock_raw = socket(AF_INET , SOCK_RAW , IPPROTO_IP); 就能接收所有种类的IP包，如前所述，这是错误的。
上述程序只能监测到输入的数据包，而且读取的数据包中已经没有了以太网头部。
只需要稍稍改进一下：
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sock_raw = socket( AF_PACKET , SOCK_RAW , htons(ETH_P_ALL)) ;
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ETH_P_IP 0X0800只接收发往目的MAC是本机的IP类型的数据帧
ETH_P_ARP 0X0806只接收发往目的MAC是本机的ARP类型的数据帧
ETH_P_RARP 0X8035只接受发往目的MAC是本机的RARP类型的数据帧
ETH_P_ALL 0X0003接收发往目的MAC是本机的所有类型(ip,arp,rarp)的数据帧，同时还可以接收从本机发出去的所有数据帧。在混杂模式打开的情况下，还会接收到发往目的MAC为非本地硬件地址的数据帧。
注意family 是AF_PACKET，这样就能监测所有输入和输出的数据包，而且不仅限于IP包（tcp/udp/icmp），如arp/rarp 包也可以监测，并且数据包还包含以太网头部。最后提一点，packet sniffer 也可以使用libpcap 库实现，著名的tcpdump 就使用了此库。
2、Tcp syn port scan
TCP 三次握手就不说了，端口扫描过程如下：
1. Send a Syn packet to a port A
2. Wait for a reply of Syn+Ack till timeout.
3. Syn+Ack reply means the port is open , Rst packet means port is
closed , and otherwise it might be inaccessible or in a filtered state.
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//Create a raw socket
int s = socket (AF_INET, SOCK_RAW , IPPROTO_TCP); if (setsockopt (s, IPPROTO_IP, IP_HDRINCL, val, sizeof (one)) < 0) { printf ("Error setting IP_HDRINCL. Error number : %d . Error message : %s \n" , errno , strerror(errno)); exit(0); } for(port = 1 ; port < 100 ; port++) { //Send the packet if ( sendto (s, datagram , sizeof(struct iphdr) + sizeof(struct tcphdr) , 0 , (struct sockaddr *) &dest, sizeof (dest)) < 0) { printf ("Error sending syn packet. Error number : %d . Error message : %s \n" , errno , strerror(errno)); exit(0); } } |
创建一个原始套接字s，开启IP_HDRINCL 选项（这两步可以直接用 int s = socket (AF_INET, SOCK_RAW, IPPROTO_RAW); ），自己封装IP 头部和tcp 头部，主要是标志位syn 置为1，然后循环端口进行发送数据包。另开一个线程创建另一个原始套接字，仿照packet sniffer 进行数据包的接收，分解tcp 头部看是否syn == 1 && ack == 1 && dest_addr == src_addr，如果是则表明端口是打开的。如果不追求效率，很简单的做法是直接用普通的套接字，循环端口去connect，成功就表明端口是打开的，只是三次握手完整了一回。
3、SYN Flood DOS Attack
仿照上面端口扫描程序，自己封装头部，主要是syn 置为1，然后在一个死循环中死命地对某个地址发送数据包。不过现在的网站一般有防火墙，我们这种小儿科程序对他们来说，跟玩一样。
4、ICMP ping flood
实际上跟SYN flood 类似的道理，不过发送的是icmp 包，即自己封装icmp 头部
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//Raw socket - if you use IPPROTO_ICMP, then kernel will fill in the correct ICMP header checksum, if IPPROTO_RAW, then it won't
int sockfd = socket (AF_INET, SOCK_RAW, IPPROTO_RAW); if (sockfd < 0) { perror("could not create socket"); return (0); } int on = 1; // We shall provide IP headers if (setsockopt (sockfd, IPPROTO_IP, IP_HDRINCL, (const char *)&on, sizeof (on)) == -1) { perror("setsockopt"); return (0); } //allow socket to send datagrams to broadcast addresses if (setsockopt (sockfd, SOL_SOCKET, SO_BROADCAST, (const char *)&on, sizeof (on)) == -1) { perror("setsockopt"); return (0); } while (1) { if ( (sent_size = sendto(sockfd, packet, packet_size, 0, (struct sockaddr *) &servaddr, sizeof (servaddr))) < 1) { perror("send failed\n"); break; } usleep(10000); //microseconds } |
附录：
1、相关头文件
#include<netinet/ip_icmp.h> //Provides declarations for icmp header
#include<netinet/udp.h> //Provides declarations for udp header
#include<netinet/tcp.h> //Provides declarations for tcp header
#include<netinet/ip.h> //Provides declarations for ip header
#include<netinet/if_ether.h> //For ETH_P_ALL
#include<net/ethernet.h> //For ether_header
2、计算校验和的函数
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/*
Function calculate checksum */ unsigned short in_cksum(unsigned short *ptr, int nbytes) { register long sum; u_short oddbyte; register u_short answer; sum = 0; while (nbytes > 1) { sum += *ptr++; nbytes -= 2; } if (nbytes == 1) { oddbyte = 0; *((u_char *) & oddbyte) = *(u_char *) ptr; sum += oddbyte; } sum = (sum >> 16) + (sum & 0xffff); sum += (sum >> 16); answer = ~sum; return (answer); } |
注意，IP头部中的校验和只校验ip头部的大小，而tcp 头部的校验和需要校验tcp头部和数据，按照封包原则，封装到TCP层的时候，ip信息还没有封装上去，但是校验值却需要马上进行计算，所以必须手工构造一个伪头部来表示ip层的信息，可以使用下面的结构体：
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struct pseudo_header //needed for checksum calculation
{ unsigned int source_address; unsigned int dest_address; unsigned char placeholder; // 0 unsigned char protocol; unsigned short tcp_length; struct tcphdr tcp; //tcp head }; |
将pseduo_header 和 use_data 都拷贝到同个缓冲区，传递给in_cksum 的ptr 为缓冲区起始地址，bytes 为总共的大小。
参考：
TCP Implementation in Linux: A Brief Tutorial.pdf
《UNP》
《TCP/IP 协议详解 卷一》
