【发布时间】：2018-01-24 22:16:25
【问题描述】：
我试图将 TCP SYN 数据包发送到我机器上端口 8000 上的服务器。然后，我想检查服务器是否以SYN ACK 响应。如果是这种情况，那么我会发回一个RST 数据包以中止连接。但是，当我嗅探我发送的SYN 数据包时，它告诉我 TCP 标头的虚假长度为 0，但事实并非如此。顺便说一句，我使用的嗅探器是tshark。这是我的代码：
在main 函数中，我运行这个：
FLAGS f = SYN;
tcp_scan("127.0.0.1",8000,f,0);
这个函数组装IP头：
struct iphdr* assemble_ip(char* dest,unsigned int proto) {
/* Assemble IP Layer */
struct iphdr* iph;
iph = malloc(sizeof(struct iphdr)); // allocate memory
if (iph == NULL) { // if the ip header is NULL
err();
return NULL;
}
srand((unsigned int)time(NULL)); // seed random number generator
/* Hardcoded values */
iph->version = 4; // the version
iph->tos = 0; // type of services
iph->ihl = 5; // internet header length
iph->id = htons(rand() % 65536); // random id
iph->ttl = rand() % 257; // ttl
iph->frag_off = 0; // fragment offset
if (iph->ttl < 64) iph->ttl += 64; // if TTL is not sufficient
iph->tot_len = htons(iph->ihl*4); // the internet header length
/* User defined values */
iph->saddr = inet_addr(client); // source address
iph->daddr = inet_addr(dest); // destination address
iph->protocol = proto; // protocol
iph->check = 0; // set to zero for later calculation
return iph;
}
此函数组装 TCP 标头：
struct tcphdr* assemble_tcp(unsigned int sport,unsigned int dport,FLAGS f) {
/* Assemble TCP layer */
struct tcphdr* tcph;
tcph = malloc(sizeof(struct tcphdr)); // allocate tcp header
if (tcph == NULL) { // if tcp is NULL
err();
return NULL;
}
bzero(tcph,sizeof(struct tcphdr));
srand((unsigned int)time(NULL)); // seed random number generator
/* Hardcoded values */
tcph->seq = htonl(rand() % 65001); // generate random sequence number
tcph->ack_seq = 0; // ack sequence should be 0
tcph->doff = 5; // set data offset
tcph->window = htons(rand() % 65536); // set window size
/* Increase values by random value above 64 */
if (ntohs(tcph->seq) < 64) tcph->seq += (rand() % 101 + 64);
if (ntohs(tcph->window) < 64) tcph->window += (rand() % 101 + 64);
/* User-defined values */
tcph->source = htons(sport); // source port
tcph->dest = htons(dport); // destination port
tcph = set_flags(tcph,f); // set the TCP flags
/* Set urgent ptr if URG flag is set*/
if (tcph->urg == 1) tcph->urg_ptr = 1;
else tcph->urg_ptr = 0;
tcph->check = 0; // set the checksum to 0 for other calculations
return tcph;
}
另外，我确实计算了标头的校验和。出于我的目的，在计算校验和时，IP 标头总是 20 字节长，因为我没有发送任何数据或选项。这意味着标题中有 10 个 16 位字。由于我没有添加任何选项或数据，因此 TCP 标头也将有 20 个字节长。代码如下：
unsigned short ip_checksum(struct iphdr* iph) {
/* Acquire IP checksum */
/*
Checksum for Internet Protocol:
One's complement of the one's complement sum of the 16 bit words in the header.
So we get the first 16 bits of the header then add it to the sum, and then
we get the next 16 bits, and add it, and so on.
...0100101010110101 -> "..." represents more bits
1111111111111111 -> this is 131071 in base 10
0000100101010110101 -> notice how the "..." bits are now 0's
*/
/* One's complement sum */
unsigned long long* ptr;
unsigned long long hdr;
unsigned short sum = 0;
unsigned long mask = 131071;
ptr = (unsigned long long*)iph; // cast structure
hdr = *ptr; // get hdr
for (int i = 0; i < 10; i++) { // 20 bytes -> 160 bits / 16 bits = 10 words
sum += (hdr & mask); // add to sum
hdr >>= 16; // shift the next 16 bits
}
sum = ~sum; // inverse
return sum;
}
TCP 校验和：
unsigned short tcp_checksum(struct tcphdr* tcph,struct iphdr* iph) {
/* Calculate TCP checksum */
struct pseudo_hdr* pseudo_hdr;
u_char* buffer;
u_char* segment;
u_char* pseudo_segment;
unsigned short sum = 0;
unsigned long mask = 131071;
unsigned long long* ptr;
unsigned long long hdr;
pseudo_hdr = malloc(sizeof(struct pseudo_hdr)); // allocate memory
buffer = malloc(32); // allocate for 32 bytes of information
if (pseudo_hdr == NULL || buffer == NULL) { // if memory wasn't allocated properly
err();
if (pseudo_hdr != NULL) free(pseudo_hdr);
if (buffer != NULL) free(buffer);
return 0;
}
pseudo_hdr->saddr = (unsigned long)iph->saddr; // we add the cast because the fields if of type u_int_32
pseudo_hdr->daddr = (unsigned long)iph->daddr; // same reason for adding the cast as above
memset(&pseudo_hdr->reserved,0,8); // set these 8 bits to 0
pseudo_hdr->proto = IPPROTO_TCP; // this will always be 6
pseudo_hdr->len = htons(tcph->doff*4); // length of tcp header
/* Place both headers into a buffer */
segment = (u_char*)tcph;
pseudo_segment = (u_char*)pseudo_hdr;
/* Concactenate */
strncat((char*)buffer,(char*)pseudo_segment,12); // first the pseudo header
strncat((char*)buffer,(char*)segment,20); // then the TCP segment
/* Calculate checksum just like IP checksum */
ptr = (unsigned long long*)buffer; // convert buffer
hdr = *ptr; // dereference for clarity in following clode
for (int i = 0; i < 16; i++) { // 32 bytes -> 256 bits / 16 bits = 16 words
sum += (hdr & mask); // apply mask to header and add to sum
hdr >>= 16; // shift the next 16 bits
}
sum = ~sum; // bitwise NOT operation
return sum;
};
这是我上面提到的所有功能：
int tcp_scan(char* ipaddr,unsigned int port,FLAGS f,unsigned int justsend) {
/* Do a TCP port scan */
u_char* buffer;
u_char recvbuf[65535];
u_char* ipbuf;
u_char* tcpbuf;
int s;
size_t bufsize;
size_t size;
struct sockaddr_in sa;
struct sockaddr_in recvstruct;
struct msghdr msg;
struct iovec iv[1];
struct iphdr* iph;
struct tcphdr* tcph;
FLAGS rst = RST;
srand((unsigned int)time(NULL)); // seed random number generator
bufsize = sizeof(struct tcphdr) + sizeof(struct iphdr); // store size in variable
buffer = malloc(bufsize); // allocate memory to buffer
iph = assemble_ip(ipaddr,IPPROTO_TCP); // set the ip address to provided and protocol as TCP
tcph = assemble_tcp(rand() % 65536,port,f); // set flag, source port as rand, and dest port as supplied port num
if (iph == NULL || tcph == NULL || buffer == NULL) { // if error occurs
err();
/* Deallocate memory to variables that still have it */
if (iph != NULL) free(iph);
if (tcph != NULL) free(tcph);
if (buffer != NULL) free(buffer);
return -1;
}
/* Now compute checksum */
iph->check = htons(ip_checksum(iph));
tcph->check = htons(tcp_checksum(tcph,iph));
/* Store headers in buffer */
ipbuf = (u_char*)iph;
tcpbuf = (u_char*)tcph;
/* Concactenate to buffer */
strncat((char*)buffer,(char*)tcpbuf,20); // copy only 20 bytes...this ensures that no extra bytes are catted
strncat((char*)buffer,(char*)ipbuf,20); // do same thing but with ip header
/* Create a socket */
s = create_socket(); // create a raw socket
if (s == -1) return -1; // if the socket wasn't able to be created
/* Clear memory */
bzero(&sa,sizeof(struct sockaddr_in));
bzero(&recvstruct,sizeof(struct sockaddr_in));
bzero(&msg,sizeof(struct msghdr));
bzero(&iv,sizeof(struct iovec));
/* For analyze_packet() */
sa.sin_family = AF_INET; // address family
sa.sin_addr.s_addr = inet_addr(ipaddr); // convert ip address
sa.sin_port = htons(port); // port number
/* For sendmsg() */
iv[0].iov_base = buffer;
iv[0].iov_len = bufsize;
msg.msg_name = &sa; // caller allocated buffer
msg.msg_namelen = sizeof(struct sockaddr_in); // specify size of buffer
msg.msg_iov = iv; // iov structure array
msg.msg_iovlen = 1; // the length of the array
msg.msg_control = NULL; // for ancillary data
msg.msg_controllen = 0; // sizeof ancillary data
if (sendmsg(s,&msg,0) == -1) {
err();
return -1;
}
printf("Sent\n");
if (justsend) return 0; // exit cleanly
bzero(&recvstruct,sizeof(struct sockaddr_in)); // clear structure
size = sizeof(struct sockaddr_in); // acquire size of recv structure
for(int i = 0; i < 100; i++) { // loop until we've received 100 packets
printf("Receiving\n");
bzero(recvbuf,65535); // clear memory
if (recvfrom(s,recvbuf,sizeof(recvbuf),0,(struct sockaddr*)&recvstruct,(socklen_t*)&size) == -1) { // recv
err();
return -1;
}
if (analyze_packet(recvbuf,sa,recvstruct) == 0) { // if packet is what we wanted
printf("\ttcp %d is open\n",port); // print out that port is opened
tcp_scan(ipaddr,port,rst,-1); // abort connection with RST flag
break;
}
}
return 0;
}
好的，现在你已经看到了，这是我用来嗅探数据包的“tshark”命令：
sudo tshark -o tcp.check_checksum:TRUE # I also wanted to check the checksum value to make sure it was OK
下面是运行程序的命令：
sudo ./netmap enp0s3 # enp0s3 is the interface I'm sending packets on
在单独的终端中运行这两个之后，tshark 提供以下输出：
1 0.000000 10.0.2.15 -> 127.0.0.1 TCP 74 31280->8000 [<None>] Seq=1 Win=0, bogus TCP header length (0, must be 20)
请注意struct iphdr 和struct tcphdr 的声明分别位于系统头文件<netinet/ip.h> 和<netinet/tcp.h> 中。
我真的不知道如何解决这个问题。事实上，我不确定是什么导致了这个问题，首先。据我所知，没有办法指定 TCP 标头的长度。任何帮助将不胜感激。
【问题讨论】：
-
转储数据包的十六进制并向我们展示。它可能就像字节序问题一样简单。
-
strncat() 不复制字节。
-
这一行：strncat((char*)buffer,(char*)pseudo_segment,12);失败的概率为 254:1，因为 buffer[] 数组，第一个字节之前没有设置为 0x00 并且数据不是 char 字符串并且可能包含一些 0x00 字节。建议使用
memcpy() -
是的，在我被通知使用
memcpy()后，我已经解决了这个问题。谢谢。