是否可以通过udp / snmp发送多少个字符？

时间：2017-08-23 18:53:05

标签： c snmp

我试图通过UDP / SNMP从服务器向客户端发送字符串。但是，如果我将长度为86的字符串复制到pdu packet->value_value一切正常，我会在客户端获取字符串。如果我只在字符串中添加一个char，则长度为87，数据包无法传递给客户端。数据包大小正确提升。是否可以发送多少个字符？我的数据包小于1500。

此代码段显示了如何将字符串复制到packet->value_value 

value_value = "asd asd asd asd asd asd asd asd asd asd asd asd asd asd asd asd asd asd asd asd asd as";
printf("String length: %d\n",strlen(value_value)); // 86 OK, 87or greater NOK
packet->value_length = strlen(value_value);
packet->value_value = malloc(packet->value_length);
strcpy(packet->value_value, value_value);

对于某些信息，这里是一个终端输出，其中包含有关数据包长度的更多信息

字符串长度为86

的输出 
src/agent.c:69:parse_request(): Input length=43
src/agent.c:116:perform_snmp_request(): OID is: 1.3.6.1.2.1.1.1.0
String length: 86
src/mibservice.c:90:snmpget(): packet->value_type=04
src/mibservice.c:91:snmpget(): packet->value_length=56
src/mibservice.c:93:snmpget(): packet->value_value=61
...
src/mibservice.c:93:snmpget(): packet->value_value=73
src/mibservice.c:95:snmpget(): Before packet->length=41
src/mibservice.c:97:snmpget(): After packet->length=127
src/mibservice.c:99:snmpget(): Before packet->pdu_length=28
src/mibservice.c:101:snmpget(): After packet->pdu_length=114
src/mibservice.c:103:snmpget(): Before packet->variablebindings_length=14
src/mibservice.c:105:snmpget(): After packet->variablebindings_length=100
src/mibservice.c:107:snmpget(): Before packet->varbind_length=12
src/mibservice.c:109:snmpget(): After packet->varbind_length=98
src/agent.c:96:create_response(): Output length=129

字符串长度为87

的输出 
src/agent.c:69:parse_request(): Input length=43
src/agent.c:116:perform_snmp_request(): OID is: 1.3.6.1.2.1.1.1.0
String length: 87
src/mibservice.c:90:snmpget(): packet->value_type=04
src/mibservice.c:91:snmpget(): packet->value_length=57
src/mibservice.c:93:snmpget(): packet->value_value=61
...
src/mibservice.c:93:snmpget(): packet->value_value=64
src/mibservice.c:95:snmpget(): Before packet->length=41
src/mibservice.c:97:snmpget(): After packet->length=128
src/mibservice.c:99:snmpget(): Before packet->pdu_length=28
src/mibservice.c:101:snmpget(): After packet->pdu_length=115
src/mibservice.c:103:snmpget(): Before packet->variablebindings_length=14
src/mibservice.c:105:snmpget(): After packet->variablebindings_length=101
src/mibservice.c:107:snmpget(): Before packet->varbind_length=12
src/mibservice.c:109:snmpget(): After packet->varbind_length=99
src/agent.c:96:create_response(): Output length=130

更新 这是一个关于我的问题的可运行的例子。数据包out_buf_0表示有效的SNMP数据包，可以通过UDP发送。数据包out_buf_1与out_buf_0相同，但最后一个字符0x64更多。此外，由于附加字符，我提高了所有长度+ 1。为什么out_buf_1不是有效的SNMP数据包/为什么它不能通过UDP发送？注意：SNMP请求无法在终端中显示，因为来自客户端的request id与out_buf_0和out_buf_1不同，请查看wireshark以查看请求/响应。

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <netdb.h>
#include <sys/socket.h>
#include <arpa/inet.h>

#define MESSAGE_MAX_LEN  1500 /* MTU, IEEE Std 802.3TM-2015 */
#define PORT 161 /* RFC 1157 */

int out_buf_0_len = 129; /* 0x7f + 2 */
char out_buf_0[] = {
0x30, /* SNMP Packet start */
0x7f, /* SNMP Packet length */
0x02, 0x01, 0x00, /* Version */
0x04, 0x06, 0x70, 0x75, 0x62, 0x6c, 0x69, 0x63, /* Community*/
0xa2, /* GetResponsePDU */
0x72, /* GetResponsePDU Length */
0x02, 0x04, 0x2c, 0x80, 0x7e, 0x2f, /* Request id */
0x02, 0x01, 0x00, /*Error status */
0x02, 0x01, 0x00, /*Error index */
0x30, /* Varbind list start */
0x64, /* Varbind list length*/
0x30, /* Varbind value start */
0x62, /* Varbind value length */
0x06, 0x08, 0x2b, 0x06, 0x01, 0x02, 0x01, 0x01, 0x01, 0x00, /* OID */
0x04,  /* Value start, type octet-string*/
0x56, /* Value length */
0x61, 0x73, 0x64, 0x20, 0x61, /* Value */
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73 };

int out_buf_1_len = 130; /* 0x80 + 2 */
char out_buf_1[] = {
0x30, /* SNMP Packet start */
0x80, /* SNMP Packet length */
0x02, 0x01, 0x00, /* Version */
0x04, 0x06, 0x70, 0x75, 0x62, 0x6c, 0x69, 0x63, /* Community*/
0xa2, /* GetResponsePDU */
0x73, /* GetResponsePDU Length */
0x02, 0x04, 0x2c, 0x80, 0x7e, 0x2f, /* Request id */
0x02, 0x01, 0x00, /*Error status */
0x02, 0x01, 0x00, /*Error index */
0x30, /* Varbind list start */
0x65, /* Varbind list length*/
0x30, /* Varbind value start */
0x63, /* Varbind value length */
0x06, 0x08, 0x2b, 0x06, 0x01, 0x02, 0x01, 0x01, 0x01, 0x00, /* OID */
0x04,  /* Value start, type octet-string*/
0x57, /* Value length */
0x61, 0x73, 0x64, 0x20, 0x61, /* Value */
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64 };

int my_socket;
struct sockaddr_in remote_addr;
int socket_create()
{
    printf("Create socket\n");
    struct sockaddr_in socket_addr;
    if ((my_socket = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
    {
        printf("Cannot create socket. Exit.\n");
        return -1;
    }
    memset((char *)&socket_addr, 0, sizeof(socket_addr));
    socket_addr.sin_family = AF_INET;
    socket_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    socket_addr.sin_port = htons(PORT);
    if (bind(my_socket, (struct sockaddr *)&socket_addr, sizeof(socket_addr)) < 0)
    {
        printf("Bind failed. Exit.\n");
        return - 1;
    }
    printf("Listen on: %s:%d\n", inet_ntoa(socket_addr.sin_addr), PORT);
    return 0;
}

socklen_t addr_len = sizeof(remote_addr);
void socket_listen(char *in_buf)
{
    int  recv_len; /* Bytes received */
    int  nbyt; /* Bytes count */
    char *out_buf[MESSAGE_MAX_LEN];
    int  out_len = 0;

    for (;;) { /* Receive snmp message from snmp manager */
        recv_len = recvfrom(my_socket, in_buf, MESSAGE_MAX_LEN, 0, (struct sockaddr *)&remote_addr, &addr_len);
        if (recv_len > 0)
            if (sendto(my_socket, out_buf_1, out_buf_1_len, 0, (struct sockaddr *)&remote_addr, addr_len) < 0)
                printf("Cannot send data to destination.\n");
    }
}

/* Disable SNMP on local machine. # systemctl stop snmpd 
 * Execute main(): gcc <filename>.c && ./a.out
 * Run SNMP Request: $ snmpget -v 1 -c public 0.0.0.0:161 1.3.6.1.2.1.1.1.0
 */
char in_buf[MESSAGE_MAX_LEN];
int main(int argc, char **argv)
{
    if (socket_create() == -1)
        exit(2);
    socket_listen(in_buf);
}

整个帧长度包括我的SNMPv1数据包out_buf_0是1368位，out_buf_1应该是1376位。

2 个答案:

答案 0 :(得分：0)

正如某人在评论中指出的那样，strcpy是邪恶的，不应该用于任何事情。你的malloc实际上分配了一个小于原始数组的数组，因为你没有考虑尾随空值。当你使用strcpy时，你写了一个带有尾部空值的已分配数组的末尾。

由于您没有包含与数据包结构相关的代码，因此我不知道它是如何被使用的，但如果此后的任何代码需要C null终止字符串，则很可能无法按预期工作。

答案 1 :(得分：0)

SNMP数据包长度0x80错误，请参阅here

  

ASN.1长度字段0x80是错误的。解码器看到了第一个   数据包的两个八位字节是0x30 0x80，并从0x80确定   应该没有更多的八位字节。然而还有更多   八位字节，因此它是无效的编码，而且不是有效的SNMP   信息。这里的0x80并不意味着内容为128个八位字节   打算，这意味着你的长度字段是0x80＆amp; ~0x80 == 0个八位字节长   跟随这个八位字节。相反，0x81意味着你的长度字段是   0x81＆amp; ~0.080 == 1跟随八位字节，然后是0x80八位字节   表示内容长度为128.

这就是SNMP数据包的样子，

char out_buf_1[] = {
0x30, /* SNMP Packet start */
0x81, 0x80, /* SNMP Packet length */
0x02, 0x01, 0x00, /* Version */
0x04, 0x06, 0x70, 0x75, 0x62, 0x6c, 0x69, 0x63, /* Community*/
0xa2, /* GetResponsePDU */
0x73, /* GetResponsePDU Length */
0x02, 0x04, 0x2c, 0x80, 0x7e, 0x2f, /* Request id */
0x02, 0x01, 0x00, /*Error status */
0x02, 0x01, 0x00, /*Error index */
0x30, /* Varbind list start */
0x65, /* Varbind list length*/
0x30, /* Varbind value start */
0x63, /* Varbind value length */
0x06, 0x08, 0x2b, 0x06, 0x01, 0x02, 0x01, 0x01, 0x01, 0x00, /* OID */
0x04,  /* Value start, type octet-string*/
0x57, /* Value length */
0x61, 0x73, 0x64, 0x20, 0x61, /* Value */
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64, 0x20, 0x61, 0x73, 0x64, 0x20, 0x61,
0x73, 0x64 };
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