我在编写小型 DNS 服务器并将其 strip 化到最低限度时偶然发现了一种奇怪的行为。该程序应监听 127.0.0.1:1337 的 DNS 查询并回复拒绝。我通过发出 dig @localhost -p 1337 foo.bar.
来测试其行为如果第48行被注释掉//char bout[bufferSize]; // <-- WTF
它就像魅力一样。
程序:
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <errno.h>
#include <unistd.h>
#include <string.h>
static int SOCKET;
void bindSocket(){
int s = socket (AF_INET, SOCK_DGRAM, 0);
if (s < 0) {
fprintf (stderr, "Could not create socket\n");
exit (EXIT_FAILURE);
}
struct sockaddr_in address;
memset((char *)&address, 0, sizeof(address));
inet_aton("127.0.0.1", &address.sin_addr);
address.sin_family = AF_INET;
address.sin_port = htons(1337);
int rc = bind ( s, (struct sockaddr *) &address, sizeof (address) );
if (rc < 0) {
fprintf (stderr, "Could not bind Socket\n %s \n", strerror(errno));
exit (EXIT_FAILURE);
}
SOCKET = s;
}
void decline(uint16_t err, char *bin, struct sockaddr *to){
char bout[12];
memset((bout + 4), 0, 8);
memcpy(bout, bin, 4);
bout[2] = (bout[2] | 0x80) & 0xFE;
bout[3] = (bout[3] | err ) & 0x7F;
sendto( SOCKET, bout, 12, 0, to, sizeof(struct sockaddr) );
}
void hereBeDragons(){
size_t bufferSize = 512;
char bin[bufferSize];
char bout[bufferSize]; // <-- WTF
struct sockaddr sender;
socklen_t len;
while(1){
memset(bin, 0, bufferSize);
int n = recvfrom( SOCKET, bin, bufferSize, 0, &sender, &len );
if (n < 0) continue;
puts("receved a query");
/* Strictly decline all invalid queries */
decline( 2, bin, &sender);
}
}
int main(){
bindSocket();
hereBeDragons();
return EXIT_FAILURE;
}
程序输出:
received a query
挖掘输出:
; <<>> DiG 9.9.5-3-Ubuntu <<>> @localhost -p 1337 foo.bar.
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 37520
;; flags: qr ad; QUERY: 0, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
;; Query time: 1 msec
;; SERVER: 127.0.0.1#1337(127.0.0.1)
;; WHEN: Fri Aug 29 21:37:46 CEST 2014
;; MSG SIZE rcvd: 12
这只是一个简化的示例,在实际代码中,bout 用于构造对传入查询的有效响应。但是当我留下它时,出现以下问题:
程序将答案发送到错误的端口,5秒后重试,程序将答案发送到正确的端口。
我做错了什么?
最佳答案
这是一个非常微妙的错误。
套接字地址长度参数,即recvfrom() 的第六个参数,必须初始化。必须设置为指示第五个参数中传递的地址缓冲区的长度。当recvfrom()返回时,长度会更新以反射(reflect)写入其中的网络地址结构的实际大小。
在调用recvfrom()之前添加:
len=sizeof(sender);
这在 recvfrom(2) man page 中有解释。 :
The argument addrlen is a value-result argument, which the caller should initialize before the call to the size of the buffer associated with src_addr, and modified on return to indicate the actual size of the source address
关于c - 启动时端口错误 (recvfrom),我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/25575477/