我在编写小型DNS服务器时发现了一个奇怪的行为,并将其划分到最低限度。该程序应该在127.0.0.1:1337上监听DNS查询并回复拒绝。我通过发行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
,它的作用就像魅力一样。启动时错误的端口(recvfrom)
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
这只是一个削减例如,在实际的代码回合被用来构建对传入查询的有效响应。但是,当我离开它时,出现以下proplem:
程序的答案发送到错误的端口,5秒后重试挖和程序发送到正确的端口。
我在做什么错?
如果你没有在函数中使用回合,那么为什么要麻烦初始化呢?或者它是不同的代码? – Arpit 2014-08-29 20:20:08
构成问题一部分的代码必须包含在问题中,而不是发布在别处。 – EJP 2014-08-29 23:10:02