ICMP“目的地不可达"udp 连接套接字上的数据包 [英] ICMP "destination unreachable" packet on udp connected socket
问题描述
我已经用这个函数创建了连接的 UDP 套接字
I've created connected UDP socket with this function
/* Creates connected udp socket */
int
udp_connect( const char *host, const char *serv)
{
int sockfd, n;
struct addrinfo hints, *res, *ressave;
bzero(&hints, sizeof(struct addrinfo));
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_DGRAM;
if ( ( n = getaddrinfo( host, serv, &hints, &res)) != 0)
err_quit( "udp_connect error for %s, %s: %s",
host, serv, gai_strerror(n));
ressave = res;
do {
sockfd = socket( res->ai_family, res->ai_socktype, res->ai_protocol);
if ( sockfd < 0)
continue; /* ignore this one */
/* The call to connect with a UDP socket does not send anything
* to the peer. If something is wrong ( the peer is unreachable
* or there is no server at the specified port), the caller
* does not discover that until it sends
* a datagram to the peer */
if ( connect( sockfd, res->ai_addr, res->ai_addrlen) == 0)
break; /* success */
Close( sockfd); /* ignore this one */
} while ( ( res = res->ai_next) != NULL);
if ( res == NULL) /* errno set from final connect() */
err_sys( "udp_connect error for %s, %s", host, serv);
freeaddrinfo( ressave);
return( sockfd);
}
我想测试当对等点实际上无法访问时它的行为方式.由于对 connect()
的调用无法生成此信息,因此我们需要实际发送一些内容.我将在以下代码段中描述我要做什么以及我会得到什么:
I would like to do a test how it behaves when peer is actually unreachable. Since call to connect()
cannot result in this information we need to actually send something. I will describe what do I do and what do I get in the following snippet:
printf( "sending to %s
", Sock_ntop_host( sa, salen));
// prints: sending to 127.0.0.1
Sendto( sockfd, "", 1, 0, sa, salen); /* send 1-byte datagram */
// prints: nbytes:1
// it is sent, I check via tcpdump or with Wireshark that datagram
// has been sent and ICMP "destination unreachable" comes back from host
printf( "sent, errno:%d,%s
", errno, strerror(errno));
// prints: sent, errno:0,Success
n = Recvfrom( sockfd, recvline, MAXLINE, 0, NULL, NULL);
// never gets here
printf( "received n=%d
", n);
Sendto
函数是 sendto
的包装器,它只打印错误并退出:
The Sendto
function is a wrapper over sendto
that just prints error and exits:
void
Sendto(int fd, const void *ptr, size_t nbytes, int flags,
const struct sockaddr *sa, socklen_t salen)
{
if ( sendto(fd, ptr, nbytes, flags, sa, salen) == -1)
exit(-1);//err_sys("sendto error");
printf( "nbytes:%d
",nbytes); // prints: nbytes:1
}
ssize_t
Recvfrom(int fd, void *ptr, size_t nbytes, int flags,
struct sockaddr *sa, socklen_t *salenptr)
{
ssize_t n;
if ( (n = recvfrom(fd, ptr, nbytes, flags, sa, salenptr)) < 0)
err_sys("recvfrom error");
return(n);
}
所以对 Recvfrom
的调用会永远阻塞,而 Sendto
会返回带有代码 Success
的 errno.我应该如何编写代码以获取有关 ICMP 响应的通知?这可能在套接字上没有超时吗?
So the call to Recvfrom
blocks forever while Sendto
returnes errno with code Success
. How then should I code this to get notification about ICMP response? is this possible without timeout on socket?
推荐答案
一旦你connect()
一个UDP socket,你就可以使用send()
和recv()
而不是 sendto()
和 recvfrom()
.recv()
将报告已连接套接字的 ICMP 错误.
Once you connect()
a UDP socket, you can use send()
and recv()
instead of sendto()
and recvfrom()
. recv()
will report an ICMP error for a connected socket.
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