问题描述
我的服务器有几个不同的 IP 地址.此时，每个 IP 都可以收到一个 UDP 请求，但回复的始终是同一个 IP，这是请求者不喜欢的.
I have a server with a couple of different IP addresses. At this point, each IP can recieve a UDP request, but it is always the same IP that replies, which the requesters do not like.
长话短说，这是所有基本代码:
To make a long story short, this is all the essential code:
int sock;
socklen_t clilen;
struct sockaddr_in serv_addr, cli_addr;
memset((char*)&serv_addr, 0, sizeof(serv_addr));
sock = socket(AF_INET, SOCK_DGRAM, 0);
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
serv_addr.sin_port = htons(PORT);
...
recvfrom(sock, buffer, BUFLEN, 0, (struct sockaddr *)&cli_addr, &clilen);
...
sendto(sock, resData, resLen, 0, (struct sockaddr *)&cli_addr, sizeof(cli_addr));
我希望能够以某种方式指定使用哪个 IP 来发回我的数据包(每个请求都可能不同)，但我不知道如何以及是否可以使用套接字来完成.我在这个领域并不是很有经验，因此非常感谢我能得到的所有帮助.
I would like to be able to specify somehow which IP is used to send back my packet (and this could differ for each and every request), but I have no idea how and if this even can be accomplished using sockets. I'm not really that experienced in this field, so all the help that I can get is greatly appreciated.
在下方编辑
我在这里接受的答案中找到了一个潜在的解决方案:如何在 Linux 中重新绑定一个 udp 套接字
I have found a potential solution in the accepted answer here: How to re bind a udp socket in Linux
但是，出现了一个新问题.我如何知道哪个 IP/接口收到了请求?这样我就可以使用该 IP/接口做出响应.
However, a new problem emerges. How do I know which IP/interface recieved the request? So that I can respond using that IP/interface.
推荐答案
我已经解决了我的问题，并且必须以良好的方式发布解决方案！我没有使用 SOCK_RAW，也没有将我的接口绑定到本地 IP 等.这是 google 上的 100 页和一些 stackoverflow 的混合体，所以我有点遗憾我没有保存链接以给出正确的学分.
I have solved my issue, and in good manner the solution must be posted! I did not use SOCK_RAW, nor did I bind my interface to a local IP or the such. This is a mixture of 100 pages on google and some stackoverflow, so I'm a bit sad that I haven't saved the links to give out the correct credits.
由于我不是专家，代码中可能存在明显的缺陷，但这是我想出的解决方案，并且它有效.我刚开始清理代码(当你尝试从 100 个不同的页面中尝试并组合它们时，它会变得很乱).无论如何，这里是:
There might be obvious flaws in the code as I am no expert, however this is the solution I came up with and it works. I just started cleaning up the code (it gets messy when you try stuff from 100 different pages and combine them). Anyways, here it is:
接收部分:
int sock;
socklen_t clilen;
struct sockaddr_in serv_addr, cli_addr;
char buffer[BUFLEN];
// Next
memset((char*)&serv_addr, 0, sizeof(serv_addr));
sock = socket(AF_INET, SOCK_DGRAM, 0);
serv_addr.sin_family = AF_INET;
//serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
serv_addr.sin_port = htons(PORT);
clilen = sizeof(cli_addr);
if (bind(sock, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) {
error("ERROR on binding");
}
bool opt = true;
setsockopt(sock, IPPROTO_IP, IP_PKTINFO, (char *)&opt, sizeof(opt));
// Packet data will be saved in buffer
bzero(buffer, BUFLEN);
struct iovec vector;
vector.iov_base = buffer;
vector.iov_len = sizeof(buffer);
struct msghdr msg;
msg.msg_name = &cli_addr;
msg.msg_namelen = sizeof(cli_addr);
msg.msg_iov = &vector;
msg.msg_iovlen = 1;
int flags = 0;
// Not sure what controlBuffer contains at this point
char controlBuffer[1024];
msg.msg_control = controlBuffer;
msg.msg_controllen = 1024;
// Recv packet
int bytes = ::recvmsg(sock, &msg, flags);
struct cmsghdr *cmsg;
struct in_pktinfo *dest_ip_ptr;
int dest_ip = 0;
// Loop through IP header messages
cmsg = CMSG_FIRSTHDR(&msg);
for ( cmsg = CMSG_FIRSTHDR(&msg);
cmsg != NULL;
cmsg = CMSG_NXTHDR( &msg, cmsg ) )
{
if (cmsg->cmsg_level != IPPROTO_IP ||
cmsg->cmsg_type != IP_PKTINFO)
{
continue;
}
// Get IP (int)
struct in_pktinfo *dest_ip_ptr = (struct in_pktinfo*)CMSG_DATA(cmsg);
dest_ip = dest_ip_ptr->ipi_addr.s_addr;
}
// Format IP
unsigned char ipParts[4];
ipParts[0] = dest_ip & 0xFF;
ipParts[1] = (dest_ip >> 8) & 0xFF;
ipParts[2] = (dest_ip >> 16) & 0xFF;
ipParts[3] = (dest_ip >> 24) & 0xFF;
发送部分:
// Build source sockaddr
struct sockaddr_in src_addr;
memset(&src_addr, 0, sizeof(struct sockaddr_in));
src_addr.sin_family = AF_INET;
// Save IP into a char array
char destIp[16];
memset(destIp, 0, sizeof(destIp));
sprintf(destIp, "%d.%d.%d.%d", ipParts[0], ipParts[1], ipParts[2], ipParts[3]);
inet_aton(destIp, &(src_addr.sin_addr));
char cmbuf[CMSG_SPACE(sizeof(struct in_pktinfo))];
bzero(buffer, BUFLEN);
int len = dp.getRaw(buffer);
struct msghdr mh;
memset(&mh, 0, sizeof(mh));
struct cmsghdr *cmsg_send;
struct in_pktinfo *pktinfo;
struct iovec iov[1];
iov[0].iov_base = buffer;
iov[0].iov_len = BUFLEN;
mh.msg_name = &cli_addr; // destination address of packet
mh.msg_namelen = sizeof(cli_addr);
mh.msg_control = cmbuf;
mh.msg_controllen = sizeof(cmbuf);
mh.msg_flags = 0;
mh.msg_iov = iov;
mh.msg_iovlen = 1;
// after initializing msghdr & control data to CMSG_SPACE(sizeof(struct in_pktinfo))
cmsg_send = CMSG_FIRSTHDR(&mh);
cmsg_send->cmsg_level = IPPROTO_IP;
cmsg_send->cmsg_type = IP_PKTINFO;
cmsg_send->cmsg_len = CMSG_LEN(sizeof(struct in_pktinfo));
pktinfo = (struct in_pktinfo*) CMSG_DATA(cmsg_send);
pktinfo->ipi_ifindex = 0;
pktinfo->ipi_spec_dst = src_addr.sin_addr;
int rc = sendmsg(sock, &mh, 0);
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