Binding a port for client tcp socket - c

Question

I have a problem about 'binging a local port for a client tcp socket'.
The code is as below:
void tcpv4_cli_connect(const char *srvhost, in_port_t srvport,
const char *clihost, in_port_t cliport)
{
struct sockaddr_in srvaddr, cliaddr;
struct in_addr inaddr;
int sockfd;
bzero(&srvaddr, sizeof(srvaddr));
inet_aton(srvhost, &inaddr);
srvaddr.sin_family = AF_INET;
srvaddr.sin_addr = inaddr;
srvaddr.sin_port = htons(srvport);
bzero(&cliaddr, sizeof(cliaddr));
inet_aton(clihost, &inaddr);
cliaddr.sin_family = AF_INET;
cliaddr.sin_addr = inaddr;
cliaddr.sin_port = htons(cliport);
sockfd = socket(AF_INET, SOCK_STREAM, 0);
bind(sockfd, (struct sockaddr *) &cliaddr, sizeof(cliaddr));
if (connect(sockfd, (struct sockaddr *) &srvaddr, sizeof(srvaddr)) != 0)
perror("Something Wrong");
return;
}
int main(int argc, char *argv[])
{
// Wrong for "220.181.111.86", but ok for "127.0.0.1"
tcpv4_cli_connect("220.181.111.86", 80, "127.0.0.1", 40888);
return 0;
}
When I do tcpv4_cli_connect("220.181.111.86", 80, "127.0.0.1", 40888) in main function, (220.181.111.86 is an address on Internet), an error will show up: Something Wrong: Invalid argument.
And if I comment bind(sockfd, (struct sockaddr *) &cliaddr, sizeof(cliaddr)) in the code, things will be fine and a random port is used for client socket.
But it is alright when I do tcpv4_cli_connect("127.0.0.1", 80, "127.0.0.1", 40888) whether or not binding a port to client socket.
What does Invalid argument mean for a connect operation? I wonder if it is only allowed to bind a specific port for the client to connect to local address? Clients can only use random port to connect to a external server?
Is there someting I misunderstood?
/br
Ruan

Answer 1

When you bind() to 127.0.0.1 (INADDR_LOOPBACK), you are binding to a loopback interface that does not have access to the outside world, only to itself, so you cannot connect() to any IP other than 127.0.0.1. If you want to bind() to a local interface when you connect() to an outside server, you have to bind to the actual IP of an interface that is connected to a network that can reach that server.
If all you want to do is bind() to a specific port, but allow the OS to pick an appropriate interface for you, then bind to 0.0.0.0 (INADDR_ANY) instead.