Question: xinetd (extended Internet services daemon) maps input to a service's standard input and output to a service's standard out.
In other words, assuming a custom TCP service written in C, data coming into port X is mapped to stdin of the service and stdout of the service is mapped to data coming out of port Y.
Again, assuming a custom TCP service written in C, is there a way for that service to determine the connecting client's IP Address?
Web Research: As of the posting of this question, there are NO other questions on Stack Exchange (or elsewhere) that specifically deal with xinetd TCP services written in C attempting to determine the connecting client's IP Address.
There are similar questions:
But none that answer the specific question detailed in this post.
FOLLOW UP NOTE: xinetd takes the socket descriptor associated with TCP port X and maps it to the service's standard input.
Being cognizant of this fact would have allowed for a better web search that resulted in the following answers:
Yes, with getpeername(2) on its stdin (0) or stdout (1) file descriptor.
Example: when run from xinetd or inetd, this will print the address of the client connected to its stdin:
#define _DEFAULT_SOURCE
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netinet/in.h>
#include <stdio.h>
#include <err.h>
#include <netdb.h>
int main(void){
union {
struct sockaddr a;
struct sockaddr_in in;
struct sockaddr_in6 in6;
} na;
char host[NI_MAXHOST], port[NI_MAXSERV]; int e;
socklen_t nl = sizeof na;
if(getpeername(0, &na.a, &nl)) err(1, "getpeername");
if(e = getnameinfo(&na.a, nl, host, sizeof host, port, sizeof port,
NI_NUMERICHOST|NI_NUMERICSERV))
errx(1, "getnameinfo: %s", gai_strerror(e));
switch(na.a.sa_family){
case AF_INET:
errx(0, "connection from %s:%s", host, port);
case AF_INET6:
errx(0, "connection from [%s]:%s", host, port);
default:
errx(0, "connection from unknown address family %d",
na.a.sa_family);
}
}