getaddrinfo.c

#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#include <stdio.h>
#include <string.h>

#define __USE_GNU
#include <dlfcn.h>

/*
    gcc -c -Wall -Wpedantic getaddrinfo.c -o getaddrinfo.o && gcc -ldl -fPIC getaddrinfo.c -shared -o getaddrinfo.so
*/

static void get_ip_str(const struct sockaddr *sa, char *s, size_t maxlen)
{
    switch(sa->sa_family) {
        case AF_INET:
            inet_ntop(AF_INET, &(((struct sockaddr_in *)sa)->sin_addr), s, maxlen);
            break;
        case AF_INET6:
            inet_ntop(AF_INET6, &(((struct sockaddr_in6 *)sa)->sin6_addr), s, maxlen);
            break;
    }
}

int (*real_getaddrinfo)(const char*, const char*, const struct addrinfo*, struct addrinfo**);

extern int getaddrinfo(const char *node, const char *service,
                const struct addrinfo *hints,
                struct addrinfo **res)
{
    *(void **)(&real_getaddrinfo) = dlsym(RTLD_NEXT, "getaddrinfo");
    int ret = real_getaddrinfo(node, service, hints, res);
    char* res_or_err = NULL;
    if (ret == 0) {
        char ip_str[INET6_ADDRSTRLEN] = "\0";
        get_ip_str((*res)->ai_addr, ip_str, sizeof(ip_str));
        res_or_err = ip_str;
    } else {
        res_or_err = strerror(ret);
    }
    fprintf(stdout, "$$$ getaddrinfo(\"%s\", \"%s\"): %d -> %s\n", node, service, ret, res_or_err);
    return ret;
}