rebind.c

#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdlib.h>
#include <dlfcn.h>
#include <string.h>
#include <stdio.h>

// expected value "REBIND_PORT=FROM:TO"
#define REBIND_PORT_ENV "REBIND_PORT"

static int(*_bind)(int sockfd, const struct sockaddr *addr, socklen_t addrlen) = NULL;

static int parse_env(in_port_t *from_port, in_port_t *to_port) {
    char *rebind_port_env = getenv(REBIND_PORT_ENV), *sep = NULL, *to_port_c = NULL;

    if (NULL == rebind_port_env) return -1; //no env var
    if (NULL == (sep = strchr(rebind_port_env,':'))) return -1; // no ':' separator
    *sep = 0;
    to_port_c = sep + 1;

    *from_port = (in_port_t)strtol(rebind_port_env, &sep, 10);
    if (*sep != '\0') return -1; //there is symbol ':', but string before it is not a number
    *to_port = (in_port_t)strtol(to_port_c, &sep, 10);
    if (*sep != '\0') return -1; //there is symbol ':', but string after it is not a number
    return 0;
}

int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen) {
    if (NULL == _bind) {
        _bind = (int (*)(int sockfd, const struct sockaddr *addr, socklen_t addrlen)) dlsym(RTLD_NEXT, "bind");
    }
    struct sockaddr_in rebindaddr;
    in_port_t from_port, to_port;

    // some different socket type or other incompatibility
    if (addr->sa_family != AF_INET || addrlen != sizeof(rebindaddr)) goto fallback;
    if (parse_env(&from_port, &to_port)) goto fallback;
    if (((struct sockaddr_in*)addr)->sin_port != htons(from_port)) goto fallback; //request port doesnt match REBIND_PORT_FROM

    memcpy(&rebindaddr, addr, addrlen);
    rebindaddr.sin_port=htons(to_port);
        
    int res = _bind(sockfd, (struct sockaddr*) &rebindaddr, sizeof(rebindaddr));
    if (to_port == 0) { // to_port == 0 means that OS chooses available port for us, looking up real value
        socklen_t rebindaddrlen = sizeof(rebindaddr);
        if (getsockname(sockfd, (struct sockaddr*) &rebindaddr, &rebindaddrlen)) {
            perror("Port was rebound, but error occured during determining final value");
        }
        to_port = ntohs(rebindaddr.sin_port);
    }
    fprintf(stderr, "librebind: Rebound port from %d to %d\n", from_port, to_port);
    fflush(stderr);
    return res;
 fallback: 
    return _bind(sockfd, addr, addrlen);
}