How to limit range of random port sockets?
on Linux, you'd do something like
sudo sysctl -w net.ipv4.ip_local_port_range="60000 61000"
instruction for changing ephemeral port range on other unices can be found for example at http://www.ncftp.com/ncftpd/doc/misc/ephemeral_ports.html
If you want to change the way a binary runs without having access to its sources, you can sometimes use a shim, a piece of code that, in your example, will replace the call on bind()
by a call to a function you provide that can manipulate the data before calling the real function. See LD_PRELOAD
in man ld.so
.
Here's some C that does exactly that, shim_bind.c, overriding the port to 7777, and assuming an AF_INET socket. Compile it with gcc -Wall -O2 -fpic -shared -ldl -o shim_bind.so shim_bind.c
and use it by putting LD_PRELOAD=shim_bind.so
in front of your command.
/*
* capture calls to a routine and replace with your code
* http://unix.stackexchange.com/a/305336/119298
* gcc -Wall -O2 -fpic -shared -ldl -o shim_bind.so shim_bind.c
* LD_PRELOAD=/path/to/shim_bind.so ./test
*/
#define _GNU_SOURCE /* needed to get RTLD_NEXT defined in dlfcn.h */
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <dlfcn.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen){
static int (*real_bind)(int sockfd, const struct sockaddr *addr,
socklen_t addrlen) = NULL;
int port = 7777;
struct sockaddr_in theaddr;
if (!real_bind) {
real_bind = dlsym(RTLD_NEXT, "bind");
char *error = dlerror();
if (error != NULL) {
fprintf(stderr, "%s\n", error);
exit(1);
}
}
fprintf(stderr, "binding: port %d\n", port);
memcpy(&theaddr, addr, sizeof(theaddr));
theaddr.sin_port = htons((unsigned short)port);
return real_bind(sockfd, (struct sockaddr*)&theaddr, addrlen);
}