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ssh-bouncer.c
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ssh-bouncer.c
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/*
* Copyright (C) 2012 Torsten Becker <torsten.becker@gmail.com>.
* All rights reserved.
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to
* deal in the Software without restriction, including without limitation the
* rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
* sell copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
* THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
* IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*
* ssh-bouncer.c, created on 2012-Jan-14.
*/
#include <stdlib.h>
// ----- Config values (who needs config files?) ------------------------------
// Configure the user which this service will run as, default is "nobody":
#ifndef __APPLE__
static const int sb_userid = 65534;
#else
static const int sb_userid = -2;
#endif
// Configure where to change the root directory to:
static const char * sb_chroot = "/var/empty";
struct sb_listen_config_t
{
unsigned short port;
const char * version_string;
};
// Define on which ports the daemon should listen and which version it
// will pretend to be running on that port:
static struct sb_listen_config_t sb_listen_config[] = {
{22, "SSH-2.0-OpenSSH_4.5p1 FreeBSD-20061110\n"},
{222, "SSH-2.0-OpenSSH_5.4p1 Debian-5\n"},
{2222, "SSH-2.0-OpenSSH_5.3\n"},
};
// The maximum number of clients that will be kept hanging, if more clients
// connect, older ones will be dropped.
static size_t sb_num_clients = 100;
// ----- SSH Bouncer Program --------------------------------------------------
#include <arpa/inet.h>
#include <assert.h>
#include <errno.h>
#include <fcntl.h>
#include <netinet/in.h>
#include <stdarg.h>
#include <stdio.h>
#include <string.h>
#include <sys/select.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <sys/uio.h>
// Linux needs __USE_BSD to get chroot(2) from here...
#define __USE_BSD
#include <unistd.h>
// Prints the error string for the last error and exits the whole program
#define SB_PRINT_ERR_DIE(MSG) { perror("[!] "MSG); exit(1); }
int sb_bound_socket(unsigned short port)
{
int sock;
struct sockaddr_in6 server_addr;
memset(&server_addr, 0, sizeof(server_addr));
sock = socket(PF_INET6, SOCK_STREAM, 0);
if(sock < 0)
SB_PRINT_ERR_DIE("Creating socked failed")
// Setting IPV6_V6ONLY to false (0) makes this a universal socket
// which will listen for IPv4 and IPv6 connections.
int fals = 0;
if(setsockopt(sock, IPPROTO_IPV6, IPV6_V6ONLY, &fals, sizeof(fals)) < 0)
SB_PRINT_ERR_DIE("setsockopt(IPV6_V6ONLY) failed")
server_addr.sin6_family = AF_INET6;
server_addr.sin6_addr = in6addr_any;
server_addr.sin6_port = htons(port);
if(bind(sock, (struct sockaddr*)&server_addr,
sizeof(struct sockaddr_in6)) < 0)
SB_PRINT_ERR_DIE("bind() failed")
if(listen(sock, 20) < 0)
SB_PRINT_ERR_DIE("listen() failed")
return sock;
}
int sb_verbose_accept(int listen_socket)
{
union {
struct sockaddr_in sin;
struct sockaddr_in6 sin6;
} client_addr;
socklen_t client_addr_len = sizeof(client_addr);
char client_addr_str[INET6_ADDRSTRLEN];
int client = accept(listen_socket, (struct sockaddr *)&client_addr,
&client_addr_len);
if(client < 0)
SB_PRINT_ERR_DIE("accept() failed")
if(fcntl(client, F_SETFL, O_NONBLOCK) < 0)
SB_PRINT_ERR_DIE("Setting nonblocking I/O failed")
// TODO: Send a email here or so
printf("Connection from %s\n",
inet_ntop(client_addr.sin.sin_family,
client_addr.sin.sin_family == AF_INET
? (const void *)&client_addr.sin.sin_addr
: (const void *)&client_addr.sin6.sin6_addr,
client_addr_str, sizeof(client_addr_str)));
return client;
}
int main(int argc, char **argv)
{
const size_t num_configs = sizeof(sb_listen_config) /
sizeof(struct sb_listen_config_t);
const size_t max_num_clients = FD_SETSIZE - num_configs;
if(sb_num_clients > max_num_clients) {
fprintf(stderr, "[!] Configured number of clients (%zu) is larger "
"than maximum available number of clients (%zu).\n",
sb_num_clients, max_num_clients);
return 1;
}
int listen_sockets[num_configs];
for(size_t i = 0; i < num_configs; ++i)
listen_sockets[i] = sb_bound_socket(sb_listen_config[i].port);
if(chroot(sb_chroot) < 0)
SB_PRINT_ERR_DIE("chroot() failed")
if(setgid(sb_userid) < 0)
SB_PRINT_ERR_DIE("Setting group id failed")
if(setuid(sb_userid) < 0)
SB_PRINT_ERR_DIE("Setting user id failed")
printf("[+] Listening on %zu sockets for maximum %zu clients.\n",
num_configs, sb_num_clients);
pid_t daemon_pid = fork();
switch(daemon_pid) {
case -1:
SB_PRINT_ERR_DIE("fork() failed")
case 0:
break;
default:
printf("[+] Daemon is PID %d\n", daemon_pid);
return 0;
}
fd_set all_sockets;
struct timeval tv;
int ready_sockets;
int max_fd = 0;
// Keep all clients in this stupid simple ring buffer, -1 means
// it's a unused slot, else it is a valid socket fd.
int client_ring[sb_num_clients];
for(size_t i = 0; i < sb_num_clients; ++i)
client_ring[i] = -1;
size_t next_free_client = 0;
char nirvana[2048];
for(;;) {
FD_ZERO(&all_sockets);
for(size_t i = 0; i < num_configs; ++i) {
FD_SET(listen_sockets[i], &all_sockets);
if(listen_sockets[i] > max_fd)
max_fd = listen_sockets[i];
}
for(size_t i = 0; i < sb_num_clients; ++i) {
int client = client_ring[i];
if(client != -1) {
FD_SET(client, &all_sockets);
if(client > max_fd)
max_fd = client;
}
}
++max_fd;
tv.tv_sec = 10;
tv.tv_usec = 0;
ready_sockets = select(max_fd, &all_sockets, NULL, NULL, &tv);
if(ready_sockets == -1)
SB_PRINT_ERR_DIE("select() failed")
else if(ready_sockets) {
// Walk over all listen sockets and accept new clients
for(size_t i = 0; i < num_configs; ++i) {
if(FD_ISSET(listen_sockets[i], &all_sockets)) {
int client = sb_verbose_accept(listen_sockets[i]);
const char * version = sb_listen_config[i].version_string;
// Try to write the fake version string...
if(write(client, version, strlen(version)) < 0) {
perror("[!] Writing version to client failed");
close(client);
}
// ... if writing succeeds, put client into the ring buffer
// of all connected clients.
else {
client_ring[next_free_client] = client;
next_free_client = (next_free_client + 1) %
sb_num_clients;
if(client_ring[next_free_client] != -1) {
close(client_ring[next_free_client]);
client_ring[next_free_client] = -1;
}
}
}
}
// Walk over all connected clients and see if there is any new
// data or if the clients is disconnected
for(size_t i = 0; i < sb_num_clients; ++i) {
int client = client_ring[i];
if(client != -1) {
if(FD_ISSET(client, &all_sockets)) {
ssize_t did_read;
// Read data into nirvana...
for(;;) {
did_read = read(client, nirvana, sizeof(nirvana));
// A error occured, hmm...
if(did_read < 0) {
// .. AH, no new data available
if(errno == EAGAIN) {
// printf("EAGAIN\n");
break;
}
perror("[!] Read error");
break;
}
// Reading 0 means, the client disconnected
else if(did_read == 0) {
// printf("NOTHING: close\n");
close(client);
client_ring[i] = -1;
break;
}
// Reading less than buffer size means, there was
// no more data available...
else if(did_read < sizeof(nirvana)) {
// printf("READ %zu\n", did_read);
break;
}
// Reading exact data means, there will be more
// or EAGAIN, this case is the loop-default...
// else {
// printf("READ EXACT\n");
// }
} // end-less read loop
} // if fd is in set
} // client != -1
} // for all connected clients
} // if socket is ready
} // infinite loop
return 0;
}