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/*
* proxy.c
*
* Binds to local port explicitly before forwarding TCP connections.
*
* Copyright (C) 2005-2011 NTT DATA CORPORATION
*
* Version: 2.5.0 2011/09/29
*
* This tool is intended to limit local port numbers that clients
* will use when connecting to servers, so that servers can enforce
* client's port number based access control.
*
* To compile this program run the following command
*
* gcc -Wall -O3 -o path_to_output proxy.c
* chown 0:0 path_to_output
*
* where the path_to_output is the location you want to place the binary.
*
* To use this program for ssh, create a file named "useproxy"
* containing a line
*
* ProxyCommand path_to_output %h %p min_port max_port
*
* where the min_port and max_port are the local port range
* the sshd will accept.
*
* Use -F option like
*
* ssh -F useproxy example.com
*
* when you run ssh.
*
* You may append to ~/.ssh/config or /etc/ssh/ssh_config
* if you want to use this tool by default.
*
* You need to turn SUID bit on (or give CAP_NET_BIND_SERVICE capability) like
*
* chmod 4755 path_to_output
*
* if you want to allow non root user to use local port less than 1024.
*
* This program is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License v2 as published by the
* Free Software Foundation.
*
* This program is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
* more details.
*
* You should have received a copy of the GNU General Public License along with
* this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA
*/
#include <arpa/inet.h>
#include <errno.h>
#include <fcntl.h>
#include <netdb.h>
#include <netinet/in.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/select.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>
#include <grp.h>
static in_addr_t get_host_by_name_alias(const char *strHostName)
{
in_addr_t IP;
IP = inet_addr(strHostName);
if (IP == INADDR_NONE) {
struct hostent *hp = gethostbyname(strHostName);
if (hp)
IP = *(in_addr_t *) hp->h_addr_list[0];
}
return IP;
}
int main(int argc, char *argv[])
{
const int remote = socket(PF_INET, SOCK_STREAM, 0);
unsigned int port;
struct sockaddr_in addr;
in_addr_t forward_connect_ip = INADDR_NONE;
unsigned short int forward_connect_port = 0;
unsigned short int forward_bind_port_min = 0;
unsigned short int forward_bind_port_max = 0;
if (argc != 5) {
fprintf(stderr, "Usage: %s forward_connect_host "
"forward_connect_port forward_bind_port_min "
"forward_bind_port_max\n", argv[0]);
return 1;
}
forward_bind_port_min = atoi(argv[3]);
forward_bind_port_max = atoi(argv[4]);
for (port = forward_bind_port_min; port <= forward_bind_port_max;
port++) {
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = htonl(INADDR_ANY);
addr.sin_port = htons(port);
if (!bind(remote, (struct sockaddr *) &addr, sizeof(addr)))
break;
}
if (port > forward_bind_port_max) {
fprintf(stderr, "ERROR: No local ports available.\n");
return 1;
}
{ /* Drop root privileges. */
const gid_t gid = -1;
setgroups(1, &gid);
setgid(-1);
setuid(-1);
}
forward_connect_ip = ntohl(get_host_by_name_alias(argv[1]));
forward_connect_port = atoi(argv[2]);
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = htonl(forward_connect_ip);
addr.sin_port = htons(forward_connect_port);
if (connect(remote, (struct sockaddr *) &addr, sizeof(addr)) ||
fcntl(0, F_SETFL, fcntl(0, F_GETFL) | O_NONBLOCK) ||
fcntl(1, F_SETFL, fcntl(1, F_GETFL) | O_NONBLOCK) ||
fcntl(remote, F_SETFL, fcntl(remote, F_GETFL) | O_NONBLOCK)) {
fprintf(stderr, "ERROR: Connecting to %u.%u.%u.%u : %s\n",
(unsigned char) (forward_connect_ip >> 24),
(unsigned char) (forward_connect_ip >> 16),
(unsigned char) (forward_connect_ip >> 8),
(unsigned char) forward_connect_ip, strerror(errno));
return 1;
}
while (1) {
fd_set rfds;
fd_set wfds;
static char local_buf[4096];
static char remote_buf[4096];
static int local_len = 0;
static int remote_len = 0;
static int local_eof = 0;
static int remote_eof = 0;
int len;
FD_ZERO(&rfds);
FD_ZERO(&wfds);
if (local_eof == 0 && local_len < sizeof(local_buf))
FD_SET(0, &rfds);
if (remote_eof == 0 && remote_len < sizeof(remote_buf))
FD_SET(remote, &rfds);
if (local_len)
FD_SET(remote, &wfds);
if (remote_len)
FD_SET(1, &wfds);
select(remote + 1, &rfds, &wfds, NULL, NULL);
if (FD_ISSET(0, &rfds)) {
len = read(0, local_buf + local_len,
sizeof(local_buf) - local_len);
if (len > 0)
local_len += len;
else if (len == 0)
local_eof = 1;
}
if (FD_ISSET(remote, &rfds)) {
len = read(remote, remote_buf + remote_len,
sizeof(remote_buf) - remote_len);
if (len > 0)
remote_len += len;
else if (len == 0)
remote_eof = 1;
}
if (FD_ISSET(remote, &wfds)) {
len = write(remote, local_buf, local_len);
if (len > 0) {
local_len -= len;
memmove(local_buf, local_buf + len, local_len);
}
}
if (FD_ISSET(1, &wfds)) {
len = write(1, remote_buf, remote_len);
if (len > 0) {
remote_len -= len;
memmove(remote_buf, remote_buf + len,
remote_len);
}
}
if (local_len == 0 && local_eof == 1) {
shutdown(remote, SHUT_WR);
local_eof = 2;
}
if (remote_len == 0 && remote_eof == 1) {
shutdown(1, SHUT_WR);
remote_eof = 2;
}
if (local_eof == 2 && remote_eof == 2)
break;
}
return 0;
}
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