path: root/ssh/server_proxy.c

#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
#include <stdbool.h>
#include <stdatomic.h>
#include <string.h>
#include <ctype.h>
#include <errno.h>
#include <assert.h>
#include <pthread.h>
#include <netdb.h>
#include <poll.h>
#include <signal.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <arpa/inet.h>
#include <libssh/server.h>
#include <libssh/callbacks.h>
#include "util.h"


#define RESOURCE_ERROR_SLEEP_MS 10000

static const bool debug_enabled = false;


__attribute__((format (printf, 1, 2)))
static void debug(const char *restrict format, ...) {
	if (debug_enabled) {
		va_list ap;
		va_start(ap, format);
		vfprintf(stderr, format, ap);
		va_end(ap);
	}
}

static void xxd(FILE *stream, const void *buf_, size_t length) {
	unsigned char *buf = (unsigned char*)buf_;

	for (size_t cursor = 0; cursor < length;) {
		fprintf(stream, "%08zx:", cursor);

		for (int i = 0; i < 16; i++) {
			if (i % 2 == 0) fprintf(stream, " ");
			if (i % 8 == 0) fprintf(stream, " ");
			if (cursor + i < length) fprintf(stream, "%02x", (unsigned)buf[cursor + i]);
			else fprintf(stream, "  ");
		}

		fprintf(stream, "  |");

		for (int i = 0; i < 16 && cursor + i < length; i++) {
			if (isprint(buf[cursor + i])) fprintf(stream, "%c", buf[cursor + i]);
			else fprintf(stream, ".");
		}

		fprintf(stream, "|\n");

		cursor += 16;
	}
}

static atomic_int g_thread_count;

struct thread_data {
	struct addrinfo backend_addr;

	int backend_fd;

	int thread_id;
	ssh_session session;
	ssh_channel channel;  // NULL before channel has been opened
	bool should_close;

	struct ssh_server_callbacks_struct server_cb;
	struct ssh_channel_callbacks_struct chan_cb;
};

///////// CHANNEL CALLBACKS //////////

static int channel_subsystem_request_cb(ssh_session session, ssh_channel channel, const char *subsystem, void *tdata_) {
	(void)session;
	(void)channel;
	struct thread_data *tdata = (struct thread_data*)tdata_;
	if (strcmp(subsystem, "tomsg") == 0) {
		debug("[%d] subsystem request: <%s>, allowing\n", tdata->thread_id, subsystem);
		return 0;
	} else {
		debug("[%d] subsystem request: <%s>, denying!\n", tdata->thread_id, subsystem);
		return 1;
	}
}

static void channel_close_cb(ssh_session session, ssh_channel channel, void *tdata_) {
	(void)session; (void)channel;
	struct thread_data *tdata = (struct thread_data*)tdata_;
	debug("[%d] channel close!\n", tdata->thread_id);
}

static void channel_eof_cb(ssh_session session, ssh_channel channel, void *tdata_) {
	(void)session; (void)channel;
	struct thread_data *tdata = (struct thread_data*)tdata_;
	debug("[%d] eof on channel, setting close flag\n", tdata->thread_id);
	tdata->should_close = true;
}

static int channel_data_cb(ssh_session session, ssh_channel channel, void *data, uint32_t len, int is_stderr, void *tdata_) {
	(void)is_stderr; (void)data; (void)channel; (void)session;
	struct thread_data *tdata = (struct thread_data*)tdata_;

	debug("[%d] data on channel (length %u):\n", tdata->thread_id, len);
	if (debug_enabled) xxd(stdout, data, len);

	// debug("[%d] echoing back!\n", tdata->thread_id);
	// if (ssh_channel_write(channel, data, len) == SSH_ERROR) {
	//     debug("[%d] write to channel failed! Setting close flag\n", tdata->thread_id);
	//     tdata->should_close = true;
	// }

	const char *start = (const char*)data;
	const char *cursor = start;
	const char *end = start + len;

	while (cursor < end) {
		ssize_t nw = write(tdata->backend_fd, cursor, end - cursor);
		if (nw < 0) {
			if (errno == EINTR) continue;
			debug("[%d] error writing to backend socket: %s\n", tdata->thread_id, strerror(errno));
			tdata->should_close = true;
			return cursor - start;
		}
		if (nw == 0) {  // should not happen?
			debug("[%d] write(2) returned 0?\n", tdata->thread_id);
			tdata->should_close = true;
			return cursor - start;
		}
		cursor += nw;
	}

	return len;
}

////////// SERVER CALLBACKS //////////

static int auth_none_cb(ssh_session session, const char *user, void *tdata_) {
	(void)session;
	struct thread_data *tdata = (struct thread_data*)tdata_;
	debug("[%d] auth none (user <%s>), accepting\n", tdata->thread_id, user);
	return SSH_AUTH_SUCCESS;
}

static int service_request_cb(ssh_session session, const char *service, void *tdata_) {
	(void)session;
	struct thread_data *tdata = (struct thread_data*)tdata_;
	if (strcmp(service, "ssh-userauth") == 0) {
		debug("[%d] ssh-userauth service request, allowing through\n", tdata->thread_id);
		return 0;
	} else {
		debug("[%d] service request <%s>, not allowing\n", tdata->thread_id, service);
		return -1;
	}
}

static ssh_channel chan_open_request_cb(ssh_session session, void *tdata_) {
	struct thread_data *tdata = (struct thread_data*)tdata_;
	if (tdata->channel == NULL) {
		ssh_channel chan = ssh_channel_new(session);
		if (chan != NULL) {
			if (ssh_set_channel_callbacks(chan, &tdata->chan_cb) == SSH_OK) {
				debug("[%d] channel open request, allowing\n", tdata->thread_id);
				tdata->channel = chan;
				return chan;
			}
			ssh_channel_close(chan);
			ssh_channel_free(chan);
		}
	}
	debug("[%d] channel open request, denying!\n", tdata->thread_id);
	return NULL;
}

static int backend_data_cb(int fd, int revents, void *tdata_) {
	struct thread_data *tdata = (struct thread_data*)tdata_;

	if (revents & (POLLERR|POLLHUP|POLLNVAL)) {
		// Print this always, because backend issues are interesting
		char descr[64] = "";
		if (revents & POLLERR) strcat(descr, "|POLLERR");
		if (revents & POLLHUP) strcat(descr, "|POLLHUP");
		if (revents & POLLNVAL) strcat(descr, "|POLLNVAL");
		printf("[%d] %s on backend\n", tdata->thread_id, descr + 1);
		close(fd);
		tdata->should_close = true;
	}

	if (revents & POLLIN) {
		char buffer[1024];
		ssize_t nr = read(fd, buffer, sizeof buffer);
		if (nr < 0) {
			if (errno == EINTR) return 0;
			// Backend issues are interesting, not noise
			printf("[%d] Error reading from backend socket: %s\n", tdata->thread_id, strerror(errno));
			tdata->should_close = true;
			return 0;
		}

		if (nr == 0) {  // eof
			tdata->should_close = true;
			return 0;
		}

		debug("[%d] data from backend (length %zu):\n", tdata->thread_id, nr);
		if (debug_enabled) xxd(stdout, buffer, nr);

		ssize_t cursor = 0;
		while (cursor < nr) {
			int nw = ssh_channel_write(tdata->channel, buffer + cursor, nr - cursor);
			if (nw == SSH_ERROR) {
				debug("[%d] Error writing to ssh channel: %s\n", tdata->thread_id, ssh_get_error(tdata->channel));
				tdata->should_close = true;
				return 0;
			}
			cursor += nw;
			debug("[%d]   forwarded %d bytes, total %zd/%zd\n",
					tdata->thread_id, nw, cursor, nr);
		}
	}

	return 0;
}

static void print_addrinfo(FILE *stream, const struct addrinfo *info) {
	if (info->ai_family == AF_INET) fprintf(stream, "inet ");
	else if (info->ai_family == AF_INET6) fprintf(stream, "inet6 ");
	else fprintf(stream, "(family=%d) ", info->ai_family);

	if (info->ai_socktype == SOCK_STREAM) fprintf(stream, "stream ");
	else if (info->ai_socktype == SOCK_DGRAM) fprintf(stream, "datagram ");
	else fprintf(stream, "(socktype=%d) ", info->ai_socktype);

	if (info->ai_protocol == IPPROTO_TCP) fprintf(stream, "TCP ");
	else if (info->ai_protocol == IPPROTO_UDP) fprintf(stream, "UDP ");
	else fprintf(stream, "(protocol=%d) ", info->ai_protocol);

	bool success = false;
	if (info->ai_family == AF_INET) {
		char addrbuf[INET_ADDRSTRLEN];
		struct sockaddr_in *sin = (struct sockaddr_in*)info->ai_addr;
		if (inet_ntop(AF_INET, &sin->sin_addr, addrbuf, INET_ADDRSTRLEN)) {
			fprintf(stream, "%s\n", addrbuf);
			success = true;
		}
	} else if (info->ai_family == AF_INET6) {
		char addrbuf[INET6_ADDRSTRLEN];
		struct sockaddr_in6 *sin = (struct sockaddr_in6*)info->ai_addr;
		if (inet_ntop(AF_INET6, &sin->sin6_addr, addrbuf, INET6_ADDRSTRLEN)) {
			fprintf(stream, "%s\n", addrbuf);
			success = true;
		}
	}
	if (!success) {
		fprintf(stream, "(unknown address format: %s)\n", strerror(errno));
	}
}

// Returns whether successful.
static bool lookup_backend(const char *host, int port, struct addrinfo *dst) {
	char port_string[16];
	sprintf(port_string, "%d", port);

	struct addrinfo hints;
	memset(&hints, 0, sizeof hints);
	hints.ai_family = AF_UNSPEC;
	hints.ai_socktype = SOCK_STREAM;
	hints.ai_flags = AI_ADDRCONFIG;

	struct addrinfo *result;
	int ret = getaddrinfo(host, port_string, &hints, &result);

	if (ret < 0) {
		fprintf(stderr, "Could not resolve backend: %s\n", gai_strerror(ret));
		return false;
	}

	int last_failure = 0;
	bool success = false;
	for (struct addrinfo *item = result; item; ) {
		debug("lookup_backend: option ");
		if (debug_enabled) print_addrinfo(stdout, item);

		int sock = socket(item->ai_family, item->ai_socktype, item->ai_protocol);
		if (sock == -1) {
			last_failure = errno;
			debug("  socket() failure: %s\n", strerror(last_failure));
			continue;
		}

		int ret = connect(sock, item->ai_addr, item->ai_addrlen);
		last_failure = errno;
		close(sock);

		if (ret == 0) {
			debug("  success!\n");
			success = true;
			// Free the rest of the linked list, keeping this item intact.
			freeaddrinfo(item->ai_next);
			*dst = *item;
			dst->ai_next = NULL;
			break;
		} else {
			debug("  connect() failure: %s\n", strerror(last_failure));
		}

		debug("  next=%p\n", item->ai_next);

		// Free this element in the linked list, but preserve (and switch to) the tail.
		struct addrinfo *next = item->ai_next;
		item->ai_next = NULL;
		freeaddrinfo(item);
		item = next;
	}

	if (success) {
		return true;
	} else {
		fprintf(stderr, "Could not connect to backend: %s\n", strerror(last_failure));
		return false;
	}
}

static int connect_backend(const struct thread_data *tdata) {
	const struct addrinfo *item = &tdata->backend_addr;
	int sock = socket(item->ai_family, item->ai_socktype, item->ai_protocol);
	if (sock == -1) return -1;

	if (connect(sock, item->ai_addr, item->ai_addrlen) == 0) {
		debug("connect_backend: sock=%d\n", sock);
		return sock;
	}

	close(sock);
	return -1;
}

static void* thread_entry(void *tdata_) {
	struct thread_data *tdata = (struct thread_data*)tdata_;
	const int tid = tdata->thread_id;
	const ssh_session session = tdata->session;
	ssh_event event = NULL;

	debug("[%d] Thread started\n", tid);

	memset(&tdata->server_cb, 0, sizeof tdata->server_cb);
	ssh_callbacks_init(&tdata->server_cb);
	tdata->server_cb.userdata = tdata;
	tdata->server_cb.auth_none_function = auth_none_cb;
	tdata->server_cb.channel_open_request_session_function = chan_open_request_cb;
	tdata->server_cb.service_request_function = service_request_cb;

	memset(&tdata->chan_cb, 0, sizeof tdata->chan_cb);
	ssh_callbacks_init(&tdata->chan_cb);
	tdata->chan_cb.userdata = tdata;
	tdata->chan_cb.channel_subsystem_request_function = channel_subsystem_request_cb;
	tdata->chan_cb.channel_close_function = channel_close_cb;
	tdata->chan_cb.channel_eof_function = channel_eof_cb;
	tdata->chan_cb.channel_data_function = channel_data_cb;

	if (ssh_set_server_callbacks(session, &tdata->server_cb) != SSH_OK) {
		debug("[%d] Failed setting server callbacks: %s\n", tid, ssh_get_error(session));
		goto cleanup;
	}

	ssh_set_auth_methods(session, SSH_AUTH_METHOD_NONE);

	if (ssh_handle_key_exchange(session) != SSH_OK) {
		debug("[%d] Key exchange failed: %s\n", tid, ssh_get_error(session));
		goto cleanup;
	}
	debug("[%d] Handled key exchange\n", tid);

	tdata->backend_fd = connect_backend(tdata);
	if (tdata->backend_fd == -1) {
		debug("[%d] Failed to connect to backend: %s\n", tid, strerror(errno));
		goto cleanup;
	}

	debug("[%d] Connected to backend (fd=%d)\n", tid, tdata->backend_fd);

	event = ssh_event_new();
	if (!event
			|| ssh_event_add_session(event, session) != SSH_OK
			|| ssh_event_add_fd(event, tdata->backend_fd, POLLIN, backend_data_cb, tdata) != SSH_OK) {
		debug("[%d] Failed to create ssh event context\n", tid);
		goto cleanup;
	}

	while (!tdata->should_close) {
		// printf("[%d] poll loop...\n", tid);
		ssh_event_dopoll(event, -1);
		int status = ssh_get_status(session);
		if (status & (SSH_CLOSED | SSH_CLOSED_ERROR)) goto cleanup;
		if (status & SSH_READ_PENDING) {
			debug("[%d] read pending?\n", tid);
		}
	}

cleanup:
	if (tdata->backend_fd != -1) close(tdata->backend_fd);
	if (event) ssh_event_free(event);
	if (tdata->channel) {
		ssh_channel_close(tdata->channel);
		ssh_channel_free(tdata->channel);
	}
	if (session) {
		ssh_disconnect(session);
		ssh_free(session);
		debug("[%d] Disconnected\n", tid);
	}
	free(tdata);
	int num_threads = atomic_fetch_sub(&g_thread_count, 1);
	debug("[%d] Exiting! (%d threads remaining)\n", tid, num_threads - 1);
	return NULL;
}

static void generate_key(const char *outfile) {
	ssh_key host_key;
	int ret = ssh_pki_generate(SSH_KEYTYPE_RSA, 4096, &host_key);
	if (ret != SSH_OK) {
		fprintf(stderr, "Key generation failed (RSA4096)!\n");
		exit(1);
	}

	ret = ssh_pki_export_privkey_file(host_key, NULL, NULL, NULL, outfile);
	if (ret != SSH_OK) {
		fprintf(stderr, "Failed to export generated host key to file '%s'; is that location accessible?\n", outfile);
		exit(1);
	}

	if (chmod(outfile, S_IRUSR | S_IWUSR) != 0) {
		fprintf(stderr, "Failed to set mode 600 on generated host key file '%s'; this is insecure!\n", outfile);
		exit(1);
	}

	printf("RSA4096 host key generated and written to '%s'.\n", outfile);
}

static void usage(const char *argv0) {
	fprintf(stderr,
			"Usage: %s <ssh host key file> <ssh port> [backendhost:port]\n"
			"       %s --generate <host key output file>\n"
			"SSH-TCP bridge for tomsg. Accepts SSH connections with a channel for subsystem\n"
			"'tomsg', and matches each SSH connection with a plain TCP connection to the\n"
			"backend server (which defaults to localhost:29536). All data is forwarded\n"
			"transparently.\n"
			"Use the '--generate' form to generate a host key for use in the main invocation\n"
			"form.\n",
			argv0, argv0);
}

int main(int argc, char **argv) {
	const char *host_key_fname;
	int ssh_port = 2222;
	const char *backend_host = "localhost";
	int backend_port = 29536;

	if (argc == 3 && strcmp(argv[1], "--generate") == 0) {
		generate_key(argv[2]);
		return 0;
	} else if (3 <= argc && argc <= 4) {
		host_key_fname = argv[1];
		char *endp;
		ssh_port = strtol(argv[2], &endp, 10);
		if (argv[2][0] == '\0' || *endp != '\0' || ssh_port < 0 || ssh_port > 65535) {
			fprintf(stderr, "Cannot parse port number from argument '%s'\n", argv[2]);
			return 1;
		}
		if (argc == 4) {
			if (!parse_host_port(argv[3], &backend_host, &backend_port)) {
				fprintf(stderr, "Cannot parse host:port from argument '%s'\n", argv[3]);
				return 1;
			}
		}
	} else {
		usage(argv[0]);
		return 1;
	}

	// We prefer to detect socket closure through return codes, not signals.
	signal(SIGPIPE, SIG_IGN);

	if (ssh_init() != SSH_OK) {
		fprintf(stderr, "Could not initialise libssh\n");
		return 1;
	}

	ssh_key host_key;
	if (ssh_pki_import_privkey_file(host_key_fname, NULL, NULL, NULL, &host_key) != SSH_OK) {
		fprintf(stderr, "Failed to read host private key file '%s'\n", host_key_fname);
		return 1;
	}

	size_t host_key_hash_length = 0;
	unsigned char *host_key_hash = NULL;
	if (ssh_get_publickey_hash(host_key, SSH_PUBLICKEY_HASH_SHA256, &host_key_hash, &host_key_hash_length) != SSH_OK) {
		fprintf(stderr, "Failed to hash host key!\n");
		return 1;
	}

	printf("Host key hash: ");
	fflush(stdout);
	ssh_print_hash(SSH_PUBLICKEY_HASH_SHA256, host_key_hash, host_key_hash_length);

	ssh_bind srvbind = ssh_bind_new();
	if (!srvbind) {
		fprintf(stderr, "Failed to create new bind socket\n");
		return 1;
	}

	bool procconfig = false;
	const char *ciphers_str = "aes256-gcm@openssh.com,aes256-ctr,aes256-cbc";
	bool ok = true;
	ok &= ssh_bind_options_set(srvbind, SSH_BIND_OPTIONS_PROCESS_CONFIG, &procconfig) == SSH_OK;
	ok &= ssh_bind_options_set(srvbind, SSH_BIND_OPTIONS_BINDPORT, &ssh_port) == SSH_OK;
	ok &= ssh_bind_options_set(srvbind, SSH_BIND_OPTIONS_IMPORT_KEY, host_key) == SSH_OK;
	ok &= ssh_bind_options_set(srvbind, SSH_BIND_OPTIONS_CIPHERS_C_S, ciphers_str) == SSH_OK;
	ok &= ssh_bind_options_set(srvbind, SSH_BIND_OPTIONS_CIPHERS_S_C, ciphers_str) == SSH_OK;

	if (!ok) {
		fprintf(stderr, "Could not set options on SSH bind socket: %s\n", ssh_get_error(srvbind));
		return 1;
	}

	if (ssh_bind_listen(srvbind) != SSH_OK) {
		fprintf(stderr, "Could not listen on SSH bind socket: %s\n", ssh_get_error(srvbind));
		return 1;
	}

	struct addrinfo backend_addr;
	if (!lookup_backend(backend_host, backend_port, &backend_addr)) {
		// Error already printed in lookup_backend
		return 1;
	}

	printf("Listening for SSH connections on port %d\n", ssh_port);
	printf("Forwarding to backend at %s:%d\n", backend_host, backend_port);

	pthread_attr_t thread_attrs;
	assert(pthread_attr_init(&thread_attrs) == 0);
	assert(pthread_attr_setdetachstate(&thread_attrs, PTHREAD_CREATE_DETACHED) == 0);

	int next_thread_id = 0;
	atomic_store(&g_thread_count, 0);

	while (true) {
		ssh_session session = ssh_new();
		if (!session) {
			fprintf(stderr, "ERROR: Cannot create new SSH session object!\n");
			usleep(1000 * RESOURCE_ERROR_SLEEP_MS);
			continue;
		}

		if (ssh_bind_accept(srvbind, session) != SSH_OK) {
			fprintf(stderr, "ERROR: Cannot accept on bind socket: %s", ssh_get_error(srvbind));
			ssh_free(session);
			usleep(1000 * RESOURCE_ERROR_SLEEP_MS);
			continue;
		}

		int num_existing_threads = atomic_fetch_add(&g_thread_count, 1);
		debug("Accepted connection, spinning up thread (currently %d threads)\n",
				num_existing_threads + 1);

		struct thread_data *tdata = calloc(1, sizeof(struct thread_data));
		if (!tdata) {
			fprintf(stderr, "ERROR: Out of memory, cannot allocate thread_data!\n");
			ssh_disconnect(session);
			ssh_free(session);
			usleep(1000 * RESOURCE_ERROR_SLEEP_MS);
			continue;
		}

		tdata->backend_addr = backend_addr;
		tdata->backend_fd = -1;
		tdata->thread_id = next_thread_id++;
		tdata->session = session;
		tdata->channel = NULL;
		tdata->should_close = false;

		pthread_t thread;
		if (pthread_create(&thread, &thread_attrs, thread_entry, tdata) != 0) {
			fprintf(stderr, "ERROR: Could not spawn thread: %s!\n", strerror(errno));
			free(tdata);
			ssh_disconnect(session);
			ssh_free(session);
			usleep(1000 * RESOURCE_ERROR_SLEEP_MS);
			continue;
		}
	}
}