SSH - a zero-heap SSH server

Layer: L5 Session · Build flags: PC_ENABLE_SSH

What this example teaches

This stands up a real SSH 2.0 server (RFC 4253/4252/4254) on port 22: it loads an RSA host key from NVS, authenticates clients by password and/or public key, and echoes channel data back. The crypto runs on the ESP32 hardware accelerator and no connection state touches the heap.

Listening on a non-HTTP protocol. The same PC can host other L5 protocols. You open an SSH listener with server.listen(port, PROTO_SSH) and then begin() (no port argument needed when you have explicitly added listeners):

server.listen(22, ConnProto::PROTO_SSH);
int32_t result = server.begin();
pc_ssh_conn_setup();   // one-time wiring of the SSH dispatcher's outbound path (after begin)

The host key lives in NVS, not in RAM. pc_ssh_rsa_load_pubkey() loads only the public half at startup; the private key is read per-signature into a stack buffer and wiped, so it is never held in static RAM. You must provision the DER key once per device (namespace ssh_host_key, key priv_der) - see docs/SSH.md:

if (pc_ssh_rsa_load_pubkey() != 0) {
    Serial.println("No SSH host key in NVS - see docs/SSH.md (Host key provisioning)");
    return;
}

Auth via callbacks. You decide who gets in. A password callback and a public-key callback are installed before begin(); the server verifies the client's signature itself once your pubkey callback accepts the key:

pc_ssh_auth_set_password_cb(ssh_password_auth);  // return true to accept user/pass
pc_ssh_auth_set_pubkey_cb(ssh_pubkey_auth);      // return true to accept (user, key blob)
pc_ssh_channel_set_data_cb(ssh_on_data);         // bytes from the client

Channel echo. The data callback receives the channel id the bytes arrived on and sends them back with pc_ssh_conn_send(slot, channel, data, len) - the skeleton for a remote console. With PC_SSH_MAX_CHANNELS > 1 the client can open several channels over one connection and each is tagged by id.

**TCP port forwarding (ssh -L).** Define PC_SSH_PORT_FORWARD (and give it channel + client-pool room) to let the board act as a local-forward tunnel: when a client opens a direct-tcpip channel, the ssh_forward owner opens the outbound TCP connection through the client transport and bridges bytes both ways. It is opt-in twice over - compiled out by default, and inert until you call pc_ssh_forward_begin() - because any authenticated client could otherwise make the board connect anywhere (an open proxy). Restrict the reachable targets with a policy callback:

static bool pc_ssh_forward_policy(const char *host, uint16_t port) {
    return port == 80 || port == 443;   // allow only outbound web
}
pc_ssh_forward_set_policy_cb(pc_ssh_forward_policy);
pc_ssh_forward_begin();                     // after pc_ssh_conn_setup()

Then ssh -L 8080:example.com:80 admin@<ip> and curl localhost:8080 reaches example.com through the board.

Hardening. Define PC_SSH_ALLOW_PASSWORD 0 to compile password auth out entirely (publickey-only), and failed attempts are bounded by SSH_MAX_AUTH_ATTEMPTS. Use a constant-time compare and real credential storage in production - the demo's strcmp is illustrative only.

Build and run

PC_ENABLE_SSH must reach the library build, so pass it as a build flag:

pio ci --board=esp32dev --project-option="framework=arduino" \
  --project-option="build_flags=-DPC_ENABLE_SSH=1" \
  --lib="." examples/L5-Session/SSH/SSH.ino

To build with port forwarding, add the forwarding flags:

pio ci --board=esp32dev --project-option="framework=arduino" \
  --project-option="build_flags=-DPC_ENABLE_SSH=1 -DPC_SSH_PORT_FORWARD=1 -DPC_SSH_MAX_CHANNELS=4 -DPC_CLIENT_CONNS=3 -DPC_SSH_FWD_MAX=3" \
  --lib="." examples/L5-Session/SSH/SSH.ino

Provision a host key first (see docs/SSH.md), then:

ssh -p 22 admin@<ip>     # password: s3cret  (type; the server echoes it back)
 
# with forwarding built in:
ssh -L 8080:example.com:80 admin@<ip>   # then in another shell:
curl http://localhost:8080/             # reaches example.com:80 through the board

Annotated source

The complete sketch (SSH.ino), reproduced verbatim with added explanatory comments:

// Copyright (C) 2026 Douglas Quigg (dstroy0) <dquigg123@gmail.com>
// SPDX-License-Identifier: AGPL-3.0-or-later
 
// Enable the SSH stack for this sketch (overrides the default-off config).
#define PC_ENABLE_SSH 1
 
// To demonstrate TCP port forwarding (ssh -L), uncomment these: the channel pool
// must hold the shell + the tunnel(s), and the outbound client pool must cover the
// concurrent forwards (PC_CLIENT_CONNS >= PC_SSH_FWD_MAX).
// #define PC_SSH_MAX_CHANNELS 4
// #define PC_SSH_PORT_FORWARD 1
// #define PC_CLIENT_CONNS 3
// #define PC_SSH_FWD_MAX 3
 
#include "protocore.h"
#include "network_drivers/physical/physical.h"
#include "network_drivers/presentation/ssh/auth/ssh_auth.h"    // pc_ssh_auth_set_*_cb
#include "network_drivers/presentation/ssh/connection/ssh_channel.h" // pc_ssh_channel_set_data_cb
#include "network_drivers/presentation/ssh/connection/ssh_conn.h"    // pc_ssh_conn_send / pc_ssh_conn_setup
#include "network_drivers/presentation/ssh/connection/ssh_forward.h" // pc_ssh_forward_begin (ssh -L)
#include "network_drivers/presentation/ssh/crypto/ssh_rsa.h"     // pc_ssh_rsa_load_pubkey
 
static const char *SSID = "YOUR_SSID";
static const char *PASSWORD = "YOUR_PASSWORD";
 
PC server;
 
// --- Authentication callbacks ----------------------------------------------
 
// Return true to accept the username/password. Use a constant-time compare and
// real credential storage in production; this is illustrative only.
static bool ssh_password_auth(const char *user, const char *pass)
{
    return strcmp(user, "admin") == 0 && strcmp(pass, "s3cret") == 0;
}
 
// Return true if (user, public-key blob) is authorized. Compare the raw blob
// against your authorized_keys (the server verifies the client's signature
// itself once you accept the key here).
static bool ssh_pubkey_auth(const char *user, const uint8_t *blob, size_t blob_len)
{
    (void)blob;
    (void)blob_len;
    // e.g. return user_key_matches(user, blob, blob_len);
    return strcmp(user, "admin") == 0; // accept any key for "admin" in this demo
}
 
// --- Channel data: echo received bytes back to the client -------------------
 
static void ssh_on_data(uint8_t slot, uint32_t channel, const uint8_t *data, size_t len)
{
    pc_ssh_conn_send(slot, channel, data, len); // echo
}
 
#if PC_SSH_PORT_FORWARD
// Forward policy: which ssh -L targets are allowed (else an open proxy for any
// authenticated client). Return true to permit a target.
static bool pc_ssh_forward_policy(const char *host, uint16_t port)
{
    return port == 80 || port == 443; // demo: allow only outbound web
}
#endif
 
void setup()
{
    Serial.begin(115200);
 
    init_wifi_physical(SSID, PASSWORD);
    Serial.print("Connecting to WiFi");
    while (!wifi_ready())
    {
        delay(250);
        Serial.print('.');
    }
    uint32_t ip = pc_net_egress_ip(); // library egress IP (network byte order), no Arduino WiFi
    Serial.printf("IP: %u.%u.%u.%u\n", (unsigned)(ip & 0xFF), (unsigned)((ip >> 8) & 0xFF),
                  (unsigned)((ip >> 16) & 0xFF), (unsigned)((ip >> 24) & 0xFF));
 
    // Load the RSA host key's public half from NVS (the private key is read
    // per-signature into a stack buffer and wiped; never held in static RAM).
    if (pc_ssh_rsa_load_pubkey() != 0)
    {
        Serial.println("No SSH host key in NVS - see docs/SSH.md (Host key provisioning)");
        return;
    }
 
    // Install SSH callbacks before begin().
    pc_ssh_auth_set_password_cb(ssh_password_auth);
    pc_ssh_auth_set_pubkey_cb(ssh_pubkey_auth);
    pc_ssh_channel_set_data_cb(ssh_on_data);
 
    // Listen for SSH on port 22 (and, optionally, HTTP on 80 alongside it).
    server.listen(22, ConnProto::PROTO_SSH);
    int32_t result = server.begin();
    if (result < 0)
    {
        Serial.printf("begin() failed (error %d)\n", result);
        return;
    }
 
    // One-time wiring of the SSH dispatcher's outbound path. Call after begin().
    pc_ssh_conn_setup();
 
#if PC_SSH_PORT_FORWARD
    // Enable ssh -L forwarding (opt-in; nothing is forwarded until this runs).
    pc_ssh_forward_set_policy_cb(pc_ssh_forward_policy);
    pc_ssh_forward_begin();
    Serial.println("SSH port forwarding enabled (ssh -L to ports 80/443)");
#endif
 
    Serial.println("SSH server started on port 22");
}
 
void loop()
{
    // Drives accept/rx for every listener, including the PROTO_SSH handshake,
    // user-auth, and channel data pumping.
    server.handle();
}