channels: implement callback for agent forwarding request

Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/callbacks.h

@@ -481,6 +481,16 @@ typedef int (*ssh_channel_pty_request_callback) (ssh_session session,
 typedef int (*ssh_channel_shell_request_callback) (ssh_session session,
                                             ssh_channel channel,
                                             void *userdata);
+/**
+ * @brief SSH auth-agent-request from the client. This request is
+ * sent by a client when agent forwarding is available.
+ * Server is free to ignore this callback, no answer is expected.
+ * @param channel the channel
+ * @param userdata Userdata to be passed to the callback function.
+ */
+typedef void (*channel_auth_agent_req_callback) (ssh_session session,
+                                            ssh_channel channel,
+                                            void *userdata);
 struct ssh_channel_callbacks_struct {
   /** DON'T SET THIS use ssh_callbacks_init() instead. */
   size_t size;
@@ -520,6 +530,10 @@ struct ssh_channel_callbacks_struct {
    * This function will be called when a client requests a shell
    */
   ssh_channel_shell_request_callback channel_shell_request_function;
+  /** This function will be called when a client requests agent
+   * authentication forwarding.
+   */
+  channel_auth_agent_req_callback channel_auth_agent_req_function;
 };
 typedef struct ssh_channel_callbacks_struct *ssh_channel_callbacks;
 

src/channels.c

@@ -833,6 +833,16 @@ SSH_PACKET_CALLBACK(channel_rcv_request) {
 	  return SSH_PACKET_USED;
 	}
 
+  if (strcmp(request, "auth-agent-req@openssh.com") == 0) {
+    SAFE_FREE(request);
+    ssh_log(session, SSH_LOG_PROTOCOL, "Received an auth-agent-req request");
+    if(ssh_callbacks_exists(channel->callbacks, channel_auth_agent_req_function)) {
+        channel->callbacks->channel_auth_agent_req_function(channel->session, channel,
+            channel->callbacks->userdata);
+    }
+    leave_function();
+    return SSH_PACKET_USED;
+  }
 	/* If we are here, that means we have a request that is not in the understood
 	 * client requests. That means we need to create a ssh message to be passed
 	 * to the user code handling ssh messages