+/**
+ * Send a sideband packet through a blocking file descriptor.
+ *
+ * \param scc fd and crypto keys.
+ * \param buf The buffer to send.
+ * \param numbytes The size of \a buf.
+ * \param band The sideband designator of this packet.
+ * \param dont_free If true, never deallocate \a buf.
+ *
+ * The nonblock flag must be disabled for the file descriptor given by \a scc.
+ *
+ * Stream cipher encryption is automatically activated if neccessary via the
+ * sideband transformation, depending on the value of \a band.
+ *
+ * \return Standard.
+ *
+ * \sa \ref send_sb_va().
+ */
+int send_sb(struct stream_cipher_context *scc, void *buf, size_t numbytes,
+ int band, bool dont_free)
+{
+ int ret;
+ struct sb_context *sbc;
+ struct iovec iov[2];
+ sb_transformation trafo = band < SBD_PROCEED? NULL : sc_trafo;
+ struct sb_buffer sbb = SBB_INIT(band, buf, numbytes);
+
+ sbc = sb_new_send(&sbb, dont_free, trafo, scc->send);
+ do {
+ ret = sb_get_send_buffers(sbc, iov);
+ ret = xwritev(scc->fd, iov, ret);
+ if (ret < 0)
+ goto fail;
+ } while (sb_sent(sbc, ret) == false);
+ return 1;
+fail:
+ sb_free(sbc);
+ return ret;
+}
+
+/**
+ * Create a variable sized buffer and send it as a sideband packet.
+ *
+ * \param scc Passed to \ref send_sb.
+ * \param band See \ref send_sb.
+ * \param fmt The format string.
+ *
+ * \return The return value of the underlying call to \ref send_sb.
+ */
+__printf_3_4 int send_sb_va(struct stream_cipher_context *scc, int band,
+ const char *fmt, ...)
+{
+ va_list ap;
+ char *msg;
+ int ret;
+
+ va_start(ap, fmt);
+ ret = xvasprintf(&msg, fmt, ap);
+ va_end(ap);
+ return send_sb(scc, msg, ret, band, false);
+}
+
+/**
+ * Send an error message to a client.
+ *
+ * \param cc Client info.
+ * \param err The (positive) error code.
+ *
+ * \return The return value of the underlying call to send_sb_va().
+ */
+int send_strerror(struct command_context *cc, int err)