+ *
+ * When NLM uses an SM_MON request to tell statd to monitor a remote,
+ * the request contains a "mon_name" argument. This is usually the
+ * "caller_name" argument of an NLMPROC_LOCK request. On Linux, the
+ * NLM can send statd the remote's IP address instead of its
+ * caller_name. The NSM protocol does not allow both the remote's
+ * caller_name and it's IP address to be sent in the same SM_MON
+ * request.
+ *
+ * The remote's caller_name is useful because it makes it simple
+ * to identify rebooting remotes by matching the "mon_name" argument
+ * they sent via an SM_NOTIFY request.
+ *
+ * The caller_name string may not be a fully qualified domain name,
+ * or even registered in the DNS database, however. Having the
+ * remote's IP address is useful because then there is no ambiguity
+ * about where to send an SM_NOTIFY after the local system reboots.
+ *
+ * Without the actual caller_name, however, statd must use an
+ * heuristic to match an incoming SM_NOTIFY request to one of the
+ * hosts it is currently monitoring. The incoming mon_name in an
+ * SM_NOTIFY address is converted to a list of IP addresses using
+ * DNS. Each mon_name on statd's monitor list is also converted to
+ * an address list, and the two lists are checked to see if there is
+ * a matching address.
+ *
+ * There are some risks to this strategy:
+ *
+ * 1. The external DNS database is not reliable. It can change
+ * over time, or the forward and reverse mappings could be
+ * inconsistent.
+ *
+ * 2. If statd's monitor list becomes substantial, finding a match
+ * can generate a not inconsequential amount of DNS traffic.
+ *
+ * 3. statd is a single-threaded service. When DNS becomes slow or
+ * unresponsive, statd also becomes slow or unresponsive.
+ *
+ * 4. If the remote does not have a DNS entry at all (or if the
+ * remote can resolve itself, but the local host can't resolve
+ * the remote's hostname), the remote cannot be monitored, and
+ * therefore NLM locking cannot be provided for that host.
+ *
+ * 5. Local DNS resolution can produce different results for the
+ * mon_name than the results the remote might see for the same
+ * query, especially if the remote did not send a caller_name
+ * or mon_name that is a fully qualified domain name.
+ *
+ * Note that a caller_name is passed from NFS client to server,
+ * but the client never knows what mon_name the server might use
+ * to notify it of a reboot. On Linux, the client extracts the
+ * server's name from the devname it was passed by the mount
+ * command. This is often not a fully-qualified domain name.