NetTime Server 3.04 Crack: How to Sync Your PC's Clock with Any Time Server
This page describes the various rate management provisions in NTPv4.Some national time metrology laboratories, including NIST and USNO, usethe NTP reference implementation in their very busy public time servers.They operate multiple servers behind load-balancing devices to supportaggregate rates up to ten thousand packets per second. The servers needto defend themselves against all manner of broken client implementationsthat can clog the server and network infrastructure. On the other hand,friendly clients need to avoid configurations that can result inunfriendly behavior.
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A review of past client abuse incidence shows the most frequent scenariois a broken client that attempts to send packets at rates of one persecond or more. On one occasion due to a defective client designover 750,000 clients demonstrated this abuse. There have been occasionswhere this abuse has persisted for days at a time. These scenarios arethe most damaging, as they can threaten not only the victim server butthe network infrastructure as well.
There are several features in the reference implementation designed todefend the servers and network against accidental or intentional floodattack. Other features are used to insure that the client is a goodcitizen, even if configured in unfriendly ways. The ground rules are:
There are two features in the reference implementation to manage theminimum average headway time between one packet and the next, and thusthe maximum average rate for each source. The transmit throttle limitsthe rate for transmit packets, while the receive discard limits the ratefor receive packets. These features make use of a pair of counters: aclient output counter for each association and a server input counterfor each distinct client IP address. For each packet received, the inputcounter increments by a value equal to the minimum average headway (MAH)and then decrements by one each second. For each packet transmitted, theoutput counter increments by the MAH and then decrements by one eachsecond. The default MAH is 8 s, but this can be changed using theaverage option of the discard command.
The reference implementation uses a special most-recently used (MRU)list of entries, one entry for each distinct client IP address found.Each entry includes the IP address, input counter and process time atthe last packet arrival. As each packet arrives, the IP source addressis compared to the IP address in each entry in turn. If a match is foundthe entry is removed and inserted first on the list. If the IP sourceaddress does not match any entry, a new entry is created and insertedfirst, possibly discarding the last entry if the list is full. Observerswill note this is the same algorithm used for page replacement invirtual memory systems. However, in the virtual memory algorithm theentry of interest is the last, whereas here the entry of interest is thefirst.
The reference implementation has a maximum MRU list size of a fewhundred entries. The national time servers operated by NIST and USNOhave an aggregate packet rate in the thousands of packets per secondfrom many thousands of customers. Under these conditions, the listoverflows after only a few seconds of traffic. However, analysis showsthat the vast majority of the abusive traffic is due to a tiny minorityof the customers, some of which send at over one packet per second. Thismeans that the few seconds retained on the list is sufficient toidentify and discard by far the majority of the abusive traffic.
Ordinarily, packets denied service are simply dropped with no furtheraction except incrementing statistics counters. Sometimes a moreproactive response is needed to cause the client to slow down. A specialpacket has been created for this purpose called the kiss-o'-death (KoD)packet. KoD packets have leap indicator 3, stratum 0 and the referenceidentifier set to a four-octet ASCII code. At present, only one codeRATE is sent by the server if the limited and kod flags of therestrict command are present and either theguard time or MAH time are violated.
A client receiving a KoD packet is expected to slow down; however, noexplicit mechanism is specified in the protocol to do this. In thereference implementation, the server sets the poll field of the KoDpacket to the greater of (a) the server MAH and (b) client packet pollfield. In response to the KoD packet, the client sets the peer pollinterval to the maximum of (a) the client MAH and (b) the server packetpoll field. This automatically increases the headway for followingclient packets.
In order to make sure the client notices the KoD packet, the server setsthe receive and transmit timestamps to the transmit timestamp of theclient packet. Thus, even if the client ignores all except thetimestamps, it cannot do any useful time computations. KoD packetsthemselves are rate limited to no more than one packet per guard time,in order to defend against flood attacks.
Mills, D.L., J. Levine, R. Schmidt and D. Plonka. Coping withoverload on the Network Time Protocol public servers. Proc. PrecisionTime and Time Interval (PTTI) Applications and Planning Meeting(Washington DC, December 2004), 5-16. Paper:PDF,Slides: PDFPowerPoint
Your choices become more interesting if you are running a smallcluster or a big server farm. Under those circumstances, it may notbe possible or even desirable to pair a UPS with every singlemachine. apcupsd supports some alternate arrangements.
The second type of configuration is the NIS (Network InformationServer) server and client. In this configuration, where one UPSpowers several computers, a copy of apcupsd running one onecomputer will act as a server while the other(s) will act asnetwork clients which poll the server for information about theUPS. Note that "NIS" is not related to Sun's directory servicealso called "NIS" or "Yellow Pages".
The third configuration is where a singlecomputer controls multiple UPSes. In this case, there are severalinstances of apcupsd on the same computer, each controlling adifferent UPS. One instance of apcupsd will run in standalone mode, andthe other instance will normally run in network mode.This type of configuration may be appropriate for large serverfarms that use one dedicated machine for monitoring anddiagnostics
Replace the serial number in quotes with the one that correspondsto your UPS. Then whenever you plug in your UPS a symlink calledups0, ups1, etc. will be created pointing to the correct hiddevnode. This technique is highly recommended if you have more thanone UPS connected to the same server since rearranging your USBcables or even upgrading the kernel can affect the order in whichdevices are detected and thus change which hiddev node correspondsto which UPS. If you use the symlink-by-serial-number approach thelink will always point to the correct device node.
NIS (Network Information Server) mode allows for communicationbetween instances of apcupsd running on different hosts. Only oneof those hosts, the server, needs to talk to the UPS directly. Theothers, clients, obtain information about the state of the UPS byquerying the server. NIS is not related to Sun's NIS/YPservices.
The NIS server is connected to the UPS and should be configuredexactly as a standalone configuration, but with NETSERVER on.In all other respects, the server should be configured instandalone mode. You may also set the NIS server specific optionsNISIP to restrict which IP address of the server which apcupsdlistens on. The default, 0.0.0.0, means to list on all of theserver host's IP addresses; NISPORT (default 3551) to set whichTCP port the server listens on; and EVENTSFILE andEVENTSFILEMAX to provide information about the last few eventsto clients. You may also need to modify your firewall rules on theserver's host to allow traffic to the NISPORT.
For the NIS client computer, you will have a configuration thatlooks something like what follows. What is important is that youget the information from an UPSCABLE ether with UPSTYPEnet over the network and you must specify the address ofa NIS server using DEVICE. The client apcupsd will then pollthe NIS server specified in DEVICE every POLLTIME seconds(formerly NETTIME).
The DEVICE is set to server-address:port, whereserver-address is the fully qualified domain name or IP addressof the apcupsd NIS server, and port is the NISPORT that theserver is listening on. The default is 3551, but older versions ofapcupsd used port 7000.
If you set POLLTIME too large, your client may not see thechange in state of the NIS server before the server has shutdown.Normally, you have at least 30 seconds of grace time between thetime the NIS server decides to shutdown and the time it no longerresponds. Your slave must poll during this interval.
Any client run using the Net driver will shutdown when its owntimers expire or when the NIS server shuts down, whichever occursfirst. This means that if you want the slave to shutdown before theserver, you need only set BATTERYLEVEL, MINUTES orTIMEOUT on the client for a faster shutdown than the valuesdefined on the NIS server. This can often be useful if the slave isless important than the master and you wish to reduce battery powerconsumption so that the master can remain up longer during a poweroutage.
NIS clients work principally by reading the STATFLAG record that issent by the NIS server (present in the output of apcaccess). Thelow 16 bits are the standard APC status flag, and the upper 16 bitsrepresent the internal state of apcupsd, so the slave can see whenthe power fails and know when to shutdown.
The difference between the NIS mode and the removed master/slavemode is that the NIS server has no explicit knowledge of theslaves. The NIS server makes its information available via the net(NIS), and the NIS slaves read it. When the NIS server is going toshutdown, it makes the information available to any NIS slave thatpolls it, but the NIS server does not explicitly call each NISslave as is the case in the Master/Slave networking describedseveral sections above.