byblock与bylayerr 与 lever 区别

来源：蜘蛛抓取(WebSpider) 时间：2014-12-09 06:58 标签： 同花顺level2
relever和lever使用上的区别？_百度知道
relever和lever使用上的区别？
都有lift up, raise等的意思。如quand on avait relevé la passerelle et que les remorqueurs avaient commencé à le tirer.穿怠扁干壮妨憋施铂渐..用..on avait levé la ...是否也通？
其实你这句话用lever也可以。作穿怠扁干壮妨憋施铂渐升起，抬起的意思讲的时候，本身relever和lever在很多时候都是可以通用。但是他们各自还有其他含义，则不可通用，比如relever有重建的意思，有记下，抄下的意思。lever有取出的意思，。
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用relever是说 重新升起舷梯，有先放下后 再升起的穿怠扁干壮妨憋施铂渐意思用lever是说升起舷梯，没有先放下的意思这是在说飞机吧？拉完客了 收梯子 然后。。。。。。如果是这样的话要用relever 习惯用法 及客观判断上都决定了用relever
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出门在外也不愁From Wikipedia, the free encyclopedia
In the context of , an application-level gateway (also known as ALG or application layer gateway) consists of a security component that augments a
employed in a . It allows customized
filters to be plugged into the
to support
for certain
"control/data" protocols such as , , , , file transfer in
applications, etc. In order for these protocols to work through NAT or a firewall, either the application has to know about an address/port number combination that allows incoming packets, or the NAT has to monitor the control traffic and open up port mappings () dynamically as required. Legitimate application data can thus be passed through the security checks of the firewall or NAT that would have otherwise restricted the traffic for not meeting its limited filter criteria.
An ALG may offer the following functions:
allowing client applications to use dynamic
TCP/ UDP ports to communicate with the known ports used by the server applications, even though a firewall configuration may allow only a limited number of known ports. In the absence of an ALG, either the ports would get blocked or the network administrator would need to explicitly open up a large number of ports in the firewall — rendering the network vulnerable to attacks on those ports.
converting the
address information found inside an application payload between the addresses acceptable by the hosts on either side of the firewall/NAT. This aspect introduces the term
for an ALG.
recognizing application-specific commands and offering granular security controls over them
synchronizing between multiple streams/sessions of data between two hosts exchanging data. For example, an
application may use separate connections for passing control commands and for exchanging data between the client and a remote server. During large file transfers, the control connection may remain idle. An ALG can prevent the control connection getting timed out by network devices before the lengthy file transfer completes.
of all the packets handled by ALGs over a given network makes this functionality possible. An ALG understands the protocol used by the specific applications that it supports.
For instance, for
(), an ALG can allow firewall traversal with SIP. If the firewall has its SIP traffic terminated on an ALG then the responsibility for permitting SIP sessions passes to the ALG instead of the firewall. An ALG can solve another major SIP headache: . Basically a NAT with a built-in ALG can rewrite information within the SIP messages and can hold address bindings until the session terminates.
An ALG is very similar to a , as it sits between the client and real server, facilitating the exchange. There seems to be an industry convention that an ALG does its job without the application being configured to use it, by intercepting the messages. A proxy, on the other hand, usually needs to be configured in the client application. The client is then explicitly aware of the proxy and connects to it, rather than the real server.
The Application Layer Gateway
provides support for third-party plugins that allow network protocols to pass through the
and work behind it and . ALG plugins can open ports and change data that is embedded in packets, such as ports and .
also includes an ALG FTP plugin. The ALG FTP plugin is designed to support active FTP sessions through the
engine in Windows. To do this, the ALG FTP plugin redirects all traffic that passes through the NAT and that is destined for port 21 (FTP control port) to a private listening port in the
range on the Microsoft loopback adapter. The ALG FTP plugin then monitors/updates traffic on the FTP control channel so that the FTP plugin can plumb port mappings through the NAT for the FTP data channels.
- ALG: official definition (refer section 2.9)
/ Network Address Translation (NAT) Router / Load-Balancing Router.

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