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至顶头条
作者:论坛整理 来源:ZDNet网络安全 2007年12月28日
关键字: telnet命令 opentelnet linux telnet telnet入侵 telnet telnet端口
IAC SB BM
The RECEIVER of the defined
DO BM) requests the sender of
3
RFC735DHC RHG 3 Nov 77 42083 Telnet Byte Macro Option
The
If the receiver absolutely insists on cancelling a given macro, the best it can do is to turn off the entire option, with IAC DONT BM, wait for an acknowledging IAC WONT BM and then restart the option, with IAC DO BM. This will reset all other macroes as well but it will allow the receiver to REFUSE with code BAD CHOICE if/when the foreign site attempts to redefine the macro in question.
3. Default:
WON'T BM -- DON'T BM
No reinterpretation of data bytes is done.
4. Motivation for the option:
Subcommands for Telnet options currently require a minimum of five characters to be sent over the network (i.e., IAC SB IAC SE). For subcommands which are employed infrequently, in absolute numbers and in relation to normal data, this overhead is tolerable.
In other cases, however, it is not. For example, data which is sent in a block- oriented fashion may need a "block separator" mark. If blocks are commonly as small as five or ten bytes, then most of the cross-net data will be control information. The BM option is intended as a simple data compression technique, to remove this overhead from the communication channel.
5. Description of the option
The option is enabled through the standard Telnet Option negotiation process. Afterwards, the SENDER of data (the side which sends the IAC WILL BM) is free to define and use mappings between single and replacement NVT characters. Except for the ability to refuse particular definitions, the receiver of data has no control over the definition and use of mappings.
The sender (of the WILL BM) is prohibited from using or redefining a
NOTE: The Telnet command character IAC (decimal 255) may be a member of a
4
RFC735DHC RHG 3 Nov 77 42083
Telnet Byte Macro Option
Within any Telnet command (i.e., any sequence beginning with IAC) macro replacement may NOT take place. Data are to be interpreted only as their normal character values. This avoids the problem of distinguishing between a character which is to be taken as a byte>, and interpreted as its corresponding
Telnet commands may be included in
Note that when skipping data due to Telnet SYNCH (INS/DM) processing, BM macro replacement should still take place, since (for example) "IAC DM" would be a valid
The
The BM option is notably inefficient with regard to problems during
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