This article http://www.ciscopress.com/articles/article.asp?p=345618&seqNum=3 describes TCP intercept mode:

One limitation of intercept mode is that any TCP options that are negotiated between the external user and the router (which normally are to the end-server device), such as RFC1323 window scaling, are not passed from the router to the internal TCP server. This is because the router does not know these options until the first three-way handshake completes with the external user and the router begins the second three-way handshake with the internal TCP server. Typically, this is not a problem because TCP allows for the dynamic negotiation of these parameters during the normal operation of the session.

What is dynamic negotiation TCP options? How can i use it? I have some TCP options that i want to work in tcp intercept mode and i don't want to use VPN softwares.

  • you might find something under waas express. Jun 9, 2015 at 17:31
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1 Answer 1


According to the current RFC, window scaling cannot be negotiated dynamically. The RFC explicitly states:

A Window Scale option in a segment without a SYN bit MUST be ignored.

I think the author of the page you quote is simply mistaken.

  • What about other options? window scale doesn't important for me. i want to use TCPCrypt options. I want to know how to negotiate custom tcp options dynamically. Mar 11, 2015 at 17:03
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    @sfallahdoost I do not know of any TCP options, that can be changed dynamically. You'd have to look into the particular standard for the options you are interested in, in order to know, if they can be changed dynamically.
    – kasperd
    Mar 11, 2015 at 17:14

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