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Using Linux (Debian, to be specific) on a laptop, I want to be able to check whether, for example, the TX or RX pairs are both connected to another machine. If, say, I've only connected RX (and am able to receive, using promiscuous mode), I want to be able to identify that TX is not connected.

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What is the reason for this? If you want to check cables, there are better alternatives like cable testers. –  SvW Mar 26 '13 at 8:35
    
I am the one plugging in the cable (I control whether TX and/or RX are connected). What I want is for the software I'm writing to be able to automatically react to both situations with different actions. –  John Locke Mar 26 '13 at 15:25

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identify that TX is not connected

There is nothing in the 802.3 standards up to 100BaseTX which would support such functionality. Of course, if you have control over both ends of the link, you could implement an echo probe which would be answered by the remote side.

For 1000BaseT things are different - there would not be such a thing as a "TX pair" or "RX pair" as all four pairs are used for transmission and reception simultaneously. As IDLE frames are continuously sent over the wire, disconnected pairs can be detected by the absence of traffic.

Additionally, some NICs may provide ways of doing cable tests, although they would be proprietary and I am not aware of any Linux implementations. This is what a Broadcom Cable Analysis dialog looks like under Windows:

enter image description here

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After two days of constant pondering, I admit that this is indeed the case. Since what follows is something of a side question, I am marking this as the answer to the initial one. However, do you think there is a way for me to somehow cheat the cable (reconnect/cut/manipulate any or all of the pairs), so that I (1) know that this is a "special cable" (recognize it with the software, for example by using what I receive, not including control over that) and (2) have the RX properly connected? –  John Locke Mar 28 '13 at 21:11

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