4 ms·

Because you cannot collect money each time customer presses physical button (or flips bit in confreg, which is cisco way of doing the same) Best solution I've

by dfox 9y ago

Because you cannot collect money each time customer presses physical button (or flips bit in confreg, which is cisco way of doing the same) Best solution I've seen from the practicality standpoint is distinct "clear password" button on HP ProCurves that you mention which works without power cycling the switch (and thus downtime). Looping ports 1 and 2 which is used by typical non-Accton chinese OEM switches seems like giant hack to me (which can even happen by accident given right environment)

dfox 9y ago
Somewhat relevant observation is that good test for various CCNwhatever wielding network administrators is presenting them with piece of cisco hardware with some "unusual" value in confreg (eg. console port set to something other than 9600 8N1).
- mschuster91 9y ago
  > console port set to something other than 9600 8N1 I have some ProCurve switches at work - they auto-detect line speed somehow by making the operator press the Return key twice. If someone can tell me how to replicate this functionality e.g. on a Linux server, here's a bounty offer for some beers...
  - dfox 9y ago
    This has to be supported by the UART hardware in order for it to work reliably. The idea is in essence that the receiver measures the length of start bit, this requires the expected character to have first transmitted bit (LSB usually) set to 1. For example this is done by almost all Hayes compatible modems and is the reason why commands start with 'AT' (or in fact, the 'A' itself). IIRC there is also somewhat reliable and simple (ie. not involving sampling the whole waveform and analysing it after the fact) for doing autodetection for random unknown data, but I cannot find the reference where I've seen it described at the moment (it was in some TI appnote or something similar)
    - dfox 9y ago
      I didn't find the appnote I mentioned, but found another that describes exactly the case of detecting baud rate from one or two CR characters (SLAA215, ie. http://www.ti.com/lit/an/slaa215/slaa215.pdf http://www.ti.com/lit/an/slaa215/slaa215.pdf) As it happens, when you set UART to 115200 then it will receive the CR character transmitted at any baud rate <115200 (with the obvious caveat that you may get additional garbage characters after that) and for standard baud rates between 9600 and 115k2 you will get unique received bit pattern. If you get zero or framing error then the baudrate is 9k6 or lower, so you change speed to 9k6 and retry the same idea (and if you care about less than 1200 you can do this for the third time, but that starts to be somewhat pointless as the really slow tty baud rates will not generate unique and repeatable bit patterns because they are too close together and are not 115k2 divided by some integer). [Edit: there is not that much special about using CR for that. You want character with LSB set and the lower hamming weight the better, CR is convinient, 0x01 would work better. Obviously the table garbage=>speed depends on what character you choose, but with some clever masking it can probably be made universal at least for every character with LSB set] It seems that I misremembered what I read about the more general auto detection. It in fact is not more general, but derived from LIN protocol and involves intentionally sending more complex synchronization pattern, consisting of break, some idle time and then 0x55. On the other the MSP430 hardware is capable of locking onto essentially any received baud rate (not only the standard ones) with sufficient precision by doing this.
  - pnutjam 9y ago
    Looks like you can use picocom to get this functionality from a Linux client, although your probably asking for this from the server. https://linux.die.net/man/8/picocom https://linux.die.net/man/8/picocom
  - linsomniac 9y ago
    In your read loop, when you get data, if it isn't a CR or a NL, change the rate and wait again. You probably don't need to cycle through them all, just the most common options like 9600, 19.2K, 115K... Depending on what you read and what speed you are at, you may be able to make more intelligent choices (a CR sent at 19.2K when you are on 9600 might look like this or that).
    - mschuster91 9y ago
      Hmm, that would be an interesting implementation idea, however the ProCurves seem to be doing this in hardware as your algorithm would require up to 6 CR presses in the worst case and the ProCurve always needs exactly two... and how do I get to modify the readline implementation of (I presume) the kernel?
      - linsomniac 9y ago
        Are you SURE they do it in hardware and not switch software? I'd bet it's in software somewhere, but the question I was trying to answer was related to how to get Linux to do it. At 115.2K, you can probably do it in two CRs, if you look at the data you see. My recollection is that if you have a rate mismatch you often get garbage characters. Just simply the number of garbage characters you get could indicate where you need to change rates to. Like, if you get 10 characters try going to 9600, if you get 5 try going to 19.2K. Just simple timing of how long you get characters might be a good representation of the speed. I don't really have any serial gear or cables handy, or I'd probably try whipping a Python program together to see if I could do it.
mschuster91 9y ago
> Best solution I've seen from the practicality standpoint is distinct "clear password" button on HP ProCurves that you mention which works without power cycling the switch (and thus downtime). No downtime == not detectable by monitoring. Port looping by accident is not a problem - when it occurs during operation the network grinds to a halt very quickly after the deed and the hellfire of flashing LEDs should show any operator what is going on, and when it happens during power-up by accident something has gone very wrong...
- dogma1138 9y ago
  It is definitely detectable by monitoring. And it can also be disabled, proliant servers also have or had since I didn’t handled them for years a password bypass button for ILO/IPMI it and it was a god send in some cases.

by dfox 9y ago

dfox 9y ago

Somewhat relevant observation is that good test for various CCNwhatever wielding network administrators is presenting them with piece of cisco hardware with some "unusual" value in confreg (eg. console port set to something other than 9600 8N1).

mschuster91 9y ago
> console port set to something other than 9600 8N1 I have some ProCurve switches at work - they auto-detect line speed somehow by making the operator press the Return key twice. If someone can tell me how to replicate this functionality e.g. on a Linux server, here's a bounty offer for some beers...
- dfox 9y ago
  This has to be supported by the UART hardware in order for it to work reliably. The idea is in essence that the receiver measures the length of start bit, this requires the expected character to have first transmitted bit (LSB usually) set to 1. For example this is done by almost all Hayes compatible modems and is the reason why commands start with 'AT' (or in fact, the 'A' itself). IIRC there is also somewhat reliable and simple (ie. not involving sampling the whole waveform and analysing it after the fact) for doing autodetection for random unknown data, but I cannot find the reference where I've seen it described at the moment (it was in some TI appnote or something similar)
  - dfox 9y ago
    I didn't find the appnote I mentioned, but found another that describes exactly the case of detecting baud rate from one or two CR characters (SLAA215, ie. http://www.ti.com/lit/an/slaa215/slaa215.pdf http://www.ti.com/lit/an/slaa215/slaa215.pdf) As it happens, when you set UART to 115200 then it will receive the CR character transmitted at any baud rate <115200 (with the obvious caveat that you may get additional garbage characters after that) and for standard baud rates between 9600 and 115k2 you will get unique received bit pattern. If you get zero or framing error then the baudrate is 9k6 or lower, so you change speed to 9k6 and retry the same idea (and if you care about less than 1200 you can do this for the third time, but that starts to be somewhat pointless as the really slow tty baud rates will not generate unique and repeatable bit patterns because they are too close together and are not 115k2 divided by some integer). [Edit: there is not that much special about using CR for that. You want character with LSB set and the lower hamming weight the better, CR is convinient, 0x01 would work better. Obviously the table garbage=>speed depends on what character you choose, but with some clever masking it can probably be made universal at least for every character with LSB set] It seems that I misremembered what I read about the more general auto detection. It in fact is not more general, but derived from LIN protocol and involves intentionally sending more complex synchronization pattern, consisting of break, some idle time and then 0x55. On the other the MSP430 hardware is capable of locking onto essentially any received baud rate (not only the standard ones) with sufficient precision by doing this.
- pnutjam 9y ago
  Looks like you can use picocom to get this functionality from a Linux client, although your probably asking for this from the server. https://linux.die.net/man/8/picocom https://linux.die.net/man/8/picocom
- linsomniac 9y ago
  In your read loop, when you get data, if it isn't a CR or a NL, change the rate and wait again. You probably don't need to cycle through them all, just the most common options like 9600, 19.2K, 115K... Depending on what you read and what speed you are at, you may be able to make more intelligent choices (a CR sent at 19.2K when you are on 9600 might look like this or that).
  - mschuster91 9y ago
    Hmm, that would be an interesting implementation idea, however the ProCurves seem to be doing this in hardware as your algorithm would require up to 6 CR presses in the worst case and the ProCurve always needs exactly two... and how do I get to modify the readline implementation of (I presume) the kernel?
    - linsomniac 9y ago
      Are you SURE they do it in hardware and not switch software? I'd bet it's in software somewhere, but the question I was trying to answer was related to how to get Linux to do it. At 115.2K, you can probably do it in two CRs, if you look at the data you see. My recollection is that if you have a rate mismatch you often get garbage characters. Just simply the number of garbage characters you get could indicate where you need to change rates to. Like, if you get 10 characters try going to 9600, if you get 5 try going to 19.2K. Just simple timing of how long you get characters might be a good representation of the speed. I don't really have any serial gear or cables handy, or I'd probably try whipping a Python program together to see if I could do it.

mschuster91 9y ago

> Best solution I've seen from the practicality standpoint is distinct "clear password" button on HP ProCurves that you mention which works without power cycling the switch (and thus downtime). No downtime == not detectable by monitoring. Port looping by accident is not a problem - when it occurs during operation the network grinds to a halt very quickly after the deed and the hellfire of flashing LEDs should show any operator what is going on, and when it happens during power-up by accident something has gone very wrong...

dogma1138 9y ago
It is definitely detectable by monitoring. And it can also be disabled, proliant servers also have or had since I didn’t handled them for years a password bypass button for ILO/IPMI it and it was a god send in some cases.