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Networking Problems with Telnet

Discussion in 'Embedded' started by rickman, Mar 31, 2014.

  1. Pray tell: how do you use a telnet client without using a TCP server?
    OK, fine.
    [examples elided.]

    You're examples demonstrate connecting to a _TCP_server_ which you
    said you didn't need...
    Grant Edwards, Apr 2, 2014
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  2. rickman

    Tauno Voipio Guest

    That is a different story - what do you think Microsoft is?
    Tauno Voipio, Apr 2, 2014
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  3. And as I already said in another post, I misread it as "Telnet server"
    not "TCP server" and I didn't even catch it on a re-read after posting.

    Simon Clubley, Apr 2, 2014
  4. rickman

    upsidedown Guest

    Windows NT4 Telnet client contained the Start/Stop logging feature,
    which the XP/Win7 didn't. In order to use Telnet efficiently, you had
    to copy the NT4 Telnet to more recent Windows versions.
    upsidedown, Apr 2, 2014
  5. rickman

    Les Cargill Guest

    There's always PuTTY. Sure, you have to download it.
    Les Cargill, Apr 3, 2014
  6. ??? Of course they did: "<ESC> set logging" toggles it on/off where
    <ESC> defaults to Ctrl-] but is selectable with "set escape {char}" or
    using the -e option on the command line.

    However, logging control does not work unless there is a logfile name
    set. You do that with "set logfile {name}" or using the -f option on
    the command line.

    George Neuner, Apr 3, 2014
  7. rickman

    rickman Guest

    Ok, I'm back to work on this issue and I read this page which seems to
    say Wireshark can't sniff local traffic. It refers you to RawCap which
    can sniff local traffic. I used RawCap on the loopback interface, IP
    address and got a small pcap file which I tried viewing using
    Wireshark. It says there are *no* packets in this file. I used the
    command line "rawcap -s 60 3 test.pcap". Is this right? Is Wireshark
    not the right tool to use to view the pcap file? It seems to be binary
    when I dump it to the screen, so a text editor won't work.
    rickman, Apr 4, 2014
  8. rickman

    rickman Guest

    On 4/2/2014 1:11 PM, George Neuner wrote:
    Sorry, I thought I had said it was a loopback with both programs running
    on the same PC. I know I mentioned that the program to monitor the
    network only seemed to have options for monitoring physical connections
    and some have said I should figure out how to enable monitoring loopback
    which I have not yet had time to do.

    I am on a router which is connected to the Internet.

    Yes, I am pretty sure I started this thread saying I have little
    knowledge of networking. Thanks to all for their help. I should get
    back to this later today.

    I still seem to have problems remembering to use the "followup" button
    rather than the "reply" button in Thunderbird. I wonder if this is
    rickman, Apr 5, 2014
  9. rickman

    Joe Chisolm Guest

    If I have time today I'll install rawcap and take a look. Wireshark
    should be able to view the pcap file unless it is not really a pcap.
    Did rawcap say anything about number of packets captured, etc?

    You might try again but this time make sure and ping localhost
    while rawcap is running just to make sure it has something to
    capture. It could be that your apps are not even getting to
    the point that they try and talk to each other, thus no
    packets. You could also start rawcap and then just try and
    telnet localhost. You should at least see the initial syn
    packet and probably a ack/reset reply if there is no telnet
    server running.

    Is there any way to run the 2 apps on different systems and
    have them talk through a lan interface? Can you configure
    them to listen/connect on different IP addresses? Might
    be easier to debug this problem if they use a real net
    Joe Chisolm, Apr 5, 2014
  10. rickman

    Joe Chisolm Guest

    I started RawCap and in another command window did "ping localhost"
    RawCap said it got 4 packets and Wireshark was able to open and read
    the resulting pcap file. It may be that RawCap does not work with
    Win 8.

    Try the test with ping and telnet to see if RawCap will even capture
    packets. If it does capture the telnet attempt but nothing from
    your app then your app is not getting to the point that it even
    tries to connect.
    Joe Chisolm, Apr 5, 2014
  11. rickman

    rickman Guest

    Ok, when I try this with ping, Rawcap says it found 8 packets and
    Wireshark is able to display them. When I use Telnet or ExtraPutty with
    my custom app, Rawcap says it found 0 packets and Wireshark sees none in
    the capture file.

    In all cases I see the expected messages in the allowed traffic list of
    the firewall and nothing related to this IP address in the blocked
    message list. I have verified that the Windows firewall is not enabled
    (at least it is reporting that it is not enabled) on both private and
    public (or whatever the two domains are called). I have a *lot* of
    interaction with the Sophos firewall with Windows 8 and I have never
    found a problem that would make me think there is any other program
    acting as a firewall.
    rickman, Apr 6, 2014
  12. Can you telnet to another host? As Simon suggested: telnet to
    www.google.com on port 80 ... if you do connect then press enter to
    get a response. If it works, you will see a page or so of html
    garbage and then the connection will close.

    Does "netstat -a -b" show that your application is listening on the
    expected port? Note that the executable name will be in square
    brackets on a separate line beneath the port information.

    George Neuner, Apr 6, 2014
  13. You will also have to enter the initial GET command after you connect,
    or at least I have always had to. If telnet connects to www.google.com
    then enter the following in uppercase:

    GET / HTTP/1.0

    and press Return _twice_.

    Simon Clubley, Apr 6, 2014
  14. rickman

    rickman Guest

    Great, this seems to be getting somewhere. This looks all wrong to
    me... Here are the significant bits.

    Active Connections

    Proto Local Address Foreign Address State
    TCP Lenni:telnet SYN_SENT

    It would appear that Win32Forth is on the right port, but the wrong IP
    address while the telnet program is on the right address, but the wrong
    port. Any idea how that could happen? Here is the command for starting
    the Telnet program.

    telnet localhost -t vtnt

    This should be operating on, right?

    The custom app uses system calls and other code I don't have source
    for... at least not at my fingertips. I will try the netstat program on
    a computer where it all works if I can find one. Thanks.
    rickman, Apr 6, 2014
  15. Disclaimer: this is based on how it works on Linux. I don't have a
    Windows box to hand (I'm at home) to double check the entry.

    The above, on Linux, would look normal to me, apart from the fact
    the outgoing connection is stuck in the SYN_SENT state.

    The means that Win32for.EXE is just listening on any
    available network interface and has not bound itself to any specific
    interface. For example, from this Linux box I am typing this from:

    Active Internet connections (servers and established)
    Proto Recv-Q Send-Q Local Address Foreign Address State
    tcp 0 0* LISTEN

    This is telling me X11 is listening for incoming connections from any

    The second entry is also ok (apart from being stuck in the SYN_SENT
    state :)). In this case, because it's a client making a outgoing
    connection, the port of interest is the port number listed under
    "Foreign Address", which is correctly listed as telnet.

    The local port, 51118, is just the local port number allocated at
    random by the local TCP stack to the client application (in this
    case telnet). The local port number will be different from the port
    number of the service you are trying to connect to.

    I would probably double check the Windows Firewall settings in Control
    Panel to make sure the firewall is not running. If it is, then you
    may have to register the Win32for.EXE as a allowed binary.

    I've had to do this for the actual FileZilla FTP server binary under
    Windows 7 (as opposed to the FileZilla manager binary which was listed

    Simon Clubley, Apr 6, 2014
  16. rickman

    rickman Guest

    Thanks for the info. I don't know what could be getting in the way. I
    *have* checked the windows firewall. It is off. Sophos reports that it
    is passing messages from these apps.

    At the time of this transaction, the firewall did block a message on

    3:15:02 AM svchost.exe IN REFUSED UDP BOOTPC Block All Activity

    I don't suspect this as a problem as I see these messages coming in often.

    This is what I see passed by the firewall...

    3:15:11 AM Win32for.exe IN TCP 51118 Allow loopback TCP
    3:15:05 AM Win32for.exe IN TCP 51118 Allow loopback TCP
    3:15:02 AM Win32for.exe IN TCP 51118 Allow loopback TCP
    3:15:02 AM telnet.exe OUT TCP Telnet Telnet

    Any idea why the packet sniffer doesn't see any of this?
    rickman, Apr 6, 2014
  17. Because I don't have any Windows 8 experience, I am now moving from what
    I know about into a guessing what might be wrong mode.

    Some generic questions:

    Did you try using the telnet client to connect to www.google.com on
    port 80 ? (That's unlikely given what you show below, but it's worth
    a try anyway just to eliminate a possible problem with the telnet

    _After_ disconnecting the PC from the network, try turning off the
    Sophos firewall temporarily and see if telnet now works.

    IIRC, someone earlier on in the thread mentioned a Windows version
    of netcat existed. You _really_ should try that instead of your
    Win32for.exe and see what happens.
    If things are getting that far, it doesn't feel like a problem with the
    telnet client as such.
    Sorry, no.

    Simon Clubley, Apr 6, 2014
  18. rickman

    rickman Guest

    I just tried this and telnet seems to be waiting for something, but
    nothing is shown. It just says, "Connecting to www.google.com..."...
    no, wait. When I hit escape it dumped a bunch of stuff that looks like
    HTML. The firewall only showed one message allowed which was from
    telnet. I didn't see the HTML traffic coming back.

    11:20:10 AM telnet.exe OUT TCP HTTP Citrix Terminal
    Services (TCP)

    I have done this before. It makes no difference.

    I'll give that a try.
    rickman, Apr 6, 2014
  19. You need to enter the string I posted in a previous message.

    After connecting, enter (in uppercase):

    GET / HTTP/1.0

    and press Return _twice_.
    Unless your firewall is logging things in a rather unusual way, that's
    about all you should see. That's a two way connection above, and the
    HTML traffic is returned in the same connection as the issued HTTP

    The fact you got this far however would seem to suggest it's not a
    problem with the telnet client itself.
    So we now know it's not the third party firewall.
    If this works, your next task would be to review the code in Win32for.exe
    which listens for and accepts incoming connections.

    The issue seems to be narrowing to either faulty code in Win32for.exe
    which just happened to work before, or something new in Windows 8
    which I am unaware of.

    For those of you with Windows 8 experience, are there any additional
    process/network containment features (in addition to the traditional
    firewall) which might need adjusting ?

    Simon Clubley, Apr 6, 2014
  20. Looks like the connection cannot be established (if the telnet client
    reports that correctly). Normally you should see
    "connected to (whatever)" message from your telnet client (i.e.
    locally generated). Wireshark should be able to see the syn-syn_ack-ack
    segment exchange to a remote host. Looks like some underlying
    networking issue, may be as hard to locate as it looks to be
    (particularly so since you did not have the problem on earlier
    OS versions).
    This looks like something locally generated, not like http coming
    from the host you are trying to connect to.

    Dimiter_Popoff, Apr 6, 2014
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