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Networking Problems with Telnet

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

  1. rickman

    rickman Guest

    I have a custom app that uses a telnet connection to the Windows telnet
    program to use as a streaming data display. This program has been
    working just fine for a number of years under XP, Vista and I think
    Windows 7. But I need to get it working under Windows 8 and the telnet
    connection won't complete.

    The custom app calls canned routines to create a socket and once started
    polls looking for a completed connection. The telnet app is then
    started, the connection completes and the two start talking. Under
    Windows 8 the two apps seem to both be doing their jobs but never
    complete the connection and time out.

    At first I had a firewall problem but after changing some settings in
    the firewall program I can now see the messages being sent by each app.
    Further if I disable the firewall entirely the connection still is not

    I don't know enough about networking to debug this further. Because the
    apps have been working just fine for a long time I don't suspect the
    custom app, but rather expect this is a matter of configuration related
    to being a Windows 8 machine.

    Anyone have a clue on how to proceed debugging this? Is there a more
    appropriate group to ask for help?
    rickman, Mar 31, 2014
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  2. rickman

    Joe Chisolm Guest

    Use wireshark or tcpdump to capture up to the timeout. Drop the
    pcap file somewhere where folks can look at it. Could be a
    routing issue.
    Joe Chisolm, Mar 31, 2014
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  3. rickman

    Tauno Voipio Guest

    You should arrange the sniffing with Wireshark on a
    known-good computer. You may need a hardware wiretap to do it.

    Check that the initial option handshake is done correctly.
    Wireshark is good in dissecting and interpreting it.

    <http://www.telnet.org/htm/dev.htm> has a good list of Telnet

    The Microsoft implementation of Telnet client has been a bit
    weird all the time. If Windows 8 accepts it, it may be worth
    trying to use a different Telnet program, e.g. puTTY.
    Tauno Voipio, Mar 31, 2014
  4. rickman

    rickman Guest

    Thanks for the suggestions. I downloaded wireshark and am trying to run
    it, but I'm not sure it will monitor these comms. The capture setup
    works on specific hardware interfaces and this traffic does not seem to
    be on a hardware interface. I tried a ping of and it doesn't
    seem to show up in the captured data at all. Did I miss something
    obvious in the setup?
    rickman, Mar 31, 2014
  5. rickman

    Paul Rubin Guest

    For you have to capture on the loopback interface rather than
    on a network interface, probably.
    Paul Rubin, Mar 31, 2014
  6. rickman

    Tim Wescott Guest

    I dunno how this is going to help, but usually when this sort of thing
    happens to or around me, it's one of the following:

    1: The interface that has died is violating some jot or tittle of the
    protocol, and previous versions of the host OS didn't care.

    2: The interface that has died was perfectly good in its day, but
    specifications have changed and now it's obsolete (IPv5 vs. IPv6?)

    3: Microsoft is staffed by lazy assholes, and they've broken something.
    (Of course, when this happens with Linux it's because everyone
    in the whole open source movement is a saint, but sometimes things
    just fall through the cracks, eh?)

    It may just be that the custom program is failing to set up one or two
    defaults that used to get by, and now don't. The suggestion to put it up
    on a known-good machine with a protocol analyzer, then compare that with
    the behavior on a known-bad machine with the same protocol analyzer is, I
    think, going to be your shortest path to success.

    Of course, if you have any need for the current version to continue
    working with old OS's, you'll have to get everything working and then do
    regression testing. Have fun!
    Tim Wescott, Mar 31, 2014
  7. I assume the telnet client has network connectivity generally aside
    from this problem? Is the application on the same or different host?

    Telnet is extremely simple - though there are terminal emulation
    issues little can go wrong with the connection itself unless the
    target host:port is wrong or being blocked by a firewall.

    All the versions of Windows that have a "user friendly" firewall block
    incoming telnet by default [telnet has long been considered a security
    risk], but they have generally allowed outgoing connections[1]. A
    loopback connection to the same host is both an outgoing and an
    incoming connection. Unfortunately I don't have Windows 8 so I can't
    tell you where to look.

    [1] from Vista onward there are separate "domain", "public" and
    "private" network profiles with differing defaults. The domain and
    private profiles assume a secure LAN and generally allow all outgoing
    traffic. The public profile is more selective.

    If this is a loopback connection to the same host then it will not
    appear on any hardware interface. You need to monitor the loopback

    FWIW, I would check the firewall settings first.

    Good luck,
    George Neuner, Apr 1, 2014
  8. rickman

    rickman Guest

    don't know. I have not tried connecting to it with anything else yet.
    What else could I try? Can two terminal programs talk to each other
    or does one have to be some sort of host?

    Someone else had suggested a couple of tests to check the connection
    using the ping command. Ping localhost seems to be translated to [::1]
    which I guess is an IPv6 address while is the IPv4 address. If
    the telnet program is connecting to localhost (I use the command line,
    "telnet localhost -t vtnt") and the custom program is connecting to I suppose they could be talking to different addresses, but
    the reports by the firewall show for both on the messages it

    Where/how is the profile set and applied? I assume this is in the
    Windows firewall? I am not using that, I am using Sophos. If I
    entirely disable the firewall all other issues go away, but the two
    programs still do not connect.

    Yes, I have not had time to deal with Wireshark any further yet. I need
    to dig in and find out how to access the loopback interface.

    Thanks to everyone who has replied. This is a big help!
    rickman, Apr 1, 2014
  9. Yes.

    On unix, you can use the netcat "nc" command to act as a "host". I
    don't know what you do under windows...
    Grant Edwards, Apr 1, 2014
  10. As Grant has said, no ... you need some kind of TCP protocol server
    for the telnet client to talk to. Unfortunately, Windows doesn't
    include anything useful by default. However, you can install "Simple
    TCPIP Services" (echo, chargen, etc.) or an actual telnet server.
    These are "features" that can be installed via the Programs/Features
    applet in the Control Panel (assuming you have your install media

    One thing you can do without fuss is use "tasklist" to see if your
    application is running and "netstat" to check if it has a listen port

    Unfortunately the netstat option to show the executable name puts the
    name on a separate line so it is hard to filter effectively ... but if
    you know the process id or port number to look for you can pipe the
    output of netstat into find.

    If you are willing to use 3rd party stuff on your Win8 host you can
    try TCPView or tcpvcon (same download)


    or Comodo Killswitch which is a ProcessExplorer work-alike which
    additionally can display netstat information. KillSwitch has to be
    installed and isn't officially supported on Win8, but AFIAK it works.


    Biggest problem with all of these tools is filtering the noise ...
    generally they show you too much information. netstat can be run
    continuously, refreshing until you stop it with Ctrl-C, but it will
    drown you in useless data unless you know exactly what process and
    port to look for and filter the output using find.

    You need to ping something external: another machine on your LAN or a
    web address (e.g., "ping www.google.com") if your host has Internet

    "pathping" and "tracert" are other handy utilities you can try (again
    on external targets).

    I'm not sure what "other issues" you have, but don't automatically
    assume that the Windows firewall is disabled under Sophos ... it might
    not be. The Win7/Win8 firewall is quite capable by itself (but hard
    to configure) and some 3rd party firewall software builds upon it
    rather than replacing it. [I don't know what Sophos does.]

    You can check if the Windows firewall is enabled using the Control
    Panel applet. If it is enabled, look under "advanced settings" to see
    if telnet is being blocked.

    WRT profiles, they are configured through the "Network and Sharing"
    applet ... the active profile for each NIC is indicated and can be
    changed (loopback always uses the private profile). The major options
    are set under "choose homegroup and sharing" (even if you don't think
    you are sharing).

    Hope this helps (or at least doesn't confuse more),
    George Neuner, Apr 2, 2014
  11. rickman

    Joe Chisolm Guest

    You might also try something like SocketSniff. It is supposed to
    capture local traffic. Not sure if it works with Win 8 though.
    Also look here. Might give some pointers for Win/local interface

    Joe Chisolm, Apr 2, 2014
  12. rickman

    Tauno Voipio Guest

    A quick Google showed several candidates for Netcat under Windows.
    Tauno Voipio, Apr 2, 2014
  13. rickman

    David Brown Guest

    I don't know whether it is due to ignorance, laziness, incompetence,
    anti-competitiveness, or just an attitude of "it works with our own
    stuff, who cares about the rest" - but Microsoft have a long history of
    getting /close/ to standard protocol support and then failing on the
    details. For many reasons, Linux folk usually have to make a much
    greater effort at following protocol standards as closely as possible.
    They might not always succeed, but it is higher priority.

    Even telnet is not immune to this. The protocol may be some 40 years
    old, but there are differences in the way Windows telnet client works
    and the way Linux telnet works. I cannot remember the details, but I
    discovered this when writing a small server program for a Linux machine,
    written in Python. All my testing was with telnet from Linux - when I
    then tested with Windows XP telnet, nothing worked. I am afraid I
    cannot remember the details of the difference. It was not hard to get
    the server code to work with Windows XP telnet - but I did have to
    change it.

    Given that, it would not surprise me to hear that one version of Windows
    telnet works, and another fails because of changes in the program from
    one Windows generation to the next.
    David Brown, Apr 2, 2014
  14. rickman

    David Brown Guest

    If I understand you correctly, both the server and the telnet client are
    on the same system. Have you tried putting them on different machines
    with different Windows versions, and tested combinations? For example,
    if XP->XP works, XP->W8 works, but W8->XP and W8->W8 fails then you know
    the change is on the W8 client side. (It doesn't necessarily mean
    that's where the /problem/ lies, just the change.)

    Also try different telnet clients, such as a Linux telnet, PuTTY, msys
    telnet, and Windows telnet programs from older versions of Windows.
    David Brown, Apr 2, 2014
  15. Actually, no you don't.

    If you read the Telnet RFC, you will see that the telnet options
    negotiation sequence is layered on top of the basic protocol and in
    the absence of such negotiations, the telnet client falls back to a
    basic telnet capabilities model.

    This model is good enough to work with many normal TCP protocols such
    as HTTP and SMTP so you can do the following (assuming your telnet
    client can specify a alternate TCP port):

    telnet www.google.co.uk 80
    Connected to www.google.co.uk (
    Escape character is '^]'.
    GET / HTTP/1.0

    HTTP/1.0 302 Found
    Cache-Control: private
    Content-Type: text/html; charset=UTF-8
    Location: http://www.google.co.uk/?gfe_rd=cr&ei=hvg7U6L_DePR8gfKKw
    Content-Length: 257
    Date: Wed, 02 Apr 2014 11:46:14 GMT
    Server: GFE/2.0
    Alternate-Protocol: 80:quic

    <HTML><HEAD><meta http-equiv="content-type" content="text/html;charset=utf-8">
    <TITLE>302 Moved</TITLE></HEAD><BODY>
    <H1>302 Moved</H1>
    The document has moved
    <A HREF="http://www.google.co.uk/?gfe_rd=cr&amp;ei=hvg7U6L_DePR8gfKKw">here</A>.
    Connection closed by foreign host.


    telnet news.eternal-september.org 119
    Connected to news.eternal-september.org (
    Escape character is '^]'.
    200 news.eternal-september.org InterNetNews NNRP server INN 2.6.0 (20140307 snapshot) ready (posting ok)
    205 Bye!
    Connection closed by foreign host.

    Simon Clubley, Apr 2, 2014
  16. Hi David,
    there are tons of "telnet options" (or sort thing) RFC-s published
    mostly decades ago. I don't know if anyone has written a client to
    follow/implement them all correctly in one place but I seriously
    doubt that. Could well be the different behaviours you have observed
    have both been OK and intended, who knows. I am not familiar
    with these options - don't remember any at the moment, really. My use of
    telnet in DPS is restricted to having a text line sent when I want it
    and receiving whatever comes my way and copying it to stdo,
    which must have been the very initial version of telnet.... I think
    I also had a "tcp segment per character" mode at some moment, not
    sure if this was ever any useful at all. But I do remember a telnet
    connection over a 100 MbpS link behaving like sort of a 2400 bpS
    one while I was trying it out... :D .

    Dimiter_Popoff, Apr 2, 2014
  17. You are conflating "TCP" with the ever expanding universe of
    application protocols. "T)ransmission C)ontrol P)rotocol" is the
    level 4 transport stream protocol defined in the standard Internet
    Protocol stack.

    That's all "TCP" is, and that's all I intended it to mean.

    Telnet can connect to any stream based IP service, however, making a
    connection doesn't mean you can do anything further with it. Telnet
    is text based - connecting to a binary interface service almost
    invariably means the connection either will be dropped immediately or
    the telnet client will crash or hang.

    Windows, by default, does not provide any text based stream services -
    the few that are bundled must be installed separately.

    Moreover, the OP has given very little information about the
    environment: e.g., it still is unclear to me whether there even is
    more than one host involved - it seems as though he is trying to make
    a loopback connection to an application/service on the same host.
    Although he has mentioned firewall software, he has said nothing about
    being on a LAN, having an Internet connection, or really anything to
    indicate that there is anything else available for telnet to connect
    to other than his own application.

    It also appears that he has limited knowledge of networking and of the
    tools Windows has available. Going only by what he has told us and
    not making assumptions, I was trying to help him using only what
    Windows provides.

    George Neuner, Apr 2, 2014
  18. But nothing provided by a default installation.
    George Neuner, Apr 2, 2014
  19. You are right and I already knew that. However, when I read that sentence
    I mentally read it as "Telnet protocol server", not "TCP protocol server".

    I didn't even catch it when I read it again after posting...


    Simon Clubley, Apr 2, 2014
  20. Yes. But the OP said this setup was working until moved onto Win8.
    AFAIK, apart from a few bug fixes, the telnet client has not changed
    since NT4. In Win7 and Win8 it isn't even installed by default.

    George Neuner, Apr 2, 2014
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