I am working on a line-oriented telnet server, not a client.
Currently, to set up the connection, I am using:
IAC DONT ECHO
IAC DONT SURPRESS-GO-AHEAD
IAC DO LINEMODE
IAC DO NAWS
Currently, the server only handles IAC AYT, and NAWS-related stuff. Anything else is rejected, like so:
IAC DONT <OPTION>
However, the server still gets characters one-at-a-time.
According to here, ECHO and SGA (at the same time) enable character-at-a-time mode. However, I have both disabled.
How can I make the client send data one line at a time?
I am using the telnet from GNU inetutils to test this.
This question's title is similar but, contrary to its title, concerns writing a telnet client.
I just checked my old telnet server source and I send WILL and WONT from the server side. I handled DO and DONT from the client.
Are you actually trying to code a true telnet server, or just a server that is compatible with telnet that does sends data a line at a time (like a text based chat or game server)? If the latter, you can do away with most of your options because in my experience, telnet clients default to line at a time.
For my game server (MUD++) all I ever did in standard game play mode was to disable / enable echo during password state in login, or kick into character mode and start a pseudo tty when the user invoked the PICO editor.
UPDATE: If you plan to seriously write a telnet server or any other network software, I really like the classic W. Richard Stevens series. TCP/IP Illustrated Volume 1, The Protocols covers telnet in a practical sense, and coupled with the RFCs like 1184 (if I recall) you'll have everything you need.
Related
I have written a basic Telnet Server in C language, and I am testing it against some telnet clients.
However, depending on the client I use (PuTTY, for example) then the client acts as Active by default.
Is there a way for the server to always mandate the client to act in passive mode?
I am asking this because, in my understanding, if the server mandates it, then I do not need to implement any protocol-specific details, only the basics to handle this mandatory passive mode. So, my Telnet Server will be at the end a simple TCP socket.
Update 1:
Here I describe the data received when a client connects to the server:
Launch the Telnet Server
Telnet Client Connects (and sends telnet protocol data)
received = {ffff1fffff20ffff18ffff27ffff01ffff03ffff03}
The data is hex encoded, and by decoding it according to the Telnet specification, the client is trying to establish some configuration before any data starts to be exchanged.
On the Telnet Server side, I simply ignore received data starting with 0xff (IAC code).
That solves the problem for a while.
Type some data on the Client side and hit enter
Telnet Server receives the data
Type some data on the Client side and hit enter
Telnet Server receives the data + telnet protocol data
Here is the received data (I sent "123456"):
received = {616263646566ffff18ffff27ffff01ffff03ffff03}
As you can see, I received what I sent, but I also got some telnet protocol stuff. That is because the Telnet Client is in Active mode.
I wish that at some intermediate step between 2 and 3, the Telnet Server could set the Telnet Client to run in Passive mode, so, I would not receive any Telnet Protocol data at step 6.
Update 2:
As requested in the comments for more details about the "Passive mode", I mean the Telnet negotiation mode: (1) Passive mode: where the Telnet Client/Server will not send any negotiation data, only user data; (2) Active mode: where the Telnet Client/Server will send the negotiation data to configure/handshake what are the features to be used/set.
And yes, I also could not find that to be specific to the protocol, only on PuTTY documentation
TL;DR: The Telnet protocol and well-known options offer no support for what you describe.
I mean the Telnet negotiation mode: (1) Passive mode: where the Telnet Client/Server will not send any negotiation data, only user data; (2) Active mode: where the Telnet Client/Server will send the negotiation data to configure/handshake what are the features to be used/set.
The Telnet specifications do not define a "passive mode" by any name. There is no defined way for one side to request that the other refrain from attempting to negotiate protocol options, much less to refrain from sending any protocol commands at all.
On the Telnet Server side, I simply ignore received data starting with 0xff (IAC code).
There are many more Telnet commands than those related to option negotiation. Ignoring all Telnet commands makes your server not only impolite, but deficient. You reference PuTTY's "passive mode", but even in that mode, PuTTY still emits Telnet commands for purposes other than option negotiation, and it likely still performs option negotiation too, albeit after allowing the server to negotiate first.
Additionally,
Option negotiation and other protocol commands are asynchronous, which is why the approach you describe doesn't stall communication altogether, but that does not make it valid to ignore protocol commands. The server should emit a response to each option negotiation command received, even if that response is negative.
And that will make it less likely for clients to make renewed attempts to negotiate the same options, but the server cannot say "never" to any option request, only "no".
Although it is not obligated to accept requests or offers to enable non-default options, the server is obligated to honor requests to disable options:
Clearly, a party may always refuse
a request to enable, and must never refuse a request to disable some option
(RFC 854, p.2)
The only way for a Telnet endpoint to receive a data byte with value 255 (decimal) is via the IAC mechanism.
If a Telnet endpoint does not respond to AYT commands, then the other endpoint may sometimes conclude that the session has been dropped when in fact it is still active.
A Telnet endpoint that ignores protocol commands does not support standard, expected terminal operations including break signals, interrupt process signals, erase character and erase line commands, and terminal synch. If yours is a special-purpose Telnet implementation then perhaps some of these do not actually require any server-side action, but the synch, at least, is about the data stream between server and client, and I don't see any way that a conforming Telnet implementation can fail to provide for it.
I have an embedded system that is programed in C. I need to do the equivalent to the DOS command Telnet. The idea is to test if the remote host is up and running.
I would like to have some orientation here like:
Open source project that I can use as a guide line (C language)
Some documentation on what the Telnet command does (so I can
implement my own)
Thanks
Update: Thank you for your valuable comments
My system connects to a host via GPRS/Ethernet/dial up/Wifi (one of them). As a developer I check if the host is ok by using my windows laptop (with a Dial up modem, GPRS modem or whatever is needed) and running Telnet like this:
telnet 192.168.0.1 8000
(non real values)
If the host is ok I got the clean screen, otherwise I got an error. That's what I need to do in code, to be able to determine if the host is up and running by using a sort of DOS telnet client command in C.
This is done once, just to check communications, after this test is cleared the real info should be sent.
I'm writing a port scanner in C and i want to detect what service is running on an open port and its version.I've already wrote the scanner code but now i have no idea about how to detect running service.
What can i do?
If you are determined to do it in your own code, you can connect to the port, see if you get any data on it, if nothing then send a few bytes, and check again.
Then match that against expected response.
to get an idea what you are looking for, you can connect manually to the port with telnet and poke at it. In many cases (a web server is an easy example) you must send some correctly formatted data in order to get a usable response.
nmap has done all this and much more (e.g. extensive checks such as looking for byte order and timing of arp traffic)
UPDATE: several people have mentioned well known ports, but that won't help you discover standard services running on nonstandard ports, such as ssh or http servers running on custom ports.
If server sends something first, use that to identify protocol.
If not, send something according to some protocol, such as http, and see what server sends back (valid response or error). You may need to make several attempts with different protocols, and a good order is important to minimize connection count.
Some protocols may be very hard to identify, and it is easy to make custom server with unique protocol you don't know about, or even hide real server under simple fake server of other proto such as http.
If you just want to know what the port usually is, check "well known ports" and official reserved ports.
Also check nmap source code.
I'm running a game website where users connect using an Adobe Flash client to a C server running on a Fedora Linux box.
Often users complain about disconnects. Usually they're "Connection reset by peer"-disconnects.
Is there any way to make the connection more stable or does it all depend on the route from the user host to my server?
One thing I tried is to make it more stable by sending PING in clear text every other minute to avoid timeout problems.
Anyone got more ideas?
You are not exhausting the number of socket/memory use/cpu that the server process is given on the server, are you?
Do check with ulimit.
Also, if possible try to trace the error message in the source code (when a RST packet is sent--), i.e. when a send() or accept() returns an error value. In such cases print a debug message into the logs; if you really fancy debugging it do a simulation of the server:
run it into debug mode on a separate machine (possibly a clone of the server)
simulate thousands of connection (or find a network harnessing program)
backtrace the call and/or sniff the connection
where are you running the server?
at home? at work? at a hosting facility?
this will make a very big difference.
Can you design your app to connect to two sockets on the server and then load balance or make it active/passive (or active/active)?
You can use SO_KEEPALIVE TCP socket option.
Hi Guys I need to write a chat server in C. It only needs to use IPC.
Could you help me on how to proceed on this. A skeleton code will help me a lot.
Write an echo server: a server that accepts one client, and repeats everything the client says back to it.
Expand this server to support multiple simultaneous connections.
Have the server echo to all connections.
Consider as commands some pattern of lines from clients -- an initial "/", say, and act on them (close the connection, name the connection, list connections, etc.) rather than echo them.
Prefix all echo'd text with the name of the client, with a default "Anonymous$N" and then the name set by a command from #4.
When receiving a new connection, have the server elicit a name from it before the server begins echoing text from it and acting on other commands.
And so on. As mentioned, Beej's Guide can help you get past #1 and #2.
EDIT: OK, you added the 'IPC' language. You can still use sockets for this over the loopback device, unless you've some special requirement that you think IPC covers. You can also use UNIX domain sockets - named pipes. perlipc discusses them with a short example, and you can continue to e.g. the GNU C library manual.