Telnet – Python Network programming
Telnet is a networking protocol that follows a client-server model. It uses TCP as its underlying communication protocol. It is typically used to start and a remote command-line session, typically on a server.
Some facts about telnet:
- Uses Transmission Control Protocol for data transmission.
- Bi-directional 8-bit protocol
- The protocol standard was initially described in RFC15 and was further extended in RFC854 and RFC855.
- It was developed in 1969.
In python, telnet communication is facilitated by the telnetlib module. The telnetlib module provides Telnet class that implements telnet protocol described in RFC 854.
The Telnet Class:
Syntax: class telnetlib.Telnet(host=None, port=0[, timeout]) :
- host(optional): it takes the server name or ip-address. Example: 127.0.0.1, “localhost”
- port(optional): takes the port number if not provided uses the default port number.
- timeout(optional): an additional parameter can be passed to specify the timeout duration else the global timeout duration is used.
If the Telnet object is created without any parameters, a connection can be established by calling the open() method. Alternatively, the user can pass the host and port details, in which case the object is returned with a connection established.
WARNING: A lot of functions in the Telnet class raise EOFError. Hence, appropriate exception handling must be done.
- Telnet.read_until(expected, timeout=None)
- Telnet.open(host, port=0[, timeout])
We have explained the usage of the functions in the following code.
When telnet was developed basic ASCII text was predominant. Present-day terminals usually used Unicode as standard. Additionally, color coding and formatting make the text that is visible on-screen very different from the byte strings that are passed via telnet. This creates confusion while using some functions of the Telnet class. This article aims to help in that respect.
Steps to writing a telnet program:
Step 1: Start a telnet server
Depending on the requirement you may have to start a telnet server or may be provided. If you have a telnet server already running proceed to step 2 else start the server.
For illustration purposes, a telnet server running on “localhost” will be used.
Step 2: Finding the magic sequence
Take a look at the following picture:
$ telnet localhost Trying ::1... Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. Linux 5.10.0-5mx-amd64 (mx) (2) mx login: pvtejeswar Password: Last login: Sun Sep 26 05:24:30 EDT 2021 from localhost on pts/2 No mail. pvtejeswar@mx:~ <========================= $
You might expect the text written at the red triangle to be: “pvtejeswar@mx:~\n$ “ but remember there is a lot of background processing and formatting going on. The text at the red triangle is infact: b”\x1b[1;35mpvtejeswar\x1b[0m@\x1b[1;36mmx\x1b[0m:\x1b[1;32m~\x1b[0m\r\r\n\x1b[1;32m$\x1b[0m “. Now you might be asking: well how do I know what this magic string looks like in my case. The following program will help you with that.
$ python3 telnet_base.py
b’\r\nLast login: Sun Sep 26 04:56:42 EDT 2021 from localhost on pts/2\r\nNo mail.\r\n\x1b[1;35mpvtejeswar\x1b[0m@\x1b[1;36mmx\x1b[0m:\x1b[1;32m~\x1b[0m\r\r\n\x1b[1;32m$\x1b[0m exit\r\nlogout\r\n’
Just by eyeballing it you may understand the output between “\r\n” and “exit” is string corresponding to “pvtejeswar@mx:~\n$ “. Now we know that all input prompts will have “pvtejeswar@mx:” which is equivalent to b”\x1b[1;35mpvtejeswar\x1b[0m@\x1b[1;36mmx\x1b[0m:”. Now we are ready to write the actual program.
Step 3: Writing the actual code.
Armed with the knowledge that we gathered from step 2 we will write the code to interact with telnet command by command:
pvtejeswar@mx:~/Desktop/telnet $ python3 telnet.py USERNAME: pvtejeswar Password: ================================================================================ output for 'ls -ltr /': ~ $ ls -ltr / total 64 lrwxrwxrwx 1 root root 8 Apr 7 23:50 sbin -> usr/sbin lrwxrwxrwx 1 root root 9 Apr 7 23:50 lib64 -> usr/lib64 lrwxrwxrwx 1 root root 7 Apr 7 23:50 lib -> usr/lib lrwxrwxrwx 1 root root 7 Apr 7 23:50 bin -> usr/bin drwxr-xr-x 2 root root 4096 Apr 7 23:50 media drwxr-xr-x 3 root root 4096 Apr 7 23:56 opt drwxr-xr-x 14 root root 4096 Apr 7 23:57 usr drwxr-xr-x 12 root root 4096 Apr 7 23:58 var drwx------ 2 root root 16384 Sep 24 21:34 lost+found drwxr-xr-x 3 root root 4096 Sep 24 21:38 home drwxr-xr-x 3 root root 4096 Sep 24 21:39 boot drwxr-xr-x 2 root root 4096 Sep 24 21:39 mnt dr-xr-xr-x 13 root root 0 Sep 24 21:39 sys dr-xr-xr-x 229 root root 0 Sep 24 21:39 proc drwx------ 7 root root 4096 Sep 25 03:24 root drwxr-xr-x 147 root root 12288 Sep 25 03:27 etc drwxr-xr-x 15 root root 3360 Sep 26 04:43 dev drwxr-xr-x 32 root root 1180 Sep 26 04:43 run drwxrwxrwt 9 root root 4096 Sep 26 05:24 tmp pvtejeswar@mx: ================================================================================ ~ $ exit logout
Telnet is an old protocol developed in 1970s. It’s not aware of for the recent day formatting and character sets used in modern-day terminals. Hence, when working with telnet we always have to keep that in mind. This guide aims to help in getting command by command interaction using telnet and provide a general overview of the telnetlib library.
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