Linux telnet client source code

How To Install and Use Telnet Client In Linux and Windows

Telnet is easiest remote management protocol used in Computer networks. It remains popular even today because of its simplicity. Telnet is a client server protocol where clients uses telnet tool which to connect remote telnet server. This telnet server may be switch, router, windows operating system, linux server or a printer. In this tutorial we will look how to install and enable telnet with some usage examples.

Install

Ubuntu, Debian, Mint, Kali

We can install telnet package with the following command in deb based Linux distributions.

Fedora, CentOS, RHEL

We can install telnet package with yum command for rpm based distributions like Fedora, centOS and RHEL.

Windows

As windows operating system do not have online package manager telnet is provided by inline. By defualt telnet server and client applications are not enabled. We will enable the client where server can be enabled too. Follow these instructions.

1. Click Start >Control Panel.
2. Click Programs and Features.
3. Click Turn Windows features on or off.
4. In the Windows Features dialog box, check the Telnet Client check box.

Windows

Security

As telnet is old fashion protocol it provides some defects. One of the most important negative issue with telnet is its security features. Telnet do not have any encryption support as builtin. So telnet traffic can be easily eavesdropped during transmission. So do not rely telnet on insecure networks.

Connect Telnet Server

We will start by simply connecting a telnet server. The simplest form is just providing the host name or IP address of the telnet server. The syntax is like below. In this example telnet server is a Ubuntu 17.10 .

Connect Telnet Server

Connect Different Telnet Port

The default port for telnet is TCP 23 . By default it is not specified during connection. But in some cases telnet server may use different then 23. We can specify the port number by adding after the host name or IP address. In this example we will connect port number 2323 .

Specify Login Name

Most cases telnet servers requires authentication. In order to authenticate ourself we need to provide some user name and password. The regular usage is providing username and then put password interactively. We will use -l which stands for login.

Interactive Shell

Telnet provides interactive shell where it provides its own environment. We can enter interactive shell just putting telnet command.

We can get help for interactive shell with help command. We can see that we will use open command and the hostname or IP address to connect telnet server.

help

We will connect to the localhost telnet server.

Open Connection

Источник

l3net – a layer 3 networking blog

A blog about IP networking, network and system administration, Linux/UNIX programming and testing. The blog also includes some small projects I run in my free time.

A Simple Telnet Client

This is a simple telnet client written in C. In the negotiation phase of the connection it will respond with WONT to any DO coming from the server, and it will encourage the server to DO anything it offers with WILL. The only exception is the screen size setting, advertised as 24 x 80 by the client.

You can download the source code for this telnet client here.

Share this:

Like this:

Related

6 thoughts on “ A Simple Telnet Client ”

Great start for a quick-and-dirty telnet client. Multiple bugs with the port though. Needs ^] handling too.

You are right, port hardcoded and bad test on argc – thanks!

How would be the server side coding for this program.
Also how to execute this program. I am new to socket programming, when I tried executing this program ,it throws an error of connection refused. Kindly help

You need to enable telnetd daemon before you run this client. If you are using Ubuntu Linux then follow these instructions.

In order to compile this program on Ubuntu Linux, simply do

sudo apt-get install build-essential

Then save this program as ‘telnet-client.c’
From the terminal, navigate to the folder

cd
gcc -g telnet-client.c -o telnet-client

Now run the program (It will ask for your Ubuntu Linux Login and password)
telnet-client 127.0.0.1 23

And you sould see something similar to this

./telnet-client 127.0.0.1 23
Connected…

Ubuntu 12.04.5 LTS
ubuntu login: virgo
Password:
Last login: Wed Oct 1 07:38:46 PDT 2014 from localhost on pts/4
Welcome to Ubuntu 12.04.5 LTS (GNU/Linux 3.13.0-36-generic x86_64)

New release ‘14.04.1 LTS’ available.
Run ‘do-release-upgrade’ to upgrade to it.

Your Hardware Enablement Stack (HWE) is supported until April 2017.

Says “network is unreachable” when I follow the above instructions even after installing telnetd

Источник

legnaleurc / telnet_client.c

/* http://l3net.wordpress.com/2012/12/09/a-simple-telnet-client/ */

# include stdio.h >

# include stdlib.h >

# include unistd.h >

# include string.h >

# include arpa/inet.h >

# include termios.h >

# include fcntl.h >

# define DO 0xfd

# define WONT 0xfc

# define WILL 0xfb

# define DONT 0xfe

# define CMD 0xff

# define CMD_ECHO 1

# define CMD_WINDOW_SIZE 31

void negotiate ( int sock, unsigned char *buf, int len) <

int i;

if (buf[ 1 ] == DO && buf[ 2 ] == CMD_WINDOW_SIZE) <

unsigned char tmp1[ 10 ] = < 255 , 251 , 31 >;

if ( send (sock, tmp1, 3 , 0 ) 0 )

exit ( 1 );

unsigned char tmp2[ 10 ] = < 255 , 250 , 31 , 0 , 80 , 0 , 24 , 255 , 240 >;

if ( send (sock, tmp2, 9 , 0 ) 0 )

exit ( 1 );

return ;

>

for (i = 0 ; i if (buf[i] == DO)

buf[i] = WONT;

else if (buf[i] == WILL)

buf[i] = DO;

>

if ( send (sock, buf, len , 0 ) 0 )

exit ( 1 );

>

static struct termios tin;

static void terminal_set ( void ) <

// save terminal configuration

tcgetattr (STDIN_FILENO, &tin);

static struct termios tlocal;

memcpy (&tlocal, &tin, sizeof (tin));

cfmakeraw (&tlocal);

tcsetattr (STDIN_FILENO,TCSANOW,&tlocal);

>

static void terminal_reset ( void ) <

// restore terminal upon exit

tcsetattr (STDIN_FILENO,TCSANOW,&tin);

>

# define BUFLEN 20

int main ( int argc , char *argv[]) <

int sock;

struct sockaddr_in server;

unsigned char buf[BUFLEN + 1 ];

int len;

int i;

if (argc 2 || argc > 3 ) <

printf ( » Usage: %s address [port] \n » , argv[ 0 ]);

return 1 ;

>

int port = 23 ;

if (argc == 3 )

port = atoi (argv[ 2 ]);

// Create socket

sock = socket (AF_INET , SOCK_STREAM , 0 );

if (sock == — 1 ) <

perror ( » Could not create socket. Error » );

return 1 ;

>

server. sin_addr . s_addr = inet_addr (argv[ 1 ]);

server. sin_family = AF_INET;

server. sin_port = htons (port);

// Connect to remote server

if ( connect (sock , ( struct sockaddr *)&server , sizeof (server)) 0 ) <

perror ( » connect failed. Error » );

return 1 ;

>

puts ( » Connected. \n » );

// set terminal

terminal_set ();

atexit (terminal_reset);

struct timeval ts;

ts. tv_sec = 1 ; // 1 second

ts. tv_usec = 0 ;

while ( 1 ) <

// select setup

fd_set fds;

FD_ZERO (&fds);

if (sock != 0 )

FD_SET (sock, &fds);

FD_SET ( 0 , &fds);

// wait for data

int nready = select (sock + 1 , &fds, ( fd_set *) 0 , ( fd_set *) 0 , &ts);

if (nready 0 ) <

perror ( » select. Error » );

return 1 ;

>

else if (nready == 0 ) <

ts. tv_sec = 1 ; // 1 second

ts. tv_usec = 0 ;

>

else if (sock != 0 && FD_ISSET (sock, &fds)) <

// start by reading a single byte

int rv;

if ((rv = recv (sock , buf , 1 , 0 )) 0 )

return 1 ;

else if (rv == 0 ) <

printf ( » Connection closed by the remote end \n\r » );

return 0 ;

>

if (buf[ 0 ] == CMD) <

// read 2 more bytes

len = recv (sock , buf + 1 , 2 , 0 );

if (len 0 )

return 1 ;

else if (len == 0 ) <

printf ( » Connection closed by the remote end \n\r » );

return 0 ;

>

negotiate (sock, buf, 3 );

>

else <

len = 1 ;

buf[len] = ‘ \0 ‘ ;

printf ( » %s » , buf);

fflush ( 0 );

>

>

else if ( FD_ISSET ( 0 , &fds)) <

buf[ 0 ] = getc (stdin); // fgets(buf, 1, stdin);

if ( send (sock, buf, 1 , 0 ) 0 )

return 1 ;

if (buf[ 0 ] == ‘ \n ‘ ) // with the terminal in raw mode we need to force a LF

putchar ( ‘ \r ‘ );

>

>

close (sock);

return 0 ;

>

You can’t perform that action at this time.

You signed in with another tab or window. Reload to refresh your session. You signed out in another tab or window. Reload to refresh your session.

Источник

Linux telnet client source code

A Quicker and better SSH/Telnet/Serial/Shell/Sftp client for DevOps.

Hello WindTerm 🌹 , hello world!

We’re just beginning! If you want a high performance text editor, you can try WindEdit.

Completely FREE for commercial and non-commercial use without limitations. All released source codes (except thirdparty directory) are provided under the terms of Apache-2.0 license.

Linux binary, MacOS binary and Windows binary: https://github.com/kingToolbox/WindTerm/releases

WindTerm is a partial open source project, and the source will be gradually opened.

Open source code includes, but is not limited to, the classes that can be used independently, such as functional, algorithms, gui widgets, etc., as well as functional libraries, such as network, protocols, etc., as well as all types that require open source according to the license.

Issues and feature requests

Any issues and feature requests are welcome.

Please click issues to commit an issue or a feature request.

Please click Discussion to discuss anything about SSH, SFtp, Shell(Linux shell, Windows cmd and powershell), Telnet, Serial and WindTerm.

Main Window (zsh):

SSH, Telnet, Tcp, Shell, Serial

• SSH v2, Telnet, Raw Tcp, Serial, Shell protocols implemented. Intro Video
• Supports SSH auto execution when session authenticated.
• Supports SSH ProxyCommand or ProxyJump. Intro Video
• Supports SSH agent forwarding. Intro Video
• Supports SSH auto login with password, public-key, keyboard-interactive, gssapi-with-mic. Intro Video
• Supports X11 forwarding. Intro Video
• Supports direct/local port forwarding, reverse/remote port forwarding and dynamic port forwarding. Intro Video
• Supports XModem, YModem and ZModem. Intro Video
• Integrated sftp, scp client, supports download, upload, remove, rename, make new file/directory and so on. Intro Video
• Integrated local file manager, supports move to, copy to, copy from, remove, rename, make new file/directory and so on.
• Supports Windows Cmd, PowerShell and Cmd, PowerShell as administrator.
• Supports Linux bash, zsh, powershell core and so on.
• Supports MacOS bash, zsh, powershell core and so on.

• Session dialog and session tree.
• Focus Mode.Intro Video
• Sync Input.Intro Video
• Enhanced protection of the session username and password.Intro Video
• Command palette.Intro Video
• Command sender.Intro Video
• Explorer Pane.Intro Video
• Quick Bar.Intro Video
• Paste Dialog.Intro Video
• Local and remote modes with vim keybindings. (Using Shift+Enter key to switch between remote and local mode) Intro Video
• Supports time stamp, folding, outlining, split views.
• Supports powerline in Linux and PowerShell, e.g. Oh-My-Zsh, Oh-My-Posh.Intro Image
• Supports color schemes like vscode. Intro Video
• Supports searching and previewing. Intro Video
• Supports highlighting the opening and closing delimiter, such as (), [], <> and the customed delimiters. Intro Video
• Supports changing the UI theme. Intro Video
• Supports setting the tab color. Intro Video
• Supports searching over the opened tabs. Intro Video
• Supports closing tabs to the right.
• Supports setting the windows transparency. Intro video
• Supports select-to-copy, right-click-to-paste or middle-click-to-paste.
• Supports searching text online with Google, Bing, Github, Stackoverflow, Wikipedia and DuckDuckGo. Intro video
• Supports hiding mouse cursor while typing.
• Supports locking screen.Intro video

• Supports vt100, vt220, vt340, vt420, vt520, xterm, xterm-256-colors.
• Supports unicode, emojis, true-color, mouse protocol, etc.
• Supports auto wrap mode. Intro Video
• Protocols and terms can be customed.
• All vttest tests have passed except Tektronix 4014.

• Supports HTTP and SOCKS5 proxy.Intro Video
• Supports Jump Server proxy.Intro Video
• Supports manual and automated session logging. Intro Video
• Rename and duplicate session. Intro Video
• Restore last sessions and layouts when restart. Intro Video

• High performance, low memory, low latency. Intro Video

The hardware used for generating the data in these benchmarks was

WindTerm1.72, WindTerm 1.2, FileZilla 3.48.1, WinSCP 5.17.2 (Build 10278) tests are performed on WSL(Ubuntu 18.04.2).

The version of clients:

Application	Version	Release Date
windterm	v1.72	2020-10-25
windterm	v1.2	2020-06-15
FileZilla	v3.48.1	2020-05-19
WinScp	v5.17.2 (Build 10278)	2020-03-09

All test data is for reference only.

5GB huge file (5,154,830 KB), generated by random data

Download Time	Download Rate	Upload Time	Upload Rate
WindTerm 1.72 (Use high speed transfer)	23s	216.3 MB/s	20s	247.0 MB/s
WindTerm 1.72	23s	214.7 MB/s	20s	244.0 MB/s
WindTerm 1.2	37s	139.3 MB/s	43s	119.9 MB/s
FileZilla	32s	161.1 MB/s	30s	171.8 MB/s
WinSCP	81s	63.7 MB/s	91s	56.7 MB/s

4400 files, 16 folders (107,042 KB), unzipped from vim-7.4.1049.zip

Download Time	Download Rate	Upload Time	Upload Rate
WindTerm 1.7	26s	3.9 MB/s	13s	8.1 MB/s
WindTerm 1.2	32s	3.4 MB/s	10s	10.7 MB/s
FileZilla	48s	2.2 MB/s	35s	3.1 MB/s
WinSCP	42s	2.6 MB/s	12s	8.9 MB/s

The hardware used for generating the data in these benchmarks was

WindTerm 1.72, rxvt, putty, xterm, Windows Terminal tests are performed on WSL(Ubuntu 18.04.2).

Iterm2, kitty, Alacritty tests are performed on MacOS shell,

The version of terminals:

Application	Version	Release Date
windterm	v1.72	2020-10-25
rxvt-unicode	v9.2.2	2016-05-14
putty	v0.71	2019-03-16
xterm	v3.30	2017-06-20
iterm2	v3.3.6	2019-10-09
alacritty	v0.5.0	2020-07-21
kitty	v0.14.6	2019-09-25
Windows Terminal	v1.3.2651.0	2020-09-22

All test data is for reference only.

Test Command: «cat ./benchmark_randomdata»

The benchmark_randomdata contains 97.6MB random text (102,401,504 bytes, 1,329,878 lines, generated and tested by random_test.sh)

In all cases, three runs were made to warm system caches. The reported numbers are the median of five runs.

1. Telnet:

Lines of scrollback	Data Rate(MB/sec)	Memory Usage(MB)
WindTerm	unlimited	52.1	106.6
rxvt	1,350,000	37.8	842.2
Putty	1,350,000	4.9	733.4
xterm	1,350,000	2.2	3328.4
Windows Terminal + telnet.exe	65,535	0.1	Not measured, use 65,535 scrollback lines setting

1. SSH:

Lines of scrollback	Data Rate(MB/sec)	Memory Usage(MB)
WindTerm	unlimited	41.8	108.5
rxvt	1,350,000	40.2	842.2
Putty	1,350,000	4.8	734.9
xterm	1,350,000	2.3	3328.4
Windows Terminal + ssh.exe	65,535	2.1	Not measured, use 65,535 scrollback lines setting

1. Shell:

Lines of scrollback	Data Rate(MB/sec)	Memory Usage(MB)
iterm2	unlimited	— (Take too long time)	more than 1300
kitty	unlimited	17.2	2655
Alacritty	100,000	41.3	—

Test command: «time seq 1 n» (n = [1000000, 2000000, 5000000, 10000000], scrollback lines: unlimited)

Time(sec)	Memory Usage(MB)
WindTerm	1.236	16.1
rxvt	5.082	633.3
putty	4.161	551.1
xterm	40.421	2500.7
iterm2	2.116	146.3
Kitty	2.535	2376.5
Alacritty	1.162	Not measured, use 100,000 scrollback lines setting
Windows Terminal + ssh.exe	23.246	Not measured, use 65,535 scrollback lines setting

Time(sec)	Memory Usage(MB)
WindTerm	2.287	24.1
rxvt	10.896	1266.6
putty	16.045	1102.6
xterm	68.154	5005.5
iterm2	4.181	383.2
Kitty	5.620	4749.9
Alacritty	2.322	Not measured, use 100,000 scrollback lines setting
Windows Terminal + ssh.exe	50.381	Not measured, use 65,535 scrollback lines setting

Time(sec)	Memory Usage(MB)
WindTerm	5.520	68.2
rxvt	27.533	3166.2
putty	45.911	2757.1
xterm	—	Out of memory
iterm2	10.805	1048.3
Kitty	—	Out of memory
Alacritty	5.799	Not measured, use 100,000 scrollback lines setting
Windows Terminal + ssh.exe	130.371	Not measured, use 65,535 scrollback lines setting

Time(sec)	Memory Usage(MB)
WindTerm	10.674	133.3
rxvt	—	Out of memory
putty	—	Out of memory
xterm	—	Out of memory
iterm2	20.468	2231.3
Kitty	—	Out of memory
Alacritty	11.598	Not measured, use 100,000 scrollback lines setting
Windows Terminal + ssh.exe	264.739	Not measured, use 65,535 scrollback lines setting

n = 10,000,000 scrollback = 30 Lines

Time(sec)	Memory Usage(MB)
WindTerm	10.167	0.7
rxvt	9.687	0.1
putty	95.382	0.4
xterm	286.510	0.1
iterm2	25.448	7.4
Kitty	16.104	0.5
Alacritty	11.798	Not measured, use zero scrollback lines setting
Windows Terminal + ssh.exe	261.096	Not measured, use zero scrollback lines setting

Linux Terminal Performance

The hardware used for generating the data in these benchmarks was

The version of terminals:

Application	Version	Release Date
Windterm	v1.9	2020-12-22
Gnome	v3.30.2	2018-10-22
Mate Terminal	v1.20.2	2019-02-11
Konsole	v18.04.0	2019-04-12
Xfce4 Terminal	v0.8.7.4	2018-5-15
QTerminal	v0.14.1	2019-01-26

All test data is for reference only.

Test Command: «cat ./benchmark_randomdata»

The benchmark_randomdata contains 97.6MB random text (102,401,504 bytes, 1,329,878 lines, generated and tested by random_test.sh)

In all cases, three runs were made to warm system caches. The reported numbers are the median of five runs.

Cost Time
WindTerm	1.976s
Gnome Terminal	9.781s
Mate Terminal	9.841s
Konsole	25.050s
xfce4 Terminal	10.520s
QTerminal	20.763s

Test command: «time seq 1 n» (n = [1000000, 2000000, 5000000, 10000000], scrollback lines: unlimited)

n	1,000,000	2,000,000	5,000,000	10,000,000	10,000,000 (scrollback lines: 100)
WindTerm	0.846s (18.6MB)	1.574s (26.6MB)	4.046s (56.4MB)	8.232s (102.2MB)	7.748s (3.4MB)
Gnome Terminal	0.920s	2.152s	5.271s	11.111s	13.109s
Mate Terminal	0.822s	1.698s	5.943s	10.920s	12.290s
Konsole	1.612s	3.199s	8.157s	16.029s	15.650s
xfce4 Terminal	0.870s	2.160s	5.866s	12.089s	13.304s
QTerminal	9.272s	18.391s	45.999s	104.277s	17.208s

Considering the network influence on the latency, the following data is from WindEdit. DIGEdit is the text component of WindTerm.

Min	Max	Avg	SD
WindEdit	1.9	7.6	2.9	0.8
Windows Notepad	0.9	16.5	7.8	1.8
GVim	0.9	10.4	2.8	1.2

Release cycle:

Features of version 2.2 (Mid-August，for reference only):

• SSH ControlMaster
• SSH Agent Forwaring
• Global Settings Dialog Box
• Multilingual User Interface
• New memory allocator and manager and garbage collector. (Postponed to v2.3)
• Search in sessions. (Postponed to v2.3)

Featurs of version 2.x:

• Shell Pane
• External tools
• UI:
  • Config dialog
• Protocols:
  • Mosh
  • Rlogin
• Session:
  • Auto Complete
  • Chat mode
  • Log viewer
• File transfer:
  • ftp, ftps
• Script, macro and plugin stystem
• More .

About

A quicker and better cross-platform SSH/Sftp/Shell/Telnet/Serial client.

Источник