Python telnetlib read_until returns cut off string

Question

[Python telnetlib read_until recv problem]

"read_until" function returns cut off string with long commands.
The commands ran perfectly, but it does not show full texts.
How can I fix this? Please help.

# my code

tn = telnetlib.Telnet(ip, 23, 5)
prompt = ']# '
tn.write('echo "this is a long command for the test purpose... > test.txt"\n')
print tn.read_until(prompt, 1)

# debug output

Telnet(192.168.220.4,23): send 'echo "this is a long command for the test purpose... > test.txt"\n'
Telnet(192.168.220.4,23): recv 'echo "this is a l'
Telnet(192.168.220.4,23): recv 'ong command for the test purpose... > te\r\x00<his is '
Telnet(192.168.220.4,23): recv 'a long command for the test purpose... > tes '
Telnet(192.168.220.4,23): recv ' \x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x08\x0 8t.txt"'
Telnet(192.168.220.4,23): recv '\r\nthis is a long command for the test purpose... >'
Telnet(192.168.220.4,23): recv ' test.txt\r\n[root@RHEL6-5 tmp]# '

# real output

<his is a long command for the test purpose... > test.txt"
this is a long command for the test purpose... > test.txt
[root@RHEL6-5 tmp]#

Answer 1

Working for me.

def set_max_window_size(tsocket, command, option):
"""
Set Window size to resolve line width issue
Set Windows size command: IAC SB NAWS <16-bit value> <16-bit value> IAC SE
--> inform the Telnet server of the window width and height.
Refer to https://www.ietf.org/rfc/rfc1073.txt
:param tsocket: telnet socket object
:param command: telnet Command
:param option: telnet option
:return: None
"""
if option == NAWS:
    naws_command = struct.pack('!BBBHHBB',
    255, 250, 31, # IAC SB NAWS
    50, 100, #width, height
    255, 240) # IAC SE
    tsocket.send(naws_command)
# -- below code taken from telnetlib source
elif command in (DO, DONT):
    tsocket.send(IAC + WONT + option)
elif command in (WILL, WONT):
    tsocket.send(IAC + DONT + option)

ip = 'x.x.x.x'
tn = telnetlib.Telnet(ip, 23, timeout=5)
socket = tn.get_socket()
tn.set_option_negotiation_callback(set_max_window_size)

tn.write('echo "this is a long command for the test purpose... > test.txt"\n')

prompt = ']# '
print tn.read_until(prompt, timeout=1)

Answer 2

The issue appears to be occurring due to line-wrapping. I found a solution where setting the window size width to near the max value allows long lines to be received without line-wrapping applied by the telnet server.

(See the Window Size Option RFC for details on setting the option https://www.ietf.org/rfc/rfc1073.txt )

import telnetlib
import struct
from telnetlib import DO, DONT, IAC, WILL, WONT, NAWS, SB, SE

MAX_WINDOW_WIDTH = 65000  # Max Value: 65535
MAX_WINDOW_HEIGHT = 5000


def set_max_window_size(tsocket, command, option):
    """
    Set Window size to resolve line width issue
    Set Windows size command: IAC SB NAWS <16-bit value> <16-bit value> IAC SE
    --> inform the Telnet server of the window width and height.
    Refer to https://www.ietf.org/rfc/rfc1073.txt
    :param tsocket: telnet socket object
    :param command: telnet Command
    :param option: telnet option
    :return: None
    """
    if option == NAWS:
        width = struct.pack('H', MAX_WINDOW_WIDTH)
        height = struct.pack('H', MAX_WINDOW_HEIGHT)
        tsocket.send(IAC + WILL + NAWS)
        tsocket.send(IAC + SB + NAWS + width + height + IAC + SE)
    # -- below code taken from telnetlib source
    elif command in (DO, DONT):
        tsocket.send(IAC + WONT + option)
    elif command in (WILL, WONT):
        tsocket.send(IAC + DONT + option)

ip = 'x.x.x.x'
tn = telnetlib.Telnet(ip, 23, timeout=5)
tn.set_option_negotiation_callback(set_max_window_size)

tn.write('echo "this is a long command for the test purpose... > test.txt"\n')

prompt = ']# '
print tn.read_until(prompt, timeout=1)