Telnet messages are being dropped

Hello. I am using a Python3.3 script that uses a telnet connection from telnetlib library.

Now, I am using the write(command) and read() functions of this connection.

So, after wiresharking this simple code:

import telnetlib
import sys
def func1(IP,user,passw):
    t=telnetlib.Telnet(IP)
    t.write(user.encode('ascii')+b'\n')
    t.write(passw.encode('ascii')+b'\n')
    return t
def func2(t,command):
t.write(command.encode('ascii')+b'\n')
print(command)
user=sys.argv[1]
passw=sys.argv[2]
IP=sys.argv[3]
t=func1(IP,user,passw)
for i in range(6):
func2(t, "message "+str(i))

I saw a weird behavior on Wireshark. After submitting the username and password, it send to the server a packet which contains only 2 messages out of six (within the ‘for loop’), and the next packet is also from my computer, resetting the telnet conection!

By adding a line to func2:

def func2(t,command):
t.write(command.encode('ascii')+b'\n')
t.read_eager()   #The new line
print(command)

Somehow it works fine.

Now, it should not be like that. If I am using the write(command) 10 times one after each other, not the way I used here, It works fine without any need of read().

Any idea?

The last two packets:

  no.    time       source  destination  protocol  info

1501    11.754366   A.B.C.A ..*.113   TELNET  Telnet Data …
1502    11.760757   A.B.C.A ..*.113   TCP esimport > telnet [RST, ACK] Seq=40 Ack=7 Win=0 Len=0