Question

I am using following script to reboot my router using Telnet:

#!/usr/bin/env python

import os
import telnetlib
from time import sleep

host = "192.168.1.1"
user = "USER"
password = "PASSWORD"
cmd = "system restart"

tn = telnetlib.Telnet(host)
sleep(1)

tn.read_until("Login: ")
tn.write(user + "\n\r")
sleep(1)

tn.read_until("Password: ")
tn.write(password + "\n\r")
sleep(1)

tn.write(cmd + "\n\r")

I don’t know why but removing "\r" breaks this code. So what does "\r" do in this script and when do I use "\r" in general?

Note: I know about "Carriage Return" but still could not figure out it is used in my script. I am running this script in Linux.

Answer 1

The '\r' character is the carriage return, and the carriage return-newline pair is both needed for newline in a network virtual terminal session.

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From the old telnet specification (RFC 854) (page 11):

The sequence "CR LF", as defined, will cause the NVT to be
positioned at the left margin of the next print line (as would,
for example, the sequence "LF CR").

However, from the latest specification (RFC5198) (page 13):

1. …

2. In Net-ASCII, CR MUST NOT appear except when immediately followed
   by either NUL or LF, with the latter (CR LF) designating the "new
   line" function. Today and as specified above, CR should
   generally appear only when followed by LF. Because page layout
   is better done in other ways, because NUL has a special
   interpretation in some programming languages, and to avoid other
   types of confusion, CR NUL should preferably be avoided as
   specified above.

3. LF CR SHOULD NOT appear except as a side-effect of multiple CR LF
   sequences (e.g., CR LF CR LF).

So newline in Telnet should always be '\r\n' but most implementations have either not been updated, or keeps the old '\n\r' for backwards compatibility.