Module telnetlib#
Module telnetlib is part of standard Python library. This is telnet client implementation.
It is also possible to connect via telnet using pexpect. The advantage of telnetlib is that this module is part of standard Python library.
Telnetlib resembles pexpect but has several differences. The most notable difference is that telnetlib requires a pass of a byte string, rather than normal one.
Connection is performed as follows:
In [1]: telnet = telnetlib.Telnet('192.168.100.1')
Method read_until #
Method read_until specifies till which line the output should be read. However, as an argument, it is necessary to pass bytes, not the usual string:
In [2]: telnet.read_until(b'Username') Out[2]: b'\r\n\r\nUser Access Verification\r\n\r\nUsername'
Method read_until returns everything it has read before specified string.
Method write#
The write method is used to transmit data. You must pass a byte string as an argument:
Read output till Password and pass the password:
In [4]: telnet.read_until(b'Password') Out[4]: b': cisco\r\nPassword' In [5]: telnet.write(b'cisco\n')
You can now specify what should be read untill prompt and then send the command:
In [6]: telnet.read_until(b'>') Out[6]: b': \r\nR1>' In [7]: telnet.write(b'sh ip int br\n')
After sending a command, you can continue to use read_until method:
In [8]: telnet.read_until(b'>') Out[8]: b'sh ip int br\r\nInterface IP-Address OK? Method Status Protocol\r\nEthernet0/0 192.168.100.1 YES NVRAM up up \r\nEthernet0/1 192.168.200.1 YES NVRAM up up \r\nEthernet0/2 19.1.1.1 YES NVRAM up up \r\nEthernet0/3 192.168.230.1 YES NVRAM up up \r\nEthernet0/3.100 10.100.0.1 YES NVRAM up up \r\nEthernet0/3.200 10.200.0.1 YES NVRAM up up \r\nEthernet0/3.300 10.30.0.1 YES NVRAM up up \r\nR1>'
Method read_very_eager #
Or use another read method read_very_eager . When using read_very_eager method, you can send multiple commands and then read all available output:
In [9]: telnet.write(b'sh arp\n') In [10]: telnet.write(b'sh clock\n') In [11]: telnet.write(b'sh ip int br\n') In [12]: all_result = telnet.read_very_eager().decode('utf-8') In [13]: print(all_result) sh arp Protocol Address Age (min) Hardware Addr Type Interface Internet 10.30.0.1 - aabb.cc00.6530 ARPA Ethernet0/3.300 Internet 10.100.0.1 - aabb.cc00.6530 ARPA Ethernet0/3.100 Internet 10.200.0.1 - aabb.cc00.6530 ARPA Ethernet0/3.200 Internet 19.1.1.1 - aabb.cc00.6520 ARPA Ethernet0/2 Internet 192.168.100.1 - aabb.cc00.6500 ARPA Ethernet0/0 Internet 192.168.100.2 124 aabb.cc00.6600 ARPA Ethernet0/0 Internet 192.168.100.3 143 aabb.cc00.6700 ARPA Ethernet0/0 Internet 192.168.100.100 160 aabb.cc80.c900 ARPA Ethernet0/0 Internet 192.168.200.1 - 0203.e800.6510 ARPA Ethernet0/1 Internet 192.168.200.100 13 0800.27ac.16db ARPA Ethernet0/1 Internet 192.168.230.1 - aabb.cc00.6530 ARPA Ethernet0/3 R1>sh clock *19:18:57.980 UTC Fri Nov 3 2017 R1>sh ip int br Interface IP-Address OK? Method Status Protocol Ethernet0/0 192.168.100.1 YES NVRAM up up Ethernet0/1 192.168.200.1 YES NVRAM up up Ethernet0/2 19.1.1.1 YES NVRAM up up Ethernet0/3 192.168.230.1 YES NVRAM up up Ethernet0/3.100 10.100.0.1 YES NVRAM up up Ethernet0/3.200 10.200.0.1 YES NVRAM up up Ethernet0/3.300 10.30.0.1 YES NVRAM up up R1> You should always set time.sleep(n) before using read_very_eager .
With read_until will be a slightly different approach. You can execute the same three commands, but then get the output one by one because of reading till prompt string:
In [14]: telnet.write(b'sh arp\n') In [15]: telnet.write(b'sh clock\n') In [16]: telnet.write(b'sh ip int br\n') In [17]: telnet.read_until(b'>') Out[17]: b'sh arp\r\nProtocol Address Age (min) Hardware Addr Type Interface\r\nInternet 10.30.0.1 - aabb.cc00.6530 ARPA Ethernet0/3.300\r\nInternet 10.100.0.1 - aabb.cc00.6530 ARPA Ethernet0/3.100\r\nInternet 10.200.0.1 - aabb.cc00.6530 ARPA Ethernet0/3.200\r\nInternet 19.1.1.1 - aabb.cc00.6520 ARPA Ethernet0/2\r\nInternet 192.168.100.1 - aabb.cc00.6500 ARPA Ethernet0/0\r\nInternet 192.168.100.2 126 aabb.cc00.6600 ARPA Ethernet0/0\r\nInternet 192.168.100.3 145 aabb.cc00.6700 ARPA Ethernet0/0\r\nInternet 192.168.100.100 162 aabb.cc80.c900 ARPA Ethernet0/0\r\nInternet 192.168.200.1 - 0203.e800.6510 ARPA Ethernet0/1\r\nInternet 192.168.200.100 15 0800.27ac.16db ARPA Ethernet0/1\r\nInternet 192.168.230.1 - aabb.cc00.6530 ARPA Ethernet0/3\r\nR1>' In [18]: telnet.read_until(b'>') Out[18]: b'sh clock\r\n*19:20:39.388 UTC Fri Nov 3 2017\r\nR1>' In [19]: telnet.read_until(b'>') Out[19]: b'sh ip int br\r\nInterface IP-Address OK? Method Status Protocol\r\nEthernet0/0 192.168.100.1 YES NVRAM up up \r\nEthernet0/1 192.168.200.1 YES NVRAM up up \r\nEthernet0/2 19.1.1.1 YES NVRAM up up \r\nEthernet0/3 192.168.230.1 YES NVRAM up up \r\nEthernet0/3.100 10.100.0.1 YES NVRAM up up \r\nEthernet0/3.200 10.200.0.1 YES NVRAM up up \r\nEthernet0/3.300 10.30.0.1 YES NVRAM up up \r\nR1>'
read_until vs read_very_eager #
An important difference between read_until and read_very_eager is how they react to the lack of output.
Method read_until waits for a certain string. By default, if it does not exist, method will “freeze”. Timeout option allows you to specify how long to wait for the desired string:
In [20]: telnet.read_until(b'>', timeout=5) Out[20]: b''
If no string appears during the specified time, an empty string is returned.
Method read_very_eager simply returns an empty string if there is no output:
In [21]: telnet.read_very_eager() Out[21]: b''
Method expect #
Method expect allows you to specify a list with regular expressions. It works like pexpect but telnetlib always has to pass a list of regular expressions.
You can then pass byte strings or compiled regular expressions:
In [22]: telnet.write(b'sh clock\n') In [23]: telnet.expect([b'[>#]']) Out[23]: (0, _sre.SRE_Match object; span=(46, 47), match=b'>'>, b'sh clock\r\n*19:35:10.984 UTC Fri Nov 3 2017\r\nR1>')
Method expect returns tuple of their three elements:
- index of matched expression
- object Match
- byte string that contains everything read till regular expression including regular expression
Accordingly, if necessary you can continue working with these elements:
In [24]: telnet.write(b'sh clock\n') In [25]: regex_idx, match, output = telnet.expect([b'[>#]']) In [26]: regex_idx Out[26]: 0 In [27]: match.group() Out[27]: b'>' In [28]: match Out[28]: _sre.SRE_Match object; span=(46, 47), match=b'>'> In [29]: match.group() Out[29]: b'>' In [30]: output Out[30]: b'sh clock\r\n*19:37:21.577 UTC Fri Nov 3 2017\r\nR1>' In [31]: output.decode('utf-8') Out[31]: 'sh clock\r\n*19:37:21.577 UTC Fri Nov 3 2017\r\nR1>'
Method close#
Method close closes connection but it’s better to open and close connection using context manager:
Using Telnet object as context manager added in version 3.6
Telnetlib usage example#
Working principle of telnetlib resembles pexpect, so the example below should be clear (2_telnetlib.py):
import telnetlib import time from pprint import pprint def to_bytes(line): return f"line>\n".encode("utf-8") def send_show_command(ip, username, password, enable, commands): with telnetlib.Telnet(ip) as telnet: telnet.read_until(b"Username") telnet.write(to_bytes(username)) telnet.read_until(b"Password") telnet.write(to_bytes(password)) index, m, output = telnet.expect([b">", b"#"]) if index == 0: telnet.write(b"enable\n") telnet.read_until(b"Password") telnet.write(to_bytes(enable)) telnet.read_until(b"#", timeout=5) telnet.write(b"terminal length 0\n") telnet.read_until(b"#", timeout=5) time.sleep(3) telnet.read_very_eager() result = <> for command in commands: telnet.write(to_bytes(command)) output = telnet.read_until(b"#", timeout=5).decode("utf-8") result[command] = output.replace("\r\n", "\n") return result if __name__ == "__main__": devices = ["192.168.100.1", "192.168.100.2", "192.168.100.3"] commands = ["sh ip int br", "sh arp"] for ip in devices: result = send_show_command(ip, "cisco", "cisco", "cisco", commands) pprint(result, width=120)
Since bytes need to be passed to write method and new line character should be added each time, a small function to_bytes is created that does the conversion to bytes and adds a new line.
'sh int desc': 'sh int desc\n' 'Interface Status Protocol Description\n' 'Et0/0 up up \n' 'Et0/1 up up \n' 'Et0/2 up up \n' 'Et0/3 up up \n' 'R1#', 'sh ip int br': 'sh ip int br\n' 'Interface IP-Address OK? Method Status Protocol\n' 'Ethernet0/0 192.168.100.1 YES NVRAM up up \n' 'Ethernet0/1 192.168.200.1 YES NVRAM up up \n' 'Ethernet0/2 unassigned YES NVRAM up up \n' 'Ethernet0/3 192.168.130.1 YES NVRAM up up \n' 'R1#'> 'sh int desc': 'sh int desc\n' 'Interface Status Protocol Description\n' 'Et0/0 up up \n' 'Et0/1 up up \n' 'Et0/2 admin down down \n' 'Et0/3 admin down down \n' 'R2#', 'sh ip int br': 'sh ip int br\n' 'Interface IP-Address OK? Method Status Protocol\n' 'Ethernet0/0 192.168.100.2 YES NVRAM up up \n' 'Ethernet0/1 unassigned YES NVRAM up up \n' 'Ethernet0/2 unassigned YES NVRAM administratively down down \n' 'Ethernet0/3 unassigned YES NVRAM administratively down down \n' 'R2#'> 'sh int desc': 'sh int desc\n' 'Interface Status Protocol Description\n' 'Et0/0 up up \n' 'Et0/1 up up \n' 'Et0/2 admin down down \n' 'Et0/3 admin down down \n' 'R3#', 'sh ip int br': 'sh ip int br\n' 'Interface IP-Address OK? Method Status Protocol\n' 'Ethernet0/0 192.168.100.3 YES NVRAM up up \n' 'Ethernet0/1 unassigned YES NVRAM up up \n' 'Ethernet0/2 unassigned YES NVRAM administratively down down \n' 'Ethernet0/3 unassigned YES NVRAM administratively down down \n' Paginated command output#
Example of using telnetlib to work with paginated output of show commands (2_telnetlib_more.py file):
import telnetlib import time from pprint import pprint import re def to_bytes(line): return f"line>\n".encode("utf-8") def send_show_command(ip, username, password, enable, command): with telnetlib.Telnet(ip) as telnet: telnet.read_until(b"Username") telnet.write(to_bytes(username)) telnet.read_until(b"Password") telnet.write(to_bytes(password)) index, m, output = telnet.expect([b">", b"#"]) if index == 0: telnet.write(b"enable\n") telnet.read_until(b"Password") telnet.write(to_bytes(enable)) telnet.read_until(b"#", timeout=5) time.sleep(3) telnet.read_very_eager() telnet.write(to_bytes(command)) result = "" while True: index, match, output = telnet.expect([b"--More--", b"#"], timeout=5) output = output.decode("utf-8") output = re.sub(" +--More--| +\x08+ +\x08+", "\n", output) result += output if index in (1, -1): break telnet.write(b" ") time.sleep(1) result.replace("\r\n", "\n") return result if __name__ == "__main__": devices = ["192.168.100.1", "192.168.100.2", "192.168.100.3"] for ip in devices: result = send_show_command(ip, "cisco", "cisco", "cisco", "sh run") pprint(result, width=120)