Parsing the output of ‘show ip dhcp snooping’ command using named groups#

Consider another example of using named groups. In this example, the task is to get from the output of ‘show ip dhcp snooping binding’ the fields: MAC address, IP address, VLAN and interface.

File dhcp_snooping.txt contains the output of command ‘show ip dhcp snooping binding’:

MacAddress          IpAddress        Lease(sec)  Type           VLAN  Interface
------------------  ---------------  ----------  -------------  ----  --------------------
00:09:BB:3D:D6:58   10.1.10.2        86250       dhcp-snooping   10    FastEthernet0/1
00:04:A3:3E:5B:69   10.1.5.2         63951       dhcp-snooping   5     FastEthernet0/10
00:05:B3:7E:9B:60   10.1.5.4         63253       dhcp-snooping   5     FastEthernet0/9
00:09:BC:3F:A6:50   10.1.10.6        76260       dhcp-snooping   10    FastEthernet0/3
Total number of bindings: 4

Let’s start with one string:

In [1]: line = '00:09:BB:3D:D6:58  10.1.10.2 86250   dhcp-snooping   10  FastEthernet0/1'

In regex terms, named groups are used for those parts of the output that need to be remembered:

In [2]: match = re.search('(?P<mac>\S+) +(?P<ip>\S+) +\d+ +\S+ +(?P<vlan>\d+) +(?P<port>\S+)', line)

Comments on regex:

(?P<mac>\S+) + - group with name ‘mac’ matches any characters except whitespace characters. regex describes a sequence of any characters before space
(?P<ip>\S+) + - the same here: a sequence of any non-whitespace characters up to space. Group name - ‘ip’
\d+ + - numerical sequence (one or more digits) followed by one or more spaces. Lease value gets here
\S+ +- sequence of any characters other than whitespace. This matches Type (in this case all of them ‘dhcp-snooping’)
(?P<vlan>\d+) + - named group ‘vlan’. Only numerical sequences with one or more characters are included here
(?P<port>\S+) - named group ‘port’. All characters except whitespace are included here

As a result, groupdict method will return such a dictionary:

In [3]: match.groupdict()
Out[3]:
{'int': 'FastEthernet0/1',
 'ip': '10.1.10.2',
 'mac': '00:09:BB:3D:D6:58',
 'vlan': '10'}

Since regex has worked well, you can create a script. In script all lines of dhcp_snooping.txt file are iterated and information about devices is displayed on the standard output stream (parse_dhcp_snooping.py):

import re

#'00:09:BB:3D:D6:58   10.1.10.2        86250       dhcp-snooping   10    FastEthernet0/1'
regex = re.compile('(?P<mac>\S+) +(?P<ip>\S+) +\d+ +\S+ +(?P<vlan>\d+) +(?P<port>\S+)')
result = []

with open('dhcp_snooping.txt') as data:
    for line in data:
        match = regex.search(line)
        if match:
            result.append(match.groupdict())

print(f'{len(result)} devices connected to switch')

for num, comp in enumerate(result, 1):
    print(f'Parameters of device {num}:')
    for key in comp:
        print(f'{key:10}: {comp[key]:10}')

Script output:

$ python parse_dhcp_snooping.py
4 devices connected to switch
Parameters of device 1:
    int:    FastEthernet0/1
    ip:    10.1.10.2
    mac:    00:09:BB:3D:D6:58
    vlan:    10
Parameters of device 2:
    int:    FastEthernet0/10
    ip:    10.1.5.2
    mac:    00:04:A3:3E:5B:69
    vlan:    5
Parameters of device 3:
    int:    FastEthernet0/9
    ip:    10.1.5.4
    mac:    00:05:B3:7E:9B:60
    vlan:    5
Parameters of device 4:
    int:    FastEthernet0/3
    ip:    10.1.10.6
    mac:    00:09:BC:3F:A6:50
    vlan:    10