In order to see the messages passed back and forth between clients and the DHCP server, we can make use of tcpdump and issue:

tcpdump -n -vvv -i eth0 port bootps or port bootpc

where:

• -n makes tcpdump not resolve IP addresses to names,
• -vvv turns all the verbosity on,
• -i eth0 makes tcpdump listen on the eth0 interface,
• port bootps or port bootpc will filter all the messages to port 67, respectively 68

Add the following snippet to the ISC DHCPd configuration (usually at /etc/dhcp/dhcpd.conf):

option ldap-server code 095 = ip-address;
option ldap-server SERVER;

where:

• SERVER is the LDAP server

in order to advertise the LDAP server on the network.

The following script by Canadian Luke will fetch the current active DHCP leases.

#!/usr/bin/env python
# Created by Canadian Luke: http://askubuntu.com/users/89412/canadian-luke
import datetime, bisect
 
DHCP_LEASE_FILE = '/var/lib/dhcp/dhcpd.leases'
 
def parse_timestamp(raw_str):
        tokens = raw_str.split()
 
        if len(tokens) == 1:
                if tokens[0].lower() == 'never':
                        return 'never';
 
                else:
                        raise Exception('Parse error in timestamp')
 
        elif len(tokens) == 3:
                return datetime.datetime.strptime(' '.join(tokens[1:]),
                        '%Y/%m/%d %H:%M:%S')
 
        else:
                raise Exception('Parse error in timestamp')
 
 
def timestamp_is_ge(t1, t2):
        if t1 == 'never':
                return True
 
        elif t2 == 'never':
                return False
 
        else:
                return t1 >= t2
 
 
def timestamp_is_lt(t1, t2):
        if t1 == 'never':
                return False
 
        elif t2 == 'never':
                return t1 != 'never'
 
        else:
                return t1 < t2
 
 
def timestamp_is_between(t, tstart, tend):
        return timestamp_is_ge(t, tstart) and timestamp_is_lt(t, tend)
 
 
def parse_hardware(raw_str):
        tokens = raw_str.split()
 
        if len(tokens) == 2:
                return tokens[1]
 
        else:
                raise Exception('Parse error in hardware')
 
 
def strip_endquotes(raw_str):
        return raw_str.strip('"')
 
 
def identity(raw_str):
        return raw_str
 
 
def parse_binding_state(raw_str):
        tokens = raw_str.split()
 
        if len(tokens) == 2:
                return tokens[1]
 
        else:
                raise Exception('Parse error in binding state')
 
 
def parse_next_binding_state(raw_str):
        tokens = raw_str.split()
 
        if len(tokens) == 3:
                return tokens[2]
 
        else:
                raise Exception('Parse error in next binding state')
 
 
def parse_rewind_binding_state(raw_str):
        tokens = raw_str.split()
 
        if len(tokens) == 3:
                return tokens[2]
 
        else:
                raise Exception('Parse error in next binding state')
 
 
def parse_leases_file(leases_file):
        valid_keys = {
                'starts':               parse_timestamp,
                'ends':                 parse_timestamp,
                'tstp':                 parse_timestamp,
                'tsfp':                 parse_timestamp,
                'atsfp':                parse_timestamp,
                'cltt':                 parse_timestamp,
                'hardware':             parse_hardware,
                'binding':              parse_binding_state,
                'next':                 parse_next_binding_state,
                'rewind':               parse_rewind_binding_state,
                'uid':                  strip_endquotes,
                'client-hostname':      strip_endquotes,
                'option':               identity,
                'set':                  identity,
                'on':                   identity,
                'abandoned':            None,
                'bootp':                None,
                'reserved':             None,
                }
 
        leases_db = {}
 
        lease_rec = {}
        in_lease = False
        in_failover = False
 
        for line in leases_file:
                if line.lstrip().startswith('#'):
                        continue
 
                tokens = line.split()
 
                if len(tokens) == 0:
                        continue
 
                key = tokens[0].lower()
 
                if key == 'lease':
                        if not in_lease:
                                ip_address = tokens[1]
 
                                lease_rec = {'ip_address' : ip_address}
                                in_lease = True
 
                        else:
                                raise Exception('Parse error in leases file')
 
                elif key == 'failover':
                        in_failover = True
                elif key == '}':
                        if in_lease:
                                for k in valid_keys:
                                        if callable(valid_keys[k]):
                                                lease_rec[k] = lease_rec.get(k, '')
                                        else:
                                                lease_rec[k] = False
 
                                ip_address = lease_rec['ip_address']
 
                                if ip_address in leases_db:
                                        leases_db[ip_address].insert(0, lease_rec)
 
                                else:
                                        leases_db[ip_address] = [lease_rec]
 
                                lease_rec = {}
                                in_lease = False
 
                        elif in_failover:
                                in_failover = False
                                continue
                        else:
                                raise Exception('Parse error in leases file')
 
                elif key in valid_keys:
                        if in_lease:
                                value = line[(line.index(key) + len(key)):]
                                value = value.strip().rstrip(';').rstrip()
 
                                if callable(valid_keys[key]):
                                        lease_rec[key] = valid_keys[key](value)
                                else:
                                        lease_rec[key] = True
 
                        else:
                                raise Exception('Parse error in leases file')
 
                else:
                        if in_lease:
                                raise Exception('Parse error in leases file')
 
        if in_lease:
                raise Exception('Parse error in leases file')
 
        return leases_db
 
 
def round_timedelta(tdelta):
        return datetime.timedelta(tdelta.days,
                tdelta.seconds + (0 if tdelta.microseconds < 500000 else 1))
 
 
def timestamp_now():
        n = datetime.datetime.utcnow()
        return datetime.datetime(n.year, n.month, n.day, n.hour, n.minute,
                n.second + (0 if n.microsecond < 500000 else 1))
 
 
def lease_is_active(lease_rec, as_of_ts):
        return timestamp_is_between(as_of_ts, lease_rec['starts'],
                lease_rec['ends'])
 
 
def ipv4_to_int(ipv4_addr):
        parts = ipv4_addr.split('.')
        return (int(parts[0]) << 24) + (int(parts[1]) << 16) + \
                (int(parts[2]) << 8) + int(parts[3])
 
 
def select_active_leases(leases_db, as_of_ts):
        retarray = []
        sortedarray = []
 
        for ip_address in leases_db:
                lease_rec = leases_db[ip_address][0]
 
                if lease_is_active(lease_rec, as_of_ts):
                        ip_as_int = ipv4_to_int(ip_address)
                        insertpos = bisect.bisect(sortedarray, ip_as_int)
                        sortedarray.insert(insertpos, ip_as_int)
                        retarray.insert(insertpos, lease_rec)
 
        return retarray
 
 
##############################################################################
 
 
try:
    dhcp_lease_file = open(DHCP_LEASE_FILE, 'r')
    leases = parse_leases_file(dhcp_lease_file)
except IOError:
    print 'Could not open lease file: ' + e
except e:
    print 'Could not parse lease file: ' + e
finally:
    dhcp_lease_file.close();
 
now = timestamp_now()
report_dataset = select_active_leases(leases, now)
 
print('+-----------------------------------------------------------------------------+')
print('| DHCPD ACTIVE LEASES REPORT                                                  |')
print('+----------------+-------------------+----------------------+-----------------+')
print('| IP Address     | MAC Address       | Expires (days,H:M:S) | Client Hostname |')
print('+----------------+-------------------+----------------------+-----------------+')
 
for lease in report_dataset:
        print('| ' + format(lease['ip_address'], '<14') + ' | ' + \
                format(lease['hardware'], '<17') + ' | ' + \
                format(str((lease['ends'] - now) if lease['ends'] != 'never' else 'never'), '>20') + ' | ' + \
                format(lease['client-hostname'], '<16') + '|')
 
print('+----------------+-------------------+----------------------+-----------------+')
print('| Total Active Leases: ' + format(str(len(report_dataset)), '<55') + '|')
print('| Report generated (UTC): ' + format(str(now), '<52') + '|')
print('+-----------------------------------------------------------------------------+')