Debug DHCP Messages

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

Broadcast LDAP Server to Clients

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.

Get Active Leases

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('+-----------------------------------------------------------------------------+')

Force DDNS Name

For dynamic DNS, ISC DHCP assigns IP addresses using names reported by the DNS client however, sometimes it is favourable to force the name to something custom using a static lease. This can be done by editing the DHCP configuration and creating a static lease with the ddns-hostname option set. For instance:

host test.internal {
    hardware ethernet 7e:fb:cf:64:51:e0;
    fixed-address 192.16.1.20;
    ddns-hostname "me";
}

will forcibly set the hostname to me.TLD (where TLD is the top level domain for which DDNS is active). If the hostname expansion is not desirable, then the full name can be spelled out and a dot appended: me..


fuss/isc_dhcpd.txt ยท Last modified: 2018/02/01 09:11 by office

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