ISC-DHCP : configure DHCP failover relationship , MCLT

# cat /etc/centos-release CentOS release 6.3 (Final) # uname -ri 2.6.32-279.11.1.el6.x86_64 x86_64

# dhcpd --version isc-dhcpd-4.1.1-P1

referred to http://www.madboa.com/geek/dhcp-failover/. ( many thx ! )

[ network topology ]

very simple

dhcpd ( primary )  -- L2SW -- Client
dhcpd ( secondary ) -----|

[ primary ]

# cat /etc/dhcp/dhcpd.conf # primary authoritative; ddns-update-style none; failover peer "dhcp-failover" {  primary; # declare this to be the primary server  address 192.168.102.100; # primary's IP  port 647;  peer address 192.168.102.101; # secondary's IP  peer port 647;  max-response-delay 30;  max-unacked-updates 10;  load balance max seconds 3;  mclt 1800;  split 128; } subnet 192.168.102.0 netmask 255.255.255.0 {  option subnet-mask 255.255.255.0;  option broadcast-address 192.168.102.255;  option routers 192.168.102.1;  option domain-name-servers 192.168.102.1;  pool {    failover peer "dhcp-failover";    default-lease-time 3600; # 1 hour    max-lease-time 86400; # 1 day    range 192.168.102.110 192.168.102.210;  } }

[ secondary ]

# cat /etc/dhcp/dhcpd.conf # secondary authoritative; ddns-update-style none; failover peer "dhcp-failover" {  secondary; # declare this to be the secondary server  address 192.168.102.101; # secondary's IP  port 647;  peer address 192.168.102.100; # primary's IP  peer port 647;  max-response-delay 30;  max-unacked-updates 10;  load balance max seconds 3; } subnet 192.168.102.0 netmask 255.255.255.0 {  option subnet-mask 255.255.255.0;  option broadcast-address 192.168.102.255;  option routers 192.168.102.1;  option domain-name-servers 192.168.102.1;  pool {    failover peer "dhcp-failover";    default-lease-time 3600; # 1 hour    max-lease-time 86400; # 1 day    range 192.168.102.110 192.168.102.210;  } }

- start DHCPD on both primar and secondary

# /etc/init.d/dhcpd start

- syslog

cofirm failover state is normal.

primary

# egrep -i normal /var/log/messages dhcpd: failover peer dhcp-failover: I move from recover-done to normal dhcpd: failover peer dhcp-failover: peer moves from recover-done to normal

secondary

# egrep normal /var/log/messages dhcpd: failover peer dhcp-failover: peer moves from recover-done to normal dhcpd: failover peer dhcp-failover: I move from recover-done to normal

[ send a DHCP discover from the client ]

DHCPD has leased 192.168.102.160 to the Client.

Client # ifconfig eth0 | grep 'inet addr'          inet addr:192.168.102.160  Bcast:192.168.102.255  Mask:255.255.255.0

the primary leased IP to the Client
log ( primary )

dhcpd: DHCPDISCOVER from 52:54:00:02:bf:f8 via eth0 dhcpd: DHCPOFFER on 192.168.102.160 to 52:54:00:02:bf:f8 via eth0 dhcpd: DHCPREQUEST for 192.168.102.160 (192.168.102.100) from 52:54:00:02:bf:f8 via eth0 dhcpd: DHCPACK on 192.168.102.160 to 52:54:00:02:bf:f8 via eth0

When configuring DHCP failover , at first DHCPD sets lease-time MCLT ( in this case 1800 seconds ) , not defai;t-lease-time ( in this case 3600 seconds )

The following is DHCP offer from the primary DHCP to the client.
lease time is 30 minutes , which is MCLT.

Your (client) IP address: 192.168.102.160 (192.168.102.160)   Option: (t=51,l=4) IP Address Lease Time = 30 minutes <- MCLT        Option: (51) IP Address Lease Time        Length: 4        Value: 00000708

- Client will send a DHCP request to the primary to extend lease time after MCLT / 2 ( 15 minites )

# tshark -r primary2.pcap Running as user "root" and group "root". This could be dangerous.  1   0.000000 192.168.102.160 -> 192.168.102.100 DHCP DHCP Request  - Transaction ID 0x1b1e6147  2   0.025141 192.168.102.100 -> 192.168.102.160 DHCP DHCP ACK      - Transaction ID 0x1b1e6147

The following is DHCP ACK from the primary to the client

Option: (t=54,l=4) DHCP Server Identifier = 192.168.102.100        Option: (54) DHCP Server Identifier        Length: 4        Value: C0A86664    Option: (t=51,l=4) IP Address Lease Time = 1 hour <- default-lease-time        Option: (51) IP Address Lease Time        Length: 4        Value: 00000E10

dhclient log ( client )

# cat /var/lib/dhclient/dhclient-eth0.leases lease {  interface "eth0";  fixed-address 192.168.102.160;  option subnet-mask 255.255.255.0;  option routers 192.168.102.1;  option dhcp-lease-time 1800; <- MCLT  option dhcp-message-type 5;  option domain-name-servers 192.168.102.1;  option dhcp-server-identifier 192.168.102.100;  option broadcast-address 192.168.102.255;  renew 1 2012/11/04 10:00:01;  rebind 1 2012/11/04 10:11:30;  expire 1 2012/11/04 10:15:15; } lease {  interface "eth0";  fixed-address 192.168.102.160;  option subnet-mask 255.255.255.0;  option routers 192.168.102.1;  option dhcp-lease-time 3600; <- default lease time  option dhcp-message-type 5;  option domain-name-servers 192.168.102.1;  option dhcp-server-identifier 192.168.102.100;  option broadcast-address 192.168.102.255;  renew 1 2012/11/04 10:24:14;  rebind 1 2012/11/04 10:52:31;  expire 1 2012/11/04 11:00:01; }