ISC-dhcp subnet limit?

Fri Jan 29 14:58:48 UTC 2016

Hi Rob,

I can't help for issue on your interface problem but I think I can help with the performance.

I used to have performance problem with my failover setup and someone at ISC told me to change some value in the code to get debug information about memory usage.

Add this to the file "includes/dhcpd.h"
#if !defined (REPORT_HASH_PERFORMANCE)
# define REPORT_HASH_PERFORMANCE 1
#endif

Compile and start the daemon and you should get something like this on screen and in the log:

dhcpd: DHCP name hash: Contents/Size (%): 106/401 (26%). Min/max: 0/2
dhcpd: DHCP code hash: Contents/Size (%): 106/254 (41%). Min/max: 0/1
dhcpd: NWIP name hash: Contents/Size (%): 11/17 (64%). Min/max: 0/2
dhcpd: NWIP code hash: Contents/Size (%): 11/17 (64%). Min/max: 0/1
dhcpd: FQDN name hash: Contents/Size (%): 8/13 (61%). Min/max: 0/2
dhcpd: FQDN code hash: Contents/Size (%): 8/13 (61%). Min/max: 0/1
dhcpd: VIVCO name hash: Contents/Size (%): 1/127 (0%). Min/max: 0/1
dhcpd: VIVCO code hash: Contents/Size (%): 1/127 (0%). Min/max: 0/1
dhcpd: VIVSO name hash: Contents/Size (%): 1/127 (0%). Min/max: 0/1
dhcpd: VIVSO code hash: Contents/Size (%): 1/127 (0%). Min/max: 0/1
dhcpd: ISC name hash: Contents/Size (%): 2/3 (66%). Min/max: 0/1
dhcpd: ISC code hash: Contents/Size (%): 2/3 (66%). Min/max: 0/1
dhcpd: Relay Agent name hash: Contents/Size (%): 5/11 (45%). Min/max: 0/1
dhcpd: Relay Agent code hash: Contents/Size (%): 5/11 (45%). Min/max: 0/1
dhcpd: Server-Config Option name hash: Contents/Size (%): 67/136 (49%). Min/max: 0/4
dhcpd: Server-Config Option code hash: Contents/Size (%): 67/136 (49%). Min/max: 0/1
dhcpd: data: hardware: no raw packet or lease is available
dhcpd: data: hardware: no raw packet or lease is available
dhcpd: data: hardware: no raw packet or lease is available
dhcpd: data: hardware: no raw packet or lease is available
dhcpd: data: hardware: no raw packet or lease is available
dhcpd: data: hardware: no raw packet or lease is available
dhcpd: Config file: /dhcpd/dhcpd.conf
dhcpd: Database file: /dhcpd/dhcpd.leases
dhcpd: PID file: /var/run/dhcpd.pid
dhcpd: Wrote 0 class decls to leases file.
dhcpd: Wrote 0 deleted host decls to leases file.
dhcpd: Wrote 0 new dynamic host decls to leases file.
dhcpd: Wrote 48578 leases to leases file.
dhcpd: Host HW hash:   Contents/Size (%): 1420/22501 (6%). Min/max: 0/4
dhcpd: Host UID hash:  No table.
dhcpd: Lease IP hash:  Contents/Size (%): 70324/100003 (70%). Min/max: 0/5
dhcpd: Lease UID hash: Contents/Size (%): 8708/100003 (8%). Min/max: 0/3
dhcpd: Lease HW hash:  Contents/Size (%): 9036/100003 (9%). Min/max: 0/3

"Lease IP hash" is where you should look.

By default the server use a lease hash size value of 100003. I had over 350000 leases so I was exceeding that value and the server was very slow to start.

To change the size you must edit the file "includes/dhcpd.h" and find LEASE_HASH_SIZE and replace the value. This value must be a prime number (I used 400009).

-# define LEASE_HASH_SIZE       100003
+# define LEASE_HASH_SIZE       400009

Compile and start again. That was the answer for me.

One other thing with the failover setup, the peer will always be in recover state when starting for the duration of the "MCLT" (time in second) in your failover definition (1800 in your case), so it will be in recover start for 30 minutes. I use 300 (5 minutes).

Le texte aurait été plus facile à écrire en français mais comme la liste est en anglais et que ça peut aider d'autres personnes alors je me suis forcé. En espérant que ce soit clair pour toi !

Denis Laventure
Université du Québec à Chicoutimi

De : dhcp-users-bounces at lists.isc.org [mailto:dhcp-users-bounces at lists.isc.org] De la part de Rob Morin
Envoyé : 27 janvier 2016 20:12
À : dhcp-users at lists.isc.org
Objet : ISC-dhcp subnet limit?

Hello all, my first post here, so please be gentle :)

I have inherited 2 dhcp servers, one primary(dhcp-1) & one secondary(dhcp-2) running isc-dhcpd-4.2.4 on Ubuntu 14.0(Trusty)

We are having a few issues, and I cannot seem to figure out whats going on. I have a few questions, maybe someone can help me with.

Is there a max limit to how many subnets can be used in the pools? As currently we are using just over 6000 subnets

Currently our secondary dhcp-server is always in recovery mode, not sure why?

Does it matter if a DISCOVER comes in on eth1 but OFFER goes out on eth0?

My primary server /etc/dhcpd.conf file

authoritative;
log-facility local7;
option domain-name "dyn";
option domain-name-servers 172.30.64.210, 172.30.64.220;
default-lease-time 1200;
max-lease-time 3600; # 1h
include "/etc/dhcp/dhcpd_pools.conf";
# Include the primary configuration
include "/etc/dhcp/dhcpd_primary.conf";

/etc/dhcp/dhcpd_primary has the following
                              ## PRIMARY
failover peer "tdl-dhcp-failover" {
  primary; # declare this to be the primary server
               address 172.30.128.9;
               port 647;
  peer address 172.30.128.10;
  peer port 647;
  max-response-delay 30;
  max-unacked-updates 10;
  load balance max seconds 3;
  mclt 1800;
  split 128;
}

Exert from dhcpd_pools file, starts like this....

subnet 10.32.0.0 netmask 255.255.255.0 {
  option routers 10.32.0.1;
  pool {
        failover peer "dhcp-failover";
        range 10.32.0.5 10.32.0.254;
  }
}

And finishes like this, with all the subnets in between...

subnet 10.57.255.0 netmask 255.255.255.0 {
  option routers 10.57.255.1;
  pool {
        failover peer "dhcp-failover";
        range 10.57.255.5 10.57.255.254;
  }
}

Example Exert from logs on both serves of a client that could not get an IP

from dhcp-1
Jan 27 18:30:31 dhcp-1 dhcpd: DHCPDISCOVER from fc:e9:98:bc:a8:7b (iPhone) via 10.50.170.1
Jan 27 18:30:31 dhcp-1 dhcpd: DHCPOFFER on 10.50.170.93 to fc:e9:98:bc:a8:7b (iPhone) via 10.50.170.1

from dhcp-2
Jan 27 18:53:55 dhcp-2 dhcpd: DHCPDISCOVER from fc:e9:98:bc:a8:7b via 10.50.170.1: peer holds all free leases
Jan 27 18:54:04 dhcp-2 dhcpd: DHCPDISCOVER from fc:e9:98:bc:a8:7b via 10.50.170.1: peer holds all free leases

Never see the ACK.

Any suggestion would be greatly appreciated.. :

Thanks...

Rob
Montreal Canada

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