DHCP, or Dynamic Host Control Protocol, is the service used to hand out IP addresses and other network settings by network appliances and servers. The DHCP Server built into OS X Server 3, installed on Yosemite running the Server app (aka Yosemite frickin’ server) is easy-to-use and fast. It’s pretty transparent, just as DHCP services should be. To install the service, open the Server app and then click on the Show button beside Advanced in the server sidebar. Then click on DHCP.
At the DHCP screen, you’ll see two tabs: Settings, used for managing the service and Clients, used to see DHCP leases in use by computers that obtain IP address information from the server. You’ll also see an ON and OFF switch, but we’re going to configure our scopes, or Networks as they appear in the Server app, before we enable the service. To configure a scope, double-click on the first entry in the Networks list.
Each scope, or Network, will have the following options:
- Name: A name for the scope, used only on the server to keep track of things.
- Lease Duration: Select an hour, a day, a week or 30 days. This is how long a lease that is provided to a client is valid before the lease expires and the client must find a new lease, either from the server you’re configuring or a different host.
- Network Interface: The network interface you’d like to share IPs over. Keep in mind that you can tag multiple VLANs on a NIC, assign each an interface in OS X and therefore provide different scopes for different VLANs with the same physical computer and NIC.
- Starting IP Address: The first IP address used. For example, if you configure a scope to go from 192.168.210.200 to 192.168.210.250 you would have 50 useable IP addresses.
- Ending IP Address: The last IP address used in a scope.
- Subnet Mask: The subnet mask used for the client configuration. This setting determines the size of the network.
- Router: The default gateway, or router for the network. Often a .1 address for the subnet used in the Starting and Ending IP address fields. Note that while in DHCP you don’t actually have to use a gateway, OS X Server does force you to do so or you cannot save changes to each scope.
- DNS: Use the Edit button for DNS to bring up a screen that allows you to configure the DNS settings provided as part of each DHCP scope you create, taking note that by default you will be handing out a server of 0.0.0.0 if you don’t configure this setting.
The DNS settings in the DHCP scope are really just the IP addresses to use for the DNS servers and the search domain. The search domain is the domain name appended to all otherwise incomplete Fully Qualified Domain Names. For example, if we use internal.krypted.lan and we have a DNS record for wiki.internal.krypted.lan then we could just type wiki into Safari to bring up the wiki server. Click the minus sign button to remove any data in these fields and then click on the plus sign to enter new values.
Click OK to save DNS settings and then OK to save each scope. Once you’ve build all required scopes, start the service. Once started, verify that a new client on the network gets an IP. Also, make sure that there are no overlapping scopes and that if you are moving a scope from one device to another (e.g. the server you’re setting up right now) that you renew all leases on client systems, most easily done using a quick reboot, or using “ipconfig /release” on a Windows computer. If you have problems with leases not renewing in OS X, check out this article I did awhile back.
So far, totally easy. Each time you make a change, the change updates a few different things. First, it updates the /etc/bootpd.plist property list, which looks something like this (note the correlation between these keys and the settings in the above screen shots.:
Settings from this file include:
- dhcp_enabled – Used to enable dhcp for each network interface. Replace the immediately below with en0 . For additional entries, duplice the string line and enter each from ifconfig that you’d like to use dhcp on.
- bootp_enabled – This can be left as Disabled or set to an array of the adapters that should be enabled if you wish to use the bootp protocol in addition to dhcp. Note that the server can do both bootp and dhcp simultaneously.
- allocate – Use the allocate key for each subnet in the Subnets array to enable each subnet once the service is enabled.
- Subnets – Use this array to create additional scopes or subnets that you will be serving up DHCP for. To do so, copy the entry in the array and paste it immediately below the existing entry. The entry is a dictionary so copy all of the data between and including the and immediately after the entry for the subnet itself.
- lease_max and lease_min – Set these integers to the time for a client to retain its dhcp lease
- name – If there are multiple subnet entries, this should be unique and reference a friendly name for the subnet itself.
- net_address – The first octets of the subnet followed by a 0. For example, assuming a /24 and 172.16.25 as the first three octets the entry would be 172.16.25.0.
- net_mask – The subnet mask clients should have
- net_range – The first entry should have the first IP in the range and the last should have the last IP in the range. For example, in the following example the addressing is 172.16.25.2 to 172.16.25.253.
- dhcp_domain_name_server – There should be a string for each DNS server supplied by dhcp in this array
- dhcp_domain_search – Each domain in the domain search field should be suppled in a string within this array, if one is needed. If not, feel free to delete the key and the array if this isn’t needed.
- dhcp_router – This entry should contain the router or default gateway used for clients on the subnet, if there is one. If not, you can delete the key and following string entries.
If you run the serveradmin command, followed by the settings verb and then the dhcp service, you’ll see the other place that gets updated:
serveradmin settings dhcp
The output indicates that:
dhcp:static_maps = _empty_array
dhcp:subnets:_array_id:B03BAE3C-AB79-4108-9E5E-F0ABAF32179E:WINS_secondary_server = ""
dhcp:subnets:_array_id:B03BAE3C-AB79-4108-9E5E-F0ABAF32179E:selected_port_name = "en0"
dhcp:subnets:_array_id:B03BAE3C-AB79-4108-9E5E-F0ABAF32179E:dhcp_router = "192.168.210.1"
dhcp:subnets:_array_id:B03BAE3C-AB79-4108-9E5E-F0ABAF32179E:dhcp_domain_name_server:_array_index:0 = "192.168.210.2"
dhcp:subnets:_array_id:B03BAE3C-AB79-4108-9E5E-F0ABAF32179E:net_mask = "255.255.255.0"
dhcp:subnets:_array_id:B03BAE3C-AB79-4108-9E5E-F0ABAF32179E:WINS_NBDD_server = ""
dhcp:subnets:_array_id:B03BAE3C-AB79-4108-9E5E-F0ABAF32179E:net_range_start = "192.168.210.200"
dhcp:subnets:_array_id:B03BAE3C-AB79-4108-9E5E-F0ABAF32179E:lease_max = 3600
dhcp:subnets:_array_id:B03BAE3C-AB79-4108-9E5E-F0ABAF32179E:dhcp_domain_search:_array_index:0 = "internal.krypted.lan"
dhcp:subnets:_array_id:B03BAE3C-AB79-4108-9E5E-F0ABAF32179E:descriptive_name = "192.168.210 Wi-Fi"
dhcp:subnets:_array_id:B03BAE3C-AB79-4108-9E5E-F0ABAF32179E:WINS_primary_server = ""
dhcp:subnets:_array_id:B03BAE3C-AB79-4108-9E5E-F0ABAF32179E:net_range_end = "192.168.210.253"
dhcp:subnets:_array_id:B03BAE3C-AB79-4108-9E5E-F0ABAF32179E:dhcp_ldap_url = _empty_array
dhcp:subnets:_array_id:B03BAE3C-AB79-4108-9E5E-F0ABAF32179E:WINS_node_type = "NOT_SET"
dhcp:subnets:_array_id:B03BAE3C-AB79-4108-9E5E-F0ABAF32179E:net_address = "192.168.210.0"
dhcp:subnets:_array_id:B03BAE3C-AB79-4108-9E5E-F0ABAF32179E:dhcp_enabled = yes
dhcp:subnets:_array_id:B03BAE3C-AB79-4108-9E5E-F0ABAF32179E:dhcp_domain_name = "internal.krypted.lan"
dhcp:subnets:_array_id:B03BAE3C-AB79-4108-9E5E-F0ABAF32179E:WINS_scope_id = ""
dhcp:subnet_defaults:logVerbosity = "MEDIUM"
dhcp:subnet_defaults:WINS_node_type_list:_array_index:0 = "BROADCAST_B_NODE"
dhcp:subnet_defaults:WINS_node_type_list:_array_index:1 = "HYBRID_H_NODE"
dhcp:subnet_defaults:WINS_node_type_list:_array_index:2 = "NOT_SET"
dhcp:subnet_defaults:WINS_node_type_list:_array_index:3 = "PEER_P_NODE"
dhcp:subnet_defaults:WINS_node_type_list:_array_index:4 = "MIXED_M_NODE"
dhcp:subnet_defaults:dhcp_domain_name = "no-dns-available.example.com"
dhcp:subnet_defaults:WINS_node_type = "NOT_SET"
dhcp:subnet_defaults:routers = _empty_dictionary
dhcp:subnet_defaults:logVerbosityList:_array_index:0 = "LOW"
dhcp:subnet_defaults:logVerbosityList:_array_index:1 = "MEDIUM"
dhcp:subnet_defaults:logVerbosityList:_array_index:2 = "HIGH"
dhcp:subnet_defaults:dhcp_domain_name_server:_array_index:0 = "192.168.210.201"
dhcp:subnet_defaults:selected_port_key = "en0"
dhcp:subnet_defaults:selected_port_key_list:_array_index:0 = "bridge0"
dhcp:subnet_defaults:selected_port_key_list:_array_index:1 = "en0"
dhcp:subnet_defaults:selected_port_key_list:_array_index:2 = "p2p0"
dhcp:subnet_defaults:selected_port_key_list:_array_index:3 = "en1"
dhcp:logging_level = "MEDIUM"
Notice the correlation between the uuid string in /etc/bootp.plist and the arrayid entry for each subnet/network/scope (too many terms referring to the same thing, ahhhh!). Using the serveradmin command you can configure a lot more than you can configure in the Server app gui. For example, on a dedicated DHCP server, you could increase logging level to HIGH (as root/with sudo of course):
serveradmin settings dhcp:logging_level = "MEDIUM"
You can also change settings within a scope. For example, if you realized that you were already using 192.168.210.200 and 201 for statically assigned IPs elsewhere you can go ahead and ssh into the server and change the first IP in a scope to 202 using the following (assuming the uuid of the domain is the same as in the previous examples):
serveradmin settings dhcp:subnets:_array_id:B03BAE3C-AB79-4108-9E5E-F0ABAF32179E:net_range_start = "192.168.210.202"
You can also obtain some really helpful information using the fullstatus verb with serveradmin:
serveradmin fullstatus dhcp
This output includes the number of active leases, path to log file (tailing that file is helpful when troubleshooting issues), static mappings (configured using the command line if needed), etc.
dhcp:state = "RUNNING"
dhcp:backendVersion = "10.5"
dhcp:timeOfModification = "2014-10-04 04:24:17 +0000"
dhcp:numDHCPActiveClients = 0
dhcp:timeOfSnapShot = "2014-10-04 04:24:19 +0000"
dhcp:dhcpLeasesArray = _empty_array
dhcp:logPaths:systemLog = "/var/log/system.log"
dhcp:numConfiguredStaticMaps = 1
dhcp:timeServiceStarted = "2014-10-04 04:24:17 +0000"
dhcp:setStateVersion = 1
dhcp:numDHCPLeases = 21
dhcp:readWriteSettingsVersion = 1
Once started, configure reservations using the /etc/bootptab file. This file should have a column for the name of a computer, the hardware type (1), the hwaddr (the MAC address) and ipaddr for the desired IP address of each entry:
# hostname hwtype hwaddr ipaddr bootfile
a.krypted.lan 1 00:00:00:aa:bb:cc 192.168.210.230
b.krypted.lan 1 00:00:00:aa:bb:cc 192.168.210.240
You can start and stop the service either using the serveradmin command:
serveradmin stop dhcp
serveradmin start dhcp
Or using the launchctl:
sudo /bin/launchctl unload -w /System/Library/LaunchDaemons/bootps.plist
sudo ; /bin/launchctl load -w /System/Library/LaunchDaemons/bootps.plist
On the clients, you can then use ifconfig followed by the getpacket verb and then an interface connected to the same network as the DHCP server in order to see the information supplied by the dhcp service, including the system that provided the DHCP lease to the client computers.