Manual:IP/DHCP Server
Applies to RouterOS: v3, v4, v5+
Summary
Standards: RFC 2131, RFC 3315, RFC 3633
Package: dhcp
The DHCP (Dynamic Host Configuration Protocol) is used for the easy distribution of IP addresses in a network. The MikroTik RouterOS implementation includes both server and client parts and is compliant with RFC 2131.
The router supports an individual server for each Ethernet-like interface. The MikroTik RouterOS DHCP server supports the basic functions of giving each requesting client an IP address/netmask lease, default gateway, domain name, DNS-server(s) and WINS-server(s) (for Windows clients) information (set up in the DHCP networks submenu)
In order for the DHCP server to work, IP pools must also be configured (do not include the DHCP server's own IP address into the pool range) and the DHCP networks.
It is also possible to hand out leases for DHCP clients using the RADIUS server; the supported parameters for a RADIUS server is as follows:
Access-Request:
- NAS-Identifier - router identity
- NAS-IP-Address - IP address of the router itself
- NAS-Port - unique session ID
- NAS-Port-Type - Ethernet
- Calling-Station-Id - client identifier (active-client-id)
- Framed-IP-Address - IP address of the client (active-address)
- Called-Station-Id - name of DHCP server
- User-Name - MAC address of the client (active-mac-address)
- Password - ""
Access-Accept:
- Framed-IP-Address - IP address that will be assigned to client
- Framed-Pool - ip pool from which to assign ip address to client
- Rate-Limit - Datarate limitation for DHCP clients. Format is: rx-rate[/tx-rate] [rx-burst-rate[/tx-burst-rate] [rx-burst-threshold[/tx-burst-threshold] [rx-burst-time[/tx-burst-time][priority] [rx-rate-min[/tx-rate-min]]]]. All rates should be numbers with optional 'k' (1,000s) or 'M' (1,000,000s). If tx-rate is not specified, rx-rate is as tx-rate too. Same goes for tx-burst-rate and tx-burst-threshold and tx-burst-time. If both rx-burst-threshold and tx-burst-threshold are not specified (but burst-rate is specified), rx-rate and tx-rate are used as burst thresholds. If both rx-burst-time and tx-burst-time are not specified, 1s is used as default. Priority takes values 1..8, where 1 implies the highest priority, but 8 - the lowest. If rx-rate-min and tx-rate-min are not specified rx-rate and tx-rate values are used. The rx-rate-min and tx-rate-min values can not exceed rx-rate and tx-rate values.
- Ascend-Data-Rate - tx/rx data rate limitation if multiple attributes are provided, first limits tx data rate, second - rx data rate. If used together with Ascend-Xmit-Rate, specifies rx rate. 0 if unlimited
- Ascend-Xmit-Rate - tx data rate limitation. It may be used to specify tx limit only instead of sending two sequential Ascend-Data-Rate attributes (in that case Ascend-Data-Rate will specify the receive rate). 0 if unlimited
- Session-Timeout - max lease time (lease-time)
Note: DHCP server requires a real interface to receive raw ethernet packets. If the interface is a Bridge interface, then the Bridge must have a real interface attached as a port to that bridge which will receive the raw ethernet packets. It cannot function correctly on a dummy (empty bridge) interface.
Quick Setup Guide
RouterOS has a built in command that lets you easily set up a DHCP server. Let's say we want to configure DHCP server on ether1 interface to lease addresses from 192.168.0.2 to 192.168.0.254 which belong to the 192.168.0.0/24 network. The gateway and DNS server is 192.168.0.1.
From /ip dhcp-server menu run setup command and follow instructions:
[admin@MikroTik] ip dhcp-server> setup
Select interface to run DHCP server on
dhcp server interface: ether1
Select network for DHCP addresses
dhcp address space: 192.168.0.0/24
Select gateway for given network
gateway for dhcp network: 192.168.0.1
Select pool of ip addresses given out by DHCP server
addresses to give out: 192.168.0.2-192.168.0.254
Select DNS servers
dns servers: 192.168.0.1
Select lease time
lease time: 3d
[admin@MikroTik] ip dhcp-server>
The wizard has made the following configuration based on the answers above:
[admin@MikroTik] ip dhcp-server> print Flags: X - disabled, I - invalid # NAME INTERFACE RELAY ADDRESS-POOL LEASE-TIME ADD-ARP 0 dhcp1 ether1 0.0.0.0 dhcp_pool1 3d no [admin@MikroTik] ip dhcp-server> network print # ADDRESS GATEWAY DNS-SERVER WINS-SERVER DOMAIN 0 192.168.0.0/24 192.168.0.1 192.168.0.1 [admin@MikroTik] ip dhcp-server> /ip pool print # NAME RANGES 0 dhcp_pool1 192.168.0.2-192.168.0.254 [admin@MikroTik] ip dhcp-server>
IPv6
Starting from v5.8 RouterOS supports IPv6 prefix delegation according to RFC 3315 and RFC 3633.
Starting from v5.9, DHCPv6 server configuration was moved to /ipv6 sub-menu. Read-more >>
General
Sub-menu: /ip dhcp-server
| Property | Description |
|---|---|
| add-arp (yes | no; Default: no) | Whether to add dynamic ARP entry. If set to no either ARP mode should be enabled on that interface or static ARP entries should be administratively defined in /ip arp submenu. |
| address-pool (string | static-only; Default: static-only) | IP pool, from which to take IP addresses for the clients. If set to static-only, then only the clients that have a static lease (added in lease submenu) will be allowed. |
| always-broadcast (yes | no; Default: no) | Always send replies as broadcasts even if destination IP is known. Will add additional load on L2 network. |
| authoritative (after-10sec-delay | after-2sec-delay | yes | no; Default: after-2sec-delay) | Option changes the way how server responds to DHCP requests:
|
| bootp-lease-time (forever | lease-time | time; Default: ) | Accepts two predefined options or time value:
|
| bootp-support (none | static | dynamic; Default: static) | Support for BOOTP clients:
|
| conflict-detection (yes | no; Default: ) | |
| delay-threshold (time | none; Default: none) | If secs field in DHCP packet is smaller than delay-threshold, then this packet is ignored. If set to none - there is no threshold (all DHCP packets are processed) |
| dhcp-option-set (name | none; Default: ) | Use custom set of DHCP options defined in option sets menu. |
| insert-queue-before (bottom | first | name; Default: ) | Specify where to place dynamic simple queue entries for static DCHP leases with rate-limit parameter set. |
| interface (string; Default: ) | Interface on which server will be running. |
| lease-script (string; Default: "") | Script that will be executed after lease is assigned or de-assigned. Internal "global" variables that can be used in the script:
|
| lease-time (time; Default: 10m) | The time that a client may use the assigned address. The client will try to renew this address after a half of this time and will request a new address after time limit expires. |
| name (string; Default: ) | Reference name |
| relay (IP; Default: 0.0.0.0) | The IP address of the relay this DHCP server should process requests from:
|
| src-address (IP; Default: 0.0.0.0) | The address which the DHCP client must send requests to in order to renew an IP address lease. If there is only one static address on the DHCP server interface and the source-address is left as 0.0.0.0, then the static address will be used. If there are multiple addresses on the interface, an address in the same subnet as the range of given addresses should be used. |
| use-framed-as-classless (yes | no; Default: ) | |
| use-radius (yes | no | accounting; Default: no) | Whether to use RADIUS server:
|
Menu specific commands
| Property | Description |
|---|---|
| setup () | Start DHCP server setup wizard, which guides you through the steps to easily create all necessary configuration. Read more>> |
Lease Store Configuration
Sub-menu: /ip dhcp-server config
This sub-menu allows the configuration of how often the DHCP leases will be stored on disk. If they would be saved on disk on every lease change, a lot of disk writes would happen which is very bad for Compact Flash (especially, if lease times are very short). To minimize writes on disk, all changes are saved on disk every store-leases-disk seconds.
Additionally leases are always stored on disk on graceful shutdown and reboot.
Note: Manual changes to leases - addition/removal of static lease, removal of dynamic lease will cause changes to be pushed for this lease to storage.
This sub-menu has only one configurable property:
| Property | Description |
|---|---|
| store-leases-disk (time | immediately | never; Default: 5m) | How frequently lease changes should be stored on disk |
Networks
Sub-menu: /ip dhcp-server network
| Property | Description |
|---|---|
| address (IP/netmask; Default: ) | the network DHCP server(s) will lease addresses from |
| boot-file-name (string; Default: ) | Boot file name |
| caps-manager (string; Default: ) | Comma-separated list of IP addresses for one or more CAPsMan system managers. DHCP Option 138 (capwap) will be used. |
| dhcp-option (string; Default: ) | Add additional DHCP options from option list. |
| dns-server (string; Default: ) | the DHCP client will use these as the default DNS servers. Two comma-separated DNS servers can be specified to be used by the DHCP client as primary and secondary DNS servers |
| domain (string; Default: ) | The DHCP client will use this as the 'DNS domain' setting for the network adapter. |
| gateway (IP; Default: 0.0.0.0) | The default gateway to be used by DHCP Client. |
| netmask (integer: 0..32; Default: 0) | The actual network mask to be used by DHCP client. If set to '0' - netmask from network address will be used. |
| next-server (IP; Default: ) | IP address of next server to use in bootstrap. |
| ntp-server (IP; Default: ) | the DHCP client will use these as the default NTP servers. Two comma-separated NTP servers can be specified to be used by the DHCP client as primary and secondary NTP servers |
| wins-server (IP; Default: ) | The Windows DHCP client will use these as the default WINS servers. Two comma-separated WINS servers can be specified to be used by the DHCP client as primary and secondary WINS servers |
Leases
Sub-menu: /ip dhcp-server lease
DHCP server lease submenu is used to monitor and manage server's leases. The issued leases are showed here as dynamic entries. You can also add static leases to issue a specific IP address to a particular client (identified by MAC address) .
Generally, the DHCP lease it allocated as follows:
- an unused lease is in waiting state
- if a client asks for an IP address, the server chooses one
- if the client receives a statically assigned address, the lease becomes offered, and then bound with the respective lease time
- if the client receives a dynamic address (taken from an IP address pool), the router sends a ping packet and waits for answer for 0.5 seconds. During this time, the lease is marked testing
- in the case where the address does not respond, the lease becomes offered and then bound with the respective lease time
- in other case, the lease becomes busy for the lease time (there is a command to retest all busy addresses), and the client's request remains unanswered (the client will try again shortly)
A client may free the leased address. The dynamic lease is removed, and the allocated address is returned to the address pool. But the static lease becomes busy until the client reacquires the address.
Note: IP addresses assigned statically are not probed!
Properties
| Property | Description |
|---|---|
| address (IP; Default: ) | Specify IP address (or ip pool) for static lease. If set to 0.0.0.0 - pool from server will be used |
| address-list (string; Default: ) | Address list to which address will be added if lease is bound. |
| always-broadcast (yes | no; Default: ) | Send all replies as broadcasts |
| block-access (yes | no; Default: no) | Block access for this client |
| client-id (string; Default: ) | If specified, must match DHCP 'client identifier' option of the request |
| lease-time (time; Default: 0s) | Time that the client may use the address. If set to 0s lease will never expire. |
| mac-address (MAC; Default: 00:00:00:00:00:00) | If specified, must match the MAC address of the client |
| src-mac-address (MAC; Default: ) | Source MAC address |
| use-src-mac (MAC; Default: ) | Use this source MAC address instead |
Warning: The always-broadcast parameter will dynamically change. For the initial DHCP discover/offer/request/ack cycle a broadcast MAC address is going to be used, for lease renewal (request and ack) an unicast MAC address will be used. In case the DHCP Server keeps receiving DHCP requests while DHCP offer has been sent, then the always-broadcast parameter will be turned on dynamically until the DHCP lease has been renewed successfully.
Read only properties
| Property | Description |
|---|---|
| active-address (IP) | Actual IP address for this lease |
| active-client-id (string) | Actual client-id of the client |
| active-mac-address (MAC) | Actual MAC address of the client |
| active-server (list) | Actual dhcp server, which serves this client |
| agent-circuit-id (string) | Circuit ID of DHCP relay agent. If each character should be valid ASCII text symbol or else this value is displayed as hex dump. |
| agent-remote-id (string) | Remote ID, set by DHCP relay agent |
| blocked ( flag ) | Whether the lease is blocked |
| expires-after (time) | Time until lease expires |
| host-name (text) | Shows host name option from last received DHCP request |
| radius (yes | no) | Shows if this dynamic lease is authenticated by RADIUS or not |
| rate-limit (string) | Sets rate limit for active lease. Format is: rx-rate[/tx-rate] [rx-burst-rate[/tx-burst-rate] [rx-burst-threshold[/tx-burst-threshold] [rx-burst-time[/tx-burst-time]]]]. All rates should be numbers with optional 'k' (1,000s) or 'M' (1,000,000s). If tx-rate is not specified, rx-rate is as tx-rate too. Same goes for tx-burst-rate and tx-burst-threshold and tx-burst-time. If both rx-burst-threshold and tx-burst-threshold are not specified (but burst-rate is specified), rx-rate and tx-rate is used as burst thresholds. If both rx-burst-time and tx-burst-time are not specified, 1s is used as default |
| server (string) | Server name which serves this client |
| status (waiting | testing | authorizing | busy | offered | bound) | Lease status:
|
Menu specific commands
| Property | Description |
|---|---|
| check-status (id) | Check status of a given busy dynamic lease, and free it in case of no response |
| make-static (id) | Convert a dynamic lease to a static one |
Alerts
Sub-menu: /ip dhcp-server alert
To find any rogue DHCP servers as soon as they appear in your network, DHCP Alert tool can be used. It will monitor the ethernet interface for all DHCP replies and check if this reply comes from a valid DHCP server. If a reply from an unknown DHCP server is detected, alert gets triggered:
[admin@MikroTik] ip dhcp-server alert>/log print
00:34:23 dhcp,critical,error,warning,info,debug dhcp alert on Public:
discovered unknown dhcp server, mac 00:02:29:60:36:E7, ip 10.5.8.236
[admin@MikroTik] ip dhcp-server alert>
When the system alerts about a rogue DHCP server, it can execute a custom script.
As DHCP replies can be unicast, the 'rogue dhcp detector' may not receive any offer to other dhcp clients at all. To deal with this, the rogue dhcp detector acts as a dhcp client as well - it sends out dhcp discover requests once a minute
Properties
| Property | Description |
|---|---|
| alert-timeout (none | time; Default: none) | Time after which alert will be forgotten. If after that time the same server is detected, new alert will be generated. If set to none timeout will never expire. |
| interface (string; Default: ) | Interface, on which to run rogue DHCP server finder. |
| on-alert (string; Default: ) | Script to run, when an unknown DHCP server is detected. |
| valid-server (string; Default: ) | List of MAC addresses of valid DHCP servers. |
Read only properties
| Property | Description |
|---|---|
| unknown-server (string) | List of MAC addresses of detected unknown DHCP servers. Server is removed from this list after alert-timeout |
Menu specific commands
| Property | Description |
|---|---|
| reset-alert (id) | Clear all alerts on an interface |
DHCP Options
Sub-menu: /ip dhcp-server option
With help of DHCP Option list, it is possible to define additional custom options for DHCP Server to advertise. Option precedence is as follows:
- radius,
- lease,
- server,
- network.
This is the order in which client option request will be filled in.
According to the DHCP protocol, a parameter is returned to the DHCP client only if it requests this parameter, specifying the respective code in DHCP request Parameter-List (code 55) attribute. If the code is not included in Parameter-List attribute, DHCP server will not send it to the DHCP client.
Properties
| Property | Description |
|---|---|
| code (integer:1..254; Default: ) | dhcp option code. All codes are available at http://www.iana.org/assignments/bootp-dhcp-parameters |
| name (string; Default: ) | Descriptive name of the option |
| value (string; Default: ) | Parameter's value.
RouterOS has predefined variables that can be used:
For example if HOSTNAME is 'kvm', then raw value will be 0x0176617264736b766d. |
| raw-value (HEX string ) | Read only field which shows raw dhcp option value (the format actually sent out) |
Example
Classless Route
A classless route adds specified route in clients routing table. In our example, it will add
- dst-address=160.0.0.0/24 gateway=10.1.101.1
- dst-address=0.0.0.0/0 gateway=10.1.101.1
According to RFC 3442: The first part is the netmask ("18" = netmask /24). Second part is
significant part of destination network ("A00000" = 160.0.0). Third part is IP address of
gateway ("0A016501" = 10.1.101.1). Then There are parts of the default route, destination netmask (0x00 = 0.0.0.0/0) followed by default route (0x0A016501 = 10.1.101.1)
/ip dhcp-server option add code=121 name=classless value=0x18A000000A016501000A016501 /ip dhcp-server network set 0 dhcp-option=classless
Result:
[admin@MikroTik] /ip route> print Flags: X - disabled, A - active, D - dynamic, C - connect, S - static, r - rip, b - bgp, o - ospf, m - mme, B - blackhole, U - unreachable, P - prohibit # DST-ADDRESS PREF-SRC GATEWAY DISTANCE 0 ADS 0.0.0.0/0 10.1.101.1 0 1 ADS 160.0.0.0/24 10.1.101.1 0
Much more robust way would be to use built in variables, previous example can be rewritten as:
/ip dhcp-server option add name=classless code=121 value="0x18A00000\$(NETWORK_GATEWAY)0x00\$(NETWORK_GATEWAY)"
Auto Proxy Config
/ip dhcp-server option add code=252 name=auto-proxy-config value="'http://autoconfig.something.lv/wpad.dat'"
DHCP Option Sets
Sub-menu: /ip dhcp-server option sets
This menu allows to combine multiple options in option sets, which later can be used to override default DHCP server option set.
| Property | Description |
|---|---|
| name (string; Default: ) | Name of the option set. |
| options (list; Default: ) | List of options defined in options menu. |
Configuration Examples
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