Skip to content
@jennifer/routeros

DHCP: server, leases, and the WAN client

File: src/topics/dhcp.j. Paths: /ip/pool, /ip/dhcp-server, /ip/dhcp-server/network, /ip/dhcp-server/lease, /ip/dhcp-client (IP_POOL_PATH, DHCP_SERVER_PATH, DHCP_NETWORK_PATH, DHCP_LEASE_PATH, DHCP_CLIENT_PATH).

Background

DHCP has two sides on a router:

  • Server (LAN side): hands addresses, gateway, and DNS to your clients. RouterOS needs three objects for this - an address pool (the range to hand out), the server itself (bound to an interface), and a network entry (gateway + DNS the clients are told). Forgetting one of the three is the classic beginner failure; routeros creates them together.
  • Client (WAN side): asks the ISP for the router's own public address. See "WAN" below.

DHCP server

jennifer
# the interface needs an address inside the network first
mt.addIpAddress($c, "192.168.77.1/24", "brlan");

mt.setupDhcp($c, "dhcplan", "brlan",
    "192.168.77.0/24",       # the network
    "192.168.77.1",          # gateway told to clients (the router itself)
    "192.168.77.10",         # range start
    "192.168.77.199",        # range end
    "192.168.77.1");         # DNS told to clients (comma list allowed)

Everything is cross-checked before anything is created: the gateway and both range ends must lie inside the network, the interface must exist, DNS entries must be IP addresses. The pool shares the server's name so the whole thing can be undone:

jennifer
mt.teardownDhcp($c, "dhcplan", "192.168.77.0/24");

(each of the three pieces is removed if present - a half-finished setup tears down too).

Other server-side functions: dhcpServers(c) → list of DhcpServer.

Leases and reservations

jennifer
# who is on the network right now?
def leases as list of mt.DhcpLease init mt.dhcpLeases($c);
for (def l in $leases) {
    io.printf("%s -> %s (%s) %s\n", $l.address, $l.mac, $l.hostName, $l.status);
}

# always give the printer the same address
mt.addStaticLease($c, "192.168.77.50", "AA:BB:CC:DD:EE:FF", "printer");

# and take the reservation away again
mt.removeLeaseByMac($c, "AA:BB:CC:DD:EE:FF");

DhcpLease.dynamic distinguishes an automatic lease from a static reservation. MAC addresses are validated (six hex pairs) and uppercased for you.

WAN: the DHCP client

jennifer
mt.setupWan($c, "ether1");                  # defaults: ISP route + ISP DNS
mt.addMasquerade($c, "ether1", "lan to internet");

def wan as mt.DhcpClient init mt.wanStatus($c, "ether1");
if ($wan.bound) {
    io.printf("online as %s via %s, lease renews in %s\n",
        $wan.address, $wan.gateway, $wan.expiresAfter);
}

setupWan is idempotent (an existing client on the interface is reused). setupWanWith(c, iface, usePeerDns, addDefaultRoute) opts out of adopting the ISP's DNS or route - useful for a backup uplink whose route you manage with addRouteWithDistance (see routing.md). Also: renewWan(c, iface) (fresh lease), dhcpClients(c), removeDhcpClient / enableDhcpClient / disableDhcpClient(c, iface).

DhcpClient.addDefaultRoute stays a string because RouterOS allows three values (yes / no / special-classless).

Pitfalls

  • The interface must have an address inside the served network - clients are told a gateway they can only reach if the router is actually there. setupDhcp's docblock reminds you; the snippet above shows the order.
  • One DHCP server per interface; RouterOS refuses a second.
  • A static lease for a device with an active dynamic lease takes effect at the next renewal, not instantly.
  • On the WAN side, bound == false with status searching... usually means the ISP link is down or the modem is not bridging.
  • Do not run a DHCP server on the WAN interface. RouterOS will happily let you annoy your ISP.