Due to an unplanned outage of my main ISP, I had to get a mobile data SIM in a hurry, to use as an LTE backup uplink for my Mikrotik hAP ac³ (the whole setup of which I’ll describe one day). Given the price discrepancy of those, I wanted to make every transferred byte count: No unnecessary update fetching or immediate download of high-res sepia-toned photos of bulldogs in tutus.

Android can advertise itself as a metered network to its (Android) clients, so how do I do the same with Router OS 7?

(router agnostic) tl;dr:

1. Make DHCP Option 43 (Vendor-Specific Option) contain the string ANDROID_METERED;
2. The option should be sent even if not requested by the client (not standard compliant, but doesn’t hurt).

To set this up with Router OS, we need to manipulate three types of objects: DHCP options, DHCP options sets, and either DHCP servers or DHCP network ranges.

First, we define the DHCP option itself.

/ip/dhcp-server/option/add name=metered code=43 value="'ANDROID_METERED'"  force=yes

Two things to note here:

1. As of ROS 7.6, the force option is not available in WebFig, so it needs to be defined on the CLI either when adding the option, or later with set. [Update: as of ROS 7.7, the option is available in WebFig]
2. For the value to be recognised as a string and encoded properly, it needs to be given with enclosing single quotes, then wrapped in double quotes (otherwise, a cryptic failure: Unknown data type! error will be given). See Properties in the doc.

Once defined, the option needs to be added to a set. While no set is defined by default, it seems that as soon as a set exists, WebFig picks it up next time any server or network is edited. I opted to create a nondescript default set, with the idea that I can add/remove the metered option as needed, and have this reflected for all networks using the set.

/ip/dhcp-server/option/sets/add name=default options=metered

All that is left is to attach the set to the desired network(s) or server(s). As I was feeling zealous, I did both, but either one of them should be sufficient.

/ip/dhcp-server/set dhcp-option-set=default [find where name="dhcp"]
/ip/dhcp-server/network/set dhcp-option-set=default [find where address=192.168.88.0/24]

And this worked nicely! On the next DHCP response, the Vendor-Option was present with the right data.

23:54:20.862628 IP (tos 0x0, ttl 16, id 0, offset 0, flags [none], proto UDP (17), length 347)
    _gateway.bootps > laptop.example.net.bootpc: [udp sum ok] BOOTP/DHCP, Reply, length 319, xid 0x00000000, Flags [none] (0x0000)
	  Your-IP laptop.example.net
	  Server-IP server.example.net
	  Client-Ethernet-Address XX:XX:XX:XX:XX:XX (oui Unknown)
	  Vendor-rfc1048 Extensions
	    Magic Cookie 0x00000000
	    DHCP-Message (53), length 1: ACK
	    Subnet-Mask (1), length 4: 255.255.255.0
	    Default-Gateway (3), length 4: _gateway
	    Domain-Name-Server (6), length 8: _gateway,192.168.88.211
	    Domain-Name (15), length 16: "example.net"
	    NTP (42), length 4: _gateway
	    Vendor-Option (43), length 15: 65.78.68.82.79.73.68.95.77.69.84.69.82.69.68
	    Lease-Time (51), length 4: 86399
	    Server-ID (54), length 4: _gateway
	    END (255), length 0

This in turn was sufficient for Android devices to consider the network as metered. Job done!