DHCP next-server

analog · Tue Nov 21, 2006 2:16 am

I am trying to use R.O.S. 2.9 to run a DHCP server for a DOCSIS cable modem system. I used to use linux running ISC DHCP, which worked fine. But the Mikrotik ROS is much better for other admin tasks. That said, I've run into a problem where the cable modems use the DHCP "next-server" option to know which TFTP server to download their config file from.

I've set DHCP option 66 and 67, but the modems are ignoring it, and instead using the next-server option, which the system is automatically setting to the IP of the Mikrotik that is the DHCP server. I need to specify another server on the internet. I've verified all of this from packet captures. See a snip here:

Capture read in Wireshark. IPs changed to protect the innocent.

DHCP (MT) server: 1.2.3.4
TFTP Server on internet: 4.3.2.1

Bootstrap Protocol
    Message type: Boot Reply (2)
    Hardware type: Ethernet
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x582f6a18
    Seconds elapsed: 0
    Bootp flags: 0x8000 (Broadcast)
        1... .... .... .... = Broadcast flag: Broadcast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0 (0.0.0.0)
    Your (client) IP address: 192.168.29.251 (192.168.29.251)
    Next server IP address: 1.2.3.4 (1.2.3.4)
    Relay agent IP address: 0.0.0.0 (0.0.0.0)
    Client MAC address: Motorola_46:11:22 (00:17:ee:46:11:22)
    Server host name not given
    Boot file name not given
    Magic cookie: (OK)
    Option: (t=53,l=1) DHCP Message Type = DHCP Offer
    Option: (t=54,l=4) Server Identifier = 1.2.3.4
    Option: (t=51,l=4) IP Address Lease Time = 1 hour
    Option: (t=1,l=4) Subnet Mask = 255.255.255.0
    Option: (t=2,l=2) Time Offset - length isn't 4
    Option: (t=3,l=4) Router = 192.168.29.1
    Option: (t=4,l=4) Time Server = 4.3.2.1
    Option: (t=7,l=4) Log Server = 4.3.2.1
    Option: (t=6,l=8) Domain Name Server
        Option: (6) Domain Name Server
        Length: 8
        Value: XXXXXXXXXXX
        IP Address: 5.6.7.8
        IP Address: 5.6.7.9
    Option: (t=66,l=11) TFTP Server Name = "4.3.2.1"
    Option: (t=67,l=3) Bootfile name = "conffile"
    End Option

Any way to change the next-server flag on the MT DHCP server?

Wed Nov 22, 2006 8:15 am

It is possible to override next-server for each ip subnet separately under
/ip dhcp-server network

analog · Thu Dec 14, 2006 11:16 pm

It is possible to override next-server for each ip subnet separately under
/ip dhcp-server network

Thanks normis I found it via SSH where you said it would be. It would very helpful if all DHCP options were available on winbox.

Even via SSH /ip dhcp-server network print doesn't show those values set. when I use /ip dhcp-server network get 0 next-server it just comes back with nothing

Fri Dec 15, 2006 8:01 am

I think, that on Winbox the following menu is available under IP ---> DHCP-server ---> Networks submenu.

analog · Thu Dec 21, 2006 8:52 pm

Yeah, that menu is there, and contains most of the info, but does not display anything about next-server nor boot-file-name