JUNOS subscriber management devices, such as MX series, can be used as DHCP-Local-Server to provide internet service for end clients. The DHCP subscriber can get its ip address and gateway either from a configure dynamic address pool or from RADIUS.

It is a challenge to assign a static IP address by RADIUS to the DHCP subscriber and make it work. As different residential-gateway devices(RGW, or say CPE) have different behaviors for DHCP, some of them do not support a netmask of 255.255.255.255, some of them do not support a gateway ip address not in the same subnet.

This article describes a solution on how to use RADIUS standard and vendor specific attributes to send the suitable DHCP options to work with most of the RGW devices.

In this article, MX will be used to represent JUNOS subscriber management device. RGW will be used to represent the end client’s modem.

This article describes a solution on how to use RADIUS standard and vendor specific attributes to send the suitable DHCP options to work with most of the RGW devices.

Usually, a standard RADIUS IETF attribute 8 and attribute 9 can be used together to assign a static IP address to the subscriber.

But the gateway IP address is an interesting part, some of the RGWs do not support a gateway in a different subnet.

There has 2 ways to handle this.

Option 1: Create pools on the MX for each subnet with a gateway IP set. The MX automatically selects the pool based on the Framed-IP-Address. The netmask and gateway IP address will also be retrieved from this pool.

Option 2: JunOS lets you set a vendor specific attribute [26-55]DHCP-Options in Access-Accept, and send raw RADIUS attributes. It supports setting the Router DHCP option that way. 

This article will focus on the option 2.

Here is from Juniper public document https://www.juniper.net/documentation/us/en/software/junos/subscriber-mgmt-sessions/topics/topic-map/radius-std-attributes-vsas-support.html

An example of freeRADIUS configure as this:

DEFAULT User-Name == "DHCP.1", Cleartext-Password := "123"

    ERX-Virtual-Router-Name = "default:vrf-broadband-cgnat",

    ERX-Local-Loopback-Interface = lo0.21,

    Framed-IP-Address = 203.46.170.52,

    Framed-IP-Netmask = 255.255.255.0,

    ERX-Dhcp-Options = 0x0304d232e001,

    Session-Timeout = 600,

    Delegated-IPv6-Prefix = 2402:1b80:14:3400::/56,

    Framed-IPv6-Prefix = 2402:1b80:12:3::34/128,band-cgnat-pd-v6-1,

    ERX-Primary-Dns = 111.222.1.1,

    ERX-Secondary-Dns = 111.222.2.2,

    ERX-Ipv6-Primary-DNS = 2403:4800:1::1,

    ERX-Ipv6-Secondary-DNS = 2403:4800:2::2,

    Service-Type = "Framed"

Explanation of the “ERX-Dhcp-Options = 0x0304d232e001”, as below.

0x — indicates it’s a hex string format. 

     Noted, some RADIUS applications may treat it as port the string, so be careful and check with the RADIUS document.

03 — DHCP option 3,

04 — length of this option,

232e001 — the IP address 210.50.224.1 in hex format. Noted, in real world, it should be a gateway IP address in the same subnet as the framed-ip-address, like 203.46.170.1.

The output as following.

op@MX> show subscribers 

Interface             IP Address/VLAN ID           User Name           LS:RI

demux0.3221225474        0x8100.76 0x8100.1           VLAN.1          default:vrf-broadband-cgnat

demux0.3221225474        203.46.170.52               DHCP.1          default:vrf-broadband-cgnat

op@MX> show subscribers extensive  

Type: DHCP

User Name: DHCP.1

IP Address: 203.46.170.52

IP Netmask: 255.255.255.0

Primary DNS Address: 111.222.1.1

Secondary DNS Address: 111.222.2.2

Logical System: default

Routing Instance: vrf-broadband-cgnat

Interface: demux0.3221225474

Interface type: Static

Interface Set: ifset-vt-nsw-ps0-1

Underlying Interface: demux0.3221225474

Dynamic Profile Name: DHCP

Dynamic Profile Version: 1

MAC Address: 00:10:94:02:9c:0b

State: Active

Radius Accounting ID: 7

Session ID: 7

PFE Flow ID: 46

Stacked VLAN Id: 76

VLAN Id: 1

Agent Remote ID: 1

Login Time: 2024-12-12 11:11:40 AEDT

Service Sessions: 1

DHCP Options: len 85

35 01 01 39 02 02 40 3d 07 01 00 10 94 02 9c 0b 33 04 00 00

00 3c 0c 34 63 6c 69 65 6e 74 5f 50 6f 72 74 20 2f 2f 34 2f

35 20 5b 36 34 3a 43 33 3a 44 36 3a 35 33 3a 32 37 3a 34 30

2f 78 65 2d 30 2f 31 2f 34 3a 30 5d 2d 36 2d 30 37 07 01 03

06 0f 21 2c 33

Server DHCP Options: len 6        

03 04 d2 32 e0 01

DHCP Header: len 44

01 01 06 00 00 56 00 45 00 00 80 00 00 00 00 00 00 00 00 00

00 00 00 00 00 00 00 00 00 10 94 02 9c 0b 00 00 00 00 00 00

00 00 00 00

Accounting interval: 3600

Access Line Attributes:

 Agent Remote ID: 1

Dynamic configuration: 

 junos-cos-shaping-rate: 115M

 junos-cos-shaping-rate-burst: 143750

 junos-interface-set-name: ifset-vt-nsw-ps0-1

No.   Time      Source        Destination      Protocol Length Info

   6 22.342323   100.72.0.1      255.255.255.255    DHCP   338  DHCP Offer  - Transaction ID 0x560045

Frame 6: 338 bytes on wire (2704 bits), 338 bytes captured (2704 bits) on interface \\.\pipe\view_capture_10-219-45-203_4_5_12122024_111231, id 0

Ethernet II, Src: 1e:9c:8c:18:c8:73 (1e:9c:8c:18:c8:73), Dst: Broadcast (ff:ff:ff:ff:ff:ff)

802.1Q Virtual LAN, PRI: 0, DEI: 0, ID: 76

802.1Q Virtual LAN, PRI: 0, DEI: 0, ID: 1

Internet Protocol Version 4, Src: 100.72.0.1, Dst: 255.255.255.255

User Datagram Protocol, Src Port: 67, Dst Port: 68

Dynamic Host Configuration Protocol (Offer)

  Message type: Boot Reply (2)

  Hardware type: Ethernet (0x01)

  Hardware address length: 6

  Hops: 0

  Transaction ID: 0x00560045

  Seconds elapsed: 0

  Bootp flags: 0x8000, Broadcast flag (Broadcast)

  Client IP address: 0.0.0.0

  Your (client) IP address: 203.46.170.52

  Next server IP address: 0.0.0.0

  Relay agent IP address: 0.0.0.0

  Client MAC address: Performa_02:9c:0b (00:10:94:02:9c:0b)

  Client hardware address padding: 00000000000000000000

  Server host name not given

  Boot file name not given

  Magic cookie: DHCP

   Option: (53) DHCP Message Type (Offer)

    Length: 1

    DHCP: Offer (2)

  Option: (51) IP Address Lease Time

    Length: 4

    IP Address Lease Time: (600s) 10 minutes

  Option: (1) Subnet Mask (255.255.255.0)

    Length: 4

    Subnet Mask: 255.255.255.0

  Option: (54) DHCP Server Identifier (100.72.0.1)

    Length: 4

    DHCP Server Identifier: 100.72.0.1

  Option: (3) Router

    Length: 4

    Router: 210.50.224.1

  Option: (6) Domain Name Server

    Length: 8

    Domain Name Server: 111.222.1.1

    Domain Name Server: 111.222.2.2

  Option: (82) Agent Information Option

    Length: 3

    Option 82 Suboption: (2) Agent Remote ID

  Option: (255) End

    Option End: 255

  Padding: 00

Noted, in this example, it’s just to demonstrate how to use [26-55] attribute to return the DHCP options. As far as the router ip address, it actually should be in the same subnet as the framed-ip-address.

For DHCPv6, [26-207] can be used.