How to add DHCP Option 66 to your Router / Firewall for 3CX

How to add DHCP Option 66 to your Router / Firewall for 3CX

DHCP Option 66 — Vendor Quick Reference (15 platforms)

About DHCP Option 66

Option 66 (TFTP Server Name, RFC 2132) is a DHCP option the server hands to a client in its lease, telling the client where to fetch a configuration or boot file. It's most commonly used for zero-touch VoIP phone provisioning — a factory-fresh phone gets an IP and the URL of its PBX/provisioning server in the same DHCP exchange, then downloads its config and firmware automatically. Despite the name, the value is a plain string — an IP, hostname, or full URL — and most modern firmware will accept all three.

Platform

Where (GUI path / config context)

How (steps / CLI)

Value format

ASUS (AsusWRT / Merlin)

No GUI support — requires JFFS + SSH (Merlin firmware recommended)

Enable JFFS and SSH under Administration → System.

SSH in and edit /jffs/configs/dnsmasq.conf.add:

dhcp-option=lan,66,pbx.example.com

Reboot to apply.

Plain string (IP, host, or URL)

Cisco IOS / IOS XE

Global config → ip dhcp pool

ip dhcp pool VOICE

  network 10.10.10.0 255.255.255.0

  option 66 ascii "http://pbx.example.com/"

Or option 66 ip 10.10.10.5 for an IP-typed value.

ascii (string/URL) or ip (TFTP server IP)

Cisco Meraki MX / MS

Security & SD-WAN → DHCP (or Switch → Routing & DHCP)

Click Add a DHCP option.

Option: Custom. Code: 66. Type: Text (hardcoded — Option 66 is always a string).

For PXE, prefer the dedicated Boot next-server / Boot filename fields.

Text only (IP entered as a string is fine)

DrayTek Vigor

LAN → General Setup → [LAN] → DHCP Server Option (button)

Click DHCP Server Option, Add a new option.

Option Number: 66, Data Type: ASCII, Data: your URL/host.

Telnet alternative: srv dhcp tftpserv <ip-or-host> (or msubnet tftp for LAN2–LAN6).

ASCII string or IP

Fortinet FortiGate

CLI: config system dhcp server (also via Network → Interfaces → DHCP)

config system dhcp server

  edit <id>

    set tftp-server "pbx.example.com"

For full URLs / non-standard values, use config options with code 66, type string.

Hostname/IP in tftp-server; string via custom option

Juniper SRX (Junos)

[edit system services dhcp-local-server] / [edit access address-assignment]

set access address-assignment pool VOICE family inet dhcp-attributes \

  next-server 10.10.10.5

set access address-assignment pool VOICE family inet dhcp-attributes \

  option 66 string "pbx.example.com"

string (URL/hostname) or next-server for IP

MikroTik (RouterOS)

IP → DHCP Server → Options and Networks tabs

/ip dhcp-server option add code=66 name=tftp value="'pbx.example.com'"

/ip dhcp-server network set [find] dhcp-option=tftp

Note: wrap string values in single quotes inside double quotes.

Quoted string ('pbx.example.com' or URL)

OPNsense

Services → ISC DHCPv4 → [Interface] (or Dnsmasq/Kea DHCP)

ISC: enter value in TFTP Server field, or add Additional BOOTP/DHCP Options with number 66.

Dnsmasq/Kea: Services → Dnsmasq DNS & DHCP → DHCP options → add option 66.

IP, hostname, or URL (Text type)

Palo Alto Networks (PAN-OS)

Network → DHCP → [Interface] → Options

Add a Custom DHCP option.

Name: TFTP Server. Option Code: 66.

Option Type: IP Address (RFC 2132) or ASCII (for URLs).

Enter the value and commit.

IP for hostname/IP; ASCII for URL

pfSense

Services → DHCP Server → [Interface]

Scroll to TFTP Server (under Other Options) and enter the value.

For full URLs (Polycom-style), use Additional BOOTP/DHCP Options: Number 66, Type Text.

IP / hostname; URL via custom Text option

SonicWall (SonicOS)

Network → System → DHCP Server → [Scope] → Advanced

Create a DHCP Option Object: Object → DHCP Option → Add.

Option Number: 66, Option Name: TFTP Server, Type: IP Address / FQDN.

Edit the scope → Advanced tab → select the option object under DHCP Generic Option Group.

IP Address or FQDN object

Sophos XG / Firewall (SFOS)

Console (option 4 — Device Console). GUI has no Option 66 field.

system dhcp dhcp-options binding add \

  dhcpname Default_DHCP_Server \

  optionname TFTP_Server_Name(66) \

  value 'pbx.example.com'

Strings in single quotes; bare IP needs no quotes.

Quoted string or plain IP

Ubiquiti EdgeRouter (EdgeOS)

CLI configuration mode

configure

set service dhcp-server shared-network-name LAN \

  subnet 192.168.1.0/24 tftp-server-name \

  'http://pbx.example.com/'

commit ; save ; exit

Quoted string (URL, hostname, or IP)

UniFi Cloud Gateway

Settings → Networks → [VLAN] → DHCP Service Management → Advanced

GUI. Use the built-in TFTP Server field (Option 66).

Or add a Custom DHCP Option: Code 66, Type Text.

URL or hostname (e.g. http://pbx.example.com/)

Windows Server DHCP role

DHCP console → IPv4 → [Scope] → Scope Options (or Server Options)

Right-click Scope Options → Configure Options.

Tick 066 Boot Server Host Name, enter value, OK.

PowerShell: Set-DhcpServerv4OptionValue -ScopeId <id> -OptionId 66 -Value "pbx.example.com"

String (FQDN preferred — see notes)

Universal notes 
•  Option 66 is defined as a string (TFTP Server Name) — IPs entered as strings are valid. 
•  Most VoIP firmware accepts full URLs (http://…); strict clients may want a bare hostname. 
•  Apply Option 66 to the LAN as well as the VoIP VLAN — factory-reset phones land on the LAN before learning the voice VLAN via LLDP/CDP. 
•  Windows DHCP only: if you enter an unresolvable FQDN in option 066, the DHCP service blocks on NetBIOS/LLMNR lookups and DHCP responses slow or fail — use an IP or a resolvable hostname.


Verify  Renew the DHCP lease on the target device, then capture the DHCP ACK (Wireshark filter: bootp.option.type == 66). Confirm the value matches what was configured, then check the provisioning server's access logs to confirm the device retrieved its config.

Not supported — consumer routers

Most consumer / ISP-supplied NBN routers do not expose custom DHCP options. If Option 66 is required, replace the router with one of the platforms above, or run a separate DHCP server (e.g. Windows Server, Raspberry Pi with dnsmasq) on the LAN and disable DHCP on the consumer router.

Platform

Status

Notes

ASUS (stock firmware)

Workaround only

Stock AsusWRT has no GUI for custom DHCP options. With Asuswrt-Merlin + JFFS + SSH you can add the option via /jffs/configs/dnsmasq.conf.add (see main table).

Netgear Nighthawk / Orbi (consumer)

Not supported

Consumer Nighthawk and Orbi firmware does not allow custom DHCP server options. Older ProSAFE business firewalls (e.g. SRX5308) did — those aren't covered here.

Telstra Smart Modem, NetComm NF/NL, ISP NBN routers (typical)

Not supported

Carrier-locked / ISP-supplied NBN modem-routers generally don't expose custom DHCP options. Run a separate DHCP server, or replace with a router from the main table.

TP-Link Archer / Deco (consumer)

Not supported

No custom DHCP option fields in the consumer Archer/Deco web UI. Officially requested on the TP-Link community and not implemented in stock firmware.

 

    • Related Articles

    • How to Autoprovision Yealink phones without DHCP Option 66

      How to Autoprovision Yealink phones without DHCP Option 66 My router doesn't have option 66 available. How can I auto provision the phones with it? You can add the auto provisioning URL directly into the handsets. http://p.sipvoice.com.au Determine ...

    • How to Autoprovision Yealink phones without DHCP Option 66

      How to Autoprovision Yealink phones without DHCP Option 66 My router doesn't have option 66 available. How can I auto provision the phones with it? You can add the auto provisioning URL directly into the handsets. http://p.sipvoice.com.au Determine ...

    • Internet Connectivity Issues caused by Draytek Router Vulnerability - Summit Internet

      Date of issue: 23/03/2025 Summit internet has seen a large number of customers with DrayTek Routers having issues with very short session times using our Internet service. It appears as though the Internet is down. Summit Internet are seeing ...

    • Summit NBN Managed Router

      Summit NBN Managed Router Port 1 : NBN Connect your NBN device via a network cable to this port. See the relevant NBN Setup guides for your connection type. Port 2 : 4G If you have a 4G fail over service, you will be provided with a 4G modem. Connect ...

    • Summit 3CX SIP Trunk Configuration

      Summit 3CX SIP Trunk Configuration This article covers the configuration of a Summit SIP trunk on 3CX PBX and the setup of inbound numbers. Requirements You will need to have the following Provided Summit Internet with the IP of your 3CX Instance ...