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The Advanced / DHCP/DNS page allows you to configure advanced settings for the DHCP and DNS services for the LAN and WAN. Most of this functionality is provided by FreshTomato's dnsmasq(uerade) module.
Use internal DNS: Makes dnsmasq(uerade) act as the DNS server on your LAN. DHCP clients will receive Tomato's IP address as the DNS server. (Default: Enabled).
Debug mode: Checking this makes Tomato write detailed information the log file.
Use received DNS with user-entered DNS: Add DNS servers received from DHCP on your WAN connection to the static (manual) DNS server list.
See Basic/Network for more information. (Default: Disabled).
Prevent DNS-rebind attacks: Enable DNS rebinding protection in dnsmasq. (Default: Enabled).
Intercept DNS port: Any DNS requests/packets sent out on UDP/TCP port 53 are redirected to the internal DNS server. Currently, only IPv4 DNS requests are intercepted. (Default: Disabled).
Ignore DHCP requests from unknown devices: dnsmasq will ignore DHCP requests from MAC addresses not listed on the Static DHCP/ARP/IPT page. These clients won't be able to obtain an IP address through DHCP. Note that this setting is also available on the Static DHCP/ARP/IPT page. (Default: Disabled).
Maximum active DHCP leases: Maximum allowable active DHCP leases at one time. (Default: 255).
Static lease time: The absolute maximum valid time for any DHCP lease.
Announce IPv6 on LAN (SLAAC): Enabling this turns on router advertisements for IPv6 Stateless Address Autoconfiguration (SLAAC) protocol. This allows hosts to self-configure an IP address with minimal contact with a server. The client sends out an RS (router solicitation) ICMP packet. The nearest router responds with a RA (router advertisement) packet. The client uses the IPv6 prefix provided in the RA packet as the first 64 bits of its address. It then derives the last 64 bits of its address using the EUI-64 process or a randomizatiion algorithm.
Announce IPv6 on LAN (DHCP): Enabling this makes Tomato turn on router advertisements using IPv6 DHCP.
DHCP IPv6 lease time: The number entered here sets the default lease time for IPv6 DHCP leases.
Mute dhcpv4 logging: Enabling this stops Tomato from logging IPv4 dhcp activity. (Default: Disabled).
Mute dhcpv6 logging: Enabling this stops Tomato from logging IPv6 dhcp activity. (Default: Disabled).
Mute RA logging: This prevents logging of Router Advertisement activity.
Custom configuration: This features allows you to add extra (custom) options to the dnsmasq configuration file.
The DHCP Client (WAN) section includes a dhcpc (dhcp client) options field.
dhcpc options: In this field you can enter custom configuration settings for the dhcp client.
Reduce Packet Size: udhcpc (the DHCP client Tomato uses to obtain a WAN IP address) has a problem. It has a DHCP discovery packet size 590 bytes long. However, DHCP relay servers can only handle DHCP discovery packets up to 576 bytes. If there are DHCP relay servers between your Tomato router and your Internet provider's DHCP server, Tomato might fail to acquire a DHCP lease on the WAN interface.
The extra bytes appeared to be entirely padding, and not necessary. Tomato developers eliminated the padding, which reduced udhcpc's DHCP discovery packet size to only 331 bytes. This 331 byte size eventually became Tomato's default setting. This way, udhcpc can succesfully obtain a DHCP lease from a provider which might have DHCP relays. It may be possible that some users may not be able to obtain a WAN IP address unless they disable this feature, however. (Default: Enabled).