DHCP Reservation

The DHCP Reservation menu contains settings to configure DHCP Reservations, Static ARP bindings and enabling/disabling of IP Traffic monitoring for clients with the above mappings.

Since FreshTomato version 2020.8, what was previously called Static DHCP is now called DHCP Reservation. Please see “Inconsistent Terminology” in this section for further clarification and differentiation of terminology.

DHCP Reservation is a simple way to ensure FreshTomato offers certain client devices the same IP address each time they request a lease. Simply enter the MAC address for a client device (which you can find on the Device List), into the MAC Address field, enter the IP Address (and optionally, Hostname) you want to assign to this device into those respective fields and click Save.

FreshTomato will then offer that IP address (and hostname) to the MAC address you specified every time it offers a lease. Thus, in general, the client device will always get that IP address whenever it requests one. That last part, “whenever it requests one” is the key part here. See the explanation of the term Hostname later on this page.

Note that you don't need to check the Bound to button to make a DHCP Reservation. Bound to is used only used to enable Static ARP binding.

When assigning Static DHCP leases, you should use an IP address within FreshTomato's main subnet, but outside the normal DHCP pool scope (assignment range). This avoids potential IP address conflicts. For example, if you have the DHCP server set to assign addresses in the range of 10.0.1.1 - 10.0.1.100, then choosing Static DHCP assignments of 10.0.1.101 - 10.0.1.254 might work well.

If you want to assign multiple hostnames to the same IP address (for example, you want the the server 10.0.1.3 to be known as both “galaxy” and “mail”, you must separate them in the hostname field with a space. A space isn't a valid DHCP Hostname character, so you must use a hyphen for a single, multi-word hostname like “My-PC”. If a client device has multiple network interfaces (for example, Ethernet and Wi-Fi) with different MAC addresses, it might not have the hostname properly assigned to both devices. You could get a “Duplicate name” error.¹⁾

If FreshTomato can't find a match for the device's Hostname (first priority) or MAC address (second priority), the server may fall back to either Dynamic or Automatic allocation. For an explanation of the term Hostname, see later on this page.

As mentioned earlier, Static DHCP offers the mapped IP address (and Hostname) to the MAC address you specified every time it offers a lease. Static DHCP does not prevent a different client from being configured with the same IP address. This is because Static DHCP only offers a static mapping to client devices which request a lease. If another device were to use Static IP, or the router or DHCP were disabled, the other device could take the IP address for itself. Similarly, if the first client for which Static DHCP were then configured for Static IP, it could claim a different IP address than the one in FreshTomato's Static DHCP mapping.

Even if everything else were working properly, only DHCP lease offers are made static. The router's IP→MAC neighbour cache (aka ARP cache) is still filled in dynamically using ARP broadcasts. Unless we change something, FreshTomato is relying on client devices to be honest about their MAC addresses. The source of ARP mapping information is assumed to be “honest” and accurate, even though that source is often the network clients themselves.

In these circumstances, there's nothing stopping unauthorized/malicious clients from pretending to be a different MAC address (ARP spoofing). ARP spoofing could even include spoofing the router or gateway's MAC address. All this could have serious consequences. This is where Static ARP becomes useful.

Technically, FreshTomato's Static DHCP function is a form of DHCP Reservation or DHCP Manual Allocation. Sometimes, confusion occurs because of imprecise or inconsistent terminology. First, it is sometimes confused with Static IP. It is not that. Static IP is the setting of an IP address manually from the client device itself. By contrast, Static DHCP involves configuring an assigned IP address for the client device within (FreshTomato's) DHCP server (for when client devices request a DHCP lease).

Second, Static DHCP is also given different names by different hardware vendors. To make things more confusing, this feature is called static DHCP assignment in DD-WRT, fixed-address in the Linux dhcp daemon (dhcpd) documentation, Address Reservation by Netgear, DHCP Reservation or Static DHCP by Cisco and Linksys and IP address reservation or MAC/IP address binding by other router vendors. Hence, one should be precise here, to reduce confusion.

FreshTomato will use input for 2 MAC addresses per IP/hostname, if the advice here is followed:

https://www.linksysinfo.org/index.php?threads/official-freshtomato-org-website.75333/post-322397

ARP is a protocol that clients use to obtain the MAC address of another client, given its IP address. ARP is used so that clients can figure out how to address network packets to another client. If a network client needs to communicate with another client, it broadcasts an ARP request across the network asking for the other client's MAC address. The “other client” should just reply honestly. With Static DHCP, only DHCP lease offers were made static. The router's IP - MAC neighbour cache (aka ARP cache) is still filled in dynamically using ARP. This means that unless we add something else, FreshTomato is relying on client devices to be honest when reporting their own MAC addresses. This has several repercussions.

Since ARP requests are broadcast across the network, they add to network traffic. Having FreshTomato as a centralized source of ARP resolution can help to limit those ARP broadcasts, reducing network traffic.

By default, ARP gets its mapping information from other clients on the network. It works in a peer-to-peer fashion. ARP mappings are assumed to be “honest” and accurate, even though the source of that information is often the network clients themselves. In that scenario, there's little stopping unathorized or malicious clients from pretending to be a different MAC address (ARP spoofing). This reduces the reliability/security of Static DHCP mappings. After all, what good is a mapping if a client can spoof another MAC address? ARP spoofing could even include spoofing the router or gateway's MAC address. That could have dangerous consequences.

Here again, Static ARP binding can help. When enabled, Static ARP binding will ignore ARP spoofing attempts. FreshTomato will ignore all (broadcast) ARP replies of devices listed in the table. Instead, FreshTomato will check the Static DHCP tables to find the MAC address that belongs to a certain IP address. We assume this information is more accurate, since the Static DHCP table is maintained by the network administrator.

MAC Address: Here you enter the MAC Address you wish to bind.

Bound To: This checkbox enables Static ARP binding for the IP - MAC address mapping. It adds a Static ARP entry for the mapping in FreshTomato's ARP table based on data it finds in the Static DHCP table. (Default: Disabled).

IP Address: Here you enter the IP address you wish to be bound to the MAC address you entered.

IP Traffic: Checking this box enables IP bandwidth Monitoring for the mapped MAC Address / IP address / Hostname combination. (Default: Disabled).

Hostname: Here you enter the (optional) DHCP Client Identifier you want mapped to the client device. This arbitrary human-readable nickname for the device makes it easier to identify it on the network. Traditionally, devices were identified within DHCP by a hardware type code and a client hardware (MAC) address. Later, the optional Hostname field allowed more freedom in mapping device names in DHCP. Ironically, many people still used the hardware type followed by the client hardware address (for example, 01 00 01 02 a0 bc d3.) as their Hostname. However, you are no longer limited to that. You can create your own naming scheme. These days, Hostnames are often the client's DNS/Netbios name. One limitation is that every client device must have a unique Hostname on the broadcast domain. Otherwise, of course, conflicts and other problems could occur.

Hostname description derived from International Engineering Task Force (IETF.ORG) RFC2131 Standards Track, DHCP Protocol, page 8

https://tools.ietf.org/html/rfc2131

Ignore DHCP Requests from unknown devices:

Enabling this function will ensure all DHCP requests from unlisted MAC addresses/Hostnames won't be offered a DHCP lease by Tomato. A MAC address is considered unknown when there is no (Static) DHCP Reservation for it. Again, this doesn't apply to any client device that has been assigned a (true) Static IP. By default, it will still be allowed on the network, unless further measures are taken.

The Ignore DHCP Requests from unknown devices function only works for devices in subnets with (subnet)mask 255.255.255.0 (previously called “Class C” subnets).

ARP only works with IPv4. IPv6 has a different protocol for IP-to-MAC Address resolution.

IPT stands for IP Traffic Monitoring. Every client device not marked as 'Disconnected' on the Status/Device List menu will be on the IP Traffic Monitoring list. Enabling this checkbox puts client devices on the IP Traffic Monitoring list even if they are inactive or disconnected.