[This document describes a field in the Princeton University Host Database. You may also view descriptions of other fields.]
For a device with more than one IP address in its Host Database entry, it is possible to assign interface-specific aliases to the second (and later) IP addresses. As with host aliases, these interface aliases appear in DNS, but appear differently, producing different results.
You might use this if your host has multiple IP addresses each providing a different service, and you need a client that specifies a particular server hostname to communicate with just one of your IP addresses, rather than any of your IP addresses. If each IP address is registered in the same Host Database entry, you would assign each IP address its own interface alias.
Resolving an interface alias in DNS returns only the IP address assigned to that one interface. It does not return any of the other IP addresses in the Host Database entry. (Specifically, looking up the IP address of an interface's first interface alias returns that interface's A-record. If an interface has more than one interface aliases, looking up the interface's additional interface aliases returns a CNAME to the interface's first interface alias; resolving that in turn returns this interface's A-record.)
Contrast resolving an interface alias with resolving a host alias: resolving a host alias returns the IP addresses assigned to all the host's interfaces, in no guaranteed order. (Specifically, resolving a host alias returns a CNAME to the host's canonical name; resolving that name in turn returns an A-record for each of the host's interfaces, in no guaranteed order.)
An interface (other than the host's first) which has no interface aliases assigned will have one generated automatically; e.g. foo-interface3.basketweaving.Princeton.EDU. That is, the name is simply the host's canonical name, with -interfaceNUMBER appended to the first component of the name. This automatically generated interface alias does not appear in the Host Database entry. It does appears in DNS (if the interface has an IP address assigned), in NIS ethers (if the interface has a hardware address), and in bootptab (used by DHCP and BootP service) if the interface has both an IP address and a hardware address.
Just like a regular host alias, an interface alias has no effect on reverse DNS resolution. That is, it doesn't affect DNS PTR records. Performing a DNS lookup on an IP address to find the corresponding hostname returns the Host Datbase entry's canonical name. That's true regardless of whether the IP address happened to have an interface alias.
The Interface Alias field cannot be specified via the Web; to set or modify it, hostmaster@princeton.edu .
Cases where an interface cannot be assigned an interface alias include:
When making up an interface alias, keep in mind that you are expected to abide by the University's acceptable use policy; that policy appears in Princeton University Information Technology Resources and Internet Access -- Guidelines for Use. The guidelines regarding civility and respect for others are particularly relevant here.
Just like an ENTRY-NAME, or ALIAS, an interface alias can consist of only letters, numbers, and dashes, and must begin with a letter. Each component (a portion between dots) must be fewer than 64 characters.
If the use of neither interface aliases nor host aliases will produce the DNS behavior you desire, then consider registering the additional IP addresses and names in separate Host Database entries. (This is typically necessary only when you find it unacceptable for the IP address' PTR records point to the entry's canonical name instead of the interface alias. In most cases, that's not an issue, and the use of interface aliases or host aliases will be sufficient.)
An example will help clarify how interface aliases behave in DNS, and how they differ from host aliases (i.e. regular aliases). Consider this Host Database entry:
Name: red
DNS Domain: Princeton.EDU
Entry Type: HOST
Alias: yellow
Alias: green
Interface[1] Type: Ethernet
Interface[1] MACaddress: 0:0:0:1:2:3
Interface[1] Subnet: bogusnet
Interface[1] IPAddress: 192.168.1.8
Interface[2] Type: Ethernet
Interface[2] Subnet: bogusnet
Interface[2] IPAddress: 192.168.1.14
Interface[2] Alias: orange
Interface[3] Type: Ethernet
Interface[3] Subnet: bogusnet
Interface[3] IPAddress: 192.168.1.10
Interface[3] Alias: blue cyan
Interface[4] Type: Ethernet
Interface[4] Subnet: bogusnet
Interface[4] IPAddress: 192.168.1.56
System Type: DELL-PC
...
The entry's canonical name is red.Princeton.EDU. It has two regular (i.e. host) aliases: yellow.Princeton.EDU and green.Princeton.EDU. It has four interfaces; all four have IP addresses, but only the first has a hardware address. The first interface has no interface aliases. The second interface has one interface alias: orange.Princeton.EDU The third interface has two interface aliases: blue.Princeton.EDU and cyan.Princeton.EDU. The fourth interface has no interface aliases.
A DNS lookup for the IP address of red.Princeton.EDU will return (in no guaranteed order) the IP addresses: 192.168.1.8, 192.168.1.14, 192.168.1.10, and 192.168.1.56.
A DNS lookup for the IP address of yellow.Princeton.EDU or green.Princeton.EDU will return (in no guaranteed order) the IP addresses: 192.168.1.8, 192.168.1.14, 192.168.1.10, and 192.168.1.56.
A DNS lookup for the IP address of orange.Princeton.EDU will return the IP address 192.168.1.14.
A DNS lookup for the IP address of blue.Princeton.EDU will return the IP address 192.168.1.10.
A DNS lookup for the IP address of cyan.Princeton.EDU will return a CNAME record pointing to blue.Princeton.EDU. Proceeding to lookup the IP address of blue.Princeton.EDU will return the IP address 192.168.1.10.
A DNS lookup for the hostname of any of the IP addresses 192.168.1.8, 192.168.1.14, 192.168.1.10, or 192.168.1.56 will return the hostname red.Princeton.EDU.