FreeBSD Network interfaces

FreeBSD Network interfaces

Introduction

So what is a network interface? In plain old english, it is a logical reference to underlying network hardware. They comprise the lowest layer of the networking subsystem, interacting with the actual transport hardware.

Network Interface Concepts

It is important to understand network interfaces as they are the key to talking to your network hardware (like Ethernet, token-ring,ATM,etc). Different network interfaces may support one or more different protocol families, such as TCP/IP, IPX, etc.

	# ifconfig -a
	xl0: flags=8843 mtu 1500
options=3
inet 205.238.129.221 netmask 0xfffffffc broadcast 205.238.129.223
inet6 fe80::250:daff:fe77:cc77%xl0 prefixlen 64 scopeid 0x1 
ether 00:50:da:77:cc:77
media: Ethernet autoselect (100baseTX )
status: active
	lp0: flags=8810 mtu 1500
	ppp0: flags=8010 mtu 1500
	sl0: flags=c010 mtu 552
	faith0: flags=8002 mtu 1500
	lo0: flags=8049 mtu 16384
inet6 ::1 prefixlen 128 
inet6 fe80::1%lo0 prefixlen 64 scopeid 0x6 
inet 127.0.0.1 netmask 0xff000000 



Ah, it appears I have several interfaces: 
xl0,lp0,ppp0,sl0,faith0,and lo0. What do they all mean? First lets talk about 
the loopback interface, lo0. This is a special interface for communicating with 
itself. It always has the IP address 127.0.0.1. All of the other interfaces 
(except xl0) will be ignored for now but I will give a brief description: 
	lp0 ->	printer interface
	ppp0 -> PPP interface
	sl0 -> slip interface
	faith0 -> IPv6 interface


Now, xl0. I chose to look at xl0 because it is the logical 
reference to my ethernet network card I have installed in my machine. This is 
not to say that every network card in FreeBSD will be referenced by xl0. Unlike 
Linux, each corresponding Ethernet chipset driver is referenced differently in 
FreeBSD. A full list is located in the kernel LINT file. I happen to be using a 
3com network card, for which the xl driver has been written. therefore, my 
network card is referenced by xl0, meaning the first 3com network card in the 
machine. If I added another 3com network card to my box, it would show up as 
xl1, add another and get xl2, etc, etc. I can get more information from the 
kernel dmesg.boot file like so: 
	# grep xl0 /var/run/dmesg.boot
	xl0: <3Com 3c905C-TX Fast Etherlink XL> port 0x9000-0x907f mem 0xf6800000-0xf680007f irq 9 
			at device 10.0 on pci1
	xl0: Ethernet address: 00:50:da:77:cc:77
	miibus0:  on xl0


This is what the kernel probe found at boot time. 
 
Lets look again at just the xl0 ifconfig output: 
	# ifconfig xl0
	xl0: flags=8843 mtu 1500
inet 205.238.129.221 netmask 0xfffffffc broadcast 205.238.129.223
inet6 fe80::250:daff:fe77:cc77%xl0 prefixlen 64 scopeid 0x1 
ether 00:50:da:77:cc:77
media: Ethernet autoselect (100baseTX )
status: active


This tells us some interesting things. The first line show 
the interface flags . The flags basically say that this 
interfaces is UP. It is a BROADCAST type interface. It's running in SIMPLEX mode 
and MULTICAST is enabled. The mtu, or Maximum Transmission Unit, is set to 1500 
bytes (standard for ethernet). The next line says inet 205.238.129.221 ... This 
is the IP address configuration line. inet (meaning IPv4 family) followed by the 
IP address, netmask and broadcast address configured on this ethernet interface. 
The next line inet6 deals with IPv6 (which I'm not covering). The next line 
ether 00:50:da:77:cc:77 tells you the ethernet MAC address. The next line media: 
... refers to the media type and option of the network card. It appears my card 
is running at 100baseTX . This was picked up by the autoselect. You 
can, however, manually set your media type and different options associated with 
media (like duplex). To see what all media types are supported by your network 
card:


	# ifconfig -m xl0
	xl0: flags=8843 mtu 1500
options=3
capability list:
	=3
inet 205.238.129.221 netmask 0xfffffffc broadcast 205.238.129.223
inet6 fe80::250:daff:fe77:cc77%xl0 prefixlen 64 scopeid 0x1 
ether 00:50:da:77:cc:77
media: Ethernet autoselect (100baseTX )
status: active
supported media:
	media autoselect
	media 100baseTX mediaopt full-duplex
	media 100baseTX
	media 10baseT/UTP mediaopt full-duplex
	media 10baseT/UTP
	media 100baseTX mediaopt hw-loopback


Take special note of the media lines down at the bottom. To 
manually set them: 
To set to 100BaseTX :

	# ifconfig xl0 media 100baseTX

Or to set to 100BaseTX and run in full-duplex:

	# ifconfig xl0 media 100baseTX mediaopt full-duplex