Linux Networking

Linux Networking

• Setting up networks with UML
• Structure of Linux Networking Code

Virtual Networks with UML

• UML can simulate an Ethernet segment with an IP multicast group
  • UML: send Ethernet packet to the group
  • UML: listen from the group

UML Networking: Step 1

• Adding a network device to UML
  • Specify a virtual Ethernet device in UML command line
  • Pick an Ethernet segment for this device (i.e. pick a multicast address)
• UML command line syntax
  • eth0=mcast,EthernetAddr,multicastAddr,port,ttl
• Example:
  • ./linux ubd0=root_fs_1 eth0=mcast,,224.1.0.0,,
  • To add an eth0 device on Ethernet segment 224.1.0.0

UML Networking: Step 2

• Configure the network interface within UML
  • Once you run UML with the eth0= argument, you get an eth0 device: ifconfig -a
  • Now, you need to know a little bit about Linux networking and network administration
• Assign it an IP address
  • ifconfig eth0 192.168.1.1 up
• To check:
  • ifconfig -a
  • route -n
  • ping 192.168.1.1

UML Networking: Step 3

• Start another UML host with the same virtual network
  • In the same Linux host or another host in the same physical network (e.g, both in UTCS net)
  • ./linux ubd0=root_fs_2 eth0=mcast,,224.1.0.0,,
  • Make sure you use the same multicast address & port!
• Within this UML, configure the network with a different IP address, but in the same subnet
  • ifconfig eth0 192.168.1.2 up

UML Networking: Step 4

• Now, you have two UML hosts connected by a virtual network
• In first host: try ping 192.168.1.2

UML Networking:

• Pick your multicast address to avoid conflict with others
• I suggest: 224.1.ab.cd, where abcd is the last 4 digits of your student-ID

More Complex Example

• Build your own network (with multiple subnets)
• Remember to use different multicast address (one per subnet)
• Set up your own routing

Networking Stack

Networking Stack Components

• Network device driver
  • Manage hardware (network interface card)
  • Deal with data-link functions (such as MAC)
  • Can be virtual (e.g., ppp0)
  • Files: drivers/net/
• Network/Transport parts
  • All different network+transport protocols
  • One directory per type: net/ (e.g, net/ipv4/ )

Input Chain and Output Chain

Data Structure: sk_buff

• Used in all network stack
• Carry the data (payload) for going through the input/output chain
  • Minimize copying: only twice (from user space to kernel space, and from/to the network hardware)
• Definition:
  • include/linux/skbuff.h
  • struct skbuff

Socket Interface

• BSD socket
  • struct socket (include/linux/net.h)
• INET socket
  • struct sock (include/net/sock.h)
• Code
  • net/socket.c
  • Start exploring from sys_socket(), sys_sendto()

Next Lecture

• Continue with Linux networking
• Network device driver
• Actual IP input/output modules