Linux Networking

Linux Networking

TCP/IP stack– kernel controls the TCP/IP protocol– Ethernet adapter is hooked to the kernel in with the

ipconfig command– ifconfig sets the ip address as well as netmask and

broadcast addresses

Network Hardware

Typically linux finds the NIC (PCI) at bootup but the administrator needs to tell linux that the card is to be enabled.

If linux doesn’t discover the NIC (network interface card) you must assign the card type, interrupt and base address for the card in the linux kernel.

Ipconfig

ifconfig eth0 addr 129.123.109.154 broadcast 129.123.7.255 netmask 255.255.255.0

Netmask forces TCP/IP to go only to the router interface for any address except those in 129.123.7.

Broadcast limits broadcasts to the 129.123.7 subnet

DHCP

Dynamic Host Configuration Protocol– Allows the client to grab TCP/IP setup information

from a centralized service– The client broadcasts the request over the ethernet

interface.– The router recognizes the DHCP request and

forwards that request to a server or system that is configured into the router as a DHCP device.

Name Service

DHCP typically assigns the nameservers for the linux box. DHCP does not assign the search domain for the box.

Name resolution configuration is done in the /etc/resolv.conf file. If the domain search field is set to usu.edu then you can access:

cc.usu.edu as just cc. If you want to contact www.cs.usu.edu you would have to use www.cs

If the domain search field is usu.edu and cs.usu.edu then you can access www.cs.usu.edu as www.

The ambiguity is: What if you want www.usu.edu and not www.cs.usu.edu.

You would then need to use the fully qualified host name to contact the site you desire.

Routers

Routers are added via DHCP or the route command. The default route for a system is the address that all of

the packets flow if they are resolved elsewhere. Other routers may be defined using appropriate

netmasks and ip addresses. Since route configuration may follow indistinct paths

the configuration may have to tell route how may hops (intermediate routers) there are between the local box and the actual ‘default’ router.

Linux Network Features

NFS– Network File System– Centralized File sharing

NIS (Yellow Pages)– Centralized password/authentication system

NFS Server Setup

NFS Server enabled RPC server enabled /etc/exportfs file created

– /usr/local –o ro sys1,sys2,sys3– /home/users –o rw sys1,sys2,sys3

NFS Client Setup

Run mountd, lockd, statd Mount the remote disk as:

– mount –o ro server.cs.usu.edu:/usr/local /usr/local

NIS Server Setup

Domainname– This is the YP domainname not necessarily the DNS

domain name– ypserve, set up the yp server as a master– ypbind, bind to the server– yppasswdd, the password changing daemon– Create the maps with ypmake

NIS Client Setup

Set the yp domainname Bind to the server

– ypbind

Add wildcard info to the /etc/passwd and /etc/group files (+::::::::)

Add wildcard to other files as needed

PPP (dialup)

PPP will automatically configure the linux box to act as a router for the dialin users.

When a user dials in, the PPP protocol typically starts immediately so the user doesn’t see a login prompt.

The PPP (CHAP/PAP) authorization is built into the protocol.

NAT (Network Address Translation) will allow the linux box to take packets from systems on an internal (non-routable) address and translate those requests to the address of the router interface.

Linux as a router

The linux box can handle up to 4 NIC (network interface cards) to act as a router for a wide area network and 3 internal networks.

With this configuration the linux box can act as a firewall. The program routed handles the packet exchange between

boards. Care must be exercised in configuring routed so that packets

are misrouted, i.e. packets are sent to the wrong interface. One of the interfaces is the WAN and should be listed as the

default router.

IP access control

Ipchains and iptables can limit packets (by address and port) in either direction (coming in to the box or going out of the box)

If a particular address needs to be filtered then ipchains or iptables can block that address and never even look a which port it’s want to attach to.

IP access can be controlled (filtered) by using ipchains or iptabels on the linux box.

Ipchains or iptables can also limit which service (port) is available to outside addresses.

Xinetd

Xinetd controls which services are accessible from the internet

The port numbers xinetd translate into service names are located in /etc/services.

Xinetd controls what process owns the service and what flags are passed to the service program.

Xinetd times outs the service program when the internet user completes to that memory and CPU time are freed.

Xinetd server programming

Xinetd redirects input that would be from stdin and takes that from the IP packets

Xinetd redirects output that would go to stdout and puts them into the IP packets.

If a program is dispatched through xinetd then all I/O can be done via stdin and stdout

Background Programs

The xinetd system has a time latency since packets need to be dispatched and the a program must start before the service can begin.

To enhance network speed a program can be started at boot time and run in the background.

These programs must communicate with the TCP/IP stack using system calls. Stdin and stdout calls will be lost or redirected to system logs and not the network.

Network Programs

High network intensive programs need to run at all times on the system. Examples are:

– Web servers (http) May run several processes to gain throughput

– Network file services– Remote Procedure Call programs– Mail

Low impact programs– telnet– ftp– Time– news

Network monitoring

ping echo test traceroute check the router path netstat

– -r show the routing table– -i shows the interfaces– -p what program is doing network stuff– No option

What is presently going on

External Tools

Sniffer, snoop, tcpdump– Look at packets on the wire

Scanner (nmap)– What ports are open?

Security checks– Mail relays, writeable anonymous ftp

Class Evaluations

Remember to do the evaluations!

December 5, 2002

The End