Www.ischool.drexel.edu INFO 320 Server Technology I Week 6 Networking 1INFO 320 week 6

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INFO 320Server Technology I

Week 6

Networking

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Overview

• Now we’ll focus on basic networking concepts for servers, and see how they’re implemented in Ubuntu– Networking overview– Interface configuration– Other networking files– Networking programs

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Networking overview

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Why network?

• Why do we need a network?

• Networking is a big problem– To solve it, make little problems out of it– That’s why networking is broken into layers,

each of which has a designated job– The message an app wants to send is broken

into packets and sent across the network

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Networking layers

• Each layer– Has a specific job to do– Has protocols associated with it– Looks at a certain kind of address

• See networking summary under INFO 330

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Networking layers

• The layers of networking spell out ‘All Turtles Need Less Protection’ *– Application– Transport– Network– Link– Physical

* I apologize for this dreadful mnemonic – let me know if you think of a better one!

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Networking protocols

• TCP and UDP are the transport layer protocols– TCP provides polite reliable delivery,

UDP is fast and dumb

• IP (Internet Protocol, v4 or v6) defines the host addresses– Other network layer protocols define how

packets are routed to get to their destination

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Addresses

• As noted, there are three kinds of networking addresses– The transport layer uses the port number

of the receiving process– The network layer uses the IP address of

the receiving host (computer)• Usually this is the only one we care about

– The link layer uses the MAC address of the receiving adapter (e.g. Ethernet interface)

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Addresses

• Notice that IP and MAC addresses belong to interfaces; each network interface has a fixed MAC address, and is assigned an IP address– Older computers typically only had one

interface (e.g. Ethernet) and therefore one IP and one MAC address

– Now your computer or router might have both wired and wireless interfaces

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Domain and host names

• Read names backward – The last part is the Top Level Domain (TLD)

(edu, com, gov, uk, etc.)– The next to last part with the TLD makes the

domain name (drexel.edu)– The full address of the computer is the host

name (www.drexel.edu, www.webmail.google.com)

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Networking and client/server

• Most protocols use the client/server model– A client process asks a server process for

information– The server process usually provides it

• Notice this is a different kind of “client/ server” from the hardware architecture– Any computer can have client and/or server

processes running on it

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Networking apps

• Key Internet apps include DNS, ARP, DHCP, and NAT

• DNS (Domain Name Service) converts hostnames (www.drexel.edu) to IP addresses; also looks up email servers

– A Berkeley Internet Name Domain (BIND) server implements DNS

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Networking apps

– A BIND server is also called a nameserver or DNS server

– The command nslookup finds DNS information for a domain name•nslookup ubuntu.com

• ARP (Address Resolution Protocol) translates MAC addresses to IP addresses

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Networking apps

• DHCP (Dynamic Host Configuration Protocol) automatically assigns IP addresses to computers within a network– If you don’t use this, you have to assign IP

addresses manually for each interface

• NAT (Network Address Translation) allows a network to use local IP addresses that aren’t visible to the outside world

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Interface configuration

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Borrowing from Debian

• Ubuntu borrows from its Debian heritage in terms of network configuration

• The primary network configuration file is/etc/network/interfaces– It contains network configuration information

for all devices on the system

• The script /etc/init.d/networking reads /etc/network/interfaces

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/etc/network/interfaces

• The details of this file are messy – see man 5 interfaces– It contains network interface configuration

information for the ifup(8) and ifdown(8) commands

– Within this file, comments must start at the beginning of the line with a #

– The file consists of zero or more "iface", "mapping", "auto" and "allow-" stanzas

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Loopback interface

• First let’s allow us to talk to ourselves – the loopback interface– When we want to talk to our own system

across the network, the loopback interface prevents us from actually leaving the computer

– It’s given a reserved IP address (?)

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Loopback interface

• The loopback is automatically configured, typically with these ‘stanzas’ in the interface file– auto lo– iface lo inet loopback

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Loopback interface

• Notice everything’s lower case– lo = name of loopback interface– ‘auto’ has it brought up automatically on boot – The ‘iface’ stanza defines the interface and

creates its settings, here to be a loopback• ‘Stanzas defining logical interfaces start with a line

consisting of the word "iface" followed by the name of the logical interface’ (from the man page)

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inet?

• Normal TCP/IPv4 networking uses ‘inet’ as the protocol type– If you’re using IPv6, use ‘inet6’ instead– Other options exist, such as ‘ipx’ for Novell

NetWare• If this were 1995, that last reference would make

sense to you

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Ethernet interfaces

• Ok, let’s try something more interesting

• How about an Ethernet interface?– Ethernet interfaces are typically given names

starting with ‘eth’, e.g. eth0, eth1

• If we’re configuring it with DHCP our life is easy– auto eth0– iface eth0 inet dhcp

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Static Ethernet interfaces

• In contrast, if we want to set up our interfaces manually, we make them static (as in static routing)– iface eth0 inet static– address 10.1.1.10– netmask 255.255.255.0– gateway 10.1.1.1

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Gateway address

• The gateway address is a key concept– It identifies the device you need to contact to

get to the rest of the world– It’s also called your first-hop router

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allow- stanzas

• ‘Lines beginning with "allow-" are used to identify interfaces that should be brought up automatically by various subsytems’ – allow-hotplug eth1

• Goes with– ifup --allow=hotplug eth0 eth1– Which will only bring up eth0 or eth1 if it is

listed in an "allow-hotplug" line

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Mapping

• ‘Stanzas beginning with the word "mapping" are used to determine how a logical interface name is chosen for a physical interface that is to be brought up’

• Mapping defines scripts that decide how an interface is to be configured

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Mapping example

• mapping eth0• script /usr/local/sbin/map-scheme• map HOME eth0-home• map WORK eth0-work• iface eth0-home inet static• address 192.168.1.1• netmask 255.255.255.0• iface eth0-work inet dhcp

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ifup and ifdown?

• ifup - bring a network interface up • ifdown - take a network interface down

• These commands do just that – take interfaces up (running) or down– ifup eth0=home – Means ‘Bring up interface eth0 as logical

interface home’

• Can use on the command line too

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ifup and ifdown?

• Other examples are simpler; bring up eth0 with– ifup eth0

• Or shut down all interfaces with– ifdown -a – How could this command be used to produce

a DoS attack?

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Wireless networking

• There are wireless options that can be included in the interface config file

• See the wireless man page for gory details

• For example, they can specify the SSID and type of networking mode– wireless-<function> <value> – wireless-essid Home – wireless-mode Ad-Hoc

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Restart to apply changes

• If you change /etc/network/interfaces, restart networking to make the changes active– sudo /etc/init.d/networking restart

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Other networking files

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Other networking files

• A few more files are important to networking, including– /etc/resolv.conf– /etc/hosts– /etc/services– /etc/protocols

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/etc/resolv.conf

• This text file is the place where your DNS servers are identified

• When you get an account with an ISP, you get two DNS server IP addresses

• This file lists them– nameserver 10.1.1.2– nameserver 10.1.1.3

• DHCP will set this file for you

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/etc/resolv.conf

• Here you can also specify the local domain name– domain drexel.edu

• This is appended when you try to go to a local server ‘myserver’ it fills out the full host name myserver.drexel.edu

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/etc/hosts

• This file was used to list all the hosts on the Internet (!)– IP_address hostname [aliases...]

• DNS made that purpose obsolete (yay!!!)

• Now it just defines localhost and loopback addresses, but it precedes DNS– Your system will consult this file before

DNS! (See /etc/host.conf to prove it)

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/etc/hosts

• Most systems have a small host table containing the name and address information for important hosts on the local network

• This is useful when DNS is not running, for example during system bootup

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/etc/hosts example

• 127.0.0.1 localhost • 192.168.1.10 foo.mydomain.org foo • 192.168.1.13 bar.mydomain.org bar • 146.82.138.7 master.debian.org master

• 209.237.226.90 www.opensource.org

• Notice that three of these lines include aliases

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/etc/services

• /etc/services lists the network services available on your system (e.g. HTTP, FTP, Telnet, etc.), and the port numbers they use

• In contrast, /etc/protocols lists the TCP/IP protocols supported by your system, and gives a description of each

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Networking programs

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Networking programs

• We’ll look at a few networking applications– dmesg– ifconfig– netstat– route– ping, finger , who , host , traceroute– wireshark, tcpdump, EtherApe, nagios3

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dmesg

• A lesser known command is dmesg

• The program helps users to print out their bootup messages – A handy diagnostic tool in case something

breaks during boot

• So what does this do?– dmesg > boot.messages

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ifconfig

• ifconfig can be used to configure network interfaces, but is being replaced by ifup and ifdown

• By itself as a command, it gives the status and data about all interfaces or a specific interface– ifconfig– ifconfig eth0

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ifconfig

• It can set the IP address of an interface– ifconfig eth0 123.45.67.89

• Or bring up an interface– ifconfig eth0 up

• It’s vaguely similar to the Windows command ipconfig (notice the spelling difference!)

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netstat

• netstat is a very powerful command, with zillions of options to get network status for interfaces, protocols, routes, groups, etc.

• For example, netstat –rn gives the routing table (-r) with numeric output of IP addresses (-n)

• The actual routing table is in the file /proc/net/route

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route

• The route command gives the routing table, or can add or delete entries from it

• For a given range of IP addresses, a routing table tells which interface a packet needs to use to get there– So the first and last columns are most

important here – for a given ‘Destination’, I should ‘Use Iface’ eth0, for example

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Routing table

• For example, ifconfig and route can be used to define a new Ethernet card, and add it to the routing table– ifconfig eth1 192.168.1.3– route add 192.168.1.3 dev eth1

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Other networking apps

• ping tells you if a host is connected to the network

• finger tells what users are online• who and w tell what users are on the local

network• host gives info about a domain• traceroute gives the path to a host

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Packet capture apps

• Many tools exist to capture packets on the network, and analyze them including– wireshark– tcpdump– EtherApe

• netstat and nagios3 do network monitoring as well

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References

• Most of the hyperlinks in this set of notes are to the corresponding man pages for Ubuntu 9.04

• Networking Summary, dated October 21, 2009 Glenn Booker

• The rest of the information is mostly from (Rankin, 2009) and (Petersen,2009)

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Www.ischool.drexel.edu INFO 320 Server Technology I Week 6 Networking 1INFO 320 week 6