Transcript Week 6

INFO 320
Server 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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