NELUG meeting 16/2/2000
- Internet connects millions of machines around the world.
- Allows machines to find/talk to each other.
- No one machine knows the whole of the network (knowledge is distributed).
- Supports the “languages” (protocols) that various applications use to talk to each other – done in layers.
applications protocol layer – http, ftp, nfs tcp udp ip icmp Hardware layer (ethernet, token ring, ppp)
- These are the user and system programs which talk to each other using IP.
- protocol layer
- These are application/domain specific languages which the various applications understand. These hide the gory details of ip, tcp and udp from the user.
- These are basic protocols that the higher level protocols use.
- The basic unit of information transmission. The higher layers use one or more ip packets to transfer data.
- internet control message protocol – a close friend of ip which is used to pass various control messages between different machines. This is normally only used by the operating system.
- hardware layer
- This is the actual hardware which is used to transmit network packets.
- Each machine on the internet has a (unique) IP address.
- Written as four digits with values between 0 & 255 e.g. 188.8.131.52.
- To talk to a machine you address packets of data with your address (source) and the targets address (destination).
Subnets (and netmasks)
- Subnets are used to group a number of machines which are directly connected together.
- A netmask defines the subnet by separating the network and subnet parts of the address parts form the host part.
- For example a netmask of 255.255.255.0 specifies a subnet which has up to 254 (0 and 255 are special addresses) hosts connected to it.
- Historically networks were classed as either class A (netmask 255.0.0.0), B (255.255.0.0) and C (255.255.255.0). These represent the way in which addresses were allocated to individual institutions. i.e a university may have a class B network allocated and it is responsible for allocating all of the addresses within that range.
- In most cases you should probably assume that you are connected to a class C network and set the netmask appropriately.
DNS – Domain Name Service
- IP addresses are not easy to remember (names are easier).
- The Domain Name Service provides a mapping from names to IP addresses.
- Makes the net more user friendly.
- Allows particular name to move between machines – e.g. to a new service provider.
- Multiple names may map to the same address (often used for web sites).
- Machines are not directly connected to all other machines.
- To talk to non local machines you go via a gateway (often an ISP).
- That gateway machine is connected to other gateways.
- Any machine can act as a gateway if it has two or more network interfaces. So to talk to machine z you may have to go via
me -> a -> b -> c -> z
me -> w -> x-> z
- Routing protocols allow machines to work out the best way to get to another machine.
- This allows problems to be worked around (i.e. broken gateway machine).
- In most cases we only need to know one gateway machine (our ISP) – this is known as the default route.
- IP (internet protocol) is the core internet message format.
- This consists of the header and a message body.
- The message body carries sub protocols.
- The most widely used are:
- tcp – transmission control protocol – a reliable bidirectional stream of data.
- udp – user datagram protocol – an unreliable packet based protocol.
tcp (also known as tcp/ip)
- tcp uses IP packets to construct a reliable bidirectional data stream.
- It handles lost, corrupted and reordered IP packets presenting a stream of data to the application.
- This is a connection oriented protocol, i.e. the user makes a connection and may then use that connection until it breaks it (or omeone else does).
- http (hypertext transmission protocol), ftp (file transfer protocol), telnet all use this protocol.
udp (also known as udp/ip)
- udp does not provide a connection oriented protocol.
- Instead each packet of data has to be individually addressed and
- The user is responsible for handling lost packets (corrupted packets are detected by the IP layer and discarded).
- This is useful where a machine must talk to multiple machines and where it does not want the overhead of a connection oriented protocol.
- Examples: nfs (network file system), tftp (trivial file transfer protocol).
- An ip address allows a packet to be delivered to a specific machine.
- But the machine must work out which application should receive that packet.
- Ports are used to do this (both tcp and udp use these).
- A port is effectively an address within a machine. They are usually specified as an ip addr/port/protocol combination i.e. 184.108.40.206:23 (tcp)
- Programs bind to a port to say that they wish to receive packets which are addressed to that port or that they wish to transmit packets from that port.
- A port is identified by a 16 bit integer e.g. 0 to 65535.
- There are a number of well known ports:
- echo – echos back everything that is sent to it
- echo – echos back everything that is sent to it
- telnet – remote terminal protocol
- smtp – simple mail transfer protocol
Note that tcp and udp have separate port numberings.
- Most systems define well known ports in the file /etc/services.
arp – address resolution protocol
- Machines on the local area network must be able to address each other directly (in terms of hardware addresses).
- arp allows machines to find others and to dynamically account for new machines which are added/removed.
- Put simply it maps ip addresses to mac (ethernet) addresses.
- Only those machines which you are currently (or have recently been) talking to are kept in the arp cache.
Diagnostic/fault finding tools
- Ping uses low level packets to talk to a machine to check if it is responding (these are not actually IP packets (they are icmp packets) but are very closely related).
- This is useful to check if things are setup correctly.
- It also helps to diagnose slow/busy links.
- Example use of ping (localhost is loopback interface which talks to your own machine)
richm@patricia richm]$ ping localhost PING localhost (127.0.0.1) from 127.0.0.1 : 56(84) bytes of data. 64 bytes from 127.0.0.1: icmp_seq=0 ttl=255 time=0.2 ms 64 bytes from 127.0.0.1: icmp_seq=1 ttl=255 time=0.2 ms 64 bytes from 127.0.0.1: icmp_seq=2 ttl=255 time=0.1 ms ... 64 bytes from 127.0.0.1: icmp_seq=8 ttl=255 time=0.1 ms --- localhost ping statistics --- 9 packets transmitted, 9 packets received, 0% packet loss round-trip min/avg/max = 0.1/0.1/0.2 ms
- Note that when using ping on a dialup connection expect to see times or 100 or 200ms.
- If a machine is very busy or there is congestion somewhere in the network some packets may get lost. This is normal but if a large percentage of packets are being lost then connection to that machine may be very difficult.
- ifconfig is used to configure network interfaces.
- It is seldom used by the user – scripts turn you configuration into appropriate ifconfig commands.
- It can be useful to look at your current network setup. e.g.
[richm@patricia richm]$ /sbin/ifconfig -a eth0 Link encap:Ethernet HWaddr 00:00:C0:A0:CE:14 inet addr:192.168.1.1 Bcast:192.168.1.255 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:0 errors:0 dropped:0 overruns:0 frame:0 TX packets:1 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:100 Interrupt:7 Base address:0x290 Memory:d0000-d2000 lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 UP LOOPBACK RUNNING MTU:3924 Metric:1 RX packets:1024 errors:0 dropped:0 overruns:0 frame:0 TX packets:1024 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0
This shows two interfaces:
- an ethernet interface
- the loopback interface – this is present on all machines and always has address 127.0.0.1 (localhost)
- netstat shows network statistics.
- with no parameters it shows the current connections (we are only concerned with “internet connections” here. UNIX domain sockets are covered in many books on networking.
- Example (from Solaris netstat):
ws-csm2:819 $ netstat -f inet TCP: IPv4 Local Address Remote Address Swind Send-Q Rwind Recv-Q State -------------------- -------------------- ----- ------ ----- ------ ------- ws-csm2.658 patricia.nfsd 8760 0 24820 0 ESTABLISHED ws-csm2.56332 patricia.32784 8760 0 24820 0 ESTABLISHED localhost.56334 localhost.32804 32768 0 32768 0 ESTABLISHED localhost.32804 localhost.56334 32768 0 32768 0 ESTABLISHED localhost.56337 localhost.56331 32768 0 32768 0 ESTABLISHED ws-csm2.56904 tux.39504 8760 0 24820 0 ESTABLISHED ws-csm2.56906 tux.44245 8760 0 24820 0 ESTABLISHED
- This shows the current routing table (where the computer will send packets based on their destination addresses) e.g.
richm@patricia richm]$ netstat -r Kernel IP routing table Destination Gateway Genmask Flags MSS Window irtt Iface 192.168.1.1 * 255.255.255.255 UH 0 0 0 eth0 192.168.1.0 * 255.255.255.0 U 0 0 0 eth0 loopback * 255.0.0.0 U 0 0 0 lo default * 0.0.0.0 U 0 0 0 eth0
- Note the “default” entry – any packet addressed to an address which does not appear in the routing table goes to the default route.
- Sometimes it is useful to use “netstat -nr” to stop and addresses being converted to machine names. Try this is netstat -r appears to hang.
- Traceroute is useful for diagnosing routing problems.
- It determines the route which a packet is taking to get to a specified machine.
- There may be 10 to 20 hops on the way to a machine.
- Some gateways are setup to not respond to traceroute. In these cases you will get a * in the listing.
# traceroute webserver traceroute to webserver (220.127.116.11): 1-30 hops, 38 byte packets 1 gateway (18.104.22.168) 2.81 ms 1.97 ms 3.49 ms 2 isp-relay1 (22.214.171.124) 14.0 ms 13.4 ms 14.1 ms 3 isp-relay2 (126.96.36.199) 17.7 ms 17.0 ms 17.7 ms 4 webserver (188.8.131.52) 24.7 ms * 19.7 ms
tcpdump (snoop is similar on Solaris)
- tcpdump analyses network packets on your local network and prints summaries of their contents.
- It is useful when looking for a subtle network problem.
- *** Care *** this program has access to all of the traffic on your network. If used inappropriately it can decode all manner of information. Unauthorised use can get you in serious trouble.
- The arp command allows you to examine the arp cache and find out the hardware addresses of local machines.
# arp -a Net to Media Table: IPv4 Device IP Address Mask Flags Phys Addr ------ -------------------- --------------- ----- --------------- hme0 ws-csm2 255.255.255.255 08:00:20:34:9a:15 hme0 patricia 255.255.255.255 00:d0:58:00:d8:e1 hme0 tux 255.255.255.255 08:00:20:89:7e:34 hme0 nelug 255.255.255.255 08:00:20:43:0f:a4
- To test that DNS lookups are working correctly nslookup can be used to perform name lookups
patricia:15 $ nslookup Default Server: ws-csm2.nelug.org Address: 184.108.40.206 > phileas Server: ws-csm2.nelug.org Address: 220.127.116.11 Name: phileas.nelug.org Address: 18.104.22.168 >