Practical TCP/IP and Ethernet Networking- P19 pot

ping www.idc-online.com This will be resolved by the domain name system There are several options available under the ping command, as shown below: C:\WINDOWS.000>ping Usage: ping [-t]

9 :)6/6[ZOROZOKY

When you have completed study of this chapter you should able to apply the following utilities:

• Ping

• ARP

• NETSTAT

• NBTSTAT

• IPCONFIG

• WINIPCFG

• tracert

• ROUTE

The TCP/IP utilities are discussed throughout the book This section is designed to bring them all together in one section for ease of reference, as they are very important in network management and troubleshooting

Most of the older utilities are DOS-based However, more and more Windows-based utilities are becoming available, many of them as freeware or shareware

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‘Pinging’ is one of the easiest ways to test connectivity across the network and confirm that an IP address is reachable The DOS ping utility (ping.exe) uses ICMP to forward an echo request packet to the destination address The destination then responds with an ICMP echo response packet Although the test seems trivial at first sight, it is a powerful diagnostic tool and can demonstrate correct operation between the Internet layers of two

:)6/6[ZOROZOKY hosts across a WAN regardless of the distance and number of intermediate routers involved

Technically speaking, the ping utility can only ‘ping’ an IP address This is due to the fact that the ICMP messages are carried within IP datagrams, which require the source and destination IP addresses in the header Without this feature, it would have been impossible to ‘ping’ across a router If, therefore, the user does not know the IP address, the name resolver on the local host system has to look it up e.g via the domain name system or in the hosts file

The IP datagram, in turn, is transported by means of a network interface layer frame (e.g Ethernet), which requires, in its header, the MAC, addresses of the source and destination nodes on the local network If this is not to be found in the ARP cache, the ARP protocol is invoked in order to obtain the MAC address The result of this action (the mapping of MAC address against IP address) is then stored in the ARP cache The easiest way to get an overall impression of the process is to capture the events described here by means of a protocol analyzer

If the IP address is known, the following format can be used:

• ping <IP Address> e.g ping 192.100.100.4

Ping 192.100.100.255 will cause all hosts on network 192.100.100.0 to respond and will cause unnecessary traffic

If the IP address is unknown, one of the following ways can be used to define the target machine:

• ping <host name> e.g ping computer1

This can be done provided computer1’s IP address has already been resolved

by NetBIOS

• ping <own machine> e.g ping 127.0.0.1

This is a reserved IP address for loopback testing

• ping <own machine> e.g ping localhost

This is a reserved name for loopback testing

• ping <domain name> e.g ping www.idc-online.com

This will be resolved by the domain name system

There are several options available under the ping command, as shown below:

C:\WINDOWS.000>ping

Usage: ping [-t] [-a] [-n count] [-l size] [-f] [-i TTL] [-v TOS] [-r count] [-s count] [[-j

[-k host-list]]

[-w timeout] destination-list

5VZOUTY

• t Ping the specified host until stopped

To see statistics and continue – type

Control-Break

• a Resolve addresses to hostnames

• n count Number of echo requests to send

• l size Send buffer size

• f Set don’t fragment flag in packet

• i TTL Time to live

• v TOS Type of service

• r count Record route for count hops

• s count Time-stamp for count hops

• j host-list Loose source route along host-list

• k host-list Strict source route along host-list

• w timeout Timeout in milliseconds to wait for each reply

C:\WINDOWS.000>

The following examples show how some of the ping options can be applied:

• Ping 193.2.45.66 -t will ‘ping’ the specified IP address repetitively until

stopped by typing Ctrl-C

• Ping 193.2.45.66 -n 10 will ‘ping’ the specified IP address 10 times instead

of the default of 4

• Ping 193.2.45.66 -l 3500 will ‘ping’ the specified IP address with 3500 bytes

of data instead of the default of 32 bytes Here are some examples of what could be learned by using the ping command

Example 1: A host with IP address 207.194.66.100 is being ‘pinged’ by another host

on the same subnet, i.e with the same NetID In this example both addresses are conventional class C addresses Note that the screen display differs between operating systems, even between Windows95 and Windows98, although the basic parameters are the same

The following response is obtained:

C:\WINDOWS.000>ping 207.194.66.100

Pinging 207.194.66.100 with 32 bytes of data:

Reply from 207.194.66.100: bytes=32 time<10ms TTL=128

Reply from 207.194.66.100: bytes=32 time=1ms TTL=128

Reply from 207.194.66.100: bytes=32 time=1ms TTL=128

Reply from 207.194.66.100: bytes=32 time=1ms TTL=128

Ping statistics for 207.194.66.100:

Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Approximate round trip times in milliseconds:

Minimum = 0ms, Maximum = 1ms, Average = 0ms

C:\WINDOWS.000>

From the result, the following can be observed:

• The ICMP message contained 32 bytes

• The average RTT (round trip time) to the target host and back is in the vicinity of 1 millisecond

• The TTL (time to live) remaining in the IP header after its return is 128 Since TTL is normally set at an initial value representing 25 (i.e 32), 26 (i.e 64) or 27 (i.e 128), it can be safely assumed that the TTL value was not altered, and hence there are no routers between the source and destination hosts

Example 2: A host with IP address 207.194.66.101 now ‘pinged’ Although this host

is, in fact, nonexistent, it seems ‘legitimate’ since the NetIDs match The originating host will therefore attempt a ping, but a timeout will occur

C:\WINDOWS.000>ping 207.194.66.101

Pinging 207.194.66.101 with 32 bytes of data:

:)6/6[ZOROZOKY Request timed out

Request timed out

Request timed out

Request timed out

Ping statistics for 207.194.66.101:

Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

Approximate round trip times in milliseconds:

Minimum = 0ms, Maximum = 0ms, Average = 0ms

C:\WINDOWS.000>

Example 3 As before, but this time the NetID differs i.e the target host is assumed to

reside on a different network Since, in this case, no default gateway has not been specified, the originating host does not even attempt to issue an ICMP message, and immediately issues a ‘host unreachable’ response

C:\WINDOWS.000>ping 208.194.66.100

Pinging 208.194.66.100 with 32 bytes of data:

Destination host unreachable

Destination host unreachable

Destination host unreachable

Destination host unreachable

Ping statistics for 208.194.66.100:

Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

Approximate round trip times in milliseconds:

Minimum = 0ms, Maximum = 0ms, Average = 0ms

C:\WINDOWS.000>

The DOS ping command is not particularly ‘user friendly’ It is, for example, not possible to ping a large number of hosts sequentially There are, however, several Windows-based Ping utilities available as freeware or shareware, of which TJPingPro is

an example

The following example shows how a block of contiguous IP addresses can be pinged with a single ‘click’, after setting up ‘start’ and ‘end’ IP addresses on the options screen

Figure 9.1

TJPingPro sequential scan (courtesy of Top Jimmy Software)

The arp utility (arp.exe) is used to display the arp cache which holds the IP to MAC address translation of hosts on the local subnet This utility is not to be confused with the ARP (address resolution protocol) that actually determines the IP to MAC address translation The ARP utility can also be used to manually add entries to the cache, using the -s option

C:\WINDOWS.000>arp

Displays and modifies the IP-to-physical address translation tables used by address

resolution protocol (ARP)

-a Displays current ARP entries by interrogating the current

If inet_addr is specified, the IP and physical addresses for only the specified computer are displayed If more than one network interface uses ARP, entries for each ARP table are displayed

inet_addr Specifies an Internet address

-N if_addr Displays the ARP entries for the network interface specified

-d Deletes the host specified by inet_addr

-s Adds the host and associates the Internet address inet_addr

with the physical address eth_addr The physical address is given as 6 hexadecimal bytes separated by hyphens The entry is permanent

eth_addr Specifies a physical address

if_addr If present, this specifies the Internet address of the interface

whose address translation table should be modified

If not present, the first applicable interface will be used

Example:

> arp -s 157.55.85.212 00-aa-00-62-c6-09 Adds a static entry

The following shows a typical display in response to the arp -a command Note the third column, which indicates type Entries in the arp cache can be entered manually as static entries, but that poses a problem as IP addresses can be changed and physical Network cards (and hence MAC addresses) can be swapped, rendering the stored IP to MAC address mapping useless unless updated For this reason the ARP protocol (not to

be confused with the utility by the same name) binds IP addresses and physical (MAC) addresses in a temporary (dynamic) way Dynamic entries are deleted from the cache after a few minutes, if not used

C:\WINDOWS.000>arp -a

Interface: 0.0.0.0 on Interface 0x1000002

Internet Address Physical Address Type

C:\WINDOWS.000>

This is used for obtaining protocol statistics and current active connections utilizing TCP/IP Nowadays there are many Windows-based utilities that can do much more; yet in

an emergency netstat is certainly better than nothing at all Here follows the netstat options

C:\WINDOWS.000>netstat /?

Displays protocol statistics and current TCP/IP network connections

NETSTAT [-a] [-e] [-n] [-s] [-p proto] [-r] [interval]

-a Displays all connections and listening ports

-e Displays Ethernet statistics This may be combined with the

-s option

-p proto Shows connections for the protocol specified by proto; proto

may be TCP or UDP If used with the -s option to display per-protocol statistics, proto may be TCP, UDP, or IP

shown for TCP, UDP and IP; the -p option may be used to specify a subset of the default

interval Re-displays selected statistics, pausing interval seconds

between each display

Press CTRL+C to stop re-displaying statistics If omitted, netstat will print the current configuration information once C:\WINDOWS.000>

In response to the netstat -e command the following packet and protocol statistics are displayed This is a summary of events on the network since the last re-boot

C:\WINDOWS.000>netstat -e

Interface Statistics

C:\WINDOWS.000>

This provides protocol statistics and current TCP/IP connections using NBT (NetBIOS over TCP/IP) This is relevant with Windows 95/98 etc, which uses NetBIOS for the upper layers of the OSI model

C:\WINDOWS.000>nbtstat /?

Displays protocol statistics and current TCP/IP connections using NBT (NetBIOS over TCP/IP)

NBTSTAT [-a RemoteName] [-A IP address] [-c] [-n] [-r]

[-R] [-s] [S] [interval] ]

-a (adapter status) Lists the remote machine’s name table given

its name

-A (Adapter status) Lists the remote machine’s name table given

its IP address

addresses

addresses

addresses to host names via the hosts file

IP address Dotted decimal representation of the IP address

Interval Re-displays selected statistics, pausing interval seconds

between each display Press Ctrl+C to stop re-displaying statistics

C:\WINDOWS.000>

This shows the entire TCP/IP configuration present in a host It also has the additional versatility of interfacing with a DHCP server to renew a leased IP address

Ipconfig will return, amongst other things, the host’s IP address, its subnet mask and default gateway

C:\WINDOWS.000>ipconfig /?

Windows 98 IP Configuration

Command line options:

/All - Display detailed information

/Batch [file] - Write to file or /WINIPCFG.OUT

/release_all - Release all adapters

C:\WINDOWS.000>

An options often used is ‘ipconfig /all’ In the case of a multi-homed host, i.e one with more than one network interface card (including dial-up modem) ‘ipconfig /all’ will display the details of each card

Note that ipconfig will list the generic name of the adapter Therefore, a 3010 3Com US Robotics 56K modem is simply listed as a PPP adapter, while a Linksys Ethernet 10BaseT/10Base2 Combo PCMCIA card is listed as a generic Novell 2000 adapter, which it emulates

C:\WINDOWS.000>ipconfig /all

Windows 98 IP Configuration

Host Name : COMPUTER100

DNS Servers :

Node Type : Broadcast

NetBIOS Scope ID :

IP Routing Enabled : No

WINS Proxy Enabled : No

NetBIOS Resolution Uses DNS : No

0 Ethernet adapter :

Description : PPP Adapter

Physical Address : 44-45-53-54-00-00

DHCP Enabled : Yes

IP Address : 0.0.0.0

Subnet Mask : 0.0.0.0

Default Gateway :

DHCP Server : 255.255.255.255

Primary WINS Server :

Secondary WINS Server :

Lease Obtained :

Lease Expires :

1 Ethernet adapter :

Description : Novell 2000 Adapter

Physical Address : 00-E0-98-71-57-AF

DHCP Enabled : No

IP Address : 207.194.66.100

Subnet Mask : 255.255.255.224

Default Gateway :

Primary WINS Server :

Secondary WINS Server :

Lease Obtained :

Lease Expires :

C:\WINDOWS.000>

Winipcfg (Windows IP Configuration) provides the same information as ‘ipconfig /all’, but in a Windows format Like ipconfig, it is capable to force a DHCP server into releasing and reissuing leased IP addresses

Figure 9.2

Windows IP configuration

It can be invoked from the DOS prompt, or from the Windows ‘run’ command Click the more details tab for an expanded view

Figure 9.3

Winipcfg display (courtesy of Microsoft Corporation)

This is often used to trace failures along a TCP/IP communications path The spelling of

the command varies slightly For UNIX it is traceroute, for Windows it is tracert

The following figure shows the tracert options

C:\WINDOWS.000>tracert

Usage: tracert [-d] [-h maximum_hops] [-j host-list] [-w timeout] target_name

Options:

-h maximum_hops Maximum number of hops to search for target

-j host-list Loose source route along host-list

-w timeout Wait timeout milliseconds for each reply

C:\WINDOWS.000>

Here follows a route trace from Perth, Australia, to a server in the USA

C:\WINDOWS.000>tracert www.idc-online.com

Tracing route to www.idc-online.com [216.55.154.228] over a maximum of 30 hops:

1 169ms 160ms 174ms slip202-135-15-3-0.sy.au.ibm.net [202.135.15.30]

2 213ms 297ms 296ms 152.158.248.250

3 624ms 589ms 533ms sfra1sr1-2-0-0-5.ca.us.prserv.net [165.87.225.46]

4 545ms 535ms 628ms sfra1sr2-101-0.ca.us.prserv.net [165.87.33.185]

5 564ms 562ms 573ms 165.87.160.193

6 558ms 564ms 573ms 114.ATM3-0.XR1.SFO1.ALTER.NET

9 504ms 534ms 511ms 297.ATM7-0.XR1.LAX2.ALTER.NET

10 500ms 478ms 491ms 195.ATM9-0-0.GW2.SDG1.ALTER.NET

11 491ms 564ms 584ms anet-gw.customer.ALTER.NET [157.130.224.154]

12 575ms 554ms 613ms www.idc-online.com [216.55.154.228]

Trace complete

C:\WINDOWS.000>

As is often the case, the DOS approach is not the user-friendliest option Notice the result when the same trace is done with TJPingPro The same TCP/IP protocols viz ARP and ICMP are still used, but now they are accessed through a third-party application program (TJPingPro) which accesses the TCP/IP stack through a WinSock interface

Figure 9.4

TJPingPro trace (courtesy of Top Jimmy Software)

The most comprehensive tracing is, however, done via application programs such as Neotrace The following figures give some of the results of a trace to the same location used for the previous two examples

Figure 9.5

NeoTrace display (courtesy NeoWorx Inc)