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Understanding networks on the host
The first step to understanding your network better is to understand the network configuration of the machine you are currently using. This will give you a number of frames of reference, such as the IP address of the current host, the DNS configuration, and what other machines you can connect to and communicate with.
Finding configuration information
Determining the current configuration of the machine you are working on gives you the base information about your environment. Your first task is to determine the IP address and network mask for the current machine. By using these two values, you can determine the address of your machine and what other machines you can connect to directly on your network (for instance, without the use of a router).
Before you determine the IP address, get the hostname for the system by using the hostname command (see Listing 1).
Listing 1. Getting the hostname
$ hostname sulaco |
The ifconfig command will display the current configuration information for all your
configured network devices when you use the -a option. For example, Listing 2 shows the output from the ifconfig command on a Solaris machine.
Listing 2. Output from ipconfig on Solaris
$ ifconfig -a
lo0: flags=2001000849<UP,LOOPBACK,RUNNING,MULTICAST,IPv4,VIRTUAL> mtu 8232 index 1
inet 127.0.0.1 netmask ff000000
pcn0: flags=201004843<UP,BROADCAST,RUNNING,MULTICAST,DHCP,IPv4,CoS>
mtu 1500 index 2
inet 192.168.1.25 netmask fffffc00 broadcast 192.168.3.255
lo0: flags=2002000849<UP,LOOPBACK,RUNNING,MULTICAST,IPv6,VIRTUAL> mtu 8252 index 1
inet6 ::1/128
pcn0: flags=202004841<UP,RUNNING,MULTICAST,DHCP,IPv6,CoS> mtu 1500 index 2
inet6 fe80::20c:29ff:fe7f:dc5/10
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You can see from this output that there is a loopback device, lo0, with the normal address of 127.0.0.1 for localhost. You can also see that the same device also has an equivalent IPv6 address.
The pcn0 device is configured with a network address of 192.168.1.25, and with a netmask of fffffc00, equivalent to 255.255.252.0. You can also see that in this case the address was set using DHCP (from the list of DHCP flags).
The netmask is particularly important, because with the netmask alone you can tell the size (in terms of registered IP addresses) of your immediate network. In this case, 255.255.252.0 equates to four class C addresses, because 256 (the maximum number of hosts) minus 252 (the number of masked hosts) equals four.
By combining the netmask with the configured IP address, you can guess the range of the IP addresses in the local network. Because IP blocks are usually split by whole groups and in sequence, you can tell that the IP address span of the network is 192.168.0.0 through 192.168.3.255. You can determine this because with a netmask of four class C addresses you would normally split the entire range (192.168.0.0-192.168.255.255) into equal blocks -- with the address prefix of 192.168.1.x it must be in the first block of four addresses.
Different operating systems output the information (and the detail) in different ways. Listing 3 shows the output from a Linux® system.
Listing 3. Output on a Linux system
eth0 Link encap:Ethernet HWaddr 00:1d:60:1b:9a:2d
inet addr:192.168.0.2 Bcast:192.168.3.255 Mask:255.255.252.0
inet6 addr: fe80::21d:60ff:fe1b:9a2d/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:2371085881 errors:36 dropped:0 overruns:0 frame:36
TX packets:2861233776 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:913269364222 (850.5 GiB) TX bytes:3093820025338 (2.8 TiB)
Interrupt:23 Base address:0x4000
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:279755697 errors:0 dropped:0 overruns:0 frame:0
TX packets:279755697 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:388038389807 (361.3 GiB) TX bytes:388038389807 (361.3 GiB)
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Listing 4 shows the output from a Mac OS X™ system.
Listing 4. Output from a Mac OS X system
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1
inet 127.0.0.1 netmask 0xff000000
inet6 ::1 prefixlen 128
gif0: flags=8010<POINTOPOINT,MULTICAST> mtu 1280
stf0: flags=0<> mtu 1280
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
inet 192.168.0.101 netmask 0xfffffc00 broadcast 192.168.3.255
ether 00:16:cb:a0:3b:cb
media: autoselect (1000baseT <full-duplex,flow-control>) status: active
supported media: autoselect 10baseT/UTP <half-duplex> 10baseT/UTP
<full-duplex> 10baseT/UTP <full-duplex,hw-loopback> 10baseT/UTP
<full-duplex,flow-control> 100baseTX <half-duplex> 100baseTX
<full-duplex> 100baseTX <full-duplex,hw-loopback> 100baseTX
<full-duplex,flow-control> 1000baseT <full-duplex> 1000baseT
<full-duplex,hw-loopback> 1000baseT <full-duplex,flow-control> none
fw0: flags=8822<BROADCAST,SMART,SIMPLEX,MULTICAST> mtu 2030
lladdr 00:17:f2:ff:fe:7b:84:d6
media: autoselect <full-duplex> status: inactive
supported media: autoselect <full-duplex>
en1: flags=8822<BROADCAST,SMART,SIMPLEX,MULTICAST> mtu 1500
ether 00:17:f2:9b:3d:38
media: autoselect (<unknown type>)
supported media: autoselect
en5: flags=8963<UP,BROADCAST,SMART,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
inet6 fe80::21c:42ff:fe00:8%en5 prefixlen 64 scopeid 0x7
inet 10.211.55.2 netmask 0xffffff00 broadcast 10.211.55.255
ether 00:1c:42:00:00:08
media: autoselect status: active
supported media: autoselect
en6: flags=8963<UP,BROADCAST,SMART,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
inet6 fe80::21c:42ff:fe00:9%en6 prefixlen 64 scopeid 0x8
inet 10.37.129.2 netmask 0xffffff00 broadcast 10.37.129.255
ether 00:1c:42:00:00:09
media: autoselect status: active
supported media: autoselect
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In all cases, you can generally find the Internet address and netmask of the connected network devices. Obviously, if you have multiple network devices then you will get the information for each device in the output, and it may be that you can reach a wide range of different networks and systems from just one machine.
Finding name resolution services
Your next step in determining the configuration of the current machine should relate to the configuration of the name service system that will convert name and domain names on your system into an IP address when you access a service on another machine.
The configuration of this on most machines is through the /etc/nsswitch.conf file, which contains a list of different naming services (hosts, users, and more) and the order in which the different services (DNS, NIS, or local files) should be used for resolution. You can see an example of this in Listing 5.
Listing 5. Resolving the name service system
passwd: files group: files hosts: files dns ipnodes: files dns networks: files protocols: files rpc: files ethers: files netmasks: files bootparams: files publickey: files netgroup: files automount: files aliases: files services: files printers: user files auth_attr: files prof_attr: files project: files tnrhtp: files tnrhdb: files |
In Listing 5, for example, the hostname information is resolved first by looking at the local files on the system (for example, /etc/hosts) and then the domain name system (DNS).
If the DNS has been configured, then the /etc/resolv.conf file will tell you which machines are being used to convert names into IP addresses. A sample of the file is shown here in Listing 6.
Listing 6. Which machines are being used to convert names into IP addresses
domain example.pri nameserver 192.168.0.2 nameserver 192.168.0.3 |
This information can be useful if you want to query these machines directly for information. You can use tools such as dig and nslookup to extract information about the name service and resolution of names and IP addresses.
Hosts outside of your network (that is, beyond the scope of your network mask in comparison to your current IP address) are sent to a router to be forwarded on to another machine. Routers can be used at all levels of your network, including between departments, different physical sites, and to public and external sites such as the Internet.
The netstat command can tell you which machines or routers are contacted when your machine wants to communicate with machines outside the 'local' network. For example, Listing 7, below, is from a Solaris machine.
Listing 7. netstat command
$ netstat -r Routing Table: IPv4 Destination Gateway Flags Ref Use Interface -------------------- -------------------- ----- ----- ---------- --------- default voyager.example.pri UG 1 139 pcn0 192.168.0.0 solaris2.example.pri U 1 447 pcn0 solaris2 solaris2 UH 1 35 lo0 Routing Table: IPv6 Destination/Mask Gateway Flags Ref Use If --------------------------- --------------------------- ----- --- ------- ----- fe80::/10 fe80::20c:29ff:fe7f:dc5 U 1 0 pcn0 solaris2 solaris2 UH 1 0 lo0 |
The default route shows the gateway (router) used to route packets that are either outside of the current network, or that are not already covered by another route for a specific IP address or IP address range.
Because you might need to determine this information in a situation where your current nameservice is not working, or not returning the right information, you can also specify the -n option to show the information using IP addresses instead of names.
The netstat command can also be used to determine what services are being shared and exposed on the current host. This includes all network services, including DNS, NFS, Web services, and other information. The information displayed is based upon the ports that are open and in the 'listening' state waiting for client connections, or ports that are already open and communicating with a client.
This information can prove invaluable, both to determine if a service is running, and as part of a standard security check to determine whether a machine is sharing or exposing itself to more risk than is necessary.
You can see an example of the output in Listing 8, here using -a to display all the open ports and services, both established (open) and listening for new connections. By default, netstat also shows the open UNIX domain sockets, which are only accessible to the current machine. For brevity these have been removed from the output.
Listing 8. Output using
-a$ netstat -a Active Internet connections (servers and established) Proto Recv-Q Send-Q Local Address Foreign Address State tcp 0 0 *:imaps *:* LISTEN tcp 0 0 *:nfs *:* LISTEN tcp 0 0 *:vmware-authd *:* LISTEN tcp 0 0 localhost:10024 *:* LISTEN tcp 0 0 localhost:10025 *:* LISTEN tcp 0 0 *:mysql *:* LISTEN tcp 0 0 *:imap *:* LISTEN tcp 0 0 localhost:783 *:* LISTEN tcp 0 0 *:sunrpc *:* LISTEN tcp 0 0 bear.example.pri:http *:* LISTEN tcp 0 0 *:cisco-sccp *:* LISTEN tcp 0 0 *:47506 *:* LISTEN tcp 0 0 *:34452 *:* LISTEN tcp 0 0 172.16.217.1:domain *:* LISTEN tcp 0 0 192.168.92.1:domain *:* LISTEN tcp 0 0 bear.example.pri:domain *:* LISTEN tcp 0 0 localhost:domain *:* LISTEN tcp 0 0 *:53941 *:* LISTEN tcp 0 0 *:3128 *:* LISTEN tcp 0 0 localhost:rndc *:* LISTEN tcp 0 0 *:smtp *:* LISTEN tcp 0 0 bear.example.pri:imap sulaco.example.p:65452 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65459 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65412 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65417 ESTABLISHED tcp 0 0 bear.example.pri:mysq bear.example.pri:35475 TIME_WAIT tcp 0 0 bear.example.pri:http sulaco.example.p:49603 FIN_WAIT2 tcp 0 0 bear.example.pri:nfs sulaco.example.p:49552 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65433 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65431 ESTABLISHED tcp 1 0 bear.example.pri:nfs sulaco.example.p:51900 CLOSE_WAIT tcp 0 0 bear.example.pri:imap sulaco.example.p:65415 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65475 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65472 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65429 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65430 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65438 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65443 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65418 ESTABLISHED tcp 0 0 bear.example.pri:nfs narcissus.exampl:62968 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65448 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65423 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65468 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65445 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65476 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65453 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65456 ESTABLISHED tcp 1 0 bear.example.pri:nfs sulaco.example.p:59172 CLOSE_WAIT tcp 0 0 bear.example.pri:imap sulaco.example.p:65416 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65439 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65441 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65446 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65470 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65450 ESTABLISHED tcp 0 0 bear.example.pri:nfs sulaco.example.p:65320 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65465 ESTABLISHED tcp 0 0 bear.example.pri:36230 solaris2.vmbear.mcs:ssh ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65421 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65464 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65474 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:64955 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65473 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65461 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65454 ESTABLISHED tcp 0 0 bear.example.pri:http sulaco.example.p:49608 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65471 ESTABLISHED tcp 0 0 localhost:50123 localhost:ssh ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65420 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65466 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65463 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65451 ESTABLISHED tcp 0 0 bear.example.pri:35471 bear.example.pri:mysql TIME_WAIT tcp 0 0 bear.example.pri:imap sulaco.example.p:65457 ESTABLISHED tcp 1 0 bear.example.pri:nfs sulaco.example.p:53877 CLOSE_WAIT tcp 0 0 bear.example.pri:imap sulaco.example.p:65432 ESTABLISHED tcp 0 0 bear.example.pri:mysql bear.example.pri:35470 TIME_WAIT tcp 0 0 bear.example.pri:imap sulaco.example.p:65467 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65414 ESTABLISHED tcp 0 0 bear.example.pri:50112 bear.example.pri:imap TIME_WAIT tcp 0 0 bear.example.pri:imap sulaco.example.p:65462 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65460 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65469 ESTABLISHED tcp 0 0 bear.example.pri:imap sulaco.example.p:65422 ESTABLISHED tcp 0 0 bear.example.pri:50110 bear.example.pri:imap TIME_WAIT tcp 0 0 bear.example.pri:50111 bear.example.pri:imap TIME_WAIT tcp 0 0 bear.example.pri:imap sulaco.example.p:65442 ESTABLISHED tcp6 0 0 [::]:imaps [::]:* LISTEN tcp6 0 0 [::]:11211 [::]:* LISTEN tcp6 0 0 [::]:imap [::]:* LISTEN tcp6 0 0 [::]:cisco-sccp [::]:* LISTEN tcp6 0 0 [::]:ssh [::]:* LISTEN tcp6 0 0 localhost:rndc [::]:* LISTEN tcp6 0 0 [::]:https [::]:* LISTEN tcp6 0 0 bear.example.pri:ssh sulaco.example.p:52786 ESTABLISHED tcp6 0 0 bear.example.pri:ssh sulaco.example.p:56220 ESTABLISHED tcp6 0 0 bear.example.pri:ssh sulaco.example.p:63895 ESTABLISHED tcp6 0 0 localhost:ssh localhost:50123 ESTABLISHED tcp6 0 0 bear.example.pri:ssh sulaco.example.p:60914 ESTABLISHED tcp6 0 0 bear.example.pri:ssh sulaco.example.p:64669 ESTABLISHED tcp6 0 0 bear.example.pri:ssh sulaco.example.p:56053 ESTABLISHED tcp6 0 0 bear.example.pri:ssh sulaco.example.p:52268 ESTABLISHED tcp6 0 0 bear.example.pri:ssh sulaco.example.p:49528 ESTABLISHED tcp6 0 0 bear.example.pri:ssh sulaco.example.p:65408 ESTABLISHED udp 0 0 *:nfs *:* udp 0 0 *:42498 *:* udp 0 0 *:54680 *:* udp 0 0 172.16.217.1:domain *:* udp 0 0 192.168.92.1:domain *:* udp 0 0 bear.example.p:domain *:* udp 0 0 localhost:domain *:* udp 0 0 *:45495 *:* udp 0 0 *:icpv2 *:* udp 0 0 *:bootps *:* udp 0 0 *:964 *:* udp 0 0 *:11211 *:* udp 0 0 *:sunrpc *:* udp 0 0 *:50042 *:* raw 0 0 *:icmp *:* 7 |
As you can see from this output, the machine is quite busy. The third column shows the hostname and port, separated by a colon, for each open connection or listening connection. If the TCP or UDP service number matches a known port number (as defined within the /etc/services file), then the service name is displayed in the output. For the host, either the hosts name, an alternative IP address, or the '*' symbol is displayed. The asterisk indicates that the service and ports are open and listening on all IP addresses.
For example, you can tell from this output that the machine is configured to support NFS, and has open (established) connections, as shown in Listing 9.
Listing 9. Machine is configured to support NFS
$ netstat -a|grep nfs tcp 0 0 *:nfs *:* LISTEN tcp 1 0 bear.example.pri:nfs sulaco.example.p:51900 CLOSE_WAIT tcp 0 0 bear.example.pri:nfs narcissus.example.p:62968 ESTABLISHED tcp 1 0 bear.example.pri:nfs sulaco.example.p:59172 CLOSE_WAIT tcp 0 0 bear.example.pri:nfs sulaco.example.p:65320 ESTABLISHED tcp 1 0 bear.example.pri:nfs sulaco.example.p:53877 CLOSE_WAIT udp 0 0 *:nfs *:* |
It is also possible using this output to see which machines are currently communicating with this machine. For example, you can extract a list of the machines connected to this one by looking at the fifth column, and then sorting and removing duplicates from the list (see Lisiting 10).
Listing 10. Extracting a list of connected machines
$ netstat -a|egrep 'tcp|udp'|grep ESTABLISHED|awk '{ print $5; }'|cut -d: -f1|sort|uniq
localhost
narcissus.mcslp.p
nautilus.wireless
polarbear.wireles
solaris2.vmbear.mcs
sulaco.mcslp.pri
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This can be useful when you suspect there is a user or computer connected to the machine that you do not recognize or don't expect.
To find out about these other machines, you need to start looking at the other computers within your network.
Table of contents
- Before you start
- Understanding networks on the host
- Finding information about other hosts
- Summary
- Resources
- About the author
