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Discover the Switch Port to which the Server is Connected A previous article, Advanced Linux Networking Tools , covered the basic usage of tcpdump and tshark to discover switch ports on a Cisco switch, but there are times a system may be connected to a different brand, or using a different protocol.  Also, wireshark is not always installed, so relying on tshark is not always the most convenient way to get the required info. By default, Cisco uses a proprietary method of communication between switches and routers called Cisco Discovery Protocol (CDP).  There is another protocol called Link Layer Discovery Protocol (LLDP), which used by other brands, so it is useful to list other options. Find switch information First, use ifconfig to find the interface names.  Then, use tcpdump to listen for packets.  Optional:  write the packet capture output to a .cap file, and use tshark to read the output. Cisco Discovery Protocol (CDP) YOUR_INTERFACE=eth0 ...

Some of the more common tools for advanced networking include: tcpdump, netstat, wireshark, and tshark. A previous article covered Basic Linux Networking Tools . Discover Switch Information Now for the advanced stuff. We use tcpdump, and write it to a file pkt.cap. Then we use tshark to read the file. How to check the router to which the linux host is connected. # tcpdump -nvi eth0 -c 1 -s 1500 -w pkt.cap ether proto 0xaa and ether dst 01:00:0c:cc:cc:cc and ether\[20\] \=\= 0x20 and ether\[21\] \=\= 0x00 # tshark -V -r pkt.cap # tcpdump -nvi eth0 -c 1 -s 1500 -w pkt.cap ether proto 0xaa and ether dst 01:00:0c:cc:cc:cc and ether\[20\] \=\= 0x20 and ether\[21\] \=\= 0x00;tshark -V -r pkt.cap tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 1500 bytes 1 packets captured 1 packets received by filter 0 packets dropped by kernel Running as user "root" and group "root". This could be dangerous. Frame 1 (416 bytes on wire, 416 bytes captured) Arrival T...

Verify Network Connection Most people, who have been around redhat linux for a while, know how to check the IP address, and MAC address using ifconfig. Next, they would typically use route -n (or netstat -r) to find the gateway, and then ping it to verify a connection. Then maybe check duplex and speed using ethtool. Check the IP Address # ifconfig eth0 eth0 Link encap:Ethernet HWaddr 00:15:17:C1:54:D4 inet addr:10.10.5.67 Bcast:10.10.5.255 Mask:255.255.255.0 inet6 addr: fe80::215:17ff:fec1:54d4/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:342967 errors:0 dropped:0 overruns:0 frame:0 TX packets:353260 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:64963573 (61.9 MiB) TX bytes:262444897 (250.2 MiB) Memory:b8820000-b8840000 Discover the Gateway Address Then, it is common to check the default route with route -n, or netstat -nr. # route -n Kernel...

When studying for a Cisco certification such as CCNA, CCNP, CCIP, or CCIE, there is nothing like having real routers and switches to play with, but it can get expensive to build a lab to study with. There is an excellent simulator that is free, but the users must supply their own Cisco IOS software. Once it is set up, the simulated routers, switches, and firewalls act as if they were real hardware. That gives the user flexibility to play with the real Cisco IOS from command prompts on the simulated hardware. What is GNS3? GNS3 is a graphical network simulator that allows you to design complex network topologies. You may run simulations or configure devices ranging from simple workstations to powerful Cisco routers. It is based on Dynamips, an IOS emulator which allows users to run IOS binary images from Cisco Systems and Pemu, a PIXOS emulator to emulate PIX firewalls. Where Can I Download GNS3? http://www.gns3.net How To Configure GNS3 This installation assumes you have a cisco rout...

An easy way to convert a subnet mask from bit notation (e.g. /24) to dotted decimal notation is to use ipcalc from any linux box. The -m option displays the subnet mask. $ ipcalc -m 10.10.10.128/25 NETMASK=255.255.255.128 $ O.K., that's fine, you say, but how do I know how many hosts are on the subnet? That's easy enough -- just use the -b option to find the broadcast address. $ ipcalc -b 10.10.10.128/25 BROADCAST=10.10.10.255 The range of IP's will be from one more than the network address up to one less than the broadcast address. In the example above the IP address range is 129-254. Often the gateway is the first or last address in the range to avoid confusion. If you don't have access to a linux box, you can always do a google search for "subnet calc," where you can probably find an online subnet calculator. For the opposite calculation - find prefix from subnet mask, use the -p option. $ ipcalc -p 10.10.10.128 255.255.255.128 PREFIX=25 $ There y...

How Do I Configure Networking in RedHat Linux? Depending on the version of RedHat, or Fedora: redhat-config-network system-config-network bash-2.05b$ which redhat-config-network /usr/bin/redhat-config-network bash-2.05b$ Network Related Files: /etc/sysconfig/network-scripts/ifcfg-eth0 /etc/sysconfig/network /etc/hosts /etc/resolv.conf For more details, remember that the Red Hat Manuals are available online: http://www.redhat.com/docs/manuals Network Info: http://www.redhat.com/docs/manuals/enterprise/RHEL-4-Manual/ref-guide/s1-networkscripts-interfaces.html rhce linux