Network is important part of the system administration. Because without a network the server have nothing to do. While administrating Linux servers troubleshooting network is very important. When a network problem occurs in a Linux server first step is checking network services and the route information. Network services can be checked with
systemcl or similar commands. Network routes can be checked with different tools but the most popular and straightforward tool is
traceroute . We will look usage examples of traceroute in this tutorial.
Networking is a stand alone profession done by its professionals. But some simple tools need to know by system administrators. Traceroute mainly uses
ICMP packets. ICMP is a protocol mainly developed for network troubleshooting and information sharing. ICMP packets are transmitted between network capable devices to share information about network traffic. ICMP packets have TTL value. TTL is time to live where holds information about hop count. Generally all network devices is one hop and as an example when a ICMP packets send to
Syntax of traceroute is like below.
traceroute [options] host [ packetlen ]
Help about traceroute commands can be get like below.
Find Route To The Host, IP Address, Website
As we stated before traceroute will find the route to the given destination like host, IP address, website etc. In this example we will route to the site named
poftut.com but the destination can be a IP address or hostname in a local area network.
$ traceroute poftut.com
This example provide information about the route. There are 18 hops to the destination. Which is shown in the first column. In the second column the hop names are provided. and in the other columns the operation metrics are provided. If we look to the example we see that
* * * lines. Those are used to inform that this hops do not provide information about themselves.
Show IP Address Do Not Resolve Host Name
In the previous example the hosts are expressed with their host names. Host names gives more clue about the host. But the disadvantage of the host name is that it should be resolved. As we know all operation is done with IP addresses but the IP addresses are expressed with their host names by using DNS. This will slow the trace operation. The
-n option can be used to disable host name resolution.
$ traceroute -n poftut.com
Set Response Time
By tracing the path the ICMP packet transmission will occur. We will send ICMP packet to the destination hop and we will wait for the answer. Sometimes this answer can came back late or never came back. This will slow our network trace operation. We can set time for the response and make our network trace operation more faster. In the example we set timeout for the trace 1 second with
$ traceroute -n -w 1 poftut.com
Set Number Of Queries Per Hop
We send ICMP packets to the hop and this hop send back responses. We call this round trip as query. Traceroute by default makes 3 query for each hop. This is changeable property. This can be changed with
-q parameter. In the following example we set query count as 1 for each hop.
$ traceroute -n -q 1 poftut.com
Set TTL For ICMP
We have previously talked about the TTL value. By default this TTL value is set as 30. This means that we can only reach up to 30 hop. Because in each hop this TTL value is incremented and after hop number 30 the TTL will be 0 and the packet will not transmitted to the next hop. In this example we will set TTL number 8 which will only reach to the first 8 hops.
$ traceroute -m 8 poftut.com
Set Start TTL
By default the traceroute command will show all hops starting from 1 to the last hop. In some situations we only need only some part of the hops starting from n’th hop. This can be set with the
-f option. In this example we will start from hop number 10 .
$ traceroute -f 10 poftut.com
Print Autonomous System (AS) Numbers
Internet is very big public network. There are a lot of parties those provides connections for each other in a mesh topology. This parties are generally ISP’s or big companies and those are called in internet terminology as Autonomous Systems. All AS’s have their unique identifier called AS numbers. While hoping in different AS’s their AS numbers can be printed with
$ traceroute -A poftut.com
We can see the AS numbers like
[AS9121] after IP address of hop.
Specify Source Interface
In enterprise environments Linux systems generally have multiple interfaces for high availability. Multiple interfaces means multiple paths to the destination. Linux have default routes which means primary interfaces for out of network destination. This is used by default but we can specify an other interfaces with the
-i option. But first we will list available interfaces with the
$ ip link
Now we will specify the network device name. But keep in mind that this options requires root privileges which can be provided with
$ sudo traceroute -i ens3 poftut.com
IP packets are firstly redirected to the host where it’s specified in the network configuration of source system. This first hop/host is called as gateway. By default this gateway is used by traceroute. We can change this gateway information with the
-g option. In this example we will set IP address
192.168.122.1 as gateway address.
$ sudo traceroute -g 192.168.122.1 poftut.com