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This chapter is from the book

This chapter is from the book

Network Troubleshooting

Most servers these days are attached to some sort of network and generally use the network to provide some sort of service. There are many different problems that can creep up on a network, so network troubleshooting skills become crucial for any system administrator. Linux provides a large set of network troubleshooting tools, and next I will discuss a few common network problems along with how to use some of the tools available for Ubuntu to track down the root cause.

Server A Can’t Talk to Server B

Probably the most common network troubleshooting scenario involves one server being unable to communicate with another server on the network. I will use an example where a server named ubuntu1 can’t access the Web service (port 80) on a second server named web1. There are any number of different problems that could cause this, so I will run step by step through tests you can perform to isolate the cause of the problem. Normally when troubleshooting a problem like this, I might skip a few of these initial steps (such as checking link) since tests further down the line will also rule them out. For instance, if I test and confirm that DNS works, I’ve proven that my host can communicate on the local network. For this guide, though, I will walk through each intermediary step to illustrate how you might test each level.

Client or Server Problem

One quick test you can perform to narrow down the cause of your problem is to go to another host on the same network and try to access the server. In my example, I would find another server on the same network as ubuntu1, such as ubuntu2, and try to access web1. If ubuntu2 also can’t access web1, then I know the problem is more likely on web1, or on the network between ubuntu1 and ubuntu2, and web1. If ubuntu2 can access web1, then I know the problem is more likely on ubuntu1. To start, let’s assume that ubuntu2 can access web1, so we will focus our troubleshooting on ubuntu1.

Is It Plugged In?

The first troubleshooting steps to perform are on the client. You first want to verify that your client’s connection to the network is healthy. To do this you can use the ethtool program to verify that your link is up (the Ethernet device is physically connected to the network), so if your Ethernet device was at eth0:

$ sudo ethtool eth0
Settings for eth0:
     Supported ports: [ TP ]
     Supported link modes:   10baseT/Half 10baseT/Full 
                               100baseT/Half 100baseT/Full 
                               1000baseT/Half 1000baseT/Full 
     Supports auto-negotiation: Yes
     Advertised link modes:  10baseT/Half 10baseT/Full 
                               100baseT/Half 100baseT/Full 
                               1000baseT/Half 1000baseT/Full 
     Advertised auto-negotiation: Yes
     Speed: 100Mb/s
     Duplex: Full
     Port: Twisted Pair
     PHYAD: 0
     Transceiver: internal
     Auto-negotiation: on
     Supports Wake-on: pg
     Wake-on: d
     Current message level: 0x000000ff (255)
     Link detected: yes

Here on the final line you can see that Link detected is set to yes so ubuntu1 is physically connected to the network. If this were set to no you would need to physically inspect ubuntu1’s network connection and make sure it is connected. Since it is physically connected, I can move on.

Is My Interface Up?

Once you have established that you are physically connected to the network, the next step is to confirm that the network interface is configured correctly on your host. The best way to check this is to run the ifconfig command with your interface as an argument, so to test eth0’s settings I would run

$ sudo ifconfig eth0
eth0      Link encap:Ethernet  HWaddr 00:17:42:1f:18:be  
          inet addr:  Bcast:  Mask:
          inet6 addr: fe80::217:42ff:fe1f:18be/64 Scope:Link
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:1 errors:0 dropped:0 overruns:0 frame:0
          TX packets:11 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:229 (229.0 B)  TX bytes:2178 (2.1 KB)

Probably the most important line in this output is the second line, which tells us our host has an IP address ( and subnet mask (255 .255.255.0) configured. Now whether these are the right settings for this host is something you will need to confirm. If the interface is not configured, try running sudo ifup eth0 and then run ifconfig again to see if the interface comes up. If the settings are wrong or the interface won’t come up, inspect /etc/network/interfaces. There you can correct any errors in the network settings. Now if the host gets its IP through DHCP, you will need to move your troubleshooting to the DHCP host to find out why you aren’t getting a lease.

Is It on the Local Network?

Once you see that the interface is up, the next step is to see if a default gateway has been set and whether you can access it. The route command will display your current routing table, including your default gateway:

$ sudo route -n
Kernel IP routing table
Destination     Gateway      Genmask          Flags Metric Ref     Use Iface        *       U     0      0        0 eth0
default           UG    100    0        0 eth0

The line you are interested in is the last line that starts with default. Here you can see that my host has a gateway of Note that I used the -n option with route so it wouldn’t try to resolve any of these IP addresses into hostnames. For one thing, the command runs more quickly, but more important, I don’t want to cloud my troubleshooting with any potential DNS errors. Now if you don’t see a default gateway configured here, and the host you want to reach is on a different subnet (say, web1, which is on, that is the likely cause of your problem. Either be sure to set the gateway in /etc/network/interfaces, or if you get your IP via DHCP, be sure it is set correctly on the DHCP server and then reset your interface with sudo service networking restart.

Once you have identified the gateway, use the ping command to confirm that you can communicate with the gateway:

$ ping -c 5
PING ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=64 time=3.13 ms
64 bytes from icmp_seq=2 ttl=64 time=1.43 ms
64 bytes from icmp_seq=3 ttl=64 time=1.79 ms
64 bytes from icmp_seq=5 ttl=64 time=1.50 ms

--- ping statistics ---
5 packets transmitted, 4 received, 20% packet loss, time 4020ms
rtt min/avg/max/mdev = 1.436/1.966/3.132/0.686 ms

As you can see, I was able to successfully ping the gateway, which means that I can at least communicate with the network. If you couldn’t ping the gateway, it could mean a few things. It could mean that your gateway is blocking ICMP packets. If so, tell your network administrator that blocking ICMP is an annoying practice with negligible security benefits and then try to ping another Linux host on the same subnet. If ICMP isn’t being blocked, then it’s possible that the switch port on your host is set to the wrong VLAN, so you will need to further inspect the switch to which it is connected.

Is DNS Working?

Once you have confirmed that you can speak to the gateway, the next thing to test is whether DNS functions. The nslookup and dig tools both can be used to troubleshoot DNS issues, but since I need to perform only basic testing at this point I will just use nslookup to see if I can resolve web1 into an IP:

$ nslookup web1

Name:   web1.example.net

In this example DNS is working. The web1 host expands into web1 .example.net and resolves to the address Of course, make sure that this IP matches the IP that web1 is supposed to have! In this case DNS works, so we can move on to the next section; however, there are also a number of ways DNS could fail.

No Name Server Configured or Inaccessible Name Server
$ nslookup web1
;; connection timed out; no servers could be reached

If you see this error, it could mean either you have no name servers configured for your host, or they are inaccessible. In either case you will need to inspect /etc/resolv.conf and see if any name servers are configured there. If you don’t see any IP addresses configured there, you will need to add a name server to the file. Otherwise, if you see something like

search example.net

you now need to start troubleshooting your connection with your name server, starting off with ping. If you can’t ping the name server and its IP address is in the same subnet (in this case is within my subnet), the name server itself could be completely down. If you can’t ping the name server and its IP address is in a different subnet, then skip ahead to the Can I Route to the Remote Host? section, only apply those troubleshooting steps to the name server’s IP. If you can ping the name server but it isn’t responding, skip ahead to the Is the Remote Port Open? section.

Missing Search Path or Name Server Problem

It is also possible that you will get the following error for your nslookup command:

$ nslookup web1

** server can't find web1: NXDOMAIN

Here you see that the server did respond, since it gave a response server can't find web1. This could mean two different things. One, it could mean that web1’s domain name is not in your DNS search path. This is set in /etc/resolv.conf in the line that begins with search. A good way to test this is to perform the same nslookup command, only use the fully qualified domain name (in this case web1.example.net). If it does resolve, then either always use the fully qualified domain name, or if you want to be able to use just the hostname, add the domain name to the search path in /etc/resolv.conf.

If even the fully qualified domain name doesn’t resolve, then the problem is on the name server. The complete method to troubleshoot all DNS issues is a bit beyond the scope of this chapter, but here are some basic pointers. If the name server is supposed to have that record, then that zone’s configuration needs to be examined. If it is a recursive name server, then you will have to test whether recursion is not working on the name server by looking up some other domain. If you can look up other domains, then you must check whether the problem is on the remote name server that does contain the zones.

Can I Route to the Remote Host?

After you have ruled out DNS issues and see that web1 is resolved into its IP, you must test whether you can route to the remote host. Assuming ICMP is enabled on your network, one quick test might be to ping web1. If you can ping the host, you know your packets are being routed there and you can move to the next section, Is the Remote Port Open? If you can’t ping web1, try to identify another host on that network and see if you can ping it. If you can, then it’s possible web1 is down or blocking your requests, so move to the next section.

If you can’t ping any hosts on the network, packets aren’t being routed correctly. One of the best tools to test routing issues is traceroute. Once you provide traceroute a host, it will test each hop between you and the host. For example, a successful traceroute between ubuntu1 and web1 would look like the following:

$ traceroute
traceroute to (, 30 hops max, 40 byte packets
 1 (  5.432 ms  5.206 ms  5.472 ms
 2  web1 (  8.039 ms  8.348 ms  8.643 ms

Here you can see that packets go from ubuntu1 to its gateway (, and then the next hop is web1. This means it’s likely that is the gateway for both subnets. On your network you might see a slightly different output if there are more routers between you and your host. If you can’t ping web1, your output would look more like the following:

$ traceroute
traceroute to (, 30 hops max, 40 byte packets
 1 (  5.432 ms  5.206 ms  5.472 ms
 2  * * *
 3  * * *

Once you start seeing asterisks in your output, you know that the problem is on your gateway. You will need to go to that router and investigate why it can’t route packets between the two networks. If instead you see something more like

$ traceroute
traceroute to (, 30 hops max, 40 byte packets
 1 (  5.432 ms  5.206 ms  5.472 ms
 1 (  3006.477 ms !H  3006.779 ms !H  3007.072 ms

then you know that the ping timed out at the gateway, so the host is likely down or inaccessible even from the same subnet. At this point if I hadn’t tried to access web1 from a machine on the same subnet as web1, I would try pings and other tests now.

Is the Remote Port Open?

So you can route to the machine but you still can’t access the Web server on port 80. The next test is to see whether the port is even open. There are a number of different ways to do this. For one, you could try telnet:

$ telnet 80
telnet: Unable to connect to remote host: Connection refused

If you see Connection refused, then either the port is down (likely Apache isn’t running on the remote host or isn’t listening on that port) or the firewall is blocking your access. If telnet can connect, then, well, you don’t have a networking problem at all. If the Web service isn’t working the way you suspected, you need to investigate your Apache configuration on web1. Instead of telnet, I prefer to use nmap to test ports because it can often detect firewalls for me. If nmap isn’t installed, run sudo apt-get install nmap to install it. To test web1 I would type the following:

$ nmap -p 80

Starting Nmap 4.62 ( http://nmap.org ) at 2009-02-05 18:49 PST
Interesting ports on web1 (
80/tcp filtered http

Aha! nmap is smart enough that it can often tell the difference between a closed port that is truly closed and a closed port behind a firewall. Now normally when a port is actually down, nmap will report it as closed. Here it reported it as filtered. What this tells me is that there is some firewall in the way that is dropping my packets to the floor. This means I need to investigate any firewall rules on my gateway ( and on web1 itself to see if port 80 is being blocked.

Test the Remote Host Locally

At this point we have either been able to narrow the problem down to a network issue or we believe the problem is on the host itself. If we think the problem is on the host itself, there are a few things we can do to test whether port 80 is available.

Test for Listening Ports

One of the first things I would do on web1 is test whether port 80 is listening. The netstat -lnp command will list all ports that are listening along with the process that has the port open. I could just run that and parse through the output for anything that is listening on port 80, or I could use grep to show me only things listening on port 80:

$ sudo netstat -lnp | grep :80
tcp      0      0*     LISTEN     919/apache

The first column tells you what protocol the port is using. The second and third columns are the receive and send queues (both set to 0 here). The column you want to pay attention to is the fourth column, as it lists the local address on which the host is listening. Here the tells us that the host is listening on all of its IPs for port 80 traffic. If Apache were listening only on web1’s Ethernet address, I would see here. The final column will tell you which process has the port open. Here I can see that Apache is running and listening. If you do not see this in your netstat output, you need to start your Apache server.

Firewall Rules

If the process is running and listening on port 80, it’s possible that web1 has some sort of firewall in place. Use the ufw command to list all of your firewall rules. If your firewall is disabled, your output would look like this:

$ sudo ufw status
Firewall not loaded

If your firewall is enabled but has no rules, it might look like this:

$ sudo ufw status
Firewall loaded

It’s possible, though, that your firewall is set to deny all packets by default even if it doesn’t list any rules. A good way to test whether a firewall is in the way is to simply disable ufw temporarily if it is enabled and see if you can connect:

$ sudo ufw disable

On the other hand, if you had a firewall rule that blocked port 80, it might look like this:

$ sudo ufw status
Firewall loaded

To           Action   From
--           ------   ----
80:tcp       DENY     Anywhere

Clearly in the latter case I would need to modify my firewall rules to allow port 80 traffic from my host. To find out more about firewall rules, review the Firewalls section of Chapter 6.

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