Hack and / - Linux Troubleshooting, Part III: Remote Networks

The Internet is down. Or, is it? Read more to find out how to know for sure.
Other Name Server Problems

Another common nslookup error you might run into is this:

$ nslookup web1

** server can't find web1: NXDOMAIN

Here my name server at responded to me but told me it couldn't find the record for server web1. This error could mean that I don't have web1's proper domain name in my DNS search path. If you don't specify a host's fully qualified domain name (for instance, web1.mysite.com) but instead use the shorthand form of the hostname, your system will check /etc/resolv.conf for domains in your DNS search path. It then will add those domains one by one to the end of your hostname to see if it resolves. The DNS search path is the line in /etc/resolv.conf that starts with the word search:

search example.net example2.net

In my case, when I search for web1's IP address, my system will first search for web1.example.net, and if that has no records, it will search for web1.example2.net. If you want to test whether this is the problem, simply run nslookup again but with the fully qualified domain name (such as web1.mysite.com). If it resolves, either make sure you always use the fully qualified domain name when you access that server, or add that domain to the search path in /etc/resolv.conf.

If you try nslookup against the fully qualified domain name and you still get the same NXDOMAIN error above, your problem is with the name server itself. Troubleshooting the full range of DNS server problems is a bit beyond what I could reasonably fit in this column, but here are a few steps to get you started. If you know your DNS server is configured to have the record you are looking for itself, you need to examine its zone records to make sure that particular hostname exists. If, on the other hand, you are searching for a domain for which you know it doesn't have a record (say, www.linuxjournal.com), it's possible your DNS server isn't allowing recursive queries from your host or at all. You can test that by trying to resolve some other remote host on the Internet. If it doesn't resolve, it's probably a recursion setting. If it does resolve, the problem might very well be with that remote site's DNS server.

Test General Internet Routing

If after all these tests you find that your DNS servers are working fine, but you still can't access the remote server, the final step is to perform another traceroute like above, only directly against the remote server. So for instance, if you wanted to test your route to www.linuxjournal.com, the traceroute might look like the following:

$ traceroute www.linuxjournal.com
traceroute to www.linuxjournal.com (, 30 hops max, 
 ↪60 byte packets
1 (  1.016 ms  2.222 ms  2.308 ms
2  75-101-46-1.dsl.static.sonic.net (  6.916 ms  
 ↪7.389 ms  8.386 ms
3  921.gig0-3.gw.sjc2.sonic.net (  11.265 ms  
 ↪12.435 ms  13.050 ms
4  108.ae0.gw.equinix-sj.sonic.net (  13.846 ms  
 ↪15.233 ms  15.390 ms
5  GIG2-0.sea-dis-2.peer1.net (  35.149 ms  
 ↪36.272 ms  36.944 ms
6  oc48.so-2-1-0.sea-coloc-dis-1.peer1.net (  
 ↪37.340 ms  27.884 ms  27.266 ms
7  10ge.ten1-2.sj-mkp16-dis-1.peer1.net (  
 ↪28.421 ms  29.014 ms 29.688 ms
8  10ge.ten1-2.sj-mkp2-dis-1.peer1.net (  
 ↪30.903 ms  31.015 ms 31.804 ms
9  10ge-ten1-3.la-600w-cor-1.peer1.net (  
 ↪40.840 ms  41.279 ms 42.069 ms
10  10ge.ten1-1.la-600w-cor-2.peer1.net (  
 ↪42.587 ms  43.710 ms 44.921 ms
11  10ge-ten1-2.dal-eqx-cor-1.peer1.net (  
 ↪81.702 ms  82.959 ms 83.934 ms
12  10ge-ten1-1.dal-eqx-cor-2.peer1.net (  
 ↪74.876 ms  72.454 ms 72.798 ms
13  10ge-ten1-3.sat-8500v-cor-2.peer1.net (  
 ↪80.224 ms  81.872 ms  82.569 ms
14 (  83.499 ms  84.162 ms  
 ↪85.048 ms
15  www.linuxjournal.com (  85.484 ms  86.461 ms  
 ↪87.153 ms

In this example, I'm 15 hops (or routers) away from the www.linuxjournal.com server. This is an example of a successful query, but if you ran the same query and noticed a number of rows of asterisks that never made it to your destination and you couldn't ping www.linuxjournal.com directly, the problem could be an Internet routing issue between you and the remote network. Unfortunately, it's probably something outside your control, but fortunately, these sorts of problems tend to resolve themselves pretty quickly, so just keep trying.

If, on the other hand, your traceroute command was successful, but the remote site still didn't work, go back to the steps I discussed in my previous column on how to use telnet and nmap to test whether a remote port is open. It actually could be that the remote server is down (hey, it happens to the best of us) or that someone has configured a firewall to block you from that remote server.

I hope this series has kindled (or rekindled) your interest in troubleshooting under Linux. One of the things I love about Linux is how little it hides from you about how it works and how many troubleshooting tools it provides when things do go wrong. If this has piqued your interest, there are many more troubleshooting avenues for you to explore—from DNS servers like I mentioned above, to troubleshooting just about any type of service. Also, if you have any other great tools or techniques you use to track down these problems, drop me a line. I'm always on the lookout for tools to solve problems faster.

Kyle Rankin is a Systems Architect in the San Francisco Bay Area and the author of a number of books, including The Official Ubuntu Server Book, Knoppix Hacks and Ubuntu Hacks. He is currently the president of the North Bay Linux Users' Group.


Kyle Rankin is Chief Security Officer at Purism, a company focused on computers that respect your privacy, security, and freedom. He is the author of many books including Linux Hardening in Hostile Networks, DevOps Troubleshooting and The Official Ubuntu