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Virtual Server not Working in F5 BigIP

OK. This posting is only directly applicable to the small number of people who run BigIP load balancers. And of that set, only a certaIn subset will likely ever have this situation. Nevertheless, it’s useful to document it. There are lessons in it for the rest of us, it shows the creative problem-solving process used in IT, or rather the creative process that should be used.

So I had a virtual server associated with a certain pool and it was operating fine for years. Then something changes. We want to associate a new name with this virtual server, but test it first, all while keeping the old name working. Well, this is a secured site by which I mean it is running https rather than http. There’s nothing intrinsic in the web site itself that ties it to a particular name. If this were a run-of-the-mill non-secure site you would solve this problem with DNS. Set up an alias and you’re good to go. But secured sites are a wee bit trickier. They present a certificate after all. And the certificate has just one name, at least ours does. Guess I can address multi-name certificates known as Subject Alternative Name CERTs in a separate post. And that name is the original DNS name. What to do? Simple. As any BigIP admin would tell you you create a new virtual server and associate it with a new IP and a new SSL profile containing the new certificate you just bought but the old pool. In DNS assign this new IP to your new DNS name. That’s all pretty straightforward

Having done all that, I blithely tested with lynx (iI’s an old curses-based browser which runs on old Unix systems. The main point is to not test with a complex browser where like Internet Explorer where you are never 100% sure if the problem lies with the browser. If I had it, I would test with curl, but it’s not on that system.). And…it hangs.

Now I’ll admit a lot of stupid things I did (which is typical of any good debugging session of an IT professional – some self-created red herrings accompany any decent sleuthing) and I ratchet up the debugging a notch. Check the web server logs. I see no log of my lynx accesses. Dig a little deeper still. Fire up a trace. Here’s a little time-saver. BigIP does have a tcpdump program, but it is a little stunted. Typically you have multiple interfaces on a BigIP. In this case I felt it pertinent to know if packets were getting to the BigIP from lynx, and then again, if those packets were leaving the BigIP and going to the web server. So the tip is that whereas a “normal” tcpdump might allow you to use the switch -i any to listen on all interfaces, that doesn’t work on BigIP. Use -i 0.0 instead. And of course restrict it somehow so that your own shell session’s packets won’t be picked up by the trace, or else you could be in for a nasty surprise of exponentially increasing traffic (a devastating situation perhaps worthy of its own blog entry!). In this case I added an expression port 443. So I have:

tcpdump -i 0.0 port 443

And, somewhat to my surprise (You should always have a hypothesis, even if it’s just a gut feeling: will this little test work, or not. Why?) not only were packets going from lynx to BigIp and then again to the web server, I could even see returned packets back from the web server to BigIp to lynx. But it was not a lot of packets. A SYN, SYN-ACK and maybe a single data packet and that’s about it. It should have been more chatty.

The more tests you can think of, the better, especially ones that emphasize the marginal differences between the thing that works and the one that doesn’t. One test along those lines: take this same virtual server and associate it with a different pool. I did that, and that test worked!

Next, I tried to access the web server using curl on the BigIP itself. I could, but not at first. First I used the local web server URL http://web_server_ip:443/. It hung my curl command, just like using lynx on the other server. Hmm. I then looked on the web server again. I notice that it has a certificate installed. Ah. So it’s actually running https. So try curl from BigIP again, but this time with the -k switch (insecure, meaning don’t verify the certificate issuer) and a url beginning with https rather than http. Bingo. It comes back with the home page. Now we’re getting somewhere.

Finally I look more closely at the virtual server setup for the old name, the one that works. I see that the server profile is SSL. It basically means that the traffic is encrypted when it hits the BigIP, and the server CERT is associated with the external name. The BigIP decrypts the traffic, then re-encrypts it before sending it along to the web server. The CERT for the second leg is a self-signed CERT and is never seen by users.

I had forgotten to set up my new test virtual server with the server SSL profile, so the second leg of traffic was not being re-encyrpted by the BigIP, even though the web server was only willing to engage in SSL communication with the BigIP. Once I updated the server profile, it all worked fine! Of course after getting the expected results from lynx I went to my desktop browser, just like a regular user, and successfully tested it there as well. You want to make sure your final tests are a realistic approximation of what the user will be doing. If that’s not all possible under your own control, bring in a user for testing.

Liked this article? Here’s another of my IT operational excellence articles that has a somewhat wider applicability.

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