Case Study: Host header, IP and Port combinations within IIS

Case Study: Host header, IP and Port combinations within IIS

  • Comments 16

 

Talking about IIS website configurations when it comes to IP Address, Port, Host headers cause a lot of confusion to many Web Administrators. Here I will be going (a bit of unnecessary details though) to explain how the configuration resolves to Websites when a web request comes to IIS.

Environment

IIS6.0

I have multiple IP addresses for my demo server (assuming you have multiple NIC cards).

Here is a list:

10.0.1.1

10.0.1.2

10.0.1.3

10.0.1.4

Here is a screenshot of how IIS manager section looks like which I will talk about later.

In the IIS manager console you may have the following scenarios:

Scenario 1:

There are multiple Websites (for our example we will take 2 here) running with following configuration:

Website                   Host Header Value                  IP Address                         Port
================================================================================
Test1                                --                                All Unassigned                        80
Test2                                --                                All Unassigned                        80

Scenario 2:

Website                   Host Header Value                  IP Address                         Port
================================================================================
Test1                                --                                10.0.1.2                        80
Test2                                --                                10.0.1.2                        80

Will this Websites run? No, only one of them can be in started mode. Rest of them will be in stopped state.
Reason: For HTTP sites one cannot have multiple sites running with the same IP/Port/HostHeader combination. For HTTPS sites you cannot have the same IP/Port combination. More specifically you cannot have the same port/IP combination for the “SecureBindings” setting (SSL settings). Technically, you could have an entirely different IP address for the SSL portion of a web site. If you mess up the SSL part, the HTTP part of the site will still start.

 

If you try to start any of the other Websites you will get an error popup like this:

Scenario 3:

Website                   Host Header Value                  IP Address                         Port
================================================================================
Test1                                --                                     10.0.1.1                             80
Test2                                --                                All Unassigned                        80

Test3                                --                                     10.0.1.3                             80 

Test4                                --                                     10.0.1.4                             80

Will these Websites start: Yes. All of the Websites will be up and running. Remember you can have only one website which can have "All Unassigned" and port XX (when there are no host headers in picture and you have all the sites running on same port XX). No two website can have exactly the same combination of Host header, IP Address and Port.

Here remember you can access the site Test1 only with the IP address 10.0.1.1. If you try to use http://localhost (or http://127.0.0.1) , it will go to some other site (discussed soon). Now using http://servername is a bit tricky. Request will actually go to the site to which your servername resolves from the user's machine. If you do a ping servername and if the IP that it resolves to is set for a specific site like Test3, Test3 will serve the page. If http://servername doesn't resolve to any of the IP addresses on which some sites are specifically configured to listen to, the request will be served by a site which is listening on "All Unassigned" (I know it may be confusing but I am giving an example before coming to theory). If none of the Websites are listening on the resolved IP or "All unassigned" then you will see a "Bad Request (Invalid Hostname)" in the browser (You may check the httperr log to confirm the same).

Let's take for our example that ping servername resolves to 10.0.1.2

So seeing the Websites configuration of Scenario 3 above, which site will should serve the page for the following URLs:

http://servername (remember servername resolves to 10.0.1.2) ?        Ans: Test2

http://10.0.1.2 ?              Ans: Test2 

http://10.0.1.3 ?              Ans: Test3

http://10.0.1.4 ?              Ans: Test4

http://10.0.1.1 ?              Ans: Test1

http://localhost ? (Remember it refers to 127.0.0.1)           Ans: Test2

I assume you are getting the logic.

So here comes the explanation:

IIS by default listens on all the IP addresses on the server on a specific port (By default 80).

If you run the command netstat -ano from the command prompt you will see an entry like 0.0.0.0:80. This means that IIS process listens on all IP addresses associated with the server on port 80. Now when you associate a website with a specific IP address and port combination it will listen only on that IP address on that port. It won't accept any request on any other IP address or port associated with the server. When you use "All Unassigned" for a website it means that this website listens on all the IP addresses associated with the server on the specified port. Hence as you may assume from above scenario any web request that is sent to the server on a specific IP address will be served by the website following the above rule...let me clear it again with the above example:

http://servername (remember servername resolves to 10.0.1.2) ?       

Ans: Test2 , Reason: When the request reaches IIS server, http.sys driver redirects the request to website Test2 because all other Websites are listening on specific IP's which do not match 10.0.1.2.

http://10.0.1.2 ?             

Ans: Test2, Reason: It will be sent to and served by Test2 because this is the website which is set to "All unassigned" which means Website is listening on all IP addresses associated with the server. Also this request won't be served by any other site because they are listening on IP's which are different from 10.0.1.2. 

http://10.0.1.3 ?              

Ans: Test3 , Reason: As expected from explanations so far this will be served by Test3 because Test3 is configured to listen to any request directed on IP 10.0.1.3.

http://10.0.1.4 ?             

Ans: Test4 , Reason: Again the same reason as above, Test4 is configured to listen on IP 10.0.1.4.

http://10.0.1.1 ?             

Ans: Test1 , Reason: Same logic as above.

http://localhost ? (Remember it refers to 127.0.0.1)          

Ans: Test2, Reason: Served by Website Test2, because the request is sent on IP 127.0.0.1 and none of the other Websites are listening on 127.0.0.1. Test2 has "All unassigned" which means it is listening on all IPs on the server including 127.0.0.1 and hence can serve the request. If for testing you change the IP configuration for a website to 127.0.0.1 then all new requests like http://localhost will be served by this specific website.

So the above scenario shows how every request will be served by some site provided we have the configuration like "All unassigned" or specific IP for the sites.

Now let's assume what will happen if we have this scenario:

Scenario 4:

Website                   Host Header Value                  IP Address                         Port
================================================================================
Test1                                --                                     10.0.1.1                             80
Test2                                --                                     10.0.1.2                             80

Test3 (Stopped)               --                                     10.0.1.3                             80 

Test4                                --                                     10.0.1.4                             80

Here we do not have any site configured with "All unassigned". Now if you try to access http://localhost or http://10.0.1.3 what should happen?

You should see "Bad Request (Invalid Hostname) in the browser. This happens because now none of the Websites are capable of serving your requests http://localhost (127.0.0.1) or http://10.0.1.3 

We have just 3 Websites (running) and they are listening on IPs 10.0.1.1, 10.0.1.2 and 10.0.1.4. Had there been a website listening on "All Unassigned" these requests would have been served by this website.

I am pretty sure you might have got rid of some of your doubts by now :) If not it's my bad.

-------------------------------------------------------------------------------------------------------------------------------------------------------

So far I had been talking about scenarios where Host header was not in picture.

Now let's assume you have host headers as well in above scenarios. Let's first see what's the use of a Host header.

IIS identifies Websites on the basis of 3 parameters: IP address, Port and Host header (if present).

Any combination of the above parameters should be able to uniquely identify a website. If there is an ambiguity IIS will throw error.

A host header is of the form www.abcd.com or abcd or abcd.com or abcd.co.in etc....

To create and host multiple Web sites, you must configure a unique identity for each site on the server. To assign a unique identity, distinguish each Web site with at least one of three unique identifiers: a host header name, an IP address, or a TCP port number.

One method for providing each site with a unique identifier is to use IIS Manager to assign multiple host header names.

Typically Host headers are set in IIS manager and its mapping to an IP is set in the DNS entry or hosts file (I assume you know hosts file is a limited simulation of DNS for a client). Users access the sites using their host headers.

A “Host” header is one header out of multiple headers that are included in an HTTP request. The headers are things like User-Agent, Accept, If-Modified-Since, Content-Length, Host, etc. As a request comes in, IIS looks at one particular header… “Host” and uses its value to decide which web site to route the request. “Host Headers” are not an IIS technology. They are an HTTP technology. Browsers typically put into the “Host” header whatever the user put into the “Address” line of the browser. If you put “http://192.25.109.11/Default.aspx” into the address line then the browser will extract the “host name” from that address and put “192.25.109.11” as the value for the “Host” header. Or maybe there is no header at all named “Host” (which makes it an HTTP/0.9 request).

Here is a screenshot of an http request for http://test1.com

and this one for http://10.0.1.1

Scenario 5:

Website                  Host Header Value                  IP Address                         Port
================================================================================
Test1                        test1.com                               10.0.1.1                             80
Test2                        test2.com                               10.0.1.1                             80

and in the DNS or hosts file you have mapping like:

10.0.1.1                test1.com

10.0.1.1                test2.com

Will these two Websites run simultaneously? Yes they will.

They will run even when they have same IP and same port because now IIS has a way of distinguishing the Websites using a 3rd parameter called host header. So it knows that when a request comes for http://test1.com and http://test2.com it can direct them to the right Websites on the basis of host headers present in the request headers.

When you access a site with host header the request is resolved to the website corresponding to DNS/Hosts mapping for the host header and not the IP address mentioned in the IIS manager. This is IMPORTANT. The request then reaches the IIS server and goes to the website which is listening on that resolved IP.

Let's see here.

DNS/Hosts setting:

10.0.1.1              test1.com                                    10.0.1.3           test3.com

10.0.1.2              test2.com                                    10.0.1.4           test4.com

Scenario 6:

Website                   Host Header Value                  IP Address                         Port
================================================================================
Test1                          test1.com                              10.0.1.1                             80
Test2                          test2.com                              10.0.1.2                             80

Test3                          test3.com                              10.0.1.3                             80 

Test4                          test4.com                              10.0.1.4                             80

Now when you try to browse to http://test3.com which site should serve the page from the above IIS configuration? Obviously Test3, now this is simple:). what happens if you try to access any of these Websites with their IP addresses? You will see "Bad Request (Invalid Hostname)". So use judiciously as to how users should access the site. If you have host header set use it instead of IP address in the URL.

If you really want to use an IP address along with an alias (a friendly name) for a website (like http://abcd.com, or http://wxyz ) , then just don’t assign the host header value in the IIS manager. And have an entry in DNS or hosts file mapping abcd.com or wxyz to the IP address for the website (there are other ways to but this neat).

Scenario 7:

Website                   Host Header Value                  IP Address                         Port
================================================================================
Test1                          test1.com                              10.0.1.1                             80
Test2                          test2.com                              10.0.1.2                             80

Test3                          test3.com                              10.0.1.1                             80 

Test4                          test4.com                              10.0.1.4                             80

Which site should serve the content when accessed with http://test3.com in the URL..............?

"Bad Request (Invalid Hostname)" in the browser. Reason being that http://test3.com resolves to IP 10.0.1.3 through DNS/Hosts and then in IIS mmc it is configured to listen on IP 10.0.1.1. Rest of the Websites work fine because both the DNS/host entry and IP in IIS mmc for the Websites match.

Scenario 8:

Website                   Host Header Value                  IP Address                         Port
================================================================================
Test1                          test1.com                              10.0.1.1                             80
Test2                          test2.com                              10.0.1.2                             80

Test3                          test3.com                              10.0.1.1                             80 

Test4                          test4.com                              All unassigned                   80

 

What site should http://test4.com resolve to..............? Test4, because it is listening on All Unassigned (which means all the IP addresses on the server) which includes DNS/hosts mapping for the site (here 10.0.1.4).

Similarly if we have       

Website                   Host Header Value                  IP Address                         Port
================================================================================

Test3                           test3.com                        All Unassigned                        80 

It should still work fine for http://test4.com and also http://test3.com

Let us consider another scenario

If you have multiple entries for a website in the IIS manager->Web Site->Advanced, you may have different host headers but should have same IP (and/or All unassigned) and TCP Port.

If you use a different port for the entries you need to add the port entry after the host header like: http://testX:port.

If you use a different IP address for the 2nd host header you may again see the blunt "Bad Request (Invalid Hostname)" error when you use this host header to access the website.

So all these explanations and pain for one reason: Avoid setting a specific IP address for the Websites in IIS mmc to avoid any conflicts or confusion with the DNS/Hosts mapping. Instead use "All unassigned" and if really required to use an IP make sure that IP address in IIS mmc and DNS/Hosts entry is same. There is no benefit to “over configuring” the IP and host header settings. It is “unnecessary administrative overhead”. For any site that don’t use SSL, assign one or more host headers and leave the IP at "All unassigned". For anything with SSL, give it a specific IP and leave the host header field blank. Leave port alone because that confuses most end users.

Hope this helps!                                                                                                              

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  • Hey Saurabh,

    This is one of the best articles, I have read in recent times. I have always been so confused with the three terms "IP Address, Port and Host Headers", but after reading your article its crystal clear to me. The way you explained it was also very good. Keep up the good work.

    Thanks,

    Prashanth

  • I thought it was difficult to understand, sorry for saying this.

  • Hi Peter,

    Thanks for bringing this to my notice. I will try reformatting the post again to make it more simple to understand.

    -Saurabh

  • Yes, please make it simple, 2 or 3 sites are enough to demonstrate.

    Waiting for update...

  • Great overivew right up to the end when you state:

    "For anything with SSL, give it a specific IP and leave the host header field blank. Leave port alone because that confuses most end users."

    Since I want to use multiple SSL sites, I could use more explanation of what this means. I only have 1 NIC card and IP to work with.

  • Hi PhilB,

    Thanks. Now since you have a limitation of 1 NIC/IP address and you want to use multiple SSL sites you may configure the sites on different ports. Host headers won't be helpful since they will be part of the encrypted request. I don't know the version of IIS in picture but if you have IIS 6.0 you may check this article http://support.microsoft.com/kb/187504/en-us

    Also if you intend to use the same certificate for multiple sites it's not feasible. In such a case you may go along with Wild card certiificates.

    Let me know if there are questions around this.

  • Dear Saurabh,

    Good article.  Yes, it's confusing, but that's why we get paid to do this stuff <grin>.   Your "Scenarios" organization was useful to me.  

    I'm also investigating multiple sites via SSL.

    I found the following reference: http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/596b9108-b1a7-494d-885d-f8941b07554c.mspx?mfr=true

    It states that with IIS 6 running on W2K3/SP1 (but not without SP1), one can run with Host Headers AND SSL.  Unfortunately, it states: "SSL host headers cannot be configured by using the IIS Manager UI." but it doesn't go on to say HOW to configure them.

    I'll be looking for this information, but if you know the answer, please feel free to share it.

    Stephen.

  • Hi Stephen,

    Oh well, you are right!. I didn't realize at the time of writing it can be confusing to a lot many people ;-).

    If you look further in the same article it has links for wildcard certificate and how you can configure it for IIS to work with SSL for multiple host headers.

    Remember that in Windows 2003  SP1 onwards you have the facility to use wildcard certificates for multiple SSL enabled websites. Your SSL websites may pick up incorrect certificate if you have multiple websites running over the same IP address and port combination and using different host headers, BUT not using the  wildcard certificates. If one wants to run multiple websites with same IP/Port combination (with obviously, different host headers) then they have to use wild card certificates and modify the metabase settings as shown below for each of the websites:

    cscript.exe adsutil.vbs set /w3svc/<site identifier>/SecureBindings ":443:<host header>"

    Here you will have to modify the metabase securebindings property for each of the  websites with their respective host headers. So as you see above we need to modify the metabase setting through a script and IIS UI doesn't help here.

    Let me know if there are any questions around it. Also have a look at this KB http://support.microsoft.com/kb/187504/en-us.

  • Thanks, Saurabh.  Good follow-up.  

    Your page is very helpful and I thank you for taking the time to post and maintain it.  Good work!

    Stephen.

  • Thanks, Saurabh.  Good follow-up.  

    Your page is very helpful and I thank you for taking the time to post and maintain it.  Good work!

    Stephen.

  • Simple and clear for new user like me. Thank and Cheers!

  • Wonderful! Finally someone has put it in plain english! I have spent two days trying to determine the appropriate host header syntax. This is the only guru who knows what he is talking about!

    Thanks dude!

  • Saurabh, really terrific article!  The different scenarios helped me think about how we have our 4 sites set up on our W2K3/IIS 6 server.  Question: is it better or worse to have sites set up within the default site?  This is a situation I inherited from the previous website manager and am wondering if it would be better to have the 4 sites separate (i.e. default site, site 2, site 3, site 4).  Again, REALLY great job on this article....Thanks!

  • Hi RedBarron, Thanks for your appreciation :-).

    Regarding your question, the setting you have if I understand correctly is that all the 4 sites that you have mentioned are actually Virtual Directories within one Website, i.e. something like below:

    -- Default Web Site

          -- VD1

          -- VD2

          -- VD3

          -- VD4

    If this is what you have then there are no specific reasons you should have separate web sites for them. Ultimately it all depends upon your architecture, application requirements etc. Let's say if you want to access the sites as www.abc.com, www.def.com etc, instead of www.default.com/abc, www.default.com/def etc. then you will have to configure them as separate web sites.

    Similarly if you want to run them under different ports/IP combinations you can put them under separate web sites. Remember, all the Virtual Directories within a Web site will have same settings for the above. I don't see a reason why you should move on to separate sites unless there are some specific requirements. Let me know if there is something specific that you were looking for that I missed.

    tnx.

  • This was a great article. Very well explain to the point with plenty of examples

    Thanks,

    Neil

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