Why DNS Based Global Server Load Balancing (GSLB) Doesn’t Work

Pete Tenereillo


Copyright Tenereillo, Inc. 2004




Fred: I’ve got a plane to catch. How long does it take to get from Hollywood to LAX?

Joe: Well, it depends which way you go.

Fred: Well, I guess I would take the freeway, right?

Joe: OK, that’s a technical question. I can answer that. The freeway route takes about 20 minutes assuming an average speed of 60MPH.

Fred: OK, thanks.

(Fred spends an hour window shopping on Rodeo Drive., then two hours driving in bumper to bumper traffic to LAX, misses his plane).

Fred: (on cell phone to Joe) The traffic was terrible, and I missed my plane! You said it would only take 20 minutes!

Joe: Oh, you didn’t ask how long it would take if there was traffic.

Fred: Is the traffic particularly bad at this time of the day?

Joe: Are you kidding? Traffic is always bad. This is LA!


A solution can be technically correct given certain assumptions, but if it ignores known details, it is not worthy of discussion. Maybe as technologists we want so much to believe that every problem has a solution, that sometimes we overlook the obvious. Maybe there are so many details to consider that we get confused. Maybe after a while we just stop caring.


DNS based Global Server Load Balancing (GSLB) solutions exist to provide features and functionality over and above what is available in standard DNS servers. This paper explains the pitfalls in using such features for the most common Internet services, including HTTP, HTTPS, FTP, streaming media, and any other application or protocol that relies on browser based client access. At this point I could add “if the solution is expected to also (simultaneously) enhance (or at least not damage!) high availability of browser-based Internet services”, but I won’t, because GSLB solutions are always expected to work well with high availability deployments. That is the obvious.

The punch line

High-availability GSLB of general Internet browser based services[1] is best accomplished by including the use of multiple A records, but the use of multiple A records debilitates DNS based global server “load balancing” (i.e. traffic control or site selection algorithms). Therefore to use global server load balancing (or multi-site traffic control) features, which are of questionable value in the first place (see Why DNS based Global Server Load Balancing (GSLB) Doesn't Work, Part II), one must accept the compromise of reduced high availability. Read on for a technical explanation about why.

The fundamental purpose of GSLB

The overwhelmingly most compelling reason that Internet sites are hosted in multiple locations is high availability. If a catastrophic event causes one of the sites to become unavailable, one or more remaining sites must be able to service requests from users, such that business can be continued. Given the following example, with servers hosted in sites in Los Angeles and New York City:

Diagram showing a Global Server Load Balancer GSLB, two sites, one in LA, one in NYC, with the equipment and/or Internet connection to the LA site as failed.

If the Internet connection fails, or the power fails, or the switching or local server load balancing (SLB) equipment fails, or a Denial of Service (DoS) attack occurs, or if a catastrophic event causes the loss of the entire site, the GSLB device should detect the failure and route requests to the remaining site, so that clients can connect successfully and business can continue.

DNS resolution

At this point a review of DNS resolution with GSLB is in order. Please feel free to skip ahead if you are a DNS GSLB guru. The following diagram steps through a client resolution of the Fully Qualified Domain Name (FQDN)  www.trapster.net.


A diagram showing a typical Global Server Load Balancing GSLB DNS name resolution process.

Site A in Los Angeles has a virtual IP address (VIP) of, and Site B in NYC has a VIP of A GSLB device is acting as the authoritative name server for www.trapster.net. Upon a DNS query for www.trapster.net, the job of the GSLB is to determine whether to return the IP address or


1)    The stub resolver (a software program running on the client computer) makes a request to the assigned local DNS server, which in this example is in the client’s Internet Service Provider (ISP) DNS server farm in Atlanta, Georgia. The client must receive either an answer, or an error. This is called a “recursive” query. Note: the stub resolver program is not capable of “digging” through the Internet to find the answer. That is the job of a DNS server.

2)    The client’s DNS server performs an “iterative” resolution on behalf of the client, querying the root name servers and eventually ending up at the authoritative name server for www.trapster.net. In this case the GSLB device is that authoritative name server.

3)    The GSLB device performs some sort of communications with software or devices at each site, gathering information such as site health, number of connections, and response time.

4)    The software or device at each site optionally performs some sort of dynamic performance measurement, such as a round trip time (RTT), or topographical footrace, or BGP hop count, back to the client’s DNS server.

5)    Using the information gathered in steps 3 and 4, the GSLB device makes a determination as to the preferred site, and returns the answer to the client’s DNS server. The answer is either IP address or IP address If the time to live (TTL) is not set to zero, the answer is cached at the client’s DNS server, so that other clients that share the server will make use of the previous calculation (and not repeat steps 2 through 4).

6)    The DNS answer is returned to the client’s stub resolver.


After DNS resolution is complete, the client makes a TCP connection to the preferred site.

Browser DNS caching

Microsoft Internet Explorer (IE), Netscape Navigator/Communicator (Netscape), other browser applications, and even some Web proxy cache applications and mail servers, have built in “DNS caches”. This DNS cache is a sort of mini database which stores a DNS answer for some period of time. DNS answers should, in general, be stored for the amount of time that is specified by the DNS server that gave the answer. This time is specified as a Time To Live (TTL). Unfortunately, the DNS caches in browser applications cannot observe the TTL. This is because a DNS query must be made via a standard available system call, gethostbyname() (or an equivalent derivative), which returns one or more IP address associated with the queried name (that system call does not allow the calling application to see the TTL). To get around this issue, the browser application developers use a configurable TTL value. In IE this value defaults to 30 minutes, and is configurable in the Windows Registry on the client. In Netscape, the value defaults to 15 minutes, and is configurable by the addition of a line of script to the prefs.js file.


The frequency of the process of DNS resolution outlined above differs depending on browser. Older browsers perform the process only one time every 30 minutes (IE) or 15 minutes (Netscape), regardless of the number of times the user/client connects to the site. Clicking on the Refresh/Reload button, even with control or other key combinations, does not affect this behavior. The only way to clear the DNS cache on these browsers is to exit and restart the browser (or reboot the computer). In most cases “restarting the browser” entails closing all browser windows, not just the windows that are open to the site in question - something a user is unlikely to do in the event of a failed connection. Microsoft issued a fix years ago, but at the time of this update (August 2007) a significant portion of browsers still in use are impacted by this problem. More information about DNS caching and browsers is available at http://www.tenereillo.com/BrowserDNSCache.htm


The browser DNS caching issue

Browser caching has extremely significant implications for GSLB. If a given site becomes unavailable because of a catastrophic failure, all clients that are currently using the site will experience connection failures until the browser DNS cache expires or they restart their browser or computer. Also, any clients that access DNS servers that have cached the IP address of the failed site will also experience connection failures. This is obviously unacceptable.


The following diagrams will help demonstrate the magnitude of this issue. Take a simple active/backup site scenario, the most simple and most popular of all GLSB configurations, and financial industry site (stock/bond trading, on-line banking, etc.):

A diagram showing a Global Server Load Balancer GSLB, two sites in an active/backup configuration, and clients happily connected to the active site.

The fictitious site www.ReallyBigWellTrustedFinancialSite.com configures Site A in Los Angeles as the primary site, with Site B as the backup site.


1)    To accomplish this, a single DNS answer, or “A record”, with the IP address of, is returned in response to a query for the FQDN www.ReallyBigWellTrustedFinancialSite.com. A GSLB device is deployed, using super dooper advanced health checking technology.

2)    Thousands of users are connected to Site A, happily conducting business. All have the IP address cached in their browsers.


Now disaster strikes, as shown in the following diagram:

A diagram showing a Global Server Load Balancer GSLB, two sites in an active/backup configuration, catastrophic failure at the active site, and existing clients unable to connect to either site for a period of up to 1/2 hour.

1)    The super dooper advanced health checking on the GSLB instantly detects a failure.

2)    The GSLB notes that Site B is still healthy, and begins returning the IP address, so as to route any new requests to site B.

3)    All existing users already have Site A, IP address, cached in their browsers. There is nothing the GSLB device can do to help these users, because clients do not make new DNS requests until the browser cache expires.


The site is effectively down for up to ½ hour for all existing clients, completely outside the control of the super dooper advanced health checking GSLB.


As if that’s not bad enough, it gets worse, as shown in the following diagram:

A diagram showing a Global Server Load Balancer GSLB, two sites in an active/backup configuration, catastrophic failure at the active site, existing clients unable to connect to either site for a period of up to 1/2 hour, and also new clients unable to connect to either site because of caching behavior at some ISPs.

1)    Some new clients share a DNS server which does not have the A record with answer cached. These clients make requests for www.ReallyBigWellTrustedFinancialSite.com.

2)    The client’s DNS server performs an iterative resolution on behalf of the client (at least the first to request), ending up at the GSLB, and the GSLB returns the A record for the healthy site, All is happy.

3)    Some other new clients share DNS servers that do have the A record with answer cached. This is either because these new requests are within the TTL window set by the GSLB, or because the DNS servers ignore low or zero TTLs (which, in fact, some DNS servers do). Because the answers are cached, the DNS servers do not query the GSLB, and do not know the site with IP address is down. These other new clients also see the entire site down for up to ½ hour, completely outside the control of the GSLB.

The solution to the browser caching issue


The long-standing solution to the browser DNS caching issue is for the authoritative name server (or GSLB) to return multiple DNS answers (or “A records”).


The use of multiple A records is not a trick of the trade, or a feature conceived by load balancing equipment vendors. The DNS protocol was designed with support for multiple A records for this very reason. Applications such as browsers and proxies and mail servers make use of that part of the DNS protocol.


The following diagrams show how this works:

A diagram showing two sites, one in LA, one in NYC, no Global Server Load Balancer GSLB, authoritative DNS configured to return multiple A records, and a client happily connected to the site in LA.

1)    The client requests a DNS resolution for the FQDN www.trapster.net.

2)    After iterative resolution (not shown), the client’s DNS server returns A records with IP addresses and then (in that order).

3)    The client makes a connection to Site A, IP


A diagram showing two sites, one in LA, one in NYC, no Global Server Load Balancer GSLB, authoritative DNS configured to return multiple A records, a client previously connected to the site in LA, catastrophic failure at the site in LA, and a client happily reconnecting to the other site in NYC.

1)    While the client is happily conducting business on Site A, a catastrophic event takes the site out completely. The connection to the site is lost.

2)    Because the second A record with address was originally sent, the client silently connects to Site B[2]. Note: Depending on the architecture of the business application, some state such as sign-on information, a shopping cart, or a financial transaction, may be lost because of the catastrophe, however the client can still connect to the replicated site and begin conducting business again.


The return of multiple A records does not require a GSLB device, but most GSLB devices are capable of returning multiple A records. All major DNS servers support the return of multiple A records, and essentially all commercial Internet sites that support browser based clients return multiple A records for this reason.

An Axiom


The only way to achieve high-availability GSLB for browser based clients is to include the use of multiple A records


There are a plethora of alternative solutions available, but none of them solve the issue effectively (see the section on “Alternative methods” below). Short of modifying the Windows Registry on every PC on the Internet that might potentially access the site, multiple A records is the only way.

Why the multiple A record solution doesn’t work with GSLB

As mentioned, DNS servers are able to return multiple A records. GSLB devices are also able to return multiple A records. Internet sites already have DNS servers. Internet site owners purchase GSLB devices, often for as much as $30,000 per unit, for the features these devices provide over and above the capabilities that come for “free” with DNS servers.


The problem:


None of these features work reliably in conjunction with multiple A records.


Not basic two site active standby, not static site preferences, not IANA table based preferences, not DNS persistence, not RTT or footraces, not geotargeting based redirection… none of them work! The following diagram will show why.


A diagram showing two sites, one in LA, one in NYC, a Global Server Load Balancer GSLB, a client in San Diego, that client's DNS server in Atlanta, and the reordering that happens on that client's DNS server, which in turn debilitates the function of the Global Server Load Balancer GSLB.


The GSLB DNS resolution takes place as in the previous example (steps such as health checking, RTT measurements, etc. removed for the sake of simplicity).

1)    Let’s say the GSLB device prefers Site A, IP address It returns the list of DNS answers in the order:



2)    The client’s DNS server receives that answer and places it into its cache. The client’s DNS server now returns the list either in the order:



Or in the order




Most all current commercial GSLB solutions return multiple A records in what is sometimes called an “ordered list”, assuming that the order will be passed all the way to the client stub resolver unchanged[3]. Unfortunately, that assumption is incorrect.


The order in which the addresses are returned can and will be changed by the client’s DNS server!


DNS servers do this to help with even distribution of traffic to multiple sites, and this is default behavior on most DNS servers at most providers[4]. It was previously thought that setting a TTL value of zero in step 1 above would prevent this re-ordering from happening, but unfortunately that is not true. Because the order of the list of A records can and will be changed, completely outside the control of a GSLB device or authoritative name server, it is not possible to deterministically prefer one site over another.

Site cookies: Guess what? Those don’t work either!

Most Web commercial sites that are hosted in multiple locations require session persistence, i.e. if a client connects to Site A, they must continue to connect to Site A for the duration of the session. Even sites that are totally synchronized work better with some level of persistence, because it is impossible to synchronize in real time.


There is a silver lining with the browser DNS caching issue. After a client resolves www.trapster.net to, say, Site A, IP address, that client will continue to connect to Site A until the browser DNS cache expires. As mentioned before, this time is 30 minutes on IE, and 15 on Netscape. Clearly this method alone will not suffice for persistence if session times are commonly greater than 30 (or 15) minutes, because the at that point the browser DNS cache will re-resolve, and the client will likely end up on the wrong site. Also, the 30 minutes and 15 minutes, respectively, are fixed timers, not inactivity timers. For example, if a user visits www.trapster.net, and then answers a phone call that takes 29 minutes, and then after the phone call returns to www.trapster.net to begin ordering some merchandise, the browser will re-resolve after only one minute, now likely directing the user to the wrong site.


This DNS client cache expiration issue is so widely known, that essentially all SLB vendors have implemented a fix for it. This method is usually called the “site cookie” method, and it is usually implemented only for HTTP (though it could work, and is implemented by at least one vendor, for some streaming media protocols). Here’s how it works:

A diagram showing two sites and a redirection scheme commonly known as the Global Server Load Balancing GSLB "site cookie" method.

1)    The client resolves www.trapster.net to Site A, IP address The client connects to Site A, and begins conducting business. In the course of the connections to Site A, the SLB at Site A inserts an HTTP cookie that indicates which site (and optionally which server) the client should connect to.

2)    After some period of time the client browser’s DNS cache expires, and the client re-resolves, this time to Site B, IP address The IP address will now be cached in the client’s browser for the next 30 (or 15) minutes. The client now makes a connection to Site B. When the client connects to Site B, it sends the site cookie indicating that the session should be at Site A.

3)    The SLB at Site B sees this cookie, and sends an HTTP redirect. The FQDN portion of the URL in the HTTP redirect cannot be www.trapster.net, because the IP address is still cached in the client’s browser. Also, the IP address probably cannot be used, because server software and SSL certificates will usually not work properly if requests do not use a DNS name. For this reason, a site-unique FQDN is almost always set up. In this case the HTTP redirect would be to, say, www-a.trapster.net (or maybe site-a.www.trapster.net).

4)    The client now reconnects to the same site using www-a.trapster.net and continues conducting business.


As shown below:

After some period of time, and especially for sites that experience long session times (such as stock trading or financial sites), a large percentage of users will be connected via the site-unique FQDN. Also, even for short session times, some users will bookmark the unique site FQDN www-a.trapster.net.


Because there are users that will access the site using www-a.trapster.net, for reliable high availability it is required that multiple A records are returned not only for www.trapster.net, but also for www-a.trapster.net. If both IP addresses and are returned in response to the query for www-a.trapster.net, the client may connect to either Site A or Site B!


Site cookies do not work properly with multiple A records, for the same reason DNS based GSLB does not work properly with multiple A records!

Is GSLB health checking beneficial?

We have established that multiple A records are necessary, but the remaining question is “are they sufficient for sites that support browser based clients?”. Browser based clients do their own sort of “health checking” when multiple A records are returned. Again, this is why multiple A records were designed into the DNS protocol. There may be a case for not returning the A record for a site that is known to be down, but there are also many cases where the A record for a dead site should be returned. This section presents such a case. Many types of failures are transient, i.e. the same problem happens at many different sites sequentially or simultaneously. Examples:

1)    A power failure may affect data centers in a region, and as the power grid is adjusted, may affect data centers in other regions.

2)    Denial of Services (DoS) attacks are usually launched against IP addresses. A DoS attack may first affect IP address, and then IP address

3)    A computer virus may first affect data center A. It may take ½ hour for site personnel to remove the virus. During this time, the virus may also affect data center B.

4)    An ISP may have an issue with routers that sequentially affects different regions of the country.

Given the previous examples, with Site A, IP address, and Site B, IP address, if an SLB or GSLB device (or even BIND with a health checking script) detects that, say, Site A is failed, should a single A record with IP address be returned? If Site B, IP address, subsequently fails, but Site A comes back up, within a ½ hour window, it would have been better to return both A records, even if health checking detected a failure. Remember, returning multiple A records is rarely a negative, as clients will silently connect to the first healthy site in the list of A records.

Alternative methods

There is a less severe sort of failure that can also occur. Servers at one of the sites may fail, while the power, Internet connection, and switching and SLB equipment remain operational. There are many commercially available solutions to that problem. These include backup redirection, triangulation, proxying, or NATing. Those are discussed here for the sake of completeness, but as this section will show, those solutions to the lesser problem of server failure do not in any way solve the more important issue of total site failure.



Triangulation is a recovery method that works for all IP protocol types.

A diagram showing two sites, a Global Server Load Balancer GSLB, and a traffic redirection method commonly known as "triangulation".

1)    Client already connected to Site A, happily surfing away.

2)    The servers at Site A now fail (but in this example the SLB, Internet connection, switches, routers, power, etc. are all OK). Software running on the SLB at Site A detects the server failure. Of course any existing TCP connections are lost, but the client will attempt to reconnect. The SLB at Site A now forwards packets that are part of a new connection request by the client to the SLB at Site B over a pre-established TCP tunnel.

3)    The SLB at Site B chooses a new server to service this client connection, and then returns the packets directly to the client, using the spoofed IP address


Backup redirection


Backup redirection works only for protocols which have application layer redirection capabilities (e.g. HTTP, HTTPS, some streaming media protocols).

A diagram showing two sites, a Global Server Load Balancer GSLB, and a traffic redirection method commonly known as "backup redirection".

1)    The client makes a request to Site A. This request is to, say, the FQDN www.trapster.net, which was resolved to the IP address

2)    The SLB at Site A, knowing that all servers at Site A have failed, issues an HTTP redirect to send the user to Site B. A different FQDN, say www-b.trapster.net, must be used. If the HTTP redirect was to www.trapster.net, the client would just use the IP address that was currently cached ( and end up back at Site A. Also, the URL in the HTTP probably cannot reference an IP address, as most servers, SSL certificates, etc. require that the site be accessed via an FQDN, not an IP address.

3)    The client now connects to Site B.


IP proxy and NAT

IP proxy (and NAT) work for all IP protocol types. They are not described in detail here. Each of these methods, upon failure of all servers at Site A, would load-balance connections from the client to a VIP on the SLB at Site B, much in the same way a local SLB would load-balance connections to local servers.


The issue with triangulation, backup redirection, IP proxy, and NAT

These methods will indeed help with disaster recovery if the Internet connection, switching equipment, power, or local SLB equipment are all operational, i.e. it is only the servers that have failed. If these methods are, however, used in conjunction with multiple A records, the value these methods add is questionable. To use these methods alone, without multiple A records, is to miss the forest for the trees. If total site loss is not a concern, there is not a strong argument for GSLB at all. It would probably be better to forget GSLB altogether, leaving behind all of its costs and complexities, and locate the sum total of all servers that would have been at two sites instead at one site, with redundant power, Internet, switching, and SLB equipment.


That said, high availability even in the case of catastrophe is the most fundamental requirement of GSLB implementations, and is almost always a specified requirement, therefore:


Triangulation, backup redirection, IP proxy, or NAT, are neither necessary or sufficient for purposes of high availability. Multiple A records are still needed for sites that support browser based clients[5].


BGP Host Route Injection


There is one more method, usually called BGP Host Route Injection (HRI), but also called “Global IP” by at least two vendors. It is not simply a backup method sometimes used in conjunction with DNS based GSLB, but rather a method used in place of DNS based GSLB. Here’s a high level overview of how it works:

A diagram showing two sites, and a Global Server Load Balancer GSLB method commonly known as "BGP Host Route Injection HRI". 

0)    (DNS resolution takes place, only one IP address,, is returned in response to a query for the FQDN www.trapster.net).

1)    Server load balancers (or routers) at Sites A and B each advertise to the Internet “I’m IP address”. Internet routers exchange metrics via BGP, and propagate this information to the router closest to the client.

2)    The client now connects to the topographically closest site.


Now all servers, or even the Internet connection, power, SLB, or switching equipment fail, or there is a catastrophic site loss.

A diagram showing two sites, a Global Server Load Balancer GSLB method commonly known as "BGP Host Route Injection HRI", and BGP route convergence after a site failure.


1)    The SLB at Site A detects the server failure, and stops advertising via BGP that it is IP address (or the SLB or connection are destroyed, in which case advertising also obviously stops).

2)    Route convergence between Internet routers occurs, so that the path to Site A is eventually removed.

3)    The client now connects to IP address, but at Site B.


Though in theory this method may look like the Holy Grail of GSLB, it is seldom implemented. Here’s why:

·      Internet routing is quite complex, and the practice of advertising the same IP address for disparate locations does not work reliably. If route maps change during a client session, packets may flow to Site A and Site B intermittently, so that the client cannot successfully connect even if both sites are functioning properly.

·      Route convergence can take considerable time. In the case of a failure, the client browser will time out, presenting a failure dialog. If the user persists in manual reattempts, eventually a connection will succeed, but it is not uncommon for this to take more than five minutes. Such down time is usually deemed unacceptable for commercial Web applications.

·      BGP route advertisements that are single IP addresses (hosts) are usually ignored by Internet routers. A possible solution is to instead advertise an entire network address, however doing so simply for the purpose of GSLB constitutes a waste of expensive IP addresses (since only one, or maybe a handful, are actually used).

·      Source IP address filters (sometimes called “bogon” filters), configured on routers for security purposes, often prevent devices from advertising IP addresses from multiple locations. This problem is often solvable by negotiations with ISPs, but is nevertheless another complication, as in practice “bogon” filters that are removed by such negotiations are often inadvertently added back by ISP personnel, causing downed sites, requiring trouble ticket calls, etc.


BGP HRI is a sound approach for closed networks, and may be made to work for some Internet applications, but is quite rare because it does not work in practice nearly as well as it does in theory.


The only way to achieve high availability GSLB for browser based clients is to return multiple A records, but returning multiple A records diminishes any possibility of deterministic site selection. Because of this features such as basic two site active-standby, DNS persistence, RTT or footrace or BGP hop count based site selection, IP address based geotargeting or IANA table based selection are all effectively useless.


The good news – now customers can use the $30,000/unit they would have spent on GSLB devices to add more servers and inter-site synchronization capabilities!

Watering it down


At the risk of helping the technically correct to obfuscate the obvious[6], the following:


At least in theory, a GSLB device could operate in a “best choices, round robin” type fashion. For example, if a FQDN is hosted in two sites in Europe and two sites in the US, the GSLB could determine that a given client is best served by the Europe sites and return only the Europe A records to that client’s DNS server. The client’s DNS server would then round robin between those. From the following diagram:

A diagram showing four sites, two in Europe (Frankfurt and Paris), and two in the US (LA and NYC), a Global Server Load Balancer GSLB, a client in London, and that client's DNS server somewhere in the UK. Also a DNS response with 2 A records being passed from the GSLB to the client's DNS server, re-ordering, and the re-ordered list being passed to the client.


0)    As before (not shown in diagram) the client makes a request for the FQDN www.trapster.net, the request ends up at the authoritative DNS server for www.trapster.net which is the GSLB device, the GSLB performs the selected algorithm and determines that Frankfurt and Paris are better choices than either LA or New York for that client.

1)    The GSLB returns both A records and

2)    The list of A records is re-ordered at the client’s DNS server, but in this case that’s OK and expected. The client connects to either Frankfurt or Paris.


For the purpose of this example, let’s arbitrarily choose the term “zone” to denote a GSLB topography. Say sites A and B above are in the Europe GSLB “zone”, and sites C and D are in the US zone. Such functionality would only work for global sites that have content replicated in at least two datacenters in every given zone that is to be GSLBed. If www.trapster.net was instead hosted in datacenters in London, New York, and Tokyo, the “best choices, round robin” solution would not be very useful. For a client in London, the GSLB would need to return A records for London (which is topographically close to the client), and also at least one of the others (New York or Tokyo, neither of which are topographically close to the client). The client would be equally likely to connect to London or one of the other sites. Clearly such complications far diminish the intended purpose of GSLB site selection algorithms. Furthermore, some site selection methods (such as footrace variants) cannot work in a “best choices, round robin” fashion (exercise left to the reader), and DNS persistence by definition does not work with “best choices, round robin”. Though such a complex implementation could be made to work for a handful of very large global sites, it is not a generally useful solution to the problem presented in this paper.



Copyright Tenereillo, Inc. 2004


[1] This paper discusses Internet behavior in the sense of the larger Internet. Certainly custom solutions can be created. For example, there is the possibility of distribution of special software to run on client computers. Such solutions are almost never practical for Internet sites, and are therefore not discussed here. Also, exceptions can be found in non-custom solutions, such as browsers or DNS servers that are configured to behave differently than described here. Again, those are very rare. Finally, there are applications that are not required to serve browser based clients. This paper does not apply to those applications.

[2] The timeliness of the connection attempt to the second site is application dependent, but essentially all browser based applications will try the second IP address after a few unanswered TCP SYN packets.

[3] Some solutions also attempt to achieve site weighting by returning duplicates. For example, to get 75% of traffic on site one they would return the ordered list {,,,}. This feature does not work reliably, as most DNS servers note the duplicates and round robin the list as {,}.

[4] The order that A records are returned by the most commonly deployed DNS server, BIND, is as follows. The first record is chosen at random. The remaining records are returned in a cyclic order. For example, if the ordered list is {,,} the first response might be {,,}, the next response to a subsequent client request, from a client that shares that name server, would be {,,}. Other DNS resolvers and servers reorder the list differently, for example the Windows XP DNS cache will reorder a response such that any subnet-adjacent IP address is returned first. This paper does not attempt to provide a canonical list of such issues. It will suffice to say that, for a number of reasons, the order in an ordered list of A records cannot be expected to be preserved.

[5] Sites that do not support browser based clients, but do support applications that do not work well with multiple A records (indeed some custom robot type client applications do not), have no other recourse but to use one of these recover methods.

[6] The “Watering it down” section was part of the original paper. I removed it because I thought it complicated the issue. Reviewers suggested that I put it back in. Done. 5/26/04.