Significance of "WARNING: recursion requested but not available" from ISP?

catkin · 10-23-2013, 02:56 AM

What is the significance of "WARNING: recursion requested but not available" from our ISP's DNS servers?

Our ISP's DNS servers are not great but, at best, are faster than any of the public DNS servers, including Google's which are so fast they must be co-located. So we have a rudimentary performance test script that every hour runs dig +nosearch "@$ip_address" www.google.com where $ip_address is cycled through a list of our ISP's and good public DNS services'. The intention is to chose the best and configure our local DNS server with them.

Almost all our ISP's servers generate "WARNING: recursion requested but not available" almost exactly 1/3rd of the time. Almost none of the public servers ever do.

How should we treat this information when choosing the "best" servers? Should we discount servers that refuse to do recursion or should we ignore the recursion messages and simply choose the fastest?

Here's a recent analysis of the results:

Code:

Results aggregated from 942 logs:

Failure codes
 T = timeout, F = not resolved, R = no recursion)


 Time (secs)          Failures  Not logged       IP address Description        
BSNL               
         .09    T:21 F:0 R:350           9    218.248.245.1 BSNL               
         .11    T:22 F:1 R:358           9  218.248.240.181 BSNL               
         .13    T:19 F:3 R:364          10  218.248.240.180 BSNL               
         .13    T:22 F:0 R:342           9  218.248.240.179 BSNL               
         .17    T:894 F:0 R:30           9    203.94.243.70 MTNL               
         .21    T:22 F:0 R:237         242    218.248.241.4 BSNL               
         .22    T:21 F:0 R:366          11  218.248.240.208 BSNL               
         .22    T:21 F:37 R:32          10   218.248.240.23 BSNL               
         .30      T:24 F:0 R:0          10          8.8.8.8 Google             
         .31      T:25 F:0 R:0          13          8.8.4.4 Google             
         .32      T:31 F:0 R:0         243    218.248.241.7 BSNL               
         .32    T:79 F:2 R:323          12  218.248.255.163 BSNL               
         .34      T:22 F:0 R:0          11    198.153.192.1 Norton             
         .38      T:48 F:0 R:0          29  218.248.255.145 BSNL               
         .40      T:24 F:0 R:0           9     156.154.70.1 DNSadvantage       
         .45      T:24 F:0 R:0          14   208.67.220.220 OpenDNS            
         .46      T:25 F:0 R:0           9   208.67.222.222 OpenDNS            
         .47      T:50 F:0 R:0          13     156.154.71.1 DNSadvantage       
         .49      T:28 F:0 R:0          12          4.2.2.3 Level 3            
         .49      T:29 F:0 R:0          14          4.2.2.5 Level 3            
         .51      T:27 F:0 R:0          11          4.2.2.4 Level 3            
         .51      T:27 F:0 R:0          13          4.2.2.2 Level 3            
         .51      T:28 F:0 R:0          11      209.244.0.3 Public Name server 
         .53      T:24 F:0 R:0          13      209.244.0.4 Public Name server 
         .54      T:26 F:0 R:0          13    198.153.194.1 Norton             
         .54      T:29 F:0 R:0          14          4.2.2.6 Level 3            
         .55    T:63 F:6 R:328          28  218.248.255.194 BSNL               
         .56      T:31 F:0 R:0          15          4.2.2.1 Level 3            
         .56   T:78 F:10 R:300          29  218.248.255.212 BSNL               
        1.13   T:96 F:14 R:301          50  218.248.255.146 BSNL               
        1.20  T:110 F:28 R:292          37  218.248.255.196 BSNL               
        1.24  T:164 F:32 R:266          56  218.248.255.147 BSNL               
        1.49   T:169 F:318 R:0          63  218.248.255.139 BSNL

druuna · 10-23-2013, 06:31 AM

Too my knowledge a recursive DNS search is one that cannot be resolved from the local DNS DB. In this case you either get a message that the query cannot be resolved or it can ask another DNS server to do the job.

Assuming the second scenario; If another DNS server is used then the one you provided, the result will be tainted.

I would assume that a recursive DNS search is probably slower due to the second DNS server that is used.

catkin · 07-15-2014, 03:59 AM

Thanks Druuna

and sorry for the delay

You are right. A recursive name server is one that will ask other name servers if it does not have the data.

Typically an ISP's name servers are recursive. In case they do not have data to answer a query (say from a simple ADSL-attached client) they ask a root server for the TLD for the authoritative name server for the domain and ask it. That process is "iterative". It is done "recursively" for the original client.

This arrangement finally made sense when I stumbled on a nice diagram illustrating the difference between recursive and iterative lookups at http://www.zytrax.com/books/dns/ch2/...html#recursive. Thanks Zytrax

Going back to my original post, BSNL (our ISP) should provide name servers that will do recursion. Not doing so is broken behaviour for the role they play.