Clients using multiple local DNS servers

Some client IP addresses in our data are associated with multiple LDNS IP addresses. This may happen due to the following reasons: (1) The first LDNS server the client contacts times out and the second LDNS server is contacted. (2) The client's LDNS server is configured by a DHCP server that assigns the LDNS server IP addresses from a set of addresses in a round-robin fashion. (3) A client may be configured to round-robin among multiple LDNS servers. (4) The client IP address is reused at different times by different users and these users may have different configurations for their LDNS servers, resulting in different associations. (5) The client IP address is that of a NAT box or a application-level proxy, so there are multiple actual clients behind this IP address using different LDNS servers. (6) The client is misconfigured.

Here we examine the distribution of the LDNS servers with which a client IP address is associated. If they all occupy the same cluster as the client, DNS-based server selection can use the local DNS server's IP address to estimate where the client is even if the client uses multiple local DNS servers. However, if they occupy multiple clusters or a single cluster different from the client, it is more difficult to use DNS-based server selection. In Table 8, we show how many clients use ten or fewer local DNS servers. In addition, we observe that some IP addresses are associated with up to 330 local DNS servers occupying up to 273 different network clusters. Further investigation shows that some of these addresses belong to cache proxies. In general, we observe that the more LDNS servers with which a client IP address is associated, the lower the percentage of associations with the client and LDNS in the same cluster. Fortunately, the majority of client IP addresses are associated with a single LDNS server. They are responsible for about 52% of the requests. However, only about 20% in this group have the client and LDNS in the same network cluster.

Table 8: Clients using ten or fewer multiple local DNS servers

# of clients	# of LDNS	% of total	% associations
(% of total)	(avg # of	HTTP	with client and
	NACs)	requests	LDNS in
			the same NAC
2,524,939 (78.064)	1 (1.0)	51.8	20.3
522,228 (16.146)	2 (1.6)	22.4	12.1
123,524 (3.819)	3 (2.1)	10.4	6.6
41,422 (1.281)	4 (2.5)	4.9	4.7
13,469 (0.416)	5 (2.9)	2.5	4.9
4,555 (0.141)	6 (3.3)	1.8	6.7
1,590 (0.049)	7 (4.1)	1.3	9.9
713 (0.022)	8 (4.7)	0.7	13.6
461 (0.014)	9 (5.5)	0.7	14.2
273 (0.008)	10 (6.1)	0.5	14.0