The name resolve functions of various libc implementations don't re-read name
server information unless explicitly told so (for example, by calling
res_init). This may cause libcurl to keep using the older server even
if DHCP has updated the server info, and this may look like a DNS cache issue
to the casual libcurl-app user.
Note that DNS entries have a "TTL" property but libcurl doesn't use that. This DNS cache timeout is entirely speculative that a name will resolve to the same address for a certain small amount of time into the future.