net: resolv.conf rotate option captured but not used

[danp]

The resolv.conf rotate option is captured when reading config but is not used by the Go stub resolver when querying.

Linux's resolv.conf(5) says:

rotate sets  RES_ROTATE in _res.options, which causes round-robin selection of name‐
       servers from among those listed.  This has the effect of spreading the  query
       load  among  all listed servers, rather than having all clients try the first
       listed server first every time.

cc @captncraig

[captncraig]

We noticed this caused an imbalance on the load our dns servers were handling. Would be really nice if the flag were honored.

[bradfitz]

/cc @mdempsky

[mdempsky]

Seems worth supporting to me. Were either of you planning to prepare a CL for this?

[captncraig]

not me.

[danp]

I can do it, sure.

[gopherbot]

CL https://golang.org/cl/29233 mentions this issue.

[captncraig]

If I were implementing this, I would consider just incrementing an atomic int at the beginning of tryOneName and using that as an offset for my iteration.