currently the DNS server query strategy was sequential, it means that lwip would query the 1st server DNS_MAX_RETRIES times before switching to the backup one. In the scenario that the main DNS server was unavailable, but the backup was available, this query stratedy would cusume much time to complete one successful domain name resolution.
I suggest the DNS query stradegy changed to Round Robin. so the query procedure was changed from "A->A->A->A->B->B->B->B" to "A->B->A->B->A->B->A->B", A and B was the main and backup DNS server. In current settings, this change could make the domain name resolution time from 6s to 1s in above scenario.
The attached file was my modification, pls have a review. There was four changes:
1, change the DNS query strategy from sequential to Round Robin;
2, for one successful DNS query, record the current server index to make the next DNS query start from this index, this would save more time if many domain name resolution happened in above case.
3, reset the current server index to the 1st valid one if servers changed.
4, change the initail DNS timeout from 1s to 2s. This could make the 1st timeout value was [1s, 2s]. If the initial timeout was set to 1s, the actually timeout value was [0, 1s], this could incur some unnecessary DNS query retransmission.