Some comments about the pull request.
One of the main goals was to preserve current API as much as possible and not to create new exported functions.

Fallback to TCP in case of truncated DNS requests is done automatically. To imitate the old behaviour macros DNS_QUERY_IGNTC should be used. To force all DNS requests to be done via TCP one should use the flag DNS_QUERY_USEVC. Names DNS_QUERY_IGNTC, DNS_QUERY_USEVC were chosen to imitate similar flags in c-ares and glibc.

Support for TCP connections in DNS server is done primarly for tests. I wasn't sure how better to implement it (in terms of API). At the moment the same function evdns_add_server_port_with_base is used to create TCP or UDP servers (to handle TCP and UDP requests at the same type one should create 2 servers). There are some other implementation variants: like create a special type for TCP servers or a new type that will handle TCP and UDP servers at the same type. I hoped the right decision about the API will be found during the discussion. Therefore to simplify things at the moment all TCP and UDP data are in the same class evdns_server_port and functions that work work with server have mixed code for TCP and UDP (which in some cases might be rather confusing). After the discussion it will be possibly e.g. to create 2 internal structure like evdns_tcp_server_port and evdns_udp_server_port which might have some mutual part evdns_server_port and make more clear distinction in code that process TCP and UDP cases.

Many thanks for this patch set!
I need to look through the code carefully, but I like the idea in general for sure (especially that everything is configurable).

Refs: #684

Hi! I've squashed all changes to one commit, rebased on master and forced pushed.

Applied, many thanks! (I fixed some small issues, you can take a look at the topic branch that contains it, and also I added the description of the PR to the topic branch merge commit)

Hi! Thanks for your help and support.