resolved: fallback to TCP if UDP fails

This is inspired by the logic in BIND [0], follow-up patches
will implement the reset of that scheme.

If we get a server error back, or if after several attempts we don't
get a reply at all, we switch from UDP to TCP for the given
server for the current and all subsequent requests. However, if
we ever successfully received a reply over UDP, we never fall
back to TCP, and once a grace-period has passed, we try to upgrade
again to using UDP. The grace-period starts off at five minutes
after the current feature level was verified and then grows
exponentially to six hours. This is to mitigate problems due
to temporary lack of network connectivity, but at the same time
avoid flooding the network with retries when the feature attempted
feature level genuinely does not work.

Note that UDP is likely much more commonly supported than TCP,
but depending on the path between the client and the server, we
may have more luck with TCP in case something is wrong. We really
do prefer UDP though, as that is much more lightweight, that is
why TCP is only the last resort.

[0]: <https://kb.isc.org/article/AA-01219/0/Refinements-to-EDNS-fallback-behavior-can-cause-different-outcomes-in-Recursive-Servers.html>

1 files changed, 5 insertions, 0 deletions

diff --git a/src/resolve/resolved-dns-scope.c b/src/resolve/resolved-dns-scope.c
index fc4ae57ce0..a8c0ae1569 100644
--- a/src/resolve/resolved-dns-scope.c
+++ b/src/resolve/resolved-dns-scope.c
@@ -243,6 +243,11 @@ static int dns_scope_socket(DnsScope *s, int type, int family, const union in_ad
                 if (!srv)
                         return -ESRCH;
 
+                srv->possible_features = dns_server_possible_features(srv);
+
+                if (type == SOCK_DGRAM && srv->possible_features < DNS_SERVER_FEATURE_LEVEL_UDP)
+                        return -EAGAIN;
+
                 sa.sa.sa_family = srv->family;
                 if (srv->family == AF_INET) {
                         sa.in.sin_port = htobe16(port);