resolved: fully support DNS search domains

This adds support for searching single-label hostnames in a set of
configured search domains.

A new object DnsQueryCandidate is added that links queries to scopes.
It keeps track of the search domain last used for a query on a specific
link. Whenever a host name was unsuccessfuly resolved on a scope all its
transactions are flushed out and replaced by a new set, with the next
search domain appended.

This also adds a new flag SD_RESOLVED_NO_SEARCH to disable search domain
behaviour. The "systemd-resolve-host" tool is updated to make this
configurable via --search=.

Fixes #1697

1 files changed, 4 insertions, 0 deletions

diff --git a/src/resolve/resolved-dns-scope.h b/src/resolve/resolved-dns-scope.h
index f69d51b203..244bb41314 100644
--- a/src/resolve/resolved-dns-scope.h
+++ b/src/resolve/resolved-dns-scope.h
@@ -59,6 +59,7 @@ struct DnsScope {
         usec_t max_rtt;
 
         Hashmap *transactions;
+        LIST_HEAD(DnsQueryCandidate, query_candidates);
 
         LIST_FIELDS(DnsScope, scopes);
 };
@@ -91,3 +92,6 @@ void dns_scope_check_conflicts(DnsScope *scope, DnsPacket *p);
 void dns_scope_dump(DnsScope *s, FILE *f);
 
 DnsSearchDomain *dns_scope_get_search_domains(DnsScope *s);
+bool dns_scope_has_search_domains(DnsScope *s);
+
+int dns_scope_name_needs_search_domain(DnsScope *s, const char *name);