tree-wide: remove Emacs lines from all files This should be handled fine now by .dir-locals.el, so need to carry that stuff in every file.

resolved: rework what ResolveHostname() with family == AF_UNSPEC means Previously, if a hostanem is resolved with AF_UNSPEC specified, this would be used as indication to resolve both an AF_INET and an AF_INET6 address. With this change this logic is altered: an AF_INET address is only resolved if there's actually a routable IPv4 address on the specific interface, and similar an AF_INET6 address is only resolved if there's a routable IPv6 address. With this in place, it's ensured that the returned data is actually connectable by applications. This logic mimics glibc's resolver behaviour. Note that if the client asks explicitly for AF_INET or AF_INET6 it will get what it asked for. This also simplifies the logic how it is determined whether a specific lookup shall take place on a scope. Specifically, the checks with dns_scope_good_key() are now moved out of the transaction code and into the query code, so that we don't even create a transaction object on a specific scope if we cannot execute the resolution on it anyway.

resolve: generate a nice clean error when clients try to resolve a name when the network is down

resolved: rework OPT RR generation logic This moves management of the OPT RR out of the scope management and into the server and packet management. There are now explicit calls for appending and truncating the OPT RR from a packet (dns_packet_append_opt() and dns_packet_truncate_opt()) as well as a call to do the right thing depending on a DnsServer's feature level (dns_server_adjust_opt()). This also unifies the code to pick a server between the TCP and UDP code paths, and makes sure the feature level used for the transaction is selected at the time the server is picked, and not changed until the next time we pick a server. The server selction code is now unified in dns_transaction_pick_server(). This all fixes problems when changing between UDP and TCP communication for the same server, and makes sure the UDP and TCP codepaths are more alike. It also makes sure we never keep the UDP port open when switchung to TCP, so that we don't have to handle incoming datagrams on the latter we don't expect. As the new code picks the DNS server at the time we make a connection, we don't need to invalidate the DNS server anymore when changing to the next one, thus dns_transaction_next_dns_server() has been removed.

resolved: name TCP and UDP socket calls uniformly Previously the calls for emitting DNS UDP packets were just called dns_{transacion|scope}_emit(), but the one to establish a DNS TCP connection was called dns_transaction_open_tcp(). Clean this up, and rename them dns_{transaction|scope}_emit_udp() and dns_transaction_open_tcp().

resolved: add code to join/leave mDNS multicast groups Per link, join the mDNS multicast groups when the scope is created, and leave it again when the scope goes away.

resolved: introduce a dnssec_mode setting per scope The setting controls which kind of DNSSEC validation is done: none at all, trusting the AD bit, or client-side validation. For now, no validation is implemented, hence the setting doesn't do much yet, except of toggling the CD bit in the generated messages if full client-side validation is requested.

resolved: rework how we allow allow queries to be dispatched to scopes Previously, we'd never do any single-label or root domain lookups via DNS, thus leaving single-label lookups to LLMNR and the search path logic in order that single-label names don't leak too easily onto the internet. With this change we open things up a bit, and only prohibit A/AAAA lookups of single-label/root domains, but allow all other lookups. This should provide similar protection, but allow us to resolve DNSKEY+DS RRs for the top-level and root domains. (This also simplifies handling of the search domain detection, and gets rid of dns_scope_has_search_domains() in favour of dns_scope_get_search_domains()).

resolved: implement minimal EDNS0 support This is a minimal implementation of RFC6891. Only default values are used, so in reality this will be a noop. EDNS0 support is dependent on the current server's feature level, so appending the OPT pseudo RR is done when the packet is emitted, rather than when it is assembled. To handle different feature levels on retransmission, we strip off the OPT RR again after sending the packet. Similarly, to how we fall back to TCP if UDP fails, we fall back to plain UDP if EDNS0 fails (but if EDNS0 ever succeeded we never fall back again, and after a timeout we will retry EDNS0).

resolved: handle properly if there are multiple transactions for the same key per scope When the zone probing code looks for a transaction to reuse it will refuse to look at transactions that have been answered from cache or the zone itself, but insist on the network. This has the effect that there might be multiple transactions around for the same key on the same scope. Previously we'd track all transactions in a hashmap, indexed by the key, which implied that there would be only one transaction per key, per scope. With this change the hashmap will only store the most recent transaction per key, and a linked list will be used to track all transactions per scope, allowing multiple per-key per-scope. Note that the linked list fields for this actually already existed in the DnsTransaction structure, but were previously unused.

dns-domain: simplify dns_name_is_root() and dns_name_is_single_label() Let's change the return value to bool. If we encounter an error while parsing, return "false" instead of the actual parsing error, after all the specified hostname does not qualify for what the function is supposed to test. Dealing with the additional error codes was always cumbersome, and easily misused, like for example in the DHCP code. Let's also rename the functions from dns_name_root() to dns_name_is_root(), to indicate that this function checks something and returns a bool. Similar for dns_name_is_signal_label().

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

resolved: add a generic DnsSearchDomain concept With this change, we add a new object to resolved, "DnsSearchDomain=" which wraps a search domain. This is then used to introduce a global search domain list, in addition to the existing per-link search domain list which is reword to make use of this new object too. This is preparation for implement proper unicast DNS search domain support.

tree-wide: sort includes in *.h This is a continuation of the previous include sort patch, which only sorted for .c files.

resolved: dump cache and zone contents to syslog on SIGUSR1

resolved: replace transaction list by hashmap Right now we keep track of ongoing transactions in a linked listed for each scope. Replace this by a hashmap that is indexed by the RR key. Given that all ongoing transactions will be placed in pretty much the same scopes usually this should optimize behaviour. We used to require a list here, since we wanted to do "superset" query checks, but this became obsolete since transactions are now single-key instead of multi-key.

resolved: only maintain one question RR key per transaction Let's simplify things and only maintain a single RR key per transaction object, instead of a full DnsQuestion. Unicast DNS and LLMNR don't support multiple questions per packet anway, and Multicast DNS suggests coalescing questions beyond a single dns query, across the whole system.

resolved: transaction - exponentially increase retry timeouts Rather than fixing this to 5s for unicast DNS and 1s for LLMNR, start at a tenth of those values and increase exponentially until the old values are reached. For LLMNR the recommended timeout for IEEE802 networks (which basically means all of the ones we care about) is 100ms, so that should be uncontroversial. For unicast DNS I have found no recommended value. However, it seems vastly more likely that hitting a 500ms timeout is casued by a packet loss, rather than the RTT genuinely being greater than 500ms, so taking this as a startnig value seems reasonable to me. In the common case this greatly reduces the latency due to normal packet loss. Moreover, once we get support for probing for features, this means that we can send more packets before degrading the feature level whilst still allowing us to settle on the correct feature level in a reasonable timeframe. The timeouts are tracked per server (or per scope for the multicast protocols), and once a server (or scope) receives a successfull package the timeout is reset. We also track the largest RTT for the given server/scope, and always start our timouts at twice the largest observed RTT.

resolved: transaction - introduce dns_transaction_emit() This function emits the UDP packet via the scope, but first it will determine the current server (and connect to it) and store the server in the transaction. This should not change the behavior, but simplifies the code.

resolved: transaction - move DNS UDP socket creation to the scope With access to the server when creating the socket, we can connect() to the server and hence simplify message sending and receiving in follow-up patches.

resolve: drop dns_scope_good_dns_server() It's not used anymore since 29815b6c608b836cada5e349d06a96b63eaa65f3, hence let's remove it from the sources.

resolved: use one UDP socket per transaction We used to have one global socket, use one per transaction instead. This has the side-effect of giving us a random UDP port per transaction, and hence increasing the entropy and making cache poisoining significantly harder to achieve. We still reuse the same port number for packets belonging to the same transaction (resent packets).

resolved: pin the server used in a transaction We want to discover information about the server and use that in when crafting packets to be resent.

remove unused includes This patch removes includes that are not used. The removals were found with include-what-you-use which checks if any of the symbols from a header is in use.

resolve: make DnsScope::conflict_queue an OrderedHashmap on_conflict_dispatch() uses hashmap_steal_first() and then does something non-trivial with it. It may care about the order.

resolved: allow passing on which protocol, family and interface to look something up Also, return on which protocol/family/interface we found something.

resolved: implement full LLMNR conflict detection logic

resolved: enforce ratelimit on LLMNR traffic

resolved: never reuse transactions for probing that are already completed based on cached data

resolved: flush cache each time we change to a different DNS server

resolved: rename resolved.h to resolved-manager.h After all it pretty much exlcusively containers definitions about the "Manager" object, hence let's call this the most obvious way.

resolved: implement LLMNR uniqueness verification

resolve: add llmnr responder side for UDP and TCP Name defending is still missing.

change type for address family to "int" Let's settle on a single type for all address family values, even if UNIX is very inconsitent on the precise type otherwise. Given that socket() is the primary entrypoint for the sockets API, and that uses "int", and "int" is relatively simple and generic, we settle on "int" for this.

resolved: add LLMNR support for looking up names

resolved: add DNS cache

resolved: support for TCP DNS queries

resolved: add a DNS client stub resolver Let's turn resolved into a something truly useful: a fully asynchronous DNS stub resolver that subscribes to network changes. (More to come: caching, LLMNR, mDNS/DNS-SD, DNSSEC, IDN, NSS module)