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resolved fixes for handling SERVFAIL errors from servers
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There might be two reasons why we get a SERVFAIL response from our selected DNS
server: because this DNS server itself is bad, or because the DNS server
actually serving the zone upstream is bad. So far we immediately downgraded our
server feature level when getting SERVFAIL, under the assumption that the first
case is the only possible case. However, this meant we'd downgrade immediately
even if we encountered the second case described above.
With this commit handling of SERVFAIL is reworked. As soon as we get a SERVFAIL
on a transaction we retry the transaction with a lower feature level, without
changing the feature level tracked for the DNS server itself. If that fails
too, we downgrade further, and so on. If during this downgrading the SERVFAIL
goes away we assume that the DNS server we are talking to is bad, but the zone
is fine and propagate the detected feature level to the information we track
about the DNS server. Should the SERVFAIL not go away this way we let the
transaction fail and accept the SERVFAIL.
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In order to improve compatibility with local clients that speak DNS directly
(and do not use NSS or our bus API) listen locally on 127.0.0.53:53 and process
any queries made that way.
Note that resolved does not implement a full DNS server on this port, but
simply enough to allow normal, local clients to resolve RRs through resolved.
Specifically it does not implement queries without the RD bit set (these are
requests where recursive lookups are explicitly disabled), and neither queries
with DNSSEC DO set in combination with DNSSEC CD (i.e. DNSSEC lookups with
validation turned off). It also refuses zone transfers and obsolete RR types.
All lookups done this way will be rejected with a clean error code, so that the
client side can repeat the query with a reduced feature set.
The code will set the DNSSEC AD flag however, depending on whether the data
resolved has been validated (or comes from a local, trusted source).
Lookups made via this mechanisms are propagated to LLMNR and mDNS as necessary,
but this is only partially useful as DNS packets cannot carry IP scope data
(i.e. the ifindex), and hence link-local addresses returned cannot be used
properly (and given that LLMNR/mDNS are mostly about link-local communication
this is quite a limitation). Also, given that DNS tends to use IDNA for
non-ASCII names, while LLMNR/mDNS uses UTF-8 lookups cannot be mapped 1:1.
In general this should improve compatibility with clients bypassing NSS but
it is highly recommended for clients to instead use NSS or our native bus API.
This patch also beefs up the DnsStream logic, as it reuses the code for local
TCP listening. DnsStream now provides proper reference counting for its
objects.
In order to avoid feedback loops resolved will no silently ignore 127.0.0.53
specified as DNS server when reading configuration.
resolved listens on 127.0.0.53:53 instead of 127.0.0.1:53 in order to leave
the latter free for local, external DNS servers or forwarders.
This also changes the "etc.conf" tmpfiles snippet to create a symlink from
/etc/resolv.conf to /usr/lib/systemd/resolv.conf by default, thus making this
stub the default mode of operation if /etc is not populated.
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Make sure we can parse DNS server addresses that use the "zone id" syntax for
local link addresses, i.e. "fe80::c256:27ff:febb:12f%wlp3s0", when reading
/etc/resolv.conf.
Also make sure we spit this out correctly again when writing /etc/resolv.conf
and via the bus.
Fixes: #3359
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This should be handled fine now by .dir-locals.el, so need to carry that
stuff in every file.
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I'm not defining _DNS_SERVER_TYPE_MAX/INVALID as usual in the enum,
because it wouldn't be used, and then gcc would complain that
various enums don't test for _DNS_SERVER_TYPE_MAX. It seems better
to define the macro rather than add assert_not_reached() in multiple
places.
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If we downgrade from DNSSEC to non-DNSSEC mode, let's log about this in a recognizable way (i.e. with a message ID),
after all, this is of major importance.
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reason to
This adds logic to downgrade the feature level more aggressively when we have reason to. Specifically:
- When we get a response packet that lacks an OPT RR for a query that had it. If so, downgrade immediately to UDP mode,
i.e. don't generate EDNS0 packets anymore.
- When we get a response which we are sure should be signed, but lacks RRSIG RRs, we downgrade to EDNS0 mode, i.e.
below DO mode, since DO is apparently not really supported.
This should increase compatibility with servers that generate non-sensical responses if they messages with OPT RRs and
suchlike, for example the situation described here:
https://open.nlnetlabs.nl/pipermail/dnssec-trigger/2014-November/000376.html
This also changes the downgrade code to explain in a debug log message why a specific downgrade happened.
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DNSSEC
Move detection into a set of new functions, that check whether one specific server can do DNSSEC, whether a server and
a specific transaction can do DNSSEC, or whether a transaction and all its auxiliary transactions could do so.
Also, do these checks both before we acquire additional RRs for the validation (so that we can skip them if the server
doesn't do DNSSEC anyway), and after we acquired them all (to see if any of the lookups changed our opinion about the
servers).
THis also tightens the checks a bit: a server that lacks TCP support is considered incompatible with DNSSEC too.
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This makes it easier to log information about a specific DnsServer object.
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This changes the DnsServer logic to count failed UDP and TCP failures separately. This is useful so that we don't end
up downgrading the feature level from one UDP level to a lower UDP level just because a TCP connection we did because
of a TC response failed.
This also adds accounting of truncated packets. If we detect incoming truncated packets, and count too many failed TCP
connections (which is the normal fall back if we get a trucnated UDP packet) we downgrade the feature level, given that
the responses at the current levels don't get through, and we somehow need to make sure they become smaller, which they
will do if we don't request DNSSEC or EDNS support.
This makes resolved work much better with crappy DNS servers that do not implement TCP and only limited UDP packet
sizes, but otherwise support DNSSEC RRs. They end up choking on the generally larger DNSSEC RRs and there's no way to
retrieve the full data.
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The name "features" suggests an orthogonal bitmap or suchlike, but the
variables really encode only a linear set of feature levels. The type
used is already called DnsServerFeatureLevel, hence fix up the variables
accordingly, too.
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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.
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This adds a mode that makes resolved automatically downgrade from DNSSEC
support to classic non-DNSSEC resolving if the configured DNS server is
not capable of DNSSEC. Enabling this mode increases compatibility with
crappy network equipment, but of course opens up the system to
downgrading attacks.
The new mode can be enabled by setting DNSSEC=downgrade-ok in
resolved.conf. DNSSEC=yes otoh remains a "strict" mode, where DNS
resolving rather fails then allow downgrading.
Downgrading is done:
- when the server does not support EDNS0+DO
- or when the server supports it but does not augment returned RRs with
RRSIGs. The latter is detected when requesting DS or SOA RRs for the
root domain (which is necessary to do proofs for unsigned data)
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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.
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This is often needed for proper DNSSEC support, and even to handle AAAA records
without falling back to TCP.
If the path between the client and server is fully compliant, this should always
work, however, that is not the case, and overlarge packets will get mysteriously
lost in some cases.
For that reason, we use a similar fallback mechanism as we do for palin EDNS0,
EDNS0+DO, etc.:
The large UDP size feature is different from the other supported feature, as we
cannot simply verify that it works based on receiving a reply (as the server
will usually send us much smaller packets than what we claim to support, so
simply receiving a reply does not mean much).
For that reason, we keep track of the largest UDP packet we ever received, as this
is the smallest known good size (defaulting to the standard 512 bytes). If
announcing the default large size of 4096 fails (in the same way as the other
features), we fall back to the known good size. The same logic of retrying after a
grace-period applies.
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This indicates that we can handle DNSSEC records (per RFC3225), even if
all we do is silently drop them. This feature requires EDNS0 support.
As we do not yet support larger UDP packets, this feature increases the
risk of getting truncated packets.
Similarly to how we fall back to plain UDP if EDNS0 fails, we will fall
back to plain EDNS0 if EDNS0+DO fails (with the same logic of remembering
success and retrying after a grace period after failure).
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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).
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Previously, we would only degrade on packet loss, but when adding EDNS0 support,
we also have to handle the case where the server replies with an explicit error.
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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>
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This copies concepts we introduced for the DnsSearchDomain stuff, and
reworks the operations on lists of dns servers to be reusable and
generic for use both with the Link and the Manager object.
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Previously, we'd keep adding new dns servers we discover to the end of
our linked list of servers. When we encountered a pre-existing server,
we'd just leave it where it was. In essence that meant that old servers
ended up at the front, and new servers at the end, but not in an order
that would reflect the configuration.
With this change we ensure that every pre-existing server we want to add
again we move to the back of the linked list, so that the order is
stable and in sync with the requested configuration.
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Previously, there was a chance of memory corruption, because when
switching to the next DNS server we didn't care whether they linked list
of DNS servers was still valid.
Clean up lifecycle of the dns server logic:
- When a DnsServer object is still in the linked list of DnsServers for
a link or the manager, indicate so with a "linked" boolean field, and
never follow the linked list if that boolean is not set.
- When picking a DnsServer to use for a link ot manager, always
explicitly take a reference.
This also rearranges some logic, to make the tracking of dns servers by
link and globally more alike.
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resolved-dns-server.c
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Let's use the same parser when parsing dns server information from
/etc/resolv.conf and our native configuration file.
Also, move all code that manages lists of dns servers to a single place.
resolved-dns-server.c
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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.
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We want to discover information about the server and use that in when crafting
packets to be resent.
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We want to reference the servers from their active transactions, so make sure
they stay around as long as the transaction does.
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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.
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It is redundant to store 'hash' and 'compare' function pointers in
struct Hashmap separately. The functions always comprise a pair.
Store a single pointer to struct hash_ops instead.
systemd keeps hundreds of hashmaps, so this saves a little bit of
memory.
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After all it pretty much exlcusively containers definitions about the
"Manager" object, hence let's call this the most obvious way.
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We now maintain two lists of DNS servers: system servers and fallback
servers.
system servers are used in combination with any per-link servers.
fallback servers are only used if there are no system servers or
per-link servers configured.
The system server list is supposed to be populated from a foreign tool's
/etc/resolv.conf (not implemented yet).
Also adds a configuration switch for LLMNR, that allows configuring
whether LLMNR shall be used simply for resolving or also for responding.
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Name defending is still missing.
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networkd will expose both statically configured DNS servers and servers
receieved over DHCP in sd_network_get_dns(), so no need to keep
the distinction in resolved.
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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.
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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)
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