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.
The LLMNR spec suggests to do do reverse address lookups by doing direct LLMNR/TCP connections to the indicated
address, instead of doing any LLMNR multicast queries. When we do this and the peer doesn't actually implement LLMNR
this will result in a TCP connection error, which we need to handle. In contrast to most LLMNR lookups this will give
us a quick response on whether we can find a suitable name. Report this as new transaction state, since this should
mostly be treated like an NXDOMAIN rcode, except that it's not one.
We already maintain statistics about positive DNSSEC proofs, and count them up by 1 for each validated RRset. Now,
update the same counters each time we validated a negative query, so that the statistics are the combined result of all
validation checks, both positive and negative.
When we restart a transaction because of an incompatible server, pick a new transaction ID.
This should increase compatibility with DNS servers that don't like if they get different requests with the same
transaction ID.
given that DNSSEC lookups may result in quite a number of auxiliary transactions, let's better be safe than sorry and
also enforce a limit on the number of total transactions, not just on the number of queries.
Previously, when getting notified about a completed auxiliary DNSSEC transaction we'd immediately act on it, and
possibly abort the main transaction. This is problematic, as DNS transactions that already completed at the time we
started using them will never get the notification event, and hence never be acted on in the same way.
Hence, introduce a new call dns_transaction_dnssec_ready() that checks the state of auxiliary DNSSEC transactions, and
returns 1 when we are ready for the actual DNSSEC validation step. Then, make sure this is invoked when the auxiliary
transactions are first acquired (and thus possibly reused) as well when the notifications explained above take place.
This fixes problems particularly when doing combined A and AAAA lookups where the auxiliary DNSSEC transactions get
reused between them, and where we got confused if we reused an auxiliary DNSSEC transaction from one when it already
got completed from the other.
If we have a signed DNAME RR response, there's no need to insist on a signature for a CNAME RR response, after all it
is unlikely to be signed, given the implicit synthethis of CNAME through DNAME RRs.
In some cases we learn something about a server's feature level through its responses. If we notice that after doing
basic checking of a response, and after collecting all auxiliary DNSSEC info the feature level of the server is lower
than where we started, restart the whole transaction.
This is useful to deal with servers that response rubbish when talked to with too high feature levels.
Let's make sure we first check if the OPT was lost in the reply, before we accept a reply as successful and use it for
verifying the current feature level.
When we restart a DNS transaction, remove all connections to any auxiliary DNSSEC transactions, after all we might
acquire completely different data this time, requiring different auxiliary DNSSEC transactions.
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.
Having this information available is useful when we need to check whether various RRs are suitable for proofs. This
information is stored in the RRs as number of labels to skip from the beginning of the owner name to reach the
synthesizing source/signer. Simple accessor calls are then added to retrieve the signer/source from the RR using this
information.
This also moves validation of a a number of RRSIG parameters into a new call dnssec_rrsig_prepare() that as side-effect
initializes the two numeric values.
Given how fragile DNS servers are with some DNS types, and given that we really should avoid confusing them with
known-weird lookups, refuse doing lookups for known-obsolete RR types.
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.
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.
If we already degraded the feature level below DO don't bother with sending requests for DS, DNSKEY, RRSIG, NSEC, NSEC3
or NSEC3PARAM RRs. After all, we cannot do DNSSEC validation then anyway, and we better not press a legacy server like
this with such modern concepts.
This also has the benefit that when we try to validate a response we received using DNSSEC, and we detect a limited
server support level while doing so, all further auxiliary DNSSEC queries will fail right-away.
UDP ICMP errors are reported to us via recvmsg() when we read a reply. Handle this properly, and consider this a lost
packet, and retry the connection.
This also adds some additional logging for invalid incoming packets.
Previously, when we couldn't connect to a DNS server via TCP we'd abort the whole transaction using a
"connection-failure" state. This change removes that, and counts failed connections as "lost packet" events, so that
we switch back to the UDP protocol again.
This implements RFC 5155, Section 8.8 and RFC 4035, Section 5.3.4:
When we receive a response with an RRset generated from a wildcard we
need to look for one NSEC/NSEC3 RR that proves that there's no explicit RR
around before we accept the wildcard RRset as response.
This patch does a couple of things: the validation calls will now
identify wildcard signatures for us, and let us know the RRSIG used (so
that the RRSIG's signer field let's us know what the wildcard was that
generate the entry). Moreover, when iterating trough the RRsets of a
response we now employ three phases instead of just two.
a) in the first phase we only look for DNSKEYs RRs
b) in the second phase we only look for NSEC RRs
c) in the third phase we look for all kinds of RRs
Phase a) is necessary, since DNSKEYs "unlock" more signatures for us,
hence we shouldn't assume a key is missing until all DNSKEY RRs have
been processed.
Phase b) is necessary since NSECs need to be validated before we can
validate wildcard RRs due to the logic explained above.
Phase c) validates everything else. This phase also handles RRsets that
cannot be fully validated and removes them or lets the transaction fail.
When validating a transaction we initially collect DNSKEY, DS, SOA RRs
in the "validated_keys" list, that we need for the proofs. This includes
DNSKEY and DS data from our trust anchor database. Quite possibly we
learn that some of these DNSKEY/DS RRs have been revoked between the
time we request and collect those additional RRs and we begin the
validation step. In this case we need to make sure that the respective
DS/DNSKEY RRs are removed again from our list. This patch adds that, and
strips known revoked trust anchor RRs from the validated list before we
begin the actual validation proof, and each time we add more DNSKEY
material to it while we are doing the proof.
Instead of first iterating through all DNSKEYs in the DnsAnswer in
dns_transaction_check_revoked_trust_anchors(), and
then doing that a second time in dns_trust_anchor_check_revoked(), do so
only once in the former, and pass the dnskey we found directly to the
latter.
There's not reason to wait for checking for revoked trust anchors until
after validation, after all revoked DNSKEYs only need to be self-signed,
but not have a full trust chain.
This way, we can be sure that all trust anchor lookups we do during
validation already honour that some keys might have been revoked.
This adds logic to detect cases like the Fritz!Box routers which serve
a private DNS domain "fritz.box" under the TLD "box" that does not
exist in the root servers. If this is detected DNSSEC validation is
turned off for this private domain, thus improving compatibility with
such private DNS zones.
This should be fairly secure as we first rely on the proof that .box
does not exist before this logic is applied. Nevertheless the logic is
only enabled for DNSSEC=allow-downgrade mode.
This logic does not work for routers that set up a full DNS zone directly
under a non-existing TLD, as in that case we cannot prove
that the domain is truly non-existing according to the root servers.
When storing negative responses, clamp the SOA minimum TTL (as suggested
by RFC2308) to the TTL of the NSEC/NSEC3 RRs we used to prove
non-existance, if it there is any.
This is necessary since otherwise an attacker might put together a faked
negative response for one of our question including a high-ttl SOA RR
for any parent zone, and we'd use trust the TTL.
Since we honour RFC5011 revoked keys it might happen we end up with an
empty trust anchor, or one where there's no entry for the root left.
With this patch the logic is changed what to do in this case.
Before this patch we'd end up requesting the root DS, which returns with
NODATA but a signed NSEC we cannot verify, since the trust anchor is
empty after all. Thus we'd return a DNSSEC result of "missing-key", as
we lack a verified version of the key.
With this patch in place, look-ups for the root DS are explicitly
recognized, and not passed on to the DNS servers. Instead, if
downgrade-ok mode is on an unsigned NODATA response is synthesized, so
that the validator code continues under the assumption the root zone was
unsigned. If downgrade-ok mode is off a new transaction failure is
generated, that makes this case recognizable.
We already try hard not to create cyclic transaction dependencies, where
a transaction requires another one for DNSSEC validation purposes, which
in turn (possibly indirectly) pulls in the original transaction again,
thus resulting in a cyclic dependency and ultimately a deadlock since
each transaction waits for another one forever.
So far we wanted to avoid such cyclic dependencies by only going "up the
tree" when requesting auxiliary RRs and only going from one RR type to
another, but never back. However this turned out to be insufficient.
Consider a domain that publishes one or more DNSKEY but which has no DS
for it. A request for the domain's DNSKEY triggers a request for the
domain's DS, which will then fail, but return an NSEC, signed by the
DNSKEY. To validate that we'd request the DNSKEY again. Thus a DNSKEY
request results in a DS request which results in the original DNSKEY
request again. If the original lookup had been a DS lookup we'd end up
in the same cyclic dependency, hence we cannot statically break one of
them, since both requests are of course fully valid. Hence, do full
cyclic dependency checking: each time we are about to add a dependency
to a transaction, check if the transaction is already a dependency of
the dependency (recursively down the tree).
If any of the transactions started by
dns_transaction_request_dnssec_keys() finishes promptly without
requiring asynchronous operation this is reported back to the issuing
transaction from the same stackframe. This might ultimately result in
this transaction to be freed while we are still in its
_request_dnssec_keys() stack frame. To avoid memory corruption block the
transaction GC while in the call, and manually issue a GC after it
returned.
With this patch resolved will properly handle revoked keys, but not
augment the locally configured trust anchor database with newly learned
keys.
Specifically, resolved now refuses validating RRsets with
revoked keys, and it will remove revoked keys from the configured trust
anchors (only until reboot).
This patch does not add logic for adding new keys to the set of trust
anchors. This is a deliberate decision as this only can work with
persistent disk storage, and would result in a different update logic
for stateful and stateless systems. Since we have to support stateless
systems anyway, and don't want to encourage two independent upgrade
paths we focus on upgrading the trust anchor database via the usual OS
upgrade logic.
Whenever a trust anchor entry is found revoked and removed from the
trust anchor a recognizable log message is written, encouraging the user
to update the trust anchor or update his operating system.
This adds negative trust anchor support and allows reading trust anchor
data from disk, from files
/etc/systemd/dnssec-trust-anchors.d/*.positive and
/etc/systemd/dnssec-trust-anchros.d/*.negative, as well as the matching
counterparts in /usr/lib and /run.
The positive trust anchor files are more or less compatible to normal
DNS zone files containing DNSKEY and DS RRs. The negative trust anchor
files contain only new-line separated hostnames for which to require no
signing.
By default no trust anchor files are installed, in which case the
compiled-in root domain DS RR is used, as before. As soon as at least
one positive root anchor for the root is defined via trust anchor files
this buil-in DS RR is not added though.
This renames dns_transaction_stop() to dns_transaction_stop_timeout()
and makes it only about stopping the transaction timeout. This is safe,
as in most occasions we call dns_transaction_stop() at the same time as
dns_transaction_close_connection() anyway, which does the rest of what
dns_transaction_stop() used to do. And in the one where we don't call
it, it's implicitly called by the UDP emission or TCP connection code.
This also closes the connections as we enter the validation phase of a
transaction, so that no further messages may be received then.
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.
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.
This new call unifies how we shut down all connection resources, such as
UDP sockets, event sources, and TCP stream objects.
This patch just adds the basic hook-up, this function will be used more
in later commits.
On LLMNR we never want to retry stream connections (since local TCP
connections should work, and we don't want to unnecessarily delay
operation), explicitly remember whether we already tried one, instead of
deriving this from a still stored stream object. This way, we can free
the stream early, without forgetting that we tried it.
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().
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)
Previously, if we couldn't reach a server via UDP we'd generate an
MAX_ATTEMPTS transaction result, but if we couldn't reach it via TCP
we'd generate a RESOURCES transaction result. While it is OK to generate
two different errors I think, "RESOURCES" is certainly a misnomer.
Introduce a new transaction result "CONNECTION_FAILURE" instead.
Previously, we'd insist on an RRSIG for all DS/NSEC/NSEC3 RRs. With this
change we don't do that anymore, but also allow unsigned DS/NSEC/NSEC3
if we can prove that the zone they are located in is unsigned.
This collects statistical data about transactions, dnssec verifications
and the cache, and exposes it over the bus. The systemd-resolve-host
tool learns new options to query these statistics and reset them.
Be stricter when searching suitable NSEC3 RRs for proof: generalize the
check we use to find suitable NSEC3 RRs, in nsec3_is_good(), and add
additional checks, such as checking whether all NSEC3 RRs use the same
parameters, have the same suffix and so on.
Let's simplify usage and memory management of DnsResourceRecord's
dns_resource_record_to_string() call: cache the formatted string as
part of the object, and return it on subsequent calls, freeing it when
the DnsResourceRecord itself is freed.
Let's make sure we propagate the DNSSEC validation status from an
auxiliary DNSSEC transaction back to the originating transaction, to
improve the error messages we generate.
We have many types of failure for a transaction, and
DNS_TRANSACTION_FAILURE was just one specific one of them, if the server
responded with a non-zero RCODE. Hence let's rename this, to indicate
which kind of failure this actually refers to.
This large patch adds a couple of mechanisms to ensure we get NSEC3 and
proof-of-unsigned support into place. Specifically:
- Each item in an DnsAnswer gets two bit flags now:
DNS_ANSWER_AUTHENTICATED and DNS_ANSWER_CACHEABLE. The former is
necessary since DNS responses might contain signed as well as unsigned
RRsets in one, and we need to remember which ones are signed and which
ones aren't. The latter is necessary, since not we need to keep track
which RRsets may be cached and which ones may not be, even while
manipulating DnsAnswer objects.
- The .n_answer_cachable of DnsTransaction is dropped now (it used to
store how many of the first DnsAnswer entries are cachable), and
replaced by the DNS_ANSWER_CACHABLE flag instead.
- NSEC3 proofs are implemented now (lacking support for the wildcard
part, to be added in a later commit).
- Support for the "AD" bit has been dropped. It's unsafe, and now that
we have end-to-end authentication we don't need it anymore.
- An auxiliary DnsTransaction of a DnsTransactions is now kept around as
least as long as the latter stays around. We no longer remove the
auxiliary DnsTransaction as soon as it completed. THis is necessary,
as we now are interested not only in the RRsets it acquired but also
in its authentication status.
A failing transaction might cause other transactions to fail too, and
thus the set of transactions to notify for a transaction might change
while we are notifying them. Protect against that.
We end up needing the stringified transaction key in many log messages,
hence let's simplify the logic and cache it inside of the transaction:
generate it the first time we need it, and reuse it afterwards. Free it
when the transaction goes away.
This also updated a couple of log messages to make use of this.
This changes answer validation to be more accepting to unordered RRs in
responses. The agorithm we now implement goes something like this:
1. populate validated keys list for this transaction from DS RRs
2. as long as the following changes the unvalidated answer list:
2a. try to validate the first RRset we find in unvalidated answer
list
2b. if that worked: add to validated answer; if DNSKEY also add to
validated keys list; remove from unvalidated answer.
2c. continue at 2a, with the next RRset, or restart from the
beginning when we hit the end
3. as long as the following changes the unvalidated answer list:
3a. try to validate the first RRset again. This will necessarily
fail, but we learn the precise error
3b. If this was a "primary" response to the question, fail the
entire transaction. "Primary" in this context means that it is
directly a response to the query, or a CNAME/DNAME for it.
3c. Otherwise, remove the RRset from the unvalidated answer list.
Note that we the too loops in 2 + 3 are actually coded as a single one,
but the dnskeys_finalized bool indicates which loop we are currently
processing.
Note that loop 2 does not drop any invalidated RRsets yet, that's
something only loop 3 does. This is because loop 2 might still encounter
additional DNSKEYS which might validate more stuff, and if we'd already
have dropped those RRsets we couldn't validate those anymore. The first
loop is hence a "constructive" loop, the second loop a "destructive"
one: the first one validates whatever is possible, the second one then
deletes whatever still isn't.
Instead of figuring out how many RRs to cache right before we do so,
determine this at the time we install the answer RRs, so that we can
still alter this as we manipulate the answer during validation.
The primary purpose of this is to pave the way so that we can drop
unsigned RRsets from the answer and invalidate the number of RRs to
cache at the same time.
Apart from dropping redundant information, this fixes an issue
where, due to broken DNS servers, we can only be certain of whether
an apparent NODATA response is in fact an NXDOMAIN response after
explicitly resolving the canonical name. This issue is outlined in
RFC2308. Moreover, by caching NXDOMAIN for an existing name, we
would mistakenly return NXDOMAIN for types which should not be
redirected. I.e., a query for AAAA on test-nx-1.jklm.no correctly
returns NXDOMAIN, but a query for CNAME should return the record
and a query for DNAME should return NODATA.
Note that this means we will not cache an NXDOMAIN response in the
presence of redirection, meaning one redundant roundtrip in case the
name is queried again.
Let's simply call it dns_transaction_prepare(), so that we have the nice
cycle for prepare() → go() → emit() → process().
After all it's pretty clear that what we prepare there, and we dont call
the others go_next_attempt(), emit_next_attempt() or
process_next_attempt().
This adds initial support for validating RRSIG/DNSKEY/DS chains when
doing lookups. Proof-of-non-existance, or proof-of-unsigned-zones is not
implemented yet.
With this change DnsTransaction objects will generate additional
DnsTransaction objects when looking for DNSKEY or DS RRs to validate an
RRSIG on a response. DnsTransaction objects are thus created for three
reasons now:
1) Because a user asked for something to be resolved, i.e. requested by
a DnsQuery/DnsQueryCandidate object.
2) As result of LLMNR RR probing, requested by a DnsZoneItem.
3) Because another DnsTransaction requires the requested RRs for
validation of its own response.
DnsTransactions are shared between all these users, and are GC
automatically as soon as all of these users don't need a specific
transaction anymore.
To unify the handling of these three reasons for existance for a
DnsTransaction, a new common naming is introduced: each DnsTransaction
now tracks its "owners" via a Set* object named "notify_xyz", containing
all owners to notify on completion.
A new DnsTransaction state is introduced called "VALIDATING" that is
entered after a response has been receieved which needs to be validated,
as long as we are still waiting for the DNSKEY/DS RRs from other
DnsTransactions.
This patch will request the DNSKEY/DS RRs bottom-up, and then validate
them top-down.
Caching of RRs is now only done after verification, so that the cache is
not poisoned with known invalid data.
The "DnsAnswer" object gained a substantial number of new calls, since
we need to add/remove RRs to it dynamically now.
This new functions exports cached records of type PTR, SRV and TXT into
an existing DnsPacket. This is used in order to fill in known records
to mDNS queries, for known answer supression.
Implement dns_transaction_make_packet_mdns(), a special version of
dns_transaction_make_packet() for mDNS which differs in many ways:
a) We coalesce queries of currently active transaction on the scope.
This is possible because mDNS actually allows many questions in a
to be sent in a single packet and it takes some burden from the
network.
b) Both A and AAAA query keys are broadcast on both IPv4 and IPv6
scopes, because other hosts might only respond on one of their
addresses but resolve both types.
c) We discard previously sent packages (t->sent) so we can start over
and coalesce pending transactions again.
For each transaction, record when the earliest point in time when the
query packet may hit the wire. This is the same time stamp for which
the timer is scheduled in retries, except for the initial query packets
which are delayed by a random jitter. In this case, we denote that the
packet may actually be sent at the nominal time, without the jitter.
Transactions that share the same timestamp will also have identical
values in this field. It is used to coalesce pending queries in a later
patch.
Split some code out of dns_transaction_go() so we can re-use it later from
different context. The new function dns_transaction_prepare_next_attempt()
takes care of preparing everything so that a new packet can conditionally
be formulated for a transaction.
This patch shouldn't cause any functional change.
mDNS packet timeouts need to be handled per transaction, not per link.
Re-use the n_attempts field for this purpose, as packets timeouts should be
determined by starting at 1 second, and doubling the value on each try.
When a jitter callback is issued instead of sending a DNS packet directly,
on_transaction_timeout() is invoked to 'retry' the transaction. However,
this function has side effects. For once, it increases the packet loss
counter on the scope, and it also unrefs/refs the server instances.
Fix this by tracking the jitter with two bool variables. One saying that
the initial jitter has been scheduled in the first place, and one that
tells us the delay packet has been sent.
This adds a new SD_RESOLVED_AUTHENTICATED flag for responses we return
on the bus. When set, then the data has been authenticated. For now this
mostly reflects the DNSSEC AD bit, if DNSSEC=trust is set. As soon as
the client-side validation is complete it will be hooked up to this flag
too.
We also set this bit whenver we generated the data ourselves, for
example, because it originates in our local LLMNR zone, or from the
built-in trust anchor database.
The "systemd-resolve-host" tool has been updated to show the flag state
for the data it shows.
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.
When doing DNSSEC lookups we need to know one or more DS or DNSKEY RRs
as trust anchors to validate lookups. With this change we add a
compiled-in trust anchor database, serving the root DS key as of today,
retrieved from:
https://data.iana.org/root-anchors/root-anchors.xml
The interface is kept generic, so that additional DS or DNSKEY RRs may
be served via the same interface, for example by provisioning them
locally in external files to support "islands" of security.
The trust anchor database becomes the fourth source of RRs we maintain,
besides, the network, the local cache, and the local zone.
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.
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).
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.
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>
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.
Let's track where the data came from: from the network, the cache or the
local zone. This is not only useful for debugging purposes, but is also
useful when the zone probing wants to ensure it's not reusing
transactions that were answered from the cache or the zone itself.
Previously we'd only store the DnsPacket in the DnsTransaction, and the
DnsQuery would then take the DnsPacket's DnsAnswer and return it. With
this change we already pull the DnsAnswer out inside the transaction.
We still store the DnsPacket in the transaction, if we have it, since we
still need to determine from which peer a response originates, to
implement caching properly. However, the DnsQuery logic doesn't care
anymore for the packet, it now only looks at answers and rcodes from the
successfuly candidate.
This also has the benefit of unifying how we propagate incoming packets,
data from the local zone or the local cache.
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
Previously, we'd always generate a packet on the wire, even for names
that are within our local zone. Shortcut this, and always check the
local zone first. This should minimize generated traffic and improve
security.
This hopefully makes this a bit more expressive and clarifies that the
fd is not used for the DNS TCP socket. This also mimics how the LLMNR
UDP fd is named in the manager object.
If we try to resoolve an LLMNR PTR RR we shall connect via TCP directly
to the specified IP address. We already refuse to do this if the address
to resolve is of a different address family as the transaction's scope.
The error returned was EAFNOSUPPORT. Let's change this to ESRCH which is
how we indicate "not server available" when connecting for LLMNR or DNS,
since that's what this really is: we have no server we could connect to
in this address family.
This allows us to ensure that no server errors are always handled the same
way.
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.
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.
It shouldn't happen that we try to resolve IPv4 addresses via LLMNR on
IPv6 and vice versa, but let's explicitly verify that we don't turn an
IPv4 LLMNR lookup into an IPv6 TCP connection.
Previously, if the event loop never ran before sd_event_now() would
fail. With this change it will instead fall back to invoking now(). This
way, the function cannot fail anymore, except for programming error when
invoking it with wrong parameters.
This takes into account the fact that many callers did not handle the
error condition correctly, and if the callers did, then they kept simply
invoking now() as fall back on their own. Hence let's shorten the code
using this call, and make things more robust, and let's just fall back
to now() internally.
Whether now() is used or the cache timestamp may still be detected via
the return value of sd_event_now(). If > 0 is returned, then the fall
back to now() was used, if == 0 is returned, then the cached value was
returned.
This patch also simplifies many of the invocations of sd_event_now():
the manual fall back to now() can be removed. Also, in cases where the
call is invoked withing void functions we can now protect the invocation
via assert_se(), acknowledging the fact that the call cannot fail
anymore except for programming errors with the parameters.
This change is inspired by #841.
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.
Let's optimize things a bit and properly compare DNS question arrays,
instead of checking if they are mutual supersets. This also makes ANY
query handling more accurate.
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.
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.
Close the socket when changing the server in a transaction, in
order for it to be reopened with the right server when we send
the next packet.
This fixes a regression where we could get stuck with a failing
server.
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).
This improves the resilience against cache poisoning by being stricter
about only accepting responses that match precisely the requst they
are in reply to.
It should be noted that we still only use one port (which is picked
at random), rather than one port for each transaction. Port
randomization would improve things further, but is not required by
the RFC.
Lennart Poettering