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resolved fixes for handling SERVFAIL errors from servers
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In some cases, caching DNS results locally is not desirable, a it makes DNS
cache poisoning attacks a tad easier and also allows users on the system to
determine whether or not a particular domain got visited by another user. Thus
provide a new "Cache" resolved.conf option to disable it.
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SERVFAIL
Some upstream DNS servers return SERVFAIL if we ask them for DNSSEC RRs, which
some forwarding DNS servers pass on to us as SERVFAIL (other though as
NOERROR...). This is should not be considered a problem, as long as the domain
in question didn't have DNSSEC enabled. Hence: when making use of auxiliary
transactions accept those that return SERVFAIL.
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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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When we return the full RR wire data, let's make sure the TTL included in it is
adjusted by the time the RR sat in the cache.
As an optimization we do this only for ResolveRecord() and not for
ResolveHostname() and friends, since adjusting the TTL means copying the RR
object, and we don#t want to do that if there's no reason to.
(ResolveHostname() and friends don't return the TTL hence there's no reason to
in that case)
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(#3553)
dns_transaction_maybe_restart() is supposed to return 1 if the the transaction
has been restarted and 0 otherwise. dns_transaction_process_dnssec() relies on
this behaviour. Before this change in case of restart we'd call
dns_transaction_go() when restarting the lookup, returning its return value
unmodified. This is wrong however, as that function returns 1 if the
transaction is pending, and 0 if it completed immediately, which is a very
different set of return values. Fix this, by always returning 1 on redirection.
The wrong return value resulted in all kinds of bad memory accesses as we might
continue processing a transaction that was redirected and completed immediately
(and thus freed).
This patch also adds comments to the two functions to clarify the return values
for the future.
Most likely fixes: #2942 #3475 #3484
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Previously, after checking the local zone for a reply and finding one we'd not
initialize the answer ifindex from that. Let's fix that.
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incapdns.net returns NXDOMAIN for the SOA of the zone itself but is not a
terminal. This is against the specs, but we really should be able to deal with
this.
Previously, when verifying whether an NXDOMAIN response for a SOA/NS lookup is
rightfully unsigned we'd issue a SOA lookup for the parent's domain, to derive
the state from that. If the parent SOA would get an NXDOMAIN, we'd continue
upwards, until we hit a signed top-level domain, which suggests that the domain
actually exists.
With this change whenver we need to authenticate an NXDOMAIN SOA reply, we'll
request the DS RR for the zone first, and use for validation, since that this
must be from the parent's zone, not the incorrect lower zone.
Fixes: #2894
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CID #1349699-1349700.
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resolved iteration fix
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Throughout the tree there's spurious use of spaces separating ++ and --
operators from their respective operands. Make ++ and -- operator
consistent with the majority of existing uses; discard the spaces.
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When a transaction is complete, and we notify its owners, make sure we deal
correctly with the requesters removing themselves from the list of owners while
we continue iterating.
This was previously already dealt with with transactions that require other
transactions for DNSSEC purposes, fix this for other possibly transaction
owners too now.
Since iterating through "Set" objects is not safe regarding removal of entries
from it, rework the logic to use two Sets, and move each entry we notified from
one set to the other set before we dispatch the notification. This move operation
requires no additional memory, and enables us to ensure that we don't notify
any object twice.
Fixes: #2676
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When the buffer is allocated on the stack we do not have to check for
failure everywhere. This is especially useful in debug statements, because
we can put dns_resource_key_to_string() call in the debug statement, and
we do not need a seperate if (log_level >= LOG_DEBUG) for the conversion.
dns_resource_key_to_string() is changed not to provide any whitespace
padding. Most callers were stripping the whitespace with strstrip(),
and it did not look to well anyway. systemd-resolve output is not column
aligned anymore.
The result of the conversion is not stored in DnsTransaction object
anymore. It is used only for debugging, so it seems fine to generate it
when needed.
Various debug statements are extended to provide more information.
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This fixes formatting of root domain in debug messages:
Old:
systemd-resolved[10049]: Requesting DS to validate transaction 19313 (., DNSKEY with key tag: 19036).
New:
systemd-resolved[10049]: Requesting DS to validate transaction 19313 (, DNSKEY with key tag: 19036).
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With two nested loops and a switch statements, it's quite hard to
understand what break and continue mean.
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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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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.
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DNS response
If we encounter NXDOMAIN, but find at least one matching RR in a response, then patch it to become SUCCESS. This should
clean up handling of CNAME/DNAMEs, and makes sure broken servers and those conforming to RFC 6604 are treated the same
way. The new behaviour opposes the logic suggested in RFC 6604, but given that some servers don't implement it
correctly, and given that in some ways the CNAME/DNAME chains will be incomplete anyway, and given that DNSSEC
generally only allows us to prove the first element of a CNAME/DNAME chain, this should simplify things for us.
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So far, abritrary NSEC and NSEC3 RRs were implicitly consider "primary" for any transaction, meaning we'd abort the
transaction immediately if we couldn't validate it. With this patch this logic is removed, and the NSEC/NSEC3 RRs will
not be considered primary anymore. This has the effect that they will be dropped from the message if they don't
validate, but processing continues. This is safe to do, as they are required anyway to validate positive wildcard and
negative responses, and if they are missing then, then message will be considered unsigned, which hence means the
outcome is effectively the same.
This is benefical in case the server sends us NSEC/NSEC3 RRs that are not directly related to the lookup we did, but
simply auxiliary information. Previously, if we couldn't authenticate those RRs we'd fail the entire lookup while with
this change we'll simply drop the auxiliary information and proceed without it.
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Whenever we encounter an OS error we did not expect, we so far put the transaction into DNS_TRANSACTION_RESOURCES
state. Rename this state to DNS_TRANSACTION_ERRNO, and save + propagate the actual system error to the caller. This
should make error messages triggered by system errors much more readable by the user.
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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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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.
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Also, don't consider RRs that aren't primary to the lookups we do as relevant to the lookups.
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an invalid packet event
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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.
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the network is down
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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.
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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.
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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.
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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.
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restart transaction
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.
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feature level
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.
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transactions
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.
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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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This should clarify that this is not regular signature-based validation, but validation through DS RR fingerprints.
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source and zone in each RR
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.
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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.
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current_feature_level
This is a follow-up for f4461e5641d53f27d6e76e0607bdaa9c0c58c1f6.
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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 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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supporting them
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.
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