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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).
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dns_transaction_request_dnssec_keys() is running
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.
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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.
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validation
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Fixes to NSEC3 proof v2
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configuration files
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.
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Instead introduce the new return-code DNSSEC_NSEC_CNAME to indicate
this condition. See RFC 6840, Section 4.3.
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traffic
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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.
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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 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.
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Indicate thar we ignore invalid messages
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Make sure we don't end up processing packets that are truncated.
Instead, actually let the TCP connection do its thing.
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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.
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Make sure to GC a transaction after dealing with a reply, even if the
transaction is not complete yet.
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No need to choke on them.
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Let's unify how we reset the answer data we collected, after all pretty
much every time we do it incompletely so far, let's fix it.
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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().
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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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via TCP
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.
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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.
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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.
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But keep track that the proof is not authenticated.
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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.
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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.
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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.
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the the bus client
It's useful to generate useful errors, so let's do that.
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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.
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OK to be unsigned
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.
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Let's be safe and explicitly avoid that we add an auxiliary transaction
dependency on ourselves.
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We need no separate timeout anymore as soon as we received a reply, as
the auxiliary transactions have their own timeouts.
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they are destroyed
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.
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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.
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Note that this is not complete yet, as we don't handle wildcard domains
correctly, nor handle domains correctly that use empty non-terminals.
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It's not OK to drop these for our proof of non-existance checks.
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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.
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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.
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Check the validity of RR types as we parse or receive data from IPC
clients, and use the same code for all of them.
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Misc resolved cache fixes
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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.
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resolved: more mDNS specific bits (3)
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RFC6762, 18.1:
In multicast query messages, the Query Identifier SHOULD be set to
zero on transmission.
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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().
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