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the results of a dns query, whether it's performed as part of one of
the standard name-resolving functions or directly by res_send, should
be a function of the query, not of the particular nameserver that
responds to it. thus, all responses which indicate a failure or
refusal by the nameserver, as opposed to a positive or negative result
for the query, should be ignored.
the strategy used is to re-issue the query immediately (but with a
limit on the number of retries, in case the server is really broken)
when a response code of 2 (server failure, typically transient) is
seen, and otherwise take no action on bad responses (which generally
indicate a misconfigured nameserver or one which the client does not
have permission to use), allowing the normal retry interval to apply
and of course accepting responses from other nameservers queried in
parallel.
empirically this matches the traditional resolver behavior for
nameservers that respond with a code of 2 in the case where there is
just a single nameserver configured. the behavior diverges when
multiple nameservers are available, since musl is querying them in
parallel. in this case we are mildly more aggressive at retrying.
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the fcntl function is heavy, so make the syscall directly instead.
also, avoid the code size and runtime overhead of querying the old
flags, since it's reasonable to assume nothing will be set on a
newly-created socket. this code is only used on old kernels which lack
proper atomic close-on-exec support, so future changes that might
invalidate such an assumption do not need to be considered.
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as usual, this is non-atomic, but better than producing an error or
failing to set the close-on-exec flag at all.
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the input name is validated, the other parameters are assumed to be
valid (the list of already compressed names are not checked for
infinite reference loops or out-of-bound offsets).
names are handled case-sensitively for now.
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trailing . should be accepted in domain name strings by convention
(RFC 1034), host name lookup accepts "." but rejects empty "", res_*
interfaces also accept empty name following existing practice.
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A domain name is at most 255 bytes long (RFC 1035), but the string
representation is two bytes smaller so the strlen maximum is 253.
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Due to an error introduced in commit fcc522c92335783293ac19df318415cd97fbf66b,
checking of the remaining output buffer space was not performed correctly,
allowing malformed input to write past the end of the buffer.
In addition, the loop detection logic failed to account for the possibility
of infinite loops with no output, which would hang the function.
The output size is now limited more strictly so only names with valid length
are accepted.
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this also affects the legacy getservbyport family, which uses
getnameinfo as its backend.
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this also affects the legacy gethostbyaddr family, which uses
getnameinfo as its backend.
some other minor changes associated with the refactoring of source
files are also made; in particular, the resolv.conf parser now uses
the same code that's used elsewhere to handle ip literals, so as a
side effect it can now accept a scope id for nameserver addressed with
link-local scope.
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the service and protocol functions are defined also in other files,
and the protocol ones are actually non-nops elsewhere, so the weak
definitions in ent.c could have prevented the strong definitions from
getting pulled in and used in some static programs.
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for all address types, a scope_id specified as a decimal value is
accepted. for addresses with link-local scope, a string containing the
interface name is also accepted.
some changes are made to error handling to avoid unwanted fallbacks in
the case where the scope_id is invalid: if an earlier name lookup
backend fails with an error rather than simply "0 results", this
failure now suppresses any later attempts with other backends.
in getnameinfo, a light "itoa" type function is added for generating
decimal scope_id results, and decimal port strings for services are
also generated using this function now so as not to pull in the
dependency on snprintf.
in netdb.h, a definition for the NI_NUMERICSCOPE flag is added. this
is required by POSIX (it was previously missing) and needed to allow
callers to suppress interface-name lookups.
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the return value is unsigned, so negative results for "errors" do not
make sense; 0 is the value reserved for when the interface name does
not exist.
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previously, all failures to obtain at least one address were treated
as nonexistant names (EAI_NONAME). this failed to account for the
possibility of transient failures (no response at all, or a response
with rcode of 2, server failure) or permanent failures that do not
indicate the nonexistence of the requested name. only an rcode of 3
should be treated as an indication of nonexistence.
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since the buffer passed always has an actual size of 512 bytes, the
maximum possible response packet size, no out-of-bounds access was
possible; however, reading past the end of the valid portion of the
packet could cause the parser to attempt to process junk as answer
content.
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the old resolver code used a function __ipparse which contained the
logic for inet_addr and inet_aton, which is needed in getaddrinfo.
this was phased out in the resolver overhaul in favor of directly
using inet_aton and inet_pton as appropriate.
this commit cleans up some stuff that was left behind.
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this is the third phase of the "resolver overhaul" project.
this commit removes all of the old dns code, and switches the
__lookup_name backend (used by getaddrinfo, etc.) and the getnameinfo
function to use the newly implemented __res_mkquery and __res_msend
interfaces. for parsing the results, a new callback-based __dns_parse
function, based on __dns_get_rr from the old dns code, is used.
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this is the second phase of the "resolver overhaul" project.
the key additions in this commit are the __res_msend and __res_mkquery
functions, which have been factored so as to provide a backend for
both the legacy res_* functions and the standard getaddrinfo and
getnameinfo functions. the latter however are still using the old
backend code; there is code duplication which still needs to be
removed, and this will be the next phase of the resolver overhaul.
__res_msend is derived from the old __dns_doqueries function, but
generalized to send arbitrary caller-provided packets in parallel
rather than producing the parallel queries itself. this allows it to
be used (completely trivially) as a backend for res_send. the
factored-out query generation code, with slightly more generality, is
now part of __res_mkquery.
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iptables and ipsec-tools among others require these to function
properly.
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this bug was introduced in the recent resolver overhaul commits. it
likely had visible symptoms. these were probably limited to wrongly
accepting truncated versions of over-long names (vs rejecting them),
as opposed to stack-based overflows or anything more severe, but no
extensive checks were made. there have been no releases where this bug
was present.
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now that host and service lookup have been separated in the backend,
there's no need for service lookup functions to pull in the host
lookup code. moreover, dynamic allocation is no longer needed, so this
function should now be async-signal-safe. it's also significantly
smaller.
one change in getservbyname is also made: knowing that getservbyname_r
needs only two character pointers in the caller-provided buffer, some
wasted bss can be avoided.
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these changes reduce the size of the function somewhat and remove many
of its dependencies, including free. in principle it should now be
async-signal-safe, but this has not been verified in detail.
minor changes to error handling are also made.
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this is the first phase of the "resolver overhaul" project.
conceptually, the results of getaddrinfo are a direct product of a
list of address results and a list of service results. the new code
makes this explicit by computing these lists separately and combining
the results. this adds support for services that have both tcp and udp
versions, where the caller has not specified which it wants, and
eliminates a number of duplicate code paths which were all producing
the final output addrinfo structures, but in subtly different ways,
making it difficult to implement any of the features which were
missing.
in addition to the above benefits, the refactoring allows for legacy
functions like gethostbyname to be implemented without using the
getaddrinfo function itself. such changes to the legacy functions have
not yet been made, however.
further improvements include matching of service alias names from
/etc/services (previously only the primary name was supported),
returning multiple results from /etc/hosts (previously only the first
matching line was honored), and support for the AI_V4MAPPED and AI_ALL
flags.
features which remain unimplemented are IDN translations (encoding
non-ASCII hostnames for DNS lookup) and the AI_ADDRCONFIG flag.
at this point, the DNS-based name resolving code is still based on the
old interfaces in __dns.c, albeit somewhat simpler in its use of them.
there may be some dead code which could already be removed, but
changes to this layer will be a later phase of the resolver overhaul.
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the other atomic FD_CLOEXEC interfaces (dup3, pipe2, socket) already
had such emulation in place. the justification for doing the emulation
here is the same as for the other functions: it allows applications to
simply use accept4 rather than having to have their own fallback code
for ENOSYS/EINVAL (which one you get is arch-specific!) and there is
no reasonable way an application could benefit from knowing the
operation is emulated/non-atomic since there is no workaround at the
application level for non-atomicity (that is the whole reason these
interfaces were added).
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based on patch by orc.
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this was unlikely to lead to any crash or dangerous behavior, but
caused adjacent string constants to be treated as part of the
protocols table, possibly returning nonsensical results for unknown
protocol names/numbers or when getprotoent was called in a loop to
enumerate all protocols.
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the type int was taken from seemingly erroneous man pages. glibc uses
in_addr_t (uint32_t), and semantically, the arguments should be
unsigned.
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based on patch by Timo Teräs; greatly simplified to use fprintf.
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at most 4 hexadecimal digits are processed in one field so the
value cannot overflow. the netdb.h header was not used.
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a v6 socket will only be used if there is at least one v6 nameserver
address. if the kernel lacks v6 support, the code will fall back to
using a v4 socket and requests to v6 servers will silently fail. when
using a v6 socket, v4 addresses are converted to v4-mapped form and
setsockopt is used to ensure that the v6 socket can accept both v4 and
v6 traffic (this is on-by-default on Linux but the default is
configurable in /proc and so it needs to be set explicitly on the
socket level). this scheme avoids increasing resource usage during
lookups and allows the existing network io loop to be used without
modification.
previously, nameservers whose address family did not match the address
family of the first-listed nameserver were simply ignored. prior to
recent __ipparse fixes, they were not ignored but erroneously parsed.
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subsequent code assumes the address family requested is either
unspecified or one of IPv4/IPv6, and could malfunction if this
constraint is not met, so other address families should be explicitly
rejected.
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This function is used by ping6 from iputils.
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inet_aton returns a boolean success value, whereas __ipparse returns 0
on success and -1 on failure. also change the conditional in inet_addr
to be consistent with other uses of __ipparse where only negative
values are treated as failure.
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* parse IPv4 dotted-decimal correctly (without strtoul, no leading zeros)
* disallow single leading ':' in IPv6 address
* allow at most 4 hex digits in IPv6 address (according to RFC 2373)
* have enough hex fields in IPv4 mapped IPv6 address
* disallow leading zeros in IPv4 mapped IPv6 address
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* allow at most 4 parts
* bounds check the parts correctly
* disallow leading whitespace and sign
* check the address family before falling back to IPv6
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despite being practically deprecated, these functions are still part
of the standard and thus cannot reside in a file that also contains
namespace pollution. this reverts some of the changes made in commit
e40f48a421a9176e3e298b5bac75f0355b219e58.
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in the case of input that does not match the expected form, the
correct return value is 0, not -1.
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off-by-one error copying the name components was yielding junk at the
beginning and truncating one character at the end (of every
component).
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there are two possible points where the length is evaluated: either
the first 'compression' jump, or the null terminator if no jumps have
taken place yet. the previous code only measured the length of the
first component.
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the duplicate code in dn_expand and its incorrect return values are
both results of the history of the code: the version in __dns.c was
originally written with no awareness of the legacy resolver API, and
was later copy-and-paste duplicated to provide the legacy API.
this commit is the first of a series that will restructure the
internal dns code to share as much code as possible with the legacy
resolver API functions.
I have also removed the loop detection logic, since the output buffer
length limit naturally prevents loops. in order to avoid long runtime
when encountering a loop if the caller provided a ridiculously long
buffer, the caller-provided length is clamped at the maximum dns name
length.
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source and dest arguments for strcpy cannot overlap, so memmove must
be used here. the length is already known from the above loop.
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based on a patch by orc. POSIX actually fails to specify the format of
the ntop conversion; presumably, any output that will correctly
round-trip back via the (well-specified) pton operation is acceptable.
the new behavior is much more convenient than the old, however.
this patch also affects getnameinfo, which is implemented in terms of
inet_ntop and which is the preferred interface for performing this
conversion.
I've also removed some inexplicable cruft (filling the buffer with 'x'
before doing anything) whose origin I was unable to track down.
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based on a patch by orc, with indexing and flow control cleaned up a
little bit. this code is all going to be replaced at some point in the
near future.
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