A relatively common situation is that the reply to a downstream query
will fit in a UDP packet when no DNSSEC RRs are present, but overflows
when the RRSIGS, NSEC ect are added. This extends the automatic
move from UDP to TCP to downstream queries which get truncated replies,
in the hope that once stripped of the DNSSEC RRs, the reply can be returned
via UDP, nwithout making the downstream retry with TCP.
If the downstream sets the DO bit, (ie it wants the DNSSEC RRs, then
this path is not taken, since the downstream will have to get a truncated
repsonse and retry to get a correct answer.
Heretofore, when a validating the result of an external query triggers
a DNSKEY or DS query and the result of that query is truncated, dnsmasq
has forced the whole validation process to move to TCP by returning a
truncated reply to the original requestor. This forces the original
requestor to retry the query in TCP mode, and the DNSSEC subqueries
also get made via TCP and everything works.
Note that in general the actual answer being validated is not large
enough to trigger truncation, and there's no reason not to return that
answer via UDP if we can validate it successfully. It follows that
a substandard client which can't do TCP queries will still work if the
answer could be returned via UDP, but fails if it gets an artifically
truncated answer and cannot move to TCP.
This patch teaches dnsmasq to move to TCP for DNSSEC queries when
validating UDP answers. That makes the substandard clients mentioned
above work, and saves a round trip even for clients that can do TCP.
The msg_controllen field passed to sendmsg is computed using the
CMSG_SPACE(), which is correct, but CMSG_SPACE() can include
padding bytes at the end of the passed structure if they are required
to align subsequent structures in the buffer. Make sure these
bytes are zeroed to avoid passing uninitiased memory to the kernel,
even though it will never touch these bytes.
Also tidy up the mashalling code in send_from to use pointers to
the structure being filled out, rather than a temporary copy which
then gets memcpy()'d into place. The DHCP sendmsg code has always
worked like this.
Thanks to Dominik Derigs for running Memcheck as submitting the
initial patch.
CAP_NET_ADMIN is needed in the DHCPv4 code to place entries into
the ARP cache. If it's configured to unconditionally broadcast
to unconfigured clients, it never touches the ARP cache and
doesn't need CAP_NET_ADMIN.
Thanks to Martin Ivičič <max.enhanced@gmail.com> for prompting this.
In generalising the RR filter code, the Dbus methods
controlling filtering A and AAAA records
got severely broken. This, and the previous commit,
fixes things.
Replies from upstream with a REFUSED rcode can result in
log messages stating that a resource limit has been exceeded,
which is not the case.
Thanks to Dominik Derigs and the Pi-hole project for
spotting this.
By calculating the hash of a DNSKEY once for each digest algo,
we reduce the hashing work from (no. DS) x (no. DNSKEY) to
(no. DNSKEY) x (no. distinct digests)
The number of distinct digests can never be more than 255 and
it's limited by which hashes we implement, so currently only 4.
An attacker can create DNSSEC signed domains which need a lot of
work to verfify. We limit the number of crypto operations to
avoid DoS attacks by CPU exhaustion.
If there are no dynamic configuration directories configured with
dhcp-hostsdir, dhcp-optsdir and hostsdir then we need to use inotify
only to track changes to resolv-files, but we don't need to do
that when DNS is disabled (port=0) or no resolv-files are configured.
It turns out that inotify slots can be a scarce resource, so not
using one when it's not needed is a Goood Thing.
Patch by HL, description above from SRK.
In extract_addresses() the "secure" argument is only set if the
whole reply is validated (ie the AD bit can be set). Even without
that, some records may be validated, and should be marked
as such in the cache.
Related, the DNS doctor code has to update the flags for individual
RRs as it works, not the global "secure" flag.
Now we can cache arbirary RRs, give more correct answers when
replying negative answers from cache.
To implement this needed the DNS-doctor code to be untangled from
find_soa(), so it should be under suspicion for any regresssions
in that department.
