** Changed in: systemd
Status: New => Fix Released
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https://bugs.launchpad.net/bugs/1811471
Title:
local resolver stub fails to handle multiple TCP dns queries
Status in systemd:
Fix Released
Status in systemd package in Ubuntu:
Fix Released
Status in systemd source package in Trusty:
Invalid
Status in systemd source package in Xenial:
Invalid
Status in systemd source package in Bionic:
Fix Released
Status in systemd source package in Cosmic:
Fix Released
Status in systemd source package in Disco:
Fix Released
Bug description:
[Impact]
The systemd local 'stub' resolver handles all local DNS queries (by
default configuration used in Ubuntu), and essentially proxies all
requests to its configured upstream DNS resolvers.
Most local DNS resolution by applications uses glibc's getaddrinfo()
function. This function is configured in various ways by the
/etc/resolv.conf file, which tells glibc what nameserver/resolver to
contact as well as how to talk to the name server.
By default, glibc performs UDP DNS queries, with a single DNS query
per UDP packet. The UDP packet size is limited per DNS spec to 512
bytes. For some DNS lookups, a 512 byte UDP packet is not large
enough to contain the entire response - for example, an A record
lookup with a large number (e.g. 30) of A record addresses. This
number of A record entries is possible in some cases of load
balancing. When the DNS UDP response size is larger than 512 bytes,
the server puts as much response as it can into the DNS UDP response,
and marks the "trunacted" flag. This lets glibc know that the DNS UDP
packet did not contain the entire response for all the A records.
When glibc sees a UDP response that is "trunacted", by default it
ignores the contents of that response and issues a new DNS query,
using TCP instead of UDP. The TCP packet size has a higher size limit
(though see bug 1804487 which is a bug in systemd's max-sizing of TCP
DNS packets), and so *should* allow glibc to receive the entire DNS
response.
However, glibc issues DNS queries for both A and AAAA records. When
it uses UDP, those DNS queries are separate (i.e. one UDP DNS packet
with a single A query, and one UDP DNS packet with a single AAAA
query). When glibc uses TCP, it puts both DNS queries into a single
TCP DNS packet - the RFC refers to this as "pipelining"
(https://tools.ietf.org/html/rfc7766#section-6.2.1.1) and states that
clients SHOULD do this, and that servers MUST expect to receive
pipelined queries and SHOULD respond to all of them. (Technically
pipelining can be separate DNS queries, one per TCP packet, but both
using the same TCP connection - but the clear intention of pipelining
is to improve TCP performance, and putting both DNS queries into a
single TCP packet is clearly more performant than using separate TCP
packets).
Unfortunately, systemd's local stub resolver has only very basic
support for TCP DNS, and it handles TCP DNS queries almost identically
to UDP DNS queries - it reads the DNS query 2-byte header (containing
the length of the query data), reads in the single DNS query data,
performs lookup and sends a response to that DNS query, and closes the
TCP connection. It does not check for "pipelined" queries in the TCP
connection.
That would be bad enough, as glibc is (rightly) expecting a response
to both its A and AAAA queries; however what glibc gets is a TCP
connection-reset error. That is because the local systemd stub
resolver has closed its TCP socket while input data was still pending
(i.e. it never even read the second pipelined DNS query). When the
kernel sees unread input bytes in a TCP connection that is closed, it
sends a TCP RST to the peer (i.e. glibc) and when the kernel sees the
RST, it dumps all data in its socket buffer and passes the ECONNRESET
error up to the application. So glibc gets nothing besides a
connection reset error.
Note also that even if the systemd local stub resolver's socket
flushes its input buffer before closing the TCP connection (which will
avoid the TCP RST), glibc still expects responses to both its A and
AAAA queries before systemd closes the TCP connection, and so a simple
change to systemd to flush the input buffer is not enough to fix the
bug (and would also not actually fix the bug since glibc would never
get the AAAA response).
[Test Case]
This can be reproduced on any system using a local systemd stub
resolver, when using an application that uses getaddrinfo() - such as
ssh, telnet, ping, etc - or with a simple C program that uses
getaddrinfo(). The dns name looked up must have enough A records to
overflow the 512 byte maximum for a UDP DNS packet; e.g.:
$ ping testing.irongiantdesign.com
ping: testing.irongiantdesign.com: Temporary failure in name resolution
Alternately, and trivially, glibc can be forced to always use TCP DNS queries
by editing the /etc/resolv.conf file and adding:
options use-vc
With that option, glibc will fail to lookup 100% of DNS names, since
all lookups will use TCP to talk to the local systemd stub resolver,
which as explained above fails to ever correctly answer glibc's
pipelined TCP DNS queries.
Note that in default Ubuntu installs, /etc/resolv.conf is a symlink to
../run/systemd/resolve/stub-resolv.conf, which systemd thinks it owns
100% - so any manual changes to the file may be overwritten at any
time. There is no way (that I can find) to tell systemd to add any
resolv.conf options (like 'use-vc') to its managed stub-resolv.conf
file, so this test case requires re-editing the /etc/resolv.conf file
intermittently, each time systemd overwrites it.
Note also that the patch used to work around this (see Other Info
below) will fix the case of lookup failures for very long A records;
but the workaround will not help at all with the test case of using
'option use-vc'. That test case will continue to fail for 100% of dns
lookups.
[Regression Potential]
To workaround this, the patch enables edns0 in systemd's stub resolver
resolv.conf file. This could cause problems for any system code that
does not expect the resolv.conf file to include a new line/option, or
could introduce problems with edns0 lookups, since glibc was not
previously using edns0.
[Other Info]
This bug exists upstream, with proposed patches to add dns tcp pipeline
support:
https://github.com/systemd/systemd/pull/11512
The specific bug of TCP DNS fallback not working for DNS responses larger
than 512 bytes can be worked around by editing the /etc/resolv.conf file to add:
options edns0
The EDNS0 option causes glibc to fall back to attempting UDP EDNS0
query (which has a higher max packet size than the default 512 byte
UDP DNS). The systemd stub resolver does support EDNS0. However,
this workaround only temporarily works - as explained above, by
default /etc/resolv.conf is a symlink to a file that systemd
overwrites intermittently, which will remove the EDNS0 option.
The upstream patch that will be used to work around this bug in exactly that
way (i.e. adding option edns0 to resolv.conf) is:
https://github.com/systemd/systemd/commit/93158c77bc69fde7cf5cff733617631c1e566fe8
That patch is already included in Debian and so no Debian bug is
required for this bug (since the only fix for this specific bug will
be sru'ing the edns0 workaround)
Since Xenial and Trusty do not use the systemd stub resolver (by
default) I marked this Invalid for those releases.
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