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Filename Latest commit message Latest commit date
Maciej Żenczykowski ba05057bd0 net/ipv4: always honour route mtu during forwarding
[ Upstream commit 02a1b175b0e92d9e0fa5df3957ade8d733ceb6a0 ]

Documentation/networking/ip-sysctl.txt:46 says:
  ip_forward_use_pmtu - BOOLEAN
    By default we don't trust protocol path MTUs while forwarding
    because they could be easily forged and can lead to unwanted
    fragmentation by the router.
    You only need to enable this if you have user-space software
    which tries to discover path mtus by itself and depends on the
    kernel honoring this information. This is normally not the case.
    Default: 0 (disabled)
    Possible values:
    0 - disabled
    1 - enabled

Which makes it pretty clear that setting it to 1 is a potential
security/safety/DoS issue, and yet it is entirely reasonable to want
forwarded traffic to honour explicitly administrator configured
route mtus (instead of defaulting to device mtu).

Indeed, I can't think of a single reason why you wouldn't want to.
Since you configured a route mtu you probably know better...

It is pretty common to have a higher device mtu to allow receiving
large (jumbo) frames, while having some routes via that interface
(potentially including the default route to the internet) specify
a lower mtu.

Note that ipv6 forwarding uses device mtu unless the route is locked
(in which case it will use the route mtu).

This approach is not usable for IPv4 where an 'mtu lock' on a route
also has the side effect of disabling TCP path mtu discovery via
disabling the IPv4 DF (don't frag) bit on all outgoing frames.

I'm not aware of a way to lock a route from an IPv6 RA, so that also
potentially seems wrong.

Signed-off-by: Maciej Żenczykowski <maze@google.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Cc: Sunmeet Gill (Sunny) <sgill@quicinc.com>
Cc: Vinay Paradkar <vparadka@qti.qualcomm.com>
Cc: Tyler Wear <twear@quicinc.com>
Cc: David Ahern <dsahern@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2020-10-29 09:57:24 +01:00
arch ARM: 8939/1: kbuild: use correct nm executable 2020-10-17 10:11:22 +02:00
block block/diskstats: more accurate approximation of io_ticks for slow disks 2020-10-07 08:01:29 +02:00
certs
crypto
Documentation block/diskstats: more accurate approximation of io_ticks for slow disks 2020-10-07 08:01:29 +02:00
drivers can: m_can_platform: don't call m_can_class_suspend in runtime suspend 2020-10-29 09:57:24 +01:00
fs reiserfs: Fix oops during mount 2020-10-17 10:11:23 +02:00
include net/ipv4: always honour route mtu during forwarding 2020-10-29 09:57:24 +01:00
init kprobes: tracing/kprobes: Fix to kill kprobes on initmem after boot 2020-10-01 13:18:23 +02:00
ipc
kernel perf: Fix task_function_call() error handling 2020-10-14 10:33:05 +02:00
lib Fonts: Support FONT_EXTRA_WORDS macros for built-in fonts 2020-10-14 10:32:56 +02:00
LICENSES
mm mm: khugepaged: recalculate min_free_kbytes after memory hotplug as expected by khugepaged 2020-10-14 10:33:05 +02:00
net net: j1939: j1939_session_fresh_new(): fix missing initialization of skbcnt 2020-10-29 09:57:24 +01:00
samples
scripts scripts/dtc: only append to HOST_EXTRACFLAGS instead of overwriting 2020-10-07 08:01:29 +02:00
security
sound
tools perf cs-etm: Move definition of 'traceid_list' global variable from header file 2020-10-17 10:11:21 +02:00
usr
virt KVM: arm64: Assume write fault on S1PTW permission fault on instruction fetch 2020-10-01 13:18:25 +02:00
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Makefile Linux 5.4.72 2020-10-17 10:11:24 +02:00
README

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.