Fw: [Bug 202355] New: UDP does not report all ICMP errors on connected sockets in violation of RFC1122 4.1.1.3

Fw: [Bug 202355] New: UDP does not report all ICMP errors on connected sockets in violation of RFC1122 4.1.1.3

[Stephen Hemminger]

Unless there is any objection, I intend to close this bug
as "that is the way Linux works, we can't break userspace"



Begin forwarded message:

Date: Mon, 21 Jan 2019 04:11:06 +0000
From: bugzilla-daemon@bugzilla.kernel.org
To: stephen@networkplumber.org
Subject: [Bug 202355] New: UDP does not report all ICMP errors on connected sockets in violation of RFC1122 4.1.1.3


https://bugzilla.kernel.org/show_bug.cgi?id=202355

            Bug ID: 202355
           Summary: UDP does not report all ICMP errors on connected
                    sockets in violation of RFC1122 4.1.1.3
           Product: Networking
           Version: 2.5
    Kernel Version: From 0.98 until v5.0-rc1
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: IPV4
          Assignee: stephen@networkplumber.org
          Reporter: linux@isomer.meta.net.nz
        Regression: No

RFC1122 section 4.1.1.3 says:

            UDP MUST pass to the application layer all ICMP error
            messages that it receives from the IP layer. 

 -- https://tools.ietf.org/html/rfc1122#page-78

However, Linux appears to try and (misapply) RFC1122 section 3.2.2.1:

            A Destination Unreachable message that is received with code
            0 (Net), 1 (Host), or 5 (Bad Source Route) may result from a
            routing transient and MUST therefore be interpreted as only
            a hint, not proof, that the specified destination is
            unreachable [IP:11].  For example, it MUST NOT be used as
            proof of a dead gateway (see Section 3.3.1).

 -- https://tools.ietf.org/html/rfc1122#page-40


Where it does not report Destination Host/Net unreachable, Source Route Failed,
and Fragmentation Required back to userspace on connect()ed sockets due to them
being considered transient.
See:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/ipv4/icmp.c#n118
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/ipv4/udp.c#n704
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/ipv4/udp.c#n722

I believe it's doing this because it incorrectly believes that these are "non
fatal" errors, and therefore should not be reported on the socket back to
userspace.  Currently this can be overridden as a side effect of setting
SO_RECVERR.  Digging around, this behaviour was added in Linux 0.98 with the
original import of the the networking stack into the Linux Kernel(!).

This differs from RFC1122, the *BSD's implementation, and differs from people's
expectations.  (Eg see the thread
https://lists.dns-oarc.net/pipermail/dns-operations/2019-January/018271.html)

Linux should be RFC1122 compliant, and report all ICMP error messages back to
userspace, *without* requiring SO_RECVERR sockopt being set.  Or, at the very
least, this discrepancy should be very clearly documented.

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