Bug 202355

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


On Mon, 21 Jan 2019 04:11:06 +0000
bugzilla-daemon@bugzilla.kernel.org wrote:

> 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.
>

Seems reasonable.

I'll investigate getting this clearly documented in the manpages.