[Bug network/30631] New: [RFE] option ends0 should allow setting optional EDNS0 buffer value

[Bug network/30631] New: [RFE] option ends0 should allow setting optional EDNS0 buffer value

[pemensik at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=30631

            Bug ID: 30631
           Summary: [RFE] option ends0 should allow setting optional EDNS0
                    buffer value
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: network
          Assignee: unassigned at sourceware dot org
          Reporter: pemensik at redhat dot com
  Target Milestone: ---

options edns0 enables outgoing queries to accept responses up to
RESOLV_EDNS_BUFFER_SIZE==1200.

I have just found systemd-resolved offers quite big EDNS0 size: 65494.

It is true on lo interface there should not be any fragmentation, which
requires low EDNS0 value (check bug #21361). There is no single value that
would work always. I think it would be great, if edns0 binary option were
expanded to numeric option like timeout:X.

That would allow to set

nameserver 127.0.0.1
options edns0:4096

But would still accept just:

nameserver 8.8.8.8
options edns0

For keeping backward compatibility. Because this actually goes over network,
where on-path MTU is not known and might be changing dynamically, low value
like 1200 is appropriate there to avoid fragmentation.

If the nameserver is on the same LAN, it might be useful to specify edns0:1500
for it. On local network only MTU can be derived from the interface leading to
the nameserver. It might delegate on-path measurements to Network Manager
daemon or whatever is configuring /etc/resolv.conf. But currently there is no
way to set custom environment or any configuration file.

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