[Bug 1804487] [NEW] systemd-resolved has issues when the answer is over 512 bytes with EDNS disabled

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[Impact]

TCP stub is cutting down the payload to 512 bytes when EDNS is disabled.
This makes non-EDNS clients (nslookup) receive a "shortened" answer even
when UDP returns a truncated reply for a new TCP query. For instance,

- If the client supports EDNS:

$ dig +noall +answer testing.irongiantdesign.com @127.0.0.53 | wc -l
30

- If the client does not support EDNS:

$ dig +noedns +noall +answer testing.irongiantdesign.com @127.0.0.53 | wc -l
29

In the second case, no-EDNS, TCP should provide the complete answer, but
it's capped at UDP's size.

[Test Case]

Query systemd-resolved with a domain name that resolves to multiple
(lots.. 30+) A records. A client with EDNS support (dig) will receive
all of them, a client without support (nslookup or dig +noedns) will
have a truncated list. Using the example above:

EDNS: dig +noall +answer testing.irongiantdesign.com @127.0.0.53 | wc -l
non-EDNS: dig +noedns +noall +answer testing.irongiantdesign.com @127.0.0.53 | wc -l

[Regression potential]

Minimal. This change only affects TCP requests, and the new size is
already used in the code for other requests.

[Other Info]

Upstream bug: https://github.com/systemd/systemd/issues/10816
Fixed upstream with commit: https://github.com/systemd/systemd/commit/e6eed9445956cfa496e1db933bfd3530db23bfce

[Original Description]

Querying a domain name that has >512 bytes in records (e.g. 30+ A
records), the number of results depends on the DNS client used:

- If the client supports EDNS:

$ dig +noall +answer testing.irongiantdesign.com @127.0.0.53 | wc -l
30

- If the client does not support EDNS:

$ dig +noedns +noall +answer testing.irongiantdesign.com @127.0.0.53 | wc -l
29

Normally a client that doesn't support EDNS would receive a truncated
reply from the initial UDP connection (limited by the spec to 512 bytes)
and a second query would be established via TCP to receive the complete
results. In this case, the number of results is the same regardless of
the protocol used (29).

Upstream bug: https://github.com/systemd/systemd/issues/10816

** Affects: systemd (Ubuntu)
     Importance: High
     Assignee: Victor Tapia (vtapia)
         Status: In Progress

** Affects: systemd (Ubuntu Xenial)
     Importance: Undecided
         Status: Invalid

** Affects: systemd (Ubuntu Bionic)
     Importance: High
     Assignee: Victor Tapia (vtapia)
         Status: In Progress

** Affects: systemd (Ubuntu Cosmic)
     Importance: High
     Assignee: Victor Tapia (vtapia)
         Status: In Progress

** Affects: systemd (Ubuntu Disco)
     Importance: High
     Assignee: Victor Tapia (vtapia)
         Status: In Progress


** Tags: patch sts
-- 
systemd-resolved has issues when the answer is over 512 bytes with EDNS disabled  
https://bugs.launchpad.net/bugs/1804487
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