Add preserve_fqdn to SYSLOG_CONFIG and toggle long/short hostname

[spandan-nexthop]

Add preserve_fqdn to SYSLOG_CONFIG and update hostname in rsyslog-config.sh such that the remote syslog messages have the long/short hostname accordingly.
Fixes #23863
[CLOSED] Original PR was #23864.

Why I did it

The original PR was #23864.
This was closed since the changes for hardcoding the hostname got committed as part of #24394

Currently, in SONiC, syslog messages are only sent with the shortened hostname, not the FQDN. It would be good to conditionally be able to toggle the hostname to the long (FQDN) or short form in remote syslog messages.

Work item tracking

• Microsoft ADO (number only):

How I did it

• Added a new leaf - preseve_fqdn - to SYSLOG_CONFIG in sonic-syslog.yang
• Since any change inside SYSLOG_CONFIG|GLOBAL automatically triggers an rsyslog-config restart, I just added a check for preserve_fqdn in rsyslog-config.sh such that it sets the hostname to the long/short form correctly.
• Note that, adding PreserveFQDN on|off in the syslog config would have been an alternate solution, but that approach requires an update and restart of rsyslog in the host AS WELL AS all the containers separately since each run their own syslog daemon. This was a more disruptive solution and we chose to not go down that path.

How to verify it

Enabling preserve_fqdn and doing a tcpdump for the syslog messages shows the correct FQDN.

host$ show syslog
SERVER IP    SOURCE IP    PORT    VRF
-----------  -----------  ------  -----
10.11.0.5    N/A          N/A     N/A
10.11.0.6    N/A          N/A     N/A

host$ sonic-db-cli CONFIG_DB HSET "SYSLOG_CONFIG|GLOBAL" "preserve_fqdn" "true"
1

(snmp)root@host:/# logger --priority CRIT "Test syslog message from snmp container MSG1"

23:50:54.629484 lo    In  IP localhost.38482 > localhost.syslog: SYSLOG user.critical, length: 104
E.....@.@..H.........R...p..<10>2025-07-31T23:50:54.629330+00:00 <host_FQDN> snmp#root: Test syslog message from snmp container MSG1
23:50:54.629610 eth0  Out IP 10.250.15.2.36470 > 10.11.0.5.syslog: SYSLOG user.critical, length: 130
E....!@.@.M"

Which release branch to backport (provide reason below if selected)

• 202305
• 202311
• 202405
• 202411
• 202505
• 202511

Tested branch (Please provide the tested image version)

Description for the changelog

Adding preserve_fqdn to SYSLOG_CONFIG to conditionally toggle the full/short hostname sent in syslog messages.

Link to config_db schema for YANG module changes

https://github.com/spandan-nexthop/sonic-buildimage/blob/spandan-nexthop.add-fqdn-syslog/src/sonic-yang-models/yang-models/sonic-syslog.yang#L184

A picture of a cute animal (not mandatory but encouraged)

[mssonicbld]

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[spandan-nexthop]

@gechiang @saiarcot895
@anders-nexthop

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[mssonicbld]

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[mssonicbld]

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[spandan-nexthop]

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[mssonicbld]

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[spandan-nexthop]

/azpw retry

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[yijingyan2]

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[mssonicbld]

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[yijingyan2]

Hi @spandan-nexthop, your build was triggered one month ago. The build has been cleaned already. Please close and reopen the PR or rebase your branch to trigger a new build. Thanks

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[yxieca]

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[mssonicbld]

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[mssonicbld]

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[mssonicbld]

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[spandan-nexthop]

/azpw retry

[mssonicbld]

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Retrying failed(or canceled) stages in build 1101788:

✅Stage Test:

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