Fluent bit syslog input should not emit IPv4-mapped IPv6-addresses when using source_address_key

[pv2b]

Bug Report

Describe the bug
When binding to :: rather than 127.0.0.1, to support dual stack operations, you would not expect the source_address_key changing, but it does, because inet_ntop doesn't properly detect IPv4-mapped IPv6 addresses, retulting in IP addresses like ::ffff:192.0.2.123 rather than 192.0.2.123 in the source_address_key output.

[0] syslog: [[1773567148.159789000, {}], {"pri"=>"13", "time"=>"2026-03-15T09:32:28.159789+00:00", "host"=>"ace7e86ecd9f", "ident"=>"fluent-bit", "pid"=>"-", "msgid"=>"-", "extradata"=>"[timeQuality tzKnown="1" isSynced="1" syncAccuracy="465500"]", "message"=>"hello there", "source"=>"udp://::ffff:127.0.0.1:56402"}]

(Also note that there are no brackets around the IPv6 literal, this is a seperate bug #11558, this bug is to make it not use IPv4-mapped IPv6-addresses at all in this case.)

To Reproduce

• Steps to reproduce the problem:

Create a fluent bit configuration like this test one:

[SERVICE]
    flush        1
    daemon       Off
    log_level    info
    parsers_file ../conf/parsers.conf

[INPUT]
    name         syslog
    tag          syslog
    listen       ::
    mode         udp
    parser       syslog-rfc5424
    source_address_key source

[OUTPUT]
    name         stdout
    match        *

Run fluent bit with this configuration using a command similar to ../build/bin/fluent-bit -c syslog-to-stdout.conf

Send a test message into the logger to IPv4 localhost using a command similar to logger -d -n 127.0.0.1 -P 5140 --rfc5424 "hello there"

[0] syslog: [[1773567148.159789000, {}], {"pri"=>"13", "time"=>"2026-03-15T09:32:28.159789+00:00", "host"=>"ace7e86ecd9f", "ident"=>"fluent-bit", "pid"=>"-", "msgid"=>"-", "extradata"=>"[timeQuality tzKnown="1" isSynced="1" syncAccuracy="465500"]", "message"=>"hello there", "source"=>"udp://::ffff:127.0.0.1:56402"}]

Expected behavior

Fluent bit should emit an IPv4 literal, not an IPv4-mapped IPv6 address. For example, the appropriate source URL would be udp://127.0.0.1:56402

Screenshots

Your Environment

• Version used: Latest "master" branch of Fluent bit as of commit acad02a)
• Configuration: See above
• Environment name and version (e.g. Kubernetes? What version?): Dev container
• Server type and version: n/a
• Operating System and version: Ubuntu 25.10
• Filters and plugins: syslog input

Additional context

This will block anyone who wants to transition from binding Fluent Bit to 0.0.0.0, to bind on :: instead, to go to dual stack, because the source address field will change.

It should possible for me to unmangle them using some kind of filter, but I shouldn't have to do it, and if I end up using a filter, it'll probably break once this is actually fixed.