pmrfc3164: accept IPv6 host tokens

Keep IPv6-based syslog sources interoperable in legacy RFC3164
pipelines without manual header cleanup.

Before: IPv6 literals in HOSTNAME stopped parsing and
After: IPv6 literals are accepted as HOSTNAME tokens.

Impact: fixes user-visible RFC3164 forwarding for IPv6 hosts.

Recognize IPv6 literals when the hostname scan hits a colon and
validate the full token with inet_pton before accepting it.
Honor permit.squarebracketsinhostname when brackets are present.
Consume the hostname token and trailing space to keep TAG parsing
in sync with RFC3164 expectations.
Add a regression test for IPv6 HOSTNAME parsing.
Tests: not run (not requested).

Fixes: rsyslog#3012
With the help of AI-Agents: GPT-5.2-Codex

Author: rgerhards

tests/Makefile.am

@@ -179,6 +179,7 @@ TESTS +=  \
 	prop-programname-with-slashes.sh \
 	hostname-with-slash-pmrfc5424.sh \
 	hostname-with-slash-pmrfc3164.sh \
+	pmrfc3164-ipv6-hostname.sh \
 	hostname-with-slash-dflt-invld.sh \
 	func-substring-invld-startpos.sh \
 	func-substring-large-endpos.sh \
@@ -2087,6 +2088,7 @@ EXTRA_DIST= \
 	func-substring-relative-endpos.sh \
 	hostname-with-slash-pmrfc5424.sh \
 	hostname-with-slash-pmrfc3164.sh \
+	pmrfc3164-ipv6-hostname.sh \
 	pmrfc3164-msgFirstSpace.sh \
 	pmrfc3164-AtSignsInHostname.sh \
 	pmrfc3164-AtSignsInHostname_off.sh \

tests/pmrfc3164-ipv6-hostname.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+## Test RFC3164 parser accepts IPv6 literals as HOSTNAME.
+. ${srcdir:=.}/diag.sh init
+generate_conf
+add_conf '
+module(load="../plugins/imtcp/.libs/imtcp")
+input(type="imtcp" port="0" listenPortFileName="'$RSYSLOG_DYNNAME'.tcpflood_port" ruleset="customparser")
+parser(name="custom.rfc3164" type="pmrfc3164" permit.squarebracketsinhostname="on")
+template(name="outfmt" type="string" string="%hostname%\n")
+
+ruleset(name="customparser" parser="custom.rfc3164") {
+	action(type="omfile" template="outfmt" file="'$RSYSLOG_OUT_LOG'")
+}
+'
+startup
+tcpflood -m1 -M "\"<129>Mar 10 01:00:00 2001:db8::1 tag: msgnum:1\""
+tcpflood -m1 -M "\"<129>Mar 10 01:00:00 [2001:db8::2] tag: msgnum:2\""
+shutdown_when_empty
+wait_shutdown
+export EXPECTED='2001:db8::1
+[2001:db8::2]'
+cmp_exact
+
+exit_test