From 3a0f498c2b21146352fb21897b250ded0861a13f Mon Sep 17 00:00:00 2001
From: Greg Sabino Mullane <greg@turnstep.com>
Date: Wed, 26 Mar 2025 12:56:49 -0400
Subject: [PATCH] Add local address to log_line_prefix with %L

This shows the server IP that a client has connected to.

Socket connections will show "[local]"

Non-client connections (e.g. background processes) will show "[none]"
---
 doc/src/sgml/config.sgml                      |  5 +++++
 src/backend/utils/error/elog.c                | 19 +++++++++++++++++++
 src/backend/utils/misc/postgresql.conf.sample |  1 +
 3 files changed, 25 insertions(+)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 69fc93dffc4..646b5e29965 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -7674,6 +7674,11 @@ local0.*    /var/log/postgresql
              <entry>Remote host name or IP address</entry>
              <entry>yes</entry>
             </row>
+            <row>
+             <entry><literal>%L</literal></entry>
+             <entry>Local address (the IP address on the server that the client connected to)</entry>
+             <entry>yes</entry>
+            </row>
             <row>
              <entry><literal>%b</literal></entry>
              <entry>Backend type</entry>
diff --git a/src/backend/utils/error/elog.c b/src/backend/utils/error/elog.c
index 860bbd40d42..16367208552 100644
--- a/src/backend/utils/error/elog.c
+++ b/src/backend/utils/error/elog.c
@@ -67,6 +67,7 @@
 #endif
 
 #include "access/xact.h"
+#include "common/ip.h"
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
 #include "mb/pg_wchar.h"
@@ -3059,6 +3061,23 @@ log_status_format(StringInfo buf, const char *format, ErrorData *edata)
 					appendStringInfoSpaces(buf,
 										   padding > 0 ? padding : -padding);
 				break;
+			case 'L':
+				{
+					char		local_host[NI_MAXHOST];
+
+					strlcpy(local_host, "[none]", sizeof(local_host));
+
+					if (MyProcPort)
+						pg_getnameinfo_all(&MyProcPort->laddr.addr, MyProcPort->laddr.salen,
+										   local_host, sizeof(local_host),
+										   NULL, 0,
+										   NI_NUMERICHOST | NI_NUMERICSERV);
+					if (padding != 0)
+						appendStringInfo(buf, "%*s", padding, local_host);
+					else
+						appendStringInfoString(buf, local_host);
+				}
+				break;
 			case 'r':
 				if (MyProcPort && MyProcPort->remote_host)
 				{
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 0b9e3066bde..5e2b64cf774 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -602,6 +602,7 @@
 					#   %d = database name
 					#   %r = remote host and port
 					#   %h = remote host
+					#   %L = local address
 					#   %b = backend type
 					#   %p = process ID
 					#   %P = process ID of parallel group leader
-- 
2.30.2