From a6c8c3ad8906958d1504aac3c2697dd8361a0f7e Mon Sep 17 00:00:00 2001
From: Gerhard Rieger <gerhard@dest-unreach.org>
Date: Thu, 16 May 2024 15:25:26 +0200
Subject: [PATCH] socat-mux.sh and socat-broker.sh use low ports when run as
 root

---
 CHANGES         |  3 +++
 socat-broker.sh | 18 ++++++++++++++++--
 socat-mux.sh    | 23 +++++++++++++++++++----
 3 files changed, 38 insertions(+), 6 deletions(-)

diff --git a/CHANGES b/CHANGES
index 88f5f1e..2f66737 100644
--- a/CHANGES
+++ b/CHANGES
@@ -26,6 +26,9 @@ Features:
 	Changed socat-chain.sh, socat-mux.sh, and socat-broker.sh to work with
 	older Socat versions.
 
+	socat-mux.sh and socat-broker.sh, when run as root, now internally use
+	low (512..1023) UDP ports to increase security.
+
 Porting:
 	Changes for building and testing on NetBSD
 
diff --git a/socat-broker.sh b/socat-broker.sh
index f8b5abb..f91b4ec 100755
--- a/socat-broker.sh
+++ b/socat-broker.sh
@@ -65,8 +65,18 @@ esac
 if [ -z "$SOCAT" ]; then SOCAT=socat; fi
 [ "$VERBOSE" ] && echo "# $0: Using executable $SOCAT" >&2
 
+# When run as root we try low ports
+LOWPORT=
+PATTERN=bound
+if [ "$(id -u)" = 0 ]; then
+    LOWPORT="lowport"
+    PATTERN="successfully prepared local socket"
+fi
+
 # We need a free UDP port (on loopback)
-PORT=$($SOCAT -d -d -T 0.000001 UDP4-RECV:0 /dev/null 2>&1 |grep bound |sed 's/.*:\([1-9][0-9]*\)$/\1/')
+if [ -z "$LOWPORT" ]; then
+    PORT=$($SOCAT -d -d -T 0.000001 UDP4-RECV:0 /dev/null 2>&1 |grep bound |sed 's/.*:\([1-9][0-9]*\)$/\1/')
+fi
 if [ -z "$PORT" ]; then
     # Probably old Socat version, use a different approach
     if type ss >/dev/null 2>&1; then
@@ -79,7 +89,11 @@ if [ -z "$PORT" ]; then
     fi
     PORT=
     while [ -z "$PORT" ] || ss -aun |grep -e ":$PORT\>" >/dev/null; do
-	PORT=$((16384+RANDOM))
+	if [ -z "$LOWPORT" ]; then
+	    PORT=$((16384+RANDOM))
+	else
+	    PORT=$((512+(RANDOM>>6) ))
+	fi
     done
 fi
 [ "$VERBOSE" ] && echo "# $0: Using UDP port $PORT" >&2
diff --git a/socat-mux.sh b/socat-mux.sh
index 1d7e0de..084c0e6 100755
--- a/socat-mux.sh
+++ b/socat-mux.sh
@@ -72,9 +72,19 @@ esac
 if [ -z "$SOCAT" ]; then SOCAT=socat; fi
 [ "$VERBOSE" ] && echo "# $0: Using executable $SOCAT" >&2
 
+# When run as root we try low ports
+LOWPORT=
+PATTERN=bound
+if [ "$(id -u)" = 0 ]; then
+    LOWPORT="lowport"
+    PATTERN="successfully prepared local socket"
+fi
+
 # We need two free UDP ports (on loopback)
-PORT1=$($SOCAT -d -d -T 0.000001 UDP4-RECV:0 /dev/null 2>&1 |grep bound |sed 's/.*:\([1-9][0-9]*\)$/\1/')
-PORT2=$($SOCAT -d -d -T 0.000001 UDP4-RECV:0 /dev/null 2>&1 |grep bound |sed 's/.*:\([1-9][0-9]*\)$/\1/')
+if [ -z "$LOWPORT" ]; then
+    PORT1=$($SOCAT -d -d -T 0.000001 UDP4-RECV:0 /dev/null 2>&1 |grep "$PATTERN" |sed 's/.*:\([1-9][0-9]*\)$/\1/')
+    PORT2=$($SOCAT -d -d -T 0.000001 UDP4-RECV:0 /dev/null 2>&1 |grep "$PATTERN" |sed 's/.*:\([1-9][0-9]*\)$/\1/')
+fi
 if [ -z "$PORT1" -o -z "$PORT2" ]; then
     # Probably old Socat version, use a different approach
     if type ss >/dev/null 2>&1; then
@@ -87,8 +97,13 @@ if [ -z "$PORT1" -o -z "$PORT2" ]; then
     fi
     PORT1= PORT2=
     while [ -z "$PORT1" -o -z "$PORT2" -o "$PORT1" = "$PORT2" ] || ss -aun |grep -e ":$PORT1\>" -e ":$PORT2\>" >/dev/null; do
-	PORT1=$((16384+RANDOM))
-	PORT2=$((16384+RANDOM))
+	if [ -z "$LOWPORT" ]; then
+	    PORT1=$((16384+RANDOM))
+	    PORT2=$((16384+RANDOM))
+	else
+	    PORT1=$((512+(RANDOM>>6) ))
+	    PORT2=$((512+(RANDOM>>6) ))
+	fi
     done
 fi
 [ "$VERBOSE" ] && echo "# $0: Using UDP ports $PORT1, $PORT2" >&2