[3/6] NETROM: Add netrom_ntop implementation.

diff --git a/Makefile b/Makefile
index 551f528b..df894d54 100644
--- a/Makefile
+++ b/Makefile
@@ -46,6 +46,9 @@  ADDLIB+=ax25_ntop.o
 #options for mpls
 ADDLIB+=mpls_ntop.o mpls_pton.o
 
+#options for NETROM
+ADDLIB+=netrom_ntop.o
+
 CC := gcc
 HOSTCC ?= $(CC)
 DEFINES += -D_GNU_SOURCE
diff --git a/include/utils.h b/include/utils.h
index 31c1e442..c685a392 100644
--- a/include/utils.h
+++ b/include/utils.h
@@ -214,6 +214,8 @@  const char *ax25_ntop(int af, const void *addr, char *str, socklen_t len);
 const char *mpls_ntop(int af, const void *addr, char *str, size_t len);
 int mpls_pton(int af, const char *src, void *addr, size_t alen);
 
+const char *netrom_ntop(int af, const void *addr, char *str, socklen_t len);
+
 extern int __iproute2_hz_internal;
 int __get_hz(void);
 
diff --git a/lib/ax25_ntop.c b/lib/ax25_ntop.c
index 48098581..cfd0e04b 100644
--- a/lib/ax25_ntop.c
+++ b/lib/ax25_ntop.c
@@ -6,13 +6,22 @@ 
 
 #include "utils.h"
 
+const char *ax25_ntop1(const ax25_address *src, char *dst, socklen_t size);
+
 /*
  * AX.25 addresses are based on Amateur radio callsigns followed by an SSID
- * like XXXXXX-SS where the callsign is up to 6 characters which are either
- * letters or digits and the SSID is a decimal number in the range 0..15.
+ * like XXXXXX-SS where the callsign consists of up to 6 ASCII characters
+ * which are either letters or digits and the SSID is a decimal number in the
+ * range 0..15.
  * Amateur radio callsigns are assigned by a country's relevant authorities
  * and are 3..6 characters though a few countries have assigned callsigns
  * longer than that.  AX.25 is not able to handle such longer callsigns.
+ * There are further restrictions on the format of valid callsigns by
+ * applicable national and international law.  Linux doesn't need to care and
+ * will happily accept anything that consists of 6 ASCII characters in the
+ * range of A-Z and 0-9 for a callsign such as the default AX.25 MAC address
+ * LINUX-1 and the default broadcast address QST-0.
+ * The SSID is just a number and not encoded in ASCII digits.
  *
  * Being based on HDLC AX.25 encodes addresses by shifting them one bit left
  * thus zeroing bit 0, the HDLC extension bit for all but the last bit of
@@ -22,14 +31,13 @@ 
  * Linux' internal representation of AX.25 addresses in Linux is very similar
  * to this on the on-air or on-the-wire format.  The callsign is padded to
  * 6 octets by adding spaces, followed by the SSID octet then all 7 octets
- * are left-shifted by one byte.
+ * are left-shifted by one bit.
  *
- * This for example turns "LINUX-1" where the callsign is LINUX and SSID is 1
- * into 98:92:9c:aa:b0:40:02.
+ * For example, for the address "LINUX-1" the callsign is LINUX and SSID is 1
+ * the internal format is 98:92:9c:aa:b0:40:02.
  */
 
-static const char *ax25_ntop1(const ax25_address *src, char *dst,
-			      socklen_t size)
+const char *ax25_ntop1(const ax25_address *src, char *dst, socklen_t size)
 {
 	char c, *s;
 	int n;
diff --git a/lib/netrom_ntop.c b/lib/netrom_ntop.c
new file mode 100644
index 00000000..3dd6cb0b
--- /dev/null
+++ b/lib/netrom_ntop.c
@@ -0,0 +1,23 @@ 
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#include <sys/socket.h>
+#include <errno.h>
+#include <linux/ax25.h>
+
+#include "utils.h"
+
+const char *ax25_ntop1(const ax25_address *src, char *dst, socklen_t size);
+
+const char *netrom_ntop(int af, const void *addr, char *buf, socklen_t buflen)
+{
+	switch (af) {
+	case AF_NETROM:
+		errno = 0;
+		return ax25_ntop1((ax25_address *)addr, buf, buflen);
+
+	default:
+		errno = EAFNOSUPPORT;
+	}
+
+	return NULL;
+}