[1/6] AX.25: Add ax25_ntop implementation.

diff --git a/Makefile b/Makefile
index 5bc11477..551f528b 100644
--- a/Makefile
+++ b/Makefile
@@ -40,6 +40,9 @@  DEFINES+=-DCONFDIR=\"$(CONFDIR)\" \
          -DNETNS_RUN_DIR=\"$(NETNS_RUN_DIR)\" \
          -DNETNS_ETC_DIR=\"$(NETNS_ETC_DIR)\"
 
+#options for AX.25
+ADDLIB+=ax25_ntop.o
+
 #options for mpls
 ADDLIB+=mpls_ntop.o mpls_pton.o
 
diff --git a/include/utils.h b/include/utils.h
index 28eaad8e..31c1e442 100644
--- a/include/utils.h
+++ b/include/utils.h
@@ -209,6 +209,8 @@  bool matches(const char *prefix, const char *string);
 int inet_addr_match(const inet_prefix *a, const inet_prefix *b, int bits);
 int inet_addr_match_rta(const inet_prefix *m, const struct rtattr *rta);
 
+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);
 
diff --git a/lib/ax25_ntop.c b/lib/ax25_ntop.c
new file mode 100644
index 00000000..48098581
--- /dev/null
+++ b/lib/ax25_ntop.c
@@ -0,0 +1,74 @@ 
+/* SPDX-License-Identifier: GPL-2.0+ */
+
+#include <errno.h>
+#include <sys/socket.h>
+#include <netax25/ax25.h>
+
+#include "utils.h"
+
+/*
+ * 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.
+ * 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.
+ *
+ * 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
+ * a packet's address field but for our purposes here we're not considering
+ * the HDLC extension bit that is it will always be zero.
+ *
+ * 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.
+ *
+ * This for example turns "LINUX-1" where the callsign is LINUX and SSID is 1
+ * into 98:92:9c:aa:b0:40:02.
+ */
+
+static const char *ax25_ntop1(const ax25_address *src, char *dst,
+			      socklen_t size)
+{
+	char c, *s;
+	int n;
+
+	for (n = 0, s = dst; n < 6; n++) {
+		c = (src->ax25_call[n] >> 1) & 0x7f;
+		if (c != ' ')
+			*s++ = c;
+	}
+
+	*s++ = '-';
+
+	n = ((src->ax25_call[6] >> 1) & 0x0f);
+	if (n > 9) {
+		*s++ = '1';
+		n -= 10;
+	}
+
+	*s++ = n + '0';
+	*s++ = '\0';
+
+	if (*dst == '\0' || *dst == '-') {
+		dst[0] = '*';
+		dst[1] = '\0';
+	}
+
+	return dst;
+}
+
+const char *ax25_ntop(int af, const void *addr, char *buf, socklen_t buflen)
+{
+	switch (af) {
+	case AF_AX25:
+		errno = 0;
+		return ax25_ntop1((ax25_address *)addr, buf, buflen);
+
+	default:
+		errno = EAFNOSUPPORT;
+	}
+
+	return NULL;
+}