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-rw-r--r--lib/ax25_ntop.c82
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diff --git a/lib/ax25_ntop.c b/lib/ax25_ntop.c
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+/* SPDX-License-Identifier: GPL-2.0+ */
+
+#include <errno.h>
+#include <sys/socket.h>
+#include <linux/ax25.h>
+
+#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 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
+ * 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 bit.
+ *
+ * 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.
+ */
+
+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;
+}