socket: Introduce osmo_sock_set_dscp() to set socket DSCP value

At least on Linux, sockets have a IP_TOS socket option that can be
configured to set the TOS.  However, TOS (of RFC791) was replaced
by the DSCP (of RFC2474) in 1998.

As the DCSP bits are only the upper 6 bits of the TOS bits, let's
introduce a helper to get, mask and set the DSCP values in the TOS
bits.

Related: OS#5136, SYS#5427
Change-Id: Ia4ba389a5b7e3e9d5f17a742a900d6fd68c08e40

1 files changed, 26 insertions, 0 deletions

diff --git a/src/socket.c b/src/socket.c
index 229f72e3..095dee6b 100644
--- a/src/socket.c
+++ b/src/socket.c
@@ -1776,6 +1776,32 @@ char *osmo_sockaddr_to_str_buf(char *buf, size_t buf_len,
 	return buf;
 }
 
+/*! Set the DSCP (differentiated services code point) of a socket.
+ *  \param[in] dscp DSCP value in range 0..63
+ *  \returns 0 on success; negative on error. */
+int osmo_sock_set_dscp(int fd, uint8_t dscp)
+{
+	uint8_t tos;
+	socklen_t tos_len = sizeof(tos);
+	int rc;
+
+	/* DSCP is a 6-bit value stored in the upper 6 bits of the 8-bit TOS */
+	if (dscp > 63)
+		return -EINVAL;
+
+	/* read the original value */
+	rc = getsockopt(fd, IPPROTO_IP, IP_TOS, &tos, &tos_len);
+	if (rc < 0)
+		return rc;
+
+	/* mask-in the DSCP into the upper 6 bits */
+	tos &= 0x03;
+	tos |= dscp << 2;
+
+	/* and write it back to the kernel */
+	return setsockopt(fd, IPPROTO_IP, IP_TOS, &tos, sizeof(tos));
+}
+
 
 #endif /* HAVE_SYS_SOCKET_H */