[PATCH 1/1] lib/socket: randomize the generated local port

[Thomas Haller]

Instead of always trying the same order of ports when
looking for an unused port, randomize the order (naively).

As libnl-1 uses the same function, it is likely that two applications
that are using both libraries generate the same ports. By chossing a
different order how to select the local port, the chances are smaller
for this to happen.

Signed-off-by: Thomas Haller <thaller at redhat.com>
---
 lib/socket.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/lib/socket.c b/lib/socket.c
index 00d8d6a..c12817e 100644
--- a/lib/socket.c
+++ b/lib/socket.c
@@ -61,12 +61,18 @@ static NL_RW_LOCK(port_map_lock);
 
 static uint32_t generate_local_port(void)
 {
-	int i, n;
+	int i, j, n;
+	static int8_t idx_state = -1;
 	uint32_t pid = getpid() & 0x3FFFFF;
 
 	nl_write_lock(&port_map_lock);
 
-	for (i = 0; i < 32; i++) {
+	if (idx_state < 0)
+		idx_state = ((unsigned) time(NULL)) % 32;
+
+	for (j = 0; j < 32; j++) {
+		i = idx_state = (idx_state + 7) % 32;
+
 		if (used_ports_map[i] == 0xFFFFFFFF)
 			continue;
 
-- 
1.9.0