If there is overlapp between ip_local_port_range and
ip_local_reserved_ports with a huge reserved block, it will affect probability
of selecting ephemeral ports, see file net/ipv4/inet_hashtables.c:723
int __inet_hash_connect(
...
for (i = 0; i < remaining; i += 2, port += 2) {
if (unlikely(port >= high))
port -= remaining;
if (inet_is_local_reserved_port(net, port))
continue;
E.g. if there is reserved block of 10000 ports, two ports right after this
block will be 5000 more likely selected than others.
If this was intended, we can/should add note into documentation as proposed
in this commit, otherwise we should think about different solution. One option
could be mapping table of continuous port ranges. Second option could be
letting user to modify step (port+=2) in above loop, e.g. using new sysctl
parameter.
Signed-off-by: Otto Hollmann <[email protected]>
---
Documentation/networking/ip-sysctl.rst | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/Documentation/networking/ip-sysctl.rst
b/Documentation/networking/ip-sysctl.rst
index c7952ac5bd2f..74c458d2686a 100644
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst
@@ -1073,7 +1073,9 @@ ip_local_reserved_ports - list of comma separated ranges
although this is redundant. However such a setting is useful
if later the port range is changed to a value that will
- include the reserved ports.
+ include the reserved ports. Also keep in mind, that overlapping
+ of these ranges may affect probability of selecting ephemeral
+ ports which are right after block of reserved ports.
Default: Empty
--
2.26.2
