bybrooklyn/linux
1
0
Fork 0
mirror of https://github.com/torvalds/linux.git synced 2026-04-18 06:44:00 -04:00
Code Issues Packages Projects Releases Wiki Activity

inet: add ip_local_port_step_width sysctl to improve port usage distribution

Browse Source 
With the current port selection algorithm, ports after a reserved port
range or long time used port are used more often than others [1]. This
causes an uneven port usage distribution. This combines with cloud
environments blocking connections between the application server and the
database server if there was a previous connection with the same source
port, leading to connectivity problems between applications on cloud
environments.

The real issue here is that these firewalls cannot cope with
standards-compliant port reuse. This is a workaround for such situations
and an improvement on the distribution of ports selected.

The proposed solution is to implement a variant of RFC 6056 Algorithm 5.
The step size is selected randomly on every connect() call ensuring it
is a coprime with respect to the size of the range of ports we want to
scan. This way, we can ensure that all ports within the range are
scanned before returning an error. To enable this algorithm, the user
must configure the new sysctl option "net.ipv4.ip_local_port_step_width".

In addition, on graphs generated we can observe that the distribution of
source ports is more even with the proposed approach. [2]

[1] https://0xffsoftware.com/port_graph_current_alg.html

[2] https://0xffsoftware.com/port_graph_random_step_alg.html

Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Link: https://patch.msgid.link/20260309023946.5473-2-fmancera@suse.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Fernando Fernandez Mancera
2026-03-09 03:39:45 +01:00
committed by Jakub Kicinski
parent ae95cbaedb
commit 7da62262ec
5 changed files with 50 additions and 3 deletions
Download Patch File Download Diff File Expand all files Collapse all files

16
Documentation/networking/ip-sysctl.rst
View File

@@ -1630,6 +1630,22 @@ ip_local_reserved_ports - list of comma separated ranges
Default: Empty
ip_local_port_step_width - INTEGER
Defines the numerical maximum increment between successive port
allocations within the ephemeral port range when an unavailable port is
reached. This can be used to mitigate accumulated nodes in port
distribution when reserved ports have been configured. Please note that
port collisions may be more frequent in a system with a very high load.
It is recommended to set this value strictly larger than the largest
contiguous block of ports configure in ip_local_reserved_ports. For
large reserved port ranges, setting this to 3x or 4x the size of the
largest block is advised. Using a value equal or greater than the local
port range size completely solves the uneven port distribution problem,
but it can degrade performance under port exhaustion situations.
Default: 0 (disabled)
ip_unprivileged_port_start - INTEGER
This is a per-namespace sysctl. It defines the first
unprivileged port in the network namespace. Privileged ports

1
Documentation/networking/net_cachelines/netns_ipv4_sysctl.rst
View File

@@ -52,6 +52,7 @@ u8 sysctl_ip_fwd_update_priority
u8 sysctl_ip_nonlocal_bind
u8 sysctl_ip_autobind_reuse
u8 sysctl_ip_dynaddr
u32 sysctl_ip_local_port_step_width
u8 sysctl_ip_early_demux read_mostly ip(6)_rcv_finish_core
u8 sysctl_raw_l3mdev_accept
u8 sysctl_tcp_early_demux read_mostly ip(6)_rcv_finish_core

1
include/net/netns/ipv4.h
View File

@@ -166,6 +166,7 @@ struct netns_ipv4 {
u8 sysctl_ip_autobind_reuse;
/* Shall we try to damage output packets if routing dev changes? */
u8 sysctl_ip_dynaddr;
u32 sysctl_ip_local_port_step_width;
#ifdef CONFIG_NET_L3_MASTER_DEV
u8 sysctl_raw_l3mdev_accept;
#endif

28
net/ipv4/inet_hashtables.c
View File

@@ -16,6 +16,7 @@
#include <linux/wait.h>
#include <linux/vmalloc.h>
#include <linux/memblock.h>
#include <linux/gcd.h>
#include <net/addrconf.h>
#include <net/inet_connection_sock.h>
@@ -1057,12 +1058,12 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
struct net *net = sock_net(sk);
struct inet_bind2_bucket *tb2;
struct inet_bind_bucket *tb;
int step, scan_step, l3mdev;
u32 index, max_rand_step;
bool tb_created = false;
u32 remaining, offset;
int ret, i, low, high;
bool local_ports;
int step, l3mdev;
u32 index;
if (port) {
local_bh_disable();
@@ -1076,6 +1077,8 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
local_ports = inet_sk_get_local_port_range(sk, &low, &high);
step = local_ports ? 1 : 2;
scan_step = step;
max_rand_step = READ_ONCE(net->ipv4.sysctl_ip_local_port_step_width);
high++; /* [32768, 60999] -> [32768, 61000[ */
remaining = high - low;
@@ -1094,9 +1097,28 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
*/
if (!local_ports)
offset &= ~1U;
if (max_rand_step && remaining > 1) {
u32 range = remaining / step;
u32 upper_bound;
upper_bound = min(range, max_rand_step);
scan_step = get_random_u32_inclusive(1, upper_bound);
while (gcd(scan_step, range) != 1) {
scan_step++;
/* if both scan_step and range are even gcd won't be 1 */
if (!(scan_step & 1) && !(range & 1))
scan_step++;
if (unlikely(scan_step > upper_bound)) {
scan_step = 1;
break;
}
}
scan_step *= step;
}
other_parity_scan:
port = low + offset;
for (i = 0; i < remaining; i += step, port += step) {
for (i = 0; i < remaining; i += step, port += scan_step) {
if (unlikely(port >= high))
port -= remaining;
if (inet_is_local_reserved_port(net, port))

7
net/ipv4/sysctl_net_ipv4.c
View File

@@ -823,6 +823,13 @@ static struct ctl_table ipv4_net_table[] = {
.mode = 0644,
.proc_handler = ipv4_local_port_range,
},
{
.procname = "ip_local_port_step_width",
.maxlen = sizeof(u32),
.data = &init_net.ipv4.sysctl_ip_local_port_step_width,
.mode = 0644,
.proc_handler = proc_douintvec,
},
{
.procname = "ip_local_reserved_ports",
.data = &init_net.ipv4.sysctl_local_reserved_ports,