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b=d8GX2OBOWzM3Xav8arbsw4w+/7MYslKkJJI5xPd1iJguX1ylyDzFSm3xnWaNyOoARwjKs3 xL/Zw/XyxbkQDvCw== Received: from imap1.dmz-prg2.suse.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by imap1.dmz-prg2.suse.org (Postfix) with ESMTPS id ABFFF3EA63; Wed, 4 Feb 2026 16:25:18 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id wlagJu5yg2mOHAAAD6G6ig (envelope-from ); Wed, 04 Feb 2026 16:25:18 +0000 Message-ID: Date: Wed, 4 Feb 2026 17:25:18 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC net-next] inet: add ip_retry_random_port sysctl to reduce sequential port retries To: netdev@vger.kernel.org Cc: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, corbet@lwn.net, ncardwell@google.com, kuniyu@google.com, dsahern@kernel.org, idosch@nvidia.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Thorsten Toepper References: <20260203175422.4620-1-fmancera@suse.de> Content-Language: en-US From: Fernando Fernandez Mancera In-Reply-To: <20260203175422.4620-1-fmancera@suse.de> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Spam-Score: -4.51 X-Spamd-Result: default: False [-4.51 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FUZZY_RATELIMITED(0.00)[rspamd.com]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; MIME_TRACE(0.00)[0:+]; ARC_NA(0.00)[]; RCPT_COUNT_TWELVE(0.00)[14]; TO_MATCH_ENVRCPT_ALL(0.00)[]; FROM_HAS_DN(0.00)[]; RCVD_TLS_ALL(0.00)[]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from]; FROM_EQ_ENVFROM(0.00)[]; TO_DN_SOME(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; RCVD_VIA_SMTP_AUTH(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; DKIM_TRACE(0.00)[suse.de:+]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo,suse.de:dkim,suse.de:mid,suse.de:email] X-Spam-Level: X-Rspamd-Action: no action X-Rspamd-Queue-Id: 8C1CD3E756 X-Rspamd-Server: rspamd1.dmz-prg2.suse.org X-Spam-Flag: NO On 2/3/26 6:54 PM, Fernando Fernandez Mancera wrote: > With the current port selection algorithm, ports after a reserved port > or long time used port are used more often than others. 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. This leads to connectivity problems between > applications on cloud environments. > > The situation is that a source tuple is usable again after being closed > for a maximum lifetime segment of two minutes while in the firewall it's > still noted as existing for 60 minutes or longer. So in case that the > port is reused for the same target tuple before the firewall cleans up, > the connection will fail due to firewall interference which itself will > reset the activity timeout in its own table. We understand the real > issue here is that these firewalls cannot cope with standards-compliant > port reuse. But this is a workaround for such situations and an > improvement on the distribution of ports selected. > > The proposed solution is instead of incrementing the port number, > performing a re-selection of a new random port within the remaining > range. This solution is configured via sysctl new option > "net.ipv4.ip_retry_random_port". > > The test run consists of two processes, a client and a server, and loops > connect to the server sending some bytes back. The results we got are > promising: > > Executed test: Current algorithm > ephemeral port range: 9000-65499 > simulated selections: 10000000 > retries during simulation: 14197718 > longest retry sequence: 5202 > > Executed test: Proposed modified algorithm > ephemeral port range: 9000-65499 > simulated selections: 10000000 > retries during simulation: 3976671 > longest retry sequence: 12 > > In addition, on graphs generated we can observe that the distribution of > source ports is more even with the proposed patch. > > Signed-off-by: Fernando Fernandez Mancera > Tested-by: Thorsten Toepper > --- > .../networking/net_cachelines/netns_ipv4_sysctl.rst | 1 + > include/net/netns/ipv4.h | 1 + > net/ipv4/inet_hashtables.c | 7 ++++++- > net/ipv4/sysctl_net_ipv4.c | 7 +++++++ > 4 files changed, 15 insertions(+), 1 deletion(-) > > diff --git a/Documentation/networking/net_cachelines/netns_ipv4_sysctl.rst b/Documentation/networking/net_cachelines/netns_ipv4_sysctl.rst > index beaf1880a19b..c4041fdca01e 100644 > --- a/Documentation/networking/net_cachelines/netns_ipv4_sysctl.rst > +++ b/Documentation/networking/net_cachelines/netns_ipv4_sysctl.rst > @@ -47,6 +47,7 @@ u8 sysctl_tcp_ecn > u8 sysctl_tcp_ecn_fallback > u8 sysctl_ip_default_ttl ip4_dst_hoplimit/ip_select_ttl > u8 sysctl_ip_no_pmtu_disc > +u8 sysctl_ip_retry_random_port > u8 sysctl_ip_fwd_use_pmtu read_mostly ip_dst_mtu_maybe_forward/ip_skb_dst_mtu > u8 sysctl_ip_fwd_update_priority ip_forward > u8 sysctl_ip_nonlocal_bind > diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h > index 2dbd46fc4734..d04b07e7c935 100644 > --- a/include/net/netns/ipv4.h > +++ b/include/net/netns/ipv4.h > @@ -156,6 +156,7 @@ struct netns_ipv4 { > > u8 sysctl_ip_default_ttl; > u8 sysctl_ip_no_pmtu_disc; > + u8 sysctl_ip_retry_random_port; > u8 sysctl_ip_fwd_update_priority; > u8 sysctl_ip_nonlocal_bind; > u8 sysctl_ip_autobind_reuse; > diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c > index f5826ec4bcaa..f1c79a7d3fd3 100644 > --- a/net/ipv4/inet_hashtables.c > +++ b/net/ipv4/inet_hashtables.c > @@ -1088,8 +1088,13 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row, > for (i = 0; i < remaining; i += step, port += step) { > if (unlikely(port >= high)) > port -= remaining; > - if (inet_is_local_reserved_port(net, port)) > + if (inet_is_local_reserved_port(net, port)) { > + if (net->ipv4.sysctl_ip_retry_random_port) { > + port = low + get_random_u32_below(remaining); > + port = ((port & 1) == step) ? port : (port - 1); The AI bot did a good observation (https://netdev-ai.bots.linux.dev/ai-review.html?id=c1544ebc-4c9d-45c5-bce9-784764102912). I think this would be better as it will keep the random scan within the same parity when needed. diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c index f1c79a7d3fd3..c9650079f9e5 100644 --- a/net/ipv4/inet_hashtables.c +++ b/net/ipv4/inet_hashtables.c @@ -1090,8 +1090,11 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row, port -= remaining; if (inet_is_local_reserved_port(net, port)) { if (net->ipv4.sysctl_ip_retry_random_port) { - port = low + get_random_u32_below(remaining); - port = ((port & 1) == step) ? port : (port - 1); + u32 candidate = low + get_random_u32_below(remaining); + + if (step == 2 && (candidate & 1) != (port & 1)) + candidate++; + port = candidate; } continue; } > + } > continue; > + } > head = &hinfo->bhash[inet_bhashfn(net, port, > hinfo->bhash_size)]; > rcu_read_lock(); > diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c > index a1a50a5c80dc..5eade7d9e4a2 100644 > --- a/net/ipv4/sysctl_net_ipv4.c > +++ b/net/ipv4/sysctl_net_ipv4.c > @@ -822,6 +822,13 @@ static struct ctl_table ipv4_net_table[] = { > .mode = 0644, > .proc_handler = ipv4_local_port_range, > }, > + { > + .procname = "ip_retry_random_port", > + .maxlen = sizeof(u8), > + .data = &init_net.ipv4.sysctl_ip_retry_random_port, > + .mode = 0644, > + .proc_handler = proc_dou8vec_minmax, > + }, > { > .procname = "ip_local_reserved_ports", > .data = &init_net.ipv4.sysctl_local_reserved_ports,