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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 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)[]; FUZZY_RATELIMITED(0.00)[rspamd.com]; RCVD_TLS_ALL(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; ARC_NA(0.00)[]; RCPT_COUNT_TWELVE(0.00)[14]; MIME_TRACE(0.00)[0:+]; RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:106:10:150:64:167:received]; MID_RHS_MATCH_FROM(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo]; RCVD_COUNT_TWO(0.00)[2]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DKIM_TRACE(0.00)[suse.de:+] X-Spam-Level: X-Rspamd-Action: no action X-Rspamd-Queue-Id: 3E2CB3E75F X-Rspamd-Server: rspamd1.dmz-prg2.suse.org X-Spam-Flag: NO On 2/4/26 5:49 PM, Eric Dumazet wrote: > On Tue, Feb 3, 2026 at 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); >> + } > > What happens when almost all ephemeral ports are in use, and > hundreds of ports are reserved ? > > Choosing a random value each time we meet a reserved port is going to > be quite expensive, > and we might return an error from this function even if there are many > available ports. > > Perhaps randomly select @step one time at the beginning of this > function so that @step/2 and @remaining/2 > are relatively prime numbers. > That actually makes sense. It would ensure all ports are visited before returning an error. Let me test this out. Thank you Eric, Fernando.