linux-kernel - [PATCH] net: Use standardized (IANA) local port range

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Date:   Fri, 21 Aug 2020 16:32:41 +0200
From:   Bart Groeneveld <avi@...tavi.nl>
To:     linux-kernel@...r.kernel.org
Cc:     Bart Groeneveld <avi@...tavi.nl>
Subject: [PATCH] net: Use standardized (IANA) local port range

IANA specifies User ports as 1024-49151,
and Private ports (local/ephemeral/dynamic/w/e) as 49152-65535 [1].

This means Linux uses 32768-49151 'illegally'.
This is not just a matter of following specifications:
IANA actually assigns numbers in this range [1].

I understand that Linux uses 61000-65535 for masquarading/NAT [2],
so I left the high value at 60999.
This means the high value still does not follow the specification,
but it also doesn't conflict with it.

This change will effectively halve the available ephemeral ports,
increasing the risk of port exhaustion. But:
a) I don't think that warrants ignoring standards.
	Consider for example setting up a (corporate) firewall blocking
	all unknown external services.
	It will only allow outgoing trafiic at port 80,443 and 49152-65535.
	A Linux computer behind such a firewall will not be able to connect
	to *any* external service *half of the time*.
	Of course, the firewall can be adjusted to also allow 32768-49151,
	but that allows computers to use some services against the policy.
b) It is only an issue with more than 11848 *outgoing* connections.
	I think that is a niche case (I know, citation needed, but still).
	If someone finds themselves in such a niche case,
	they can still modify ip_local_port_range.

This patch keeps the low and high value at different parity,
as to optimize port assignment [3].

[1]: https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.txt
[2]: https://marc.info/?l=linux-kernel&m=117900026927289
[3]: See for example commit 1580ab63fc9a03593072cc5656167a75c4f1d173

Signed-off-by: Bart Groeneveld <avi@...tavi.nl>
---
 Documentation/networking/ip-sysctl.rst | 4 ++--
 net/ipv4/af_inet.c                     | 2 +-
 net/ipv4/inet_connection_sock.c        | 2 +-
 net/ipv4/inet_hashtables.c             | 2 +-
 4 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index 837d51f9e1fa..5048b326f773 100644
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst
@@ -1024,7 +1024,7 @@ ip_local_port_range - 2 INTEGERS
 	If possible, it is better these numbers have different parity
 	(one even and one odd value).
 	Must be greater than or equal to ip_unprivileged_port_start.
-	The default values are 32768 and 60999 respectively.
+	The default values are 49152 and 60999 respectively.
 
 ip_local_reserved_ports - list of comma separated ranges
 	Specify the ports which are reserved for known third-party
@@ -1047,7 +1047,7 @@ ip_local_reserved_ports - list of comma separated ranges
 	ip_local_port_range, e.g.::
 
 	    $ cat /proc/sys/net/ipv4/ip_local_port_range
-	    32000	60999
+	    49152	60999
 	    $ cat /proc/sys/net/ipv4/ip_local_reserved_ports
 	    8080,9148
 
diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index 4307503a6f0b..f95a9ffffdc9 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -1838,7 +1838,7 @@ static __net_init int inet_init_net(struct net *net)
 	 * Set defaults for local port range
 	 */
 	seqlock_init(&net->ipv4.ip_local_ports.lock);
-	net->ipv4.ip_local_ports.range[0] =  32768;
+	net->ipv4.ip_local_ports.range[0] =  49152;
 	net->ipv4.ip_local_ports.range[1] =  60999;
 
 	seqlock_init(&net->ipv4.ping_group_range.lock);
diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
index b457dd2d6c75..322bcfce0737 100644
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -196,7 +196,7 @@ inet_csk_find_open_port(struct sock *sk, struct inet_bind_bucket **tb_ret, int *
 	attempt_half = (sk->sk_reuse == SK_CAN_REUSE) ? 1 : 0;
 other_half_scan:
 	inet_get_local_port_range(net, &low, &high);
-	high++; /* [32768, 60999] -> [32768, 61000[ */
+	high++; /* [49152, 60999] -> [49152, 61000[ */
 	if (high - low < 4)
 		attempt_half = 0;
 	if (attempt_half) {
diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
index 239e54474b65..547b95a4891a 100644
--- a/net/ipv4/inet_hashtables.c
+++ b/net/ipv4/inet_hashtables.c
@@ -695,7 +695,7 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
 	l3mdev = inet_sk_bound_l3mdev(sk);
 
 	inet_get_local_port_range(net, &low, &high);
-	high++; /* [32768, 60999] -> [32768, 61000[ */
+	high++; /* [49152, 60999] -> [49152, 61000[ */
 	remaining = high - low;
 	if (likely(remaining > 1))
 		remaining &= ~1U;
-- 
2.28.0