On Fri, 28 Aug 2020 22:44:47 +0200 Bart Groeneveld <avi@xxxxxxxxxx> wrote: > 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 ("tcp/dccp: better use of ephemeral ports in connect()") > > Signed-off-by: Bart Groeneveld <avi@xxxxxxxxxx> Changing the default range impacts existing users. Since Linux has been doing this for so long, I don't think just because a standards body decided to reserve some space is sufficient justification to do this.
