Request for more intelligent local port allocation algorithm

Paul devgs at
Wed Feb 6 15:13:20 UTC 2019

Hi dev team,

It's not a secret that when application is trying to establish new TCP connection, without
first binding a socket to specific local interface address, OS handles that automatically.
Unfortunately there is a catch, that lies in a different logic of local port allocation: 
(1) when socket is bound before connect() vs (2) when it is not. When allocating the port  
in in_pcb_lport() by checking whether different ports are free, using in_pcblookup_local(),
the behaviour is following:

(1) Bound, ie laddr is assigned with specific address: 
    Port is considered occupied only if there is a PCBs that matches both laddr and lport

(2) Not bound, ie laddr == INADDR_ANY: 
    Port is considered occupied if there is any PCBs that only matches lport. What this  
    means is that in order to allocate a port none of the all available local addresses  
    should have it allocated, even though this requirement is ridiculous, since we are 
    allocating only one PCB

Looking though the code, it seems that (2) is due to the fact that tcp_connect() first 
allocates the port, indirectly through the call to in_pcbbind() and only then allocates
the actual local address, also indirectly, though the call to in_pcbconnect_setup(), that
in turn calls in_pcbladdr(). So, probably, in order to guarantee that in_pcbconnect_setup()
will not fail we make sure that all range of local addresses are available, no matter 
which one of them is actually selected by in_pcbladdr()?

In real world, this creates serious problems for servers that have a lot of outgoing 
connections, for example nginx proxy with a lot of open HTTP2 connections. In order to 
avoid this limitation we have created workarounds within the nginx config as well as 
within our  own software, basically by having 50 local addresses and only following the 
scenario (1). Alas, all of the built-in Unix utilities as well as other software always  
follow scenario (2). As the result given large number of connections there may be points
in time, when whole range of ports is occupied by at least one local address. Even worse is  
the outcome of such condition: when in_pcb_lport() travels over the range of possible port 
numbers, making myriad of calls to in_pcblookup_local(), some  kind of important lock is 
being held withing the kernel. So important that it leads to a complete lock of the system.
Even the direct terminal access is not available: it is not responsive. The more calls to 
connect through scenario (2) there are the longer it takes the system to unfreeze. Given 
some circumstances, the only option is hard reset.

Is it possible to somehow update the code that does connect via scenario (2) to enable
more intelligent port allocation, like for example allocating local address and port simultaneously  

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