Debian Stretch source tcp port is *always* even

Question

While debugging some strange behaviour with an Azure load-balancer, I noticed that my local Debian Stretch TCP stack was only establishing TCP connections with an even-numbered port. I do not start a single TCP handshake with an odd source port. Is that intended?

Answer 1

It's to reduce contention between connect() and bind() (appeared in Linux 4.2; Jessie has 3.16 and Stretch has 4.9):

commit 07f4c90062f8fc7c8c26f8f95324cbe8fa3145a5
Author: Eric Dumazet 
Date:   Sun May 24 14:49:35 2015 -0700

    tcp/dccp: try to not exhaust ip_local_port_range in connect()

    A long standing problem on busy servers is the tiny available TCP port
    range (/proc/sys/net/ipv4/ip_local_port_range) and the default
    sequential allocation of source ports in connect() system call.

    If a host is having a lot of active TCP sessions, chances are
    very high that all ports are in use by at least one flow,
    and subsequent bind(0) attempts fail, or have to scan a big portion of
    space to find a slot.

    In this patch, I changed the starting point in __inet_hash_connect()
    so that we try to favor even [1] ports, leaving odd ports for bind()
    users.

    We still perform a sequential search, so there is no guarantee, but
    if connect() targets are very different, end result is we leave
    more ports available to bind(), and we spread them all over the range,
    lowering time for both connect() and bind() to find a slot.

    This strategy only works well if /proc/sys/net/ipv4/ip_local_port_range
    is even, ie if start/end values have different parity.

    Therefore, default /proc/sys/net/ipv4/ip_local_port_range was changed to
    32768 - 60999 (instead of 32768 - 61000)

    There is no change on security aspects here, only some poor hashing
    schemes could be eventually impacted by this change.

    [1] : The odd/even property depends on ip_local_port_range values parity

You may also want to see the followup commit 1580ab63fc9a03593072cc5656167a75c4f1d173.