[Bug 1176046] Re: isc-dhcp dhclient listens on extra random ports

[Eric Desrochers]

I was able to identify from where the port range list is initialize, and the decision is taken by binary package "libdns162" (source package "bind9")[1] which dhclient relies on.
Basically, it randomly takes any port available between 1024 and 65535[2]

So there is a port randomization mechanism made by the library that take
the decision instead of, for instance in some other cases, where the
decision is leave it to the kernel[3].

In this particular case both sysctl options :
net.ipv4.ip_local_port_range
net.ipv4.ip_local_reserved_ports

have no effect on port assignation decision.

[1] - https://github.com/wklaebe/bind9/blob/ac1dcdd124a5abdec4969e2c33836d863bf73aa7/lib/dns/dispatch.c#L1921
[2] - https://tools.ietf.org/html/rfc6335
[3] - Example : net.ipv4.ip_local_port_range = 32768 60999

Regards,

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https://bugs.launchpad.net/bugs/1176046

Title:
  isc-dhcp dhclient listens on extra random ports

Status in isc-dhcp package in Ubuntu:
  In Progress

Bug description:
  Ubuntu 13.04 Server 64-bit.  Fresh install.  Only one network adapter.

  dhclient process is listening on two randomly chosen udp ports in
  addition to the usual port 68.  This appears to be a bug in the
  discovery code for probing information on interfaces in the system.

  Initial research of the code also suggested omapi, but adding omapi
  port 9999 to /etc/dhcp/dhclient.conf only opened a forth port with the
  two random udp ports still enabled.

  Version of included distro dhclient was 4.2.4.  I also tested with the
  latest isc-dhclient-4.2.5-P1 and got the same results.

  Debian has the same bug:
  http://forums.debian.net/viewtopic.php?f=10&t=95273&p=495605#p495605

  One impact of these random ports is that security hardening becomes
  more difficult.  The purpose of these random ports and security
  implications are unknown.

  
  Example netstat -lnp  output:

  udp        0      0 0.0.0.0:21117           0.0.0.0:*                           2659/dhclient   
  udp        0      0 0.0.0.0:68              0.0.0.0:*                           2659/dhclient   
  udp6       0      0 :::45664                :::*                                2659/dhclient

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