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

[Eric Desrochers]

Rectification, after more investigation here what is happening :

Trusty got the random port right away because NSUPDATE is enable at
compile time in package "isc-dhcp-client"

In Xenial and late (including debian) with commit
[b1d5bab630a29f2ca94c458f9402aa43cc5975c4], a new pkg binary package is
introduced "isc-dhcp-client-ddns" set as optional.

#debian/control:
+Package: isc-dhcp-client-ddns
+Priority: optional
+Architecture: any
+Depends:
+ ${misc:Depends},
+ ${shlibs:Depends},
+ isc-dhcp-client (= ${binary:Version}),
+Enhances: isc-dhcp-client
+Description: Dynamic DNS (DDNS) enabled DHCP client
+ This a Dynamic DNS enabled version of the DHCP client.


So basically the random port still exist in Xenial and late, but it requires, the "isc-dhcp-client-ddns" package (optional) to be installed. So basically, it is split probably to accommodate the folks having this problem, and in the same time it allows peoples that really needs this feature to use it by installing this additionnal new package.

On Xenial :
** Without "isc-dhcp-client-ddns" **
# lsof -i udp
COMMAND  PID USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
dhclient 140 root    6u  IPv4 344477      0t0  UDP *:bootpc 


** With "isc-dhcp-client-ddns" **
# lsof -i udp
COMMAND  PID USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
dhclient 140 root    6u  IPv4 336839      0t0  UDP *:bootpc 
dhclient 140 root   20u  IPv4 336827      0t0  UDP *:65239 
dhclient 140 root   21u  IPv6 336828      0t0  UDP *:12817 


Eric

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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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