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

Thu Mar 2 19:10:48 UTC 2017

Feedback from the field about the test package of "isc-dhcp-client-
noddns" I made for someone interested to give it a try before the SRU:

"... we've tried it out and didn’t notice anything unusual, but I
suspect we would only see problems if there are any if we ran it 1000s
of times in ci.

I also don’t see any additional random ports consumed which is good:

# lsof -p 485
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
dhclient 485 root cwd DIR 202,1 4096 2 /
dhclient 485 root rtd DIR 202,1 4096 2 /
dhclient 485 root txt REG 202,1 7543253 9727 /sbin/dhclient
dhclient 485 root mem REG 202,1 43616 8991 /lib/x86_64-linux-gnu/libnss_files-2.19.so
dhclient 485 root mem REG 202,1 1840928 8950 /lib/x86_64-linux-gnu/libc-2.19.so
dhclient 485 root mem REG 202,1 149120 8938 /lib/x86_64-linux-gnu/ld-2.19.so
dhclient 485 root 0u CHR 1,3 0t0 6 /dev/null
dhclient 485 root 1u CHR 1,3 0t0 6 /dev/null
dhclient 485 root 2u CHR 1,3 0t0 6 /dev/null
dhclient 485 root 3w REG 202,1 1461 135467 /var/lib/dhcp/dhclient.eth0.leases
dhclient 485 root 4u pack 9509 0t0 ALL type=SOCK_RAW
dhclient 485 root 5u IPv4 9512 0t0 UDP *:bootpc
dhclient 485 root 6u unix 0xffff8800e7069000 0t0 15863 socket
"

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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:
  Fix Released
Status in isc-dhcp source package in Trusty:
  In Progress
Status in isc-dhcp source package in Xenial:
  Fix Released
Status in isc-dhcp source package in Yakkety:
  Fix Released

Bug description:
  [Impact]

  In trusty, there is only 1 version of dhclient, including #define NSUPDATE, which introduce DDNS functionnality.
  The DDNS functionnality, generate 2 random extra ports between 1024-65535.

  Impact reported by users :

  "One impact of these random ports is that security hardening becomes more difficult. The purpose of these random ports and security implications are unknown."
  "We have software that was using one of the lower udp ports but it happened to collide with dhclient which seems to allocate 2 random ports."

  There is a randomization mechanism in libdns that prevent dhclient to take the sysctl values into account (net.ipv4.ip_local_port_range & net.ipv4.ip_local_reserved_ports) to workaround this, and after discussion isc-dhcp upstream doesn't want to rely on kernel for randomization.

  There is no realtime configuration to disable the feature or workaround this. The only possible way is at compile time.

  I also talk with upstream maintainers, and there is no way they will
  accept to reduce the range (1024-65535) for security reason. Reducing
  the port range may facilitate the spoofing.

  Xenial has separated dhclient in two packages :

  isc-dhcp-client pkg : dhclient with DDNS functionality disabled (no random extra ports)
  isc-dhcp-client-ddns : dhclient with DDNS functionality enabled (with random extra ports)

  The goal here is to reproduce the same situation in Trusty, for this
  bug to be less painful for at least users that doesn't require DDNS
  functionnality.

  [Test Case]

  Run a Trusty image with following package :
  isc-dhcp-client
  isc-dhcp-common

  ```
  dhclient 1110 root 6u IPv4 11535 0t0 UDP *:bootpc
  dhclient 1110 root 20u IPv4 11516 0t0 UDP *:64589 # <----------- extra random port
  dhclient 1110 root 21u IPv6 11517 0t0 UDP *:7749  # <----------- extra random port
  ```

  [Regression Potential] 

  * none expected

  I did the split such that users will automatically get isc-dhcp-client-ddns installed but users bothered by this bug then will have the option to switch to the one without it by uninstalling (isc-dhcp-client-ddns), 
  so existing Trusty users can continue to use this DDNS functionality after the SRU without any necessary intervention.

  With  isc-dhcp-client-ddns :
  dhclient 1110 root 6u IPv4 11535 0t0 UDP *:bootpc
  dhclient 1110 root 20u IPv4 11516 0t0 UDP *:64589 # <----------- extra random port
  dhclient 1110 root 21u IPv6 11517 0t0 UDP *:7749  # <----------- extra random port

  Without isc-dhcp-client-ddns :
  dhclient 1110 root 6u IPv4 11535 0t0 UDP *:bootpc

  Note that this is how Xenial does it.

  [Other Info]

   * See : https://bugs.launchpad.net/ubuntu/+source/isc-dhcp/+bug/1176046/comments/19 to look at my discussion with rbasak on if that approach would be acceptable for SRU.

  [Original 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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