[Bug 1670303] Re: dhcpd does not respect ip_local_port _range or ip_local_reserved_ports

[Brian Candler]

WORKAROUND: if the other application which needs to bind to a UDP port
can be configured to bind to a specific interface, then you can make
isc-dhcp-server bind to a different interface in dhcpd.conf. e.g.

ddns-local-address4 127.0.0.1;
ddns-local-address6 ::1;

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Title:
  dhcpd does not respect ip_local_port _range or ip_local_reserved_ports

Status in isc-dhcp package in Ubuntu:
  New

Bug description:
  When isc-dhcp-server starts up, in addition to listening on port 67,
  it binds to a random UDP port on an IPv4 socket and another on an IPv6
  socket:

  # netstat -naup | grep dhcp
  udp        0      0 0.0.0.0:11075           0.0.0.0:*                           8188/dhcpd
  udp        0      0 0.0.0.0:67              0.0.0.0:*                           8188/dhcpd
  udp6       0      0 :::10800                :::*                                8188/dhcpd
  #

  (I am guessing this is for making outbound DNS queries?)  However,
  this prevented a later application of mine from working, as it wanted
  to bind to port 11075 for accepting incoming data.

  Simply doing "service isc-dhcp-server restart" makes it choose new
  ports, but this problem may occur again in the future.

  In the default configuration, I believe ephemeral ports should only
  use 32768 and above:

  # cat /proc/sys/net/ipv4/ip_local_port_range
  32768    60999
  # cat /proc/sys/net/ipv4/ip_local_reserved_ports

  #

  I also tried setting a reservation, and this was not respected either.

  # sysctl net.ipv4.ip_local_reserved_ports="10000-59999"
  net.ipv4.ip_local_reserved_ports = 10000-59999

  After restarting dhcpd:

  # netstat -naup | grep dhcp
  udp        0      0 0.0.0.0:50610           0.0.0.0:*                           4592/dhcpd
  udp        0      0 0.0.0.0:67              0.0.0.0:*                           4592/dhcpd
  udp6       0      0 :::28891                :::*                                4592/dhcpd

  
  I can find no way to tell isc-dhcp-server which port range to use. Setting "omapi-port" in dhcpd.conf makes it listen for *TCP* connections on the given port, and does not affect the UDP behaviour.

  I don't know if this is a problem with the application (explicitly
  picking a local port), the resolver library (ditto), or the kernel
  (ignoring its own ip_local_port_range)

  ProblemType: Bug
  DistroRelease: Ubuntu 16.04
  Package: isc-dhcp-server 4.3.3-5ubuntu12.6
  ProcVersionSignature: Ubuntu 4.4.0-64.85-generic 4.4.44
  Uname: Linux 4.4.0-64-generic x86_64
  ApportVersion: 2.20.1-0ubuntu2.5
  Architecture: amd64
  Date: Mon Mar  6 09:30:29 2017
  DhServerLeases:
   
  InstallationDate: Installed on 2017-03-04 (2 days ago)
  InstallationMedia: Ubuntu-Server 16.04.2 LTS "Xenial Xerus" - Release amd64 (20170215.8)
  ProcEnviron:
   SHELL=/bin/bash
   TERM=xterm-256color
   PATH=(custom, no user)
   LANG=en_US
   LANGUAGE=en_US:
  SourcePackage: isc-dhcp
  UpgradeStatus: No upgrade log present (probably fresh install)
  mtime.conffile..etc.dhcp.dhcpd.conf: 2017-03-04T09:46:07.987046

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