Public bug reported:
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
** Affects: isc-dhcp (Ubuntu)
Importance: Undecided
Status: New
** Tags: amd64 apport-bug xenial
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https://bugs.launchpad.net/bugs/1670303
Title:
dhcpd does not respect ip_local_port _range or ip_local_reserved_ports
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