strace shows that dhcpd is binding to explicit ports which it has chosen
itself:
setsockopt(20, SOL_SOCKET, SO_TIMESTAMP, [1], 4) = 0
setsockopt(20, SOL_IP, IP_MTU_DISCOVER, [0], 4) = 0
getsockopt(20, SOL_SOCKET, SO_RCVBUF, [212992], [4]) = 0
setsockopt(20, SOL_IP, IP_RECVTOS, [1], 4) = 0
bind(20, {sa_family=AF_INET, sin_port=htons(15101),
sin_addr=inet_addr("0.0.0.0")}, 16) = 0
socket(PF_INET6, SOCK_DGRAM, IPPROTO_UDP) = 5
fcntl(5, F_DUPFD, 20) = 21
close(5) = 0
fcntl(21, F_GETFL) = 0x2 (flags O_RDWR)
fcntl(21, F_SETFL, O_RDWR|O_NONBLOCK) = 0
setsockopt(21, SOL_IPV6, IPV6_MTU, [1280], 4) = 0
setsockopt(21, SOL_SOCKET, SO_TIMESTAMP, [1], 4) = 0
setsockopt(21, SOL_IPV6, IPV6_RECVPKTINFO, [1], 4) = 0
setsockopt(21, SOL_IPV6, IPV6_MTU_DISCOVER, [0], 4) = 0
getsockopt(21, SOL_SOCKET, SO_RCVBUF, [212992], [4]) = 0
setsockopt(21, SOL_IPV6, IPV6_RECVTCLASS, [1], 4) = 0
setsockopt(21, SOL_IPV6, IPV6_V6ONLY, [1], 4) = 0
bind(21, {sa_family=AF_INET6, sin6_port=htons(11709), inet_pton(AF_INET6, "::",
&sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, 28) = 0
Aha - from man dhcpd:
When DDNS is enabled at compile time (see includes/site.h)
the server will open both a v4 and a v6 UDP socket on
random ports. These ports are opened even if DDNS is disabled
in the configuration file.
In the source code, the README says:
A fully-featured implementation of dynamic DNS updates is included in
this release. It uses libraries from BIND and, to avoid issues with
different versions, includes the necessary BIND version. The appropriate
BIND libraries will be compiled and installed in the bind subdirectory
as part of the make step.
However, debian/patches/system-bind.patch causes it to link against the
system bind library; this seems to be -ldns-export from package libdns-
export162
apt-get source libdns-export162 pulls in bind9. The CHANGES file says:
4109. [port] linux: support reading the local port range from
net.ipv4.ip_local_port_range. [RT # 39379]
However, the strace output from dhcpd shows no attempt to read
ip_local_port_range.
I see the bind code includes these fallback values:
#define ISC_NET_PORTRANGELOW 1024
#define ISC_NET_PORTRANGEHIGH 65535
ISTM that a safer bound would be 49152-65535 (IANA dynamic port range),
or at least the linux default of "32768 60999"
But the fundamental problem here is that for some reason, when dhcpd
links into this code, it isn't making use of the detection of available
Linux port range.
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You received this bug notification because you are a member of Ubuntu
Touch seeded packages, which is subscribed to isc-dhcp in Ubuntu.
https://bugs.launchpad.net/bugs/1670303
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
To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/isc-dhcp/+bug/1670303/+subscriptions
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