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

Mon Mar 6 10:24:50 UTC 2017

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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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

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