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Message #13508
[Bug 1176046] Re: isc-dhcp dhclient listens on extra random ports
** Changed in: isc-dhcp (Ubuntu Xenial)
Status: Fix Released => In Progress
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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:
In Progress
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]
I did the split such that Trusty users will automatically get "isc-
dhcp-client-ddns" installed but users bothered by this bug will have
the option to switch to "isc-dhcp-client-noddns".
Existing Trusty users can continue to use this DDNS functionality
after the SRU without any necessary intervention.
With isc-dhcp-client:
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
With isc-dhcp-client-noddns :
dhclient 1110 root 6u IPv4 11535 0t0 UDP *:bootpc
Xenial also has both distinct dhclient binary package but in the
opposite way. We have decided to use the opposite way approach for not
impacting actual Trusty users by changing the nature of isc-dhcp-
client itself.
Caribou and I, slashd, have also tested a couple of release upgrades from Trusty to Xenial with both scenarios :
1 - Trusty upgrade to Xenial with "isc-dhcp-client-ddns"
2 - Trusty upgrade to Xenial with "isc-dhcp-client-noddns"
and both scenarios worked as expected for caribou and I. (See comment
#42)
Results :
===
** Upgrade tested with isc-dhcp-client **
# dpkg -l
ii isc-dhcp-client 4.2.4-7ubuntu12.8 amd64 ISC DHCP client
ii isc-dhcp-common 4.2.4-7ubuntu12.8 amd64 common files used by all the isc-dhcp* packages
# netstat -anputa | grep -i dhclient
udp 0 0 0.0.0.0:20114 0.0.0.0:* 632/dhclient
udp 0 0 0.0.0.0:68 0.0.0.0:* 632/dhclient
udp6 0 0 :::52249 :::* 632/dhclient
After successful upgrade Trusty (14.04.5) -> Xenial (16.04.2)
ii isc-dhcp-client 4.3.3-5ubuntu12.7 amd64 DHCP client for automatically obtaining an IP address
ii isc-dhcp-common 4.3.3-5ubuntu12.7 amd64 common files used by all of the isc-dhcp packages
# netstat -anputa | grep -i dhclient
udp 0 0 0.0.0.0:68 0.0.0.0:* 633/dhclient
** Upgrade tested with isc-dhcp-noddns (4.2.4-7ubuntu12.9) **
# dpkg -l
ii isc-dhcp-client-noddns 4.2.4-7ubuntu12.9 amd64 Dynamic DNS (DDNS) disabled DHCP client
# netstat -anputa | grep dhclient
udp 0 0 0.0.0.0:68 0.0.0.0:* 682/dhclient
After successful upgrade Trusty (14.04.5) -> Xenial (16.04.2)
# dpkg -l | grep -i dhcp
ii isc-dhcp-client 4.3.3-5ubuntu12.7 amd64 DHCP client for automatically obtaining an IP address
rc isc-dhcp-client-noddns 4.2.4-7ubuntu12.9 amd64 Dynamic DNS (DDNS) disabled DHCP client
ii isc-dhcp-common 4.3.3-5ubuntu12.6 amd64 common files used by all of the isc-dhcp packages
# netstat -anputa | grep dhclient
udp 0 0 0.0.0.0:68 0.0.0.0:* 639/dhclient
===
For ~ubuntu-sru, the above tests/scenarios will also be part of the
testing phase once the package will be found in -proposed and I'll
make sure they all succeed before tagging the bug to "verfication-
done".
[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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