[Bug 2024929] [NEW] dhclient does not reup valid_lft on service restart, kernel reaps IP

[Steven Haber]

Public bug reported:

Between Bionic and Focal, dhclient was patched to set the valid_lft on
ipv4 addresses, which is a timer in the kernel that tells the kernel
when to reap the IP. dhclient then is supposed to issue repeated `ip
addr add` commands to reset this lft and prevent the kernel from
releasing the IP. However, if you restart the dhclient service, it
acquires a lease and then does *not* reup the lft. It only reups after a
lease the currently running service knows about expires. So if you
restart the dhclient service on a cadence that is faster than the DHCP
leases in your network environment, you never see a lease expire during
the lifetime of the service, which means the kernel will eventually rip
the address out from under the server, causing a network outage. In some
environments, the DCHP lease can be longer than a service restart
cadence, and this bug can be very severe.

I'm using the most up-to-date version of Focal's dhclient package.

I'm not familiar with how the patch management for Ubuntu works, but the
bug was introduced here:

commit 41013cf19647ec323a7100fb61d50779d8c8a205 (tag: import/4.4.1-2)
Author: Michael Gilbert <mgilbert at debian.org>
Date:   Tue Dec 11 03:55:12 2018 +0000

    4.4.1-2 (patches unapplied)

    Imported using git-ubuntu import.

These changes specifically:

diff --git a/debian/dhclient-script.linux b/debian/dhclient-script.linux
index 9b0d3f89..f9b734ab 100644
--- a/debian/dhclient-script.linux
+++ b/debian/dhclient-script.linux
@@ -246,6 +246,8 @@ case "$reason" in
             # new IP has been leased or leased IP changed => set it
             ip -4 addr add ${new_ip_address}${new_subnet_mask:+/$new_subnet_mask} \
                 ${new_broadcast_address:+broadcast $new_broadcast_address} \
+                ${new_dhcp_lease_time:+valid_lft $new_dhcp_lease_time} \
+                ${new_dhcp_lease_time:+preferred_lft $new_dhcp_lease_time} \
                 dev ${interface} label ${interface}

             if [ -n "$new_interface_mtu" ]; then
@@ -277,6 +279,12 @@ case "$reason" in
                        fi
                    done
            fi
+        else # RENEW||REBIND
+            ip -4 addr change ${new_ip_address}${new_subnet_mask:+/$new_subnet_mask} \
+                ${new_broadcast_address:+broadcast $new_broadcast_address} \
+                ${new_dhcp_lease_time:+valid_lft $new_dhcp_lease_time} \
+                ${new_dhcp_lease_time:+preferred_lft $new_dhcp_lease_time} \
+                dev ${interface} label ${interface}
         fi

         if [ -n "$alias_ip_address" ] &&
@@ -323,6 +331,8 @@ case "$reason" in
         # set IP from recorded lease
         ip -4 addr add ${new_ip_address}${new_subnet_mask:+/$new_subnet_mask} \
             ${new_broadcast_address:+broadcast $new_broadcast_address} \
+            ${new_dhcp_lease_time:+valid_lft $new_dhcp_lease_time} \
+            ${new_dhcp_lease_time:+preferred_lft $new_dhcp_lease_time} \
             dev ${interface} label ${interface}

         if [ -n "$new_interface_mtu" ]; then

** Affects: isc-dhcp (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/2024929

Title:
  dhclient does not reup valid_lft on service restart, kernel reaps IP

Status in isc-dhcp package in Ubuntu:
  New

Bug description:
  Between Bionic and Focal, dhclient was patched to set the valid_lft on
  ipv4 addresses, which is a timer in the kernel that tells the kernel
  when to reap the IP. dhclient then is supposed to issue repeated `ip
  addr add` commands to reset this lft and prevent the kernel from
  releasing the IP. However, if you restart the dhclient service, it
  acquires a lease and then does *not* reup the lft. It only reups after
  a lease the currently running service knows about expires. So if you
  restart the dhclient service on a cadence that is faster than the DHCP
  leases in your network environment, you never see a lease expire
  during the lifetime of the service, which means the kernel will
  eventually rip the address out from under the server, causing a
  network outage. In some environments, the DCHP lease can be longer
  than a service restart cadence, and this bug can be very severe.

  I'm using the most up-to-date version of Focal's dhclient package.

  I'm not familiar with how the patch management for Ubuntu works, but
  the bug was introduced here:

  commit 41013cf19647ec323a7100fb61d50779d8c8a205 (tag: import/4.4.1-2)
  Author: Michael Gilbert <mgilbert at debian.org>
  Date:   Tue Dec 11 03:55:12 2018 +0000

      4.4.1-2 (patches unapplied)

      Imported using git-ubuntu import.

  These changes specifically:

  diff --git a/debian/dhclient-script.linux b/debian/dhclient-script.linux
  index 9b0d3f89..f9b734ab 100644
  --- a/debian/dhclient-script.linux
  +++ b/debian/dhclient-script.linux
  @@ -246,6 +246,8 @@ case "$reason" in
               # new IP has been leased or leased IP changed => set it
               ip -4 addr add ${new_ip_address}${new_subnet_mask:+/$new_subnet_mask} \
                   ${new_broadcast_address:+broadcast $new_broadcast_address} \
  +                ${new_dhcp_lease_time:+valid_lft $new_dhcp_lease_time} \
  +                ${new_dhcp_lease_time:+preferred_lft $new_dhcp_lease_time} \
                   dev ${interface} label ${interface}

               if [ -n "$new_interface_mtu" ]; then
  @@ -277,6 +279,12 @@ case "$reason" in
                          fi
                      done
              fi
  +        else # RENEW||REBIND
  +            ip -4 addr change ${new_ip_address}${new_subnet_mask:+/$new_subnet_mask} \
  +                ${new_broadcast_address:+broadcast $new_broadcast_address} \
  +                ${new_dhcp_lease_time:+valid_lft $new_dhcp_lease_time} \
  +                ${new_dhcp_lease_time:+preferred_lft $new_dhcp_lease_time} \
  +                dev ${interface} label ${interface}
           fi

           if [ -n "$alias_ip_address" ] &&
  @@ -323,6 +331,8 @@ case "$reason" in
           # set IP from recorded lease
           ip -4 addr add ${new_ip_address}${new_subnet_mask:+/$new_subnet_mask} \
               ${new_broadcast_address:+broadcast $new_broadcast_address} \
  +            ${new_dhcp_lease_time:+valid_lft $new_dhcp_lease_time} \
  +            ${new_dhcp_lease_time:+preferred_lft $new_dhcp_lease_time} \
               dev ${interface} label ${interface}

           if [ -n "$new_interface_mtu" ]; then

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