install of "openvpn-dco-dkms" fails on Debian 11 with kernel "5.10.0-36"

[keros]

I did a upgrade of my Debian 11 machine (apt update and apt dist-upgrade).
This upgraded my Kernel from "5.10.0-35" to "5.10.0-36" (linux-image-5.10.0-36-amd64).

Openvpn and the dco package are installed via the build.openvpn.net/debian/openvpn/stable repository:

# dpkg -l | grep openvpn
ii  openvpn                        2.6.15-bullseye0                   amd64        virtual private network daemon
ii  openvpn-dco-dkms               0.2.20250922-bullseye0             all          DCO (Data-Channel Offload) kernel module for OpenVPN)

With kernel "5.10.0-36" the dkms build fails ("5.10.0-34" and "5.10.0-35" work without an error):

# apt install openvpn-dco-dkms
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
openvpn-dco-dkms is already the newest version (0.2.20250922-bullseye0).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
1 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n] y
Setting up openvpn-dco-dkms (0.2.20250922-bullseye0) ...
Removing old ovpn-dco-0.2.20250922 DKMS files...

-------- Uninstall Beginning --------
Module:  ovpn-dco
Version: 0.2.20250922
Kernel:  5.10.0-35-amd64 (x86_64)
-------------------------------------

Status: Before uninstall, this module version was ACTIVE on this kernel.

ovpn-dco-v2.ko:
 - Uninstallation
   - Deleting from: /lib/modules/5.10.0-35-amd64/updates/dkms/
 - Original module
   - No original module was found for this module on this kernel.
   - Use the dkms install command to reinstall any previous module version.

depmod...

DKMS: uninstall completed.

------------------------------
Deleting module version: 0.2.20250922
completely from the DKMS tree.
------------------------------
Done.
Loading new ovpn-dco-0.2.20250922 DKMS files...
Building for 5.10.0-35-amd64 5.10.0-36-amd64
Building initial module for 5.10.0-35-amd64
Done.

ovpn-dco-v2.ko:
Running module version sanity check.
 - Original module
   - No original module exists within this kernel
 - Installation
   - Installing to /lib/modules/5.10.0-35-amd64/updates/dkms/

depmod...

DKMS: install completed.
Building initial module for 5.10.0-36-amd64
Error! Bad return status for module build on kernel: 5.10.0-36-amd64 (x86_64)
Consult /var/lib/dkms/ovpn-dco/0.2.20250922/build/make.log for more information.
dpkg: error processing package openvpn-dco-dkms (--configure):
 installed openvpn-dco-dkms package post-installation script subprocess returned error exit status 10
Errors were encountered while processing:
 openvpn-dco-dkms
E: Sub-process /usr/bin/dpkg returned an error code (1)

# cat /var/lib/dkms/ovpn-dco/0.2.20250922/build/make.log
DKMS make.log for ovpn-dco-0.2.20250922 for kernel 5.10.0-36-amd64 (x86_64)
Tue Nov  4 12:05:37 CET 2025
/var/lib/dkms/ovpn-dco/0.2.20250922/build/gen-compat-autoconf.sh /var/lib/dkms/ovpn-dco/0.2.20250922/build/compat-autoconf.h
make -C /lib/modules/5.10.0-36-amd64/build M=/var/lib/dkms/ovpn-dco/0.2.20250922/build PWD=/var/lib/dkms/ovpn-dco/0.2.20250922/build REVISION=0.2.20250922 CONFIG_OVPN_DCO_V2=m INSTALL_MOD_DIR=updates/	modules
make[1]: Entering directory '/usr/src/linux-headers-5.10.0-36-amd64'
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20250922/build/drivers/net/ovpn-dco/main.o
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20250922/build/drivers/net/ovpn-dco/bind.o
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h: In function ‘ovpn_udp_tunnel6_xmit_skb’:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:42: warning: passing argument 5 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                          ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:149:46: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  149 |     struct net_device *dev, struct in6_addr *saddr,
      |                             ~~~~~~~~~~~~~~~~~^~~~~
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:49: warning: passing argument 6 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                                 ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:150:22: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  150 |     struct in6_addr *daddr,
      |     ~~~~~~~~~~~~~~~~~^~~~~
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h: In function ‘ovpn_udp_tunnel6_xmit_skb’:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:42: warning: passing argument 5 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                          ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:149:46: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  149 |     struct net_device *dev, struct in6_addr *saddr,
      |                             ~~~~~~~~~~~~~~~~~^~~~~
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:49: warning: passing argument 6 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                                 ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:150:22: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  150 |     struct in6_addr *daddr,
      |     ~~~~~~~~~~~~~~~~~^~~~~
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20250922/build/drivers/net/ovpn-dco/crypto.o
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20250922/build/drivers/net/ovpn-dco/ovpn.o
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h: In function ‘ovpn_udp_tunnel6_xmit_skb’:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:42: warning: passing argument 5 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                          ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:149:46: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  149 |     struct net_device *dev, struct in6_addr *saddr,
      |                             ~~~~~~~~~~~~~~~~~^~~~~
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:49: warning: passing argument 6 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                                 ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:150:22: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  150 |     struct in6_addr *daddr,
      |     ~~~~~~~~~~~~~~~~~^~~~~
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h: In function ‘ovpn_udp_tunnel6_xmit_skb’:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:42: warning: passing argument 5 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                          ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:149:46: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  149 |     struct net_device *dev, struct in6_addr *saddr,
      |                             ~~~~~~~~~~~~~~~~~^~~~~
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:49: warning: passing argument 6 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                                 ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:150:22: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  150 |     struct in6_addr *daddr,
      |     ~~~~~~~~~~~~~~~~~^~~~~
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20250922/build/drivers/net/ovpn-dco/peer.o
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20250922/build/drivers/net/ovpn-dco/sock.o
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h: In function ‘ovpn_udp_tunnel6_xmit_skb’:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:42: warning: passing argument 5 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                          ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:149:46: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  149 |     struct net_device *dev, struct in6_addr *saddr,
      |                             ~~~~~~~~~~~~~~~~~^~~~~
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:49: warning: passing argument 6 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                                 ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:150:22: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  150 |     struct in6_addr *daddr,
      |     ~~~~~~~~~~~~~~~~~^~~~~
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h: In function ‘ovpn_udp_tunnel6_xmit_skb’:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:42: warning: passing argument 5 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                          ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:149:46: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  149 |     struct net_device *dev, struct in6_addr *saddr,
      |                             ~~~~~~~~~~~~~~~~~^~~~~
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:49: warning: passing argument 6 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                                 ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:150:22: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  150 |     struct in6_addr *daddr,
      |     ~~~~~~~~~~~~~~~~~^~~~~
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20250922/build/drivers/net/ovpn-dco/stats.o
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20250922/build/drivers/net/ovpn-dco/netlink.o
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h: In function ‘ovpn_udp_tunnel6_xmit_skb’:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:42: warning: passing argument 5 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                          ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:149:46: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  149 |     struct net_device *dev, struct in6_addr *saddr,
      |                             ~~~~~~~~~~~~~~~~~^~~~~
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:49: warning: passing argument 6 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                                 ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:150:22: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  150 |     struct in6_addr *daddr,
      |     ~~~~~~~~~~~~~~~~~^~~~~
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h: In function ‘ovpn_udp_tunnel6_xmit_skb’:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:42: warning: passing argument 5 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                          ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:149:46: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  149 |     struct net_device *dev, struct in6_addr *saddr,
      |                             ~~~~~~~~~~~~~~~~~^~~~~
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:49: warning: passing argument 6 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                                 ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:150:22: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  150 |     struct in6_addr *daddr,
      |     ~~~~~~~~~~~~~~~~~^~~~~
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20250922/build/drivers/net/ovpn-dco/crypto_aead.o
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20250922/build/drivers/net/ovpn-dco/pktid.o
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h: In function ‘ovpn_udp_tunnel6_xmit_skb’:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:42: warning: passing argument 5 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                          ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:149:46: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  149 |     struct net_device *dev, struct in6_addr *saddr,
      |                             ~~~~~~~~~~~~~~~~~^~~~~
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:49: warning: passing argument 6 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                                 ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:150:22: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  150 |     struct in6_addr *daddr,
      |     ~~~~~~~~~~~~~~~~~^~~~~
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20250922/build/drivers/net/ovpn-dco/tcp.o
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h: In function ‘ovpn_udp_tunnel6_xmit_skb’:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:42: warning: passing argument 5 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                          ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:149:46: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  149 |     struct net_device *dev, struct in6_addr *saddr,
      |                             ~~~~~~~~~~~~~~~~~^~~~~
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:49: warning: passing argument 6 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                                 ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:150:22: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  150 |     struct in6_addr *daddr,
      |     ~~~~~~~~~~~~~~~~~^~~~~
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20250922/build/drivers/net/ovpn-dco/udp.o
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h: In function ‘ovpn_udp_tunnel6_xmit_skb’:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:42: warning: passing argument 5 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                          ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:149:46: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  149 |     struct net_device *dev, struct in6_addr *saddr,
      |                             ~~~~~~~~~~~~~~~~~^~~~~
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:49: warning: passing argument 6 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                                 ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:150:22: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  150 |     struct in6_addr *daddr,
      |     ~~~~~~~~~~~~~~~~~^~~~~
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/drivers/net/ovpn-dco/tcp.c: In function ‘ovpn_tcp_init’:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:158:26: error: ‘struct proto’ has no member named ‘stream_memory_read’; did you mean ‘stream_memory_free’?
  158 | #define sock_is_readable stream_memory_read
      |                          ^~~~~~~~~~~~~~~~~~
/var/lib/dkms/ovpn-dco/0.2.20250922/build/drivers/net/ovpn-dco/tcp.c:483:16: note: in expansion of macro ‘sock_is_readable’
  483 |  ovpn_tcp_prot.sock_is_readable = ovpn_tcp_sock_is_readable;
      |                ^~~~~~~~~~~~~~~~
make[4]: *** [/usr/src/linux-headers-5.10.0-36-common/scripts/Makefile.build:291: /var/lib/dkms/ovpn-dco/0.2.20250922/build/drivers/net/ovpn-dco/tcp.o] Fehler 1
make[4]: *** Es wird auf noch nicht beendete Prozesse gewartet....
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h: In function ‘ovpn_udp_tunnel6_xmit_skb’:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:42: warning: passing argument 5 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                          ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:149:46: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  149 |     struct net_device *dev, struct in6_addr *saddr,
      |                             ~~~~~~~~~~~~~~~~~^~~~~
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:69:49: warning: passing argument 6 of ‘udp_tunnel6_xmit_skb’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   69 |  udp_tunnel6_xmit_skb(dst, sk, skb, dev, saddr, daddr, prio, ttl, label,
      |                                                 ^~~~~
In file included from /var/lib/dkms/ovpn-dco/0.2.20250922/build/linux-compat.h:45,
                 from <command-line>:
/usr/src/linux-headers-5.10.0-36-common/include/net/udp_tunnel.h:150:22: note: expected ‘struct in6_addr *’ but argument is of type ‘const struct in6_addr *’
  150 |     struct in6_addr *daddr,
      |     ~~~~~~~~~~~~~~~~~^~~~~
make[3]: *** [/usr/src/linux-headers-5.10.0-36-common/scripts/Makefile.build:508: /var/lib/dkms/ovpn-dco/0.2.20250922/build/drivers/net/ovpn-dco] Fehler 2
make[2]: *** [/usr/src/linux-headers-5.10.0-36-common/Makefile:1875: /var/lib/dkms/ovpn-dco/0.2.20250922/build] Fehler 2
make[1]: *** [/usr/src/linux-headers-5.10.0-36-common/Makefile:192: __sub-make] Error 2
make[1]: Leaving directory '/usr/src/linux-headers-5.10.0-36-amd64'
make: *** [Makefile:51: all] Error 2

Current workaround was to downgrade the kernel:

apt install linux-headers-5.10.0-35-amd64 linux-headers-5.10.0-35-common linux-image-5.10.0-35-amd64
apt purge linux-headers-5.10.0-36-amd64 linux-headers-5.10.0-36-common linux-image-5.10.0-36-amd64
reboot

If more information or tests are needed please feel free to ask.

[ordex]

from "-35" to "-36" Debian changed the mainline kernel from "5.10.237" to "5.10.244". Weird that a breaking change was backported to this maintenance version, but I'll need to double check.

[ordex]

indeed this commit

commit c83455d7de0baa77989ae68b8c6f2a4f33965763
Author: Cong Wang <cong.wang@bytedance.com>
Date:   Fri Oct 8 13:33:03 2021 -0700

    net: Rename ->stream_memory_read to ->sock_is_readable

was merged in 5.10.239 and it is now causing the issue you are seeing. Will tackle that in our compat header.

FTR: the original commit was merged in 5.15 for the first time and its ID is 7b50ecfcc6cdfe87488576bc3ed443dc8d083b90

[ordex]

I think this is fixed at this point

[keros]

Hi

Thank you for your work. I just tried it again with the current kernel:

# apt install linux-headers-5.10.0-37-amd64 linux-headers-5.10.0-37-common linux-image-5.10.0-37-amd64
# reboot
# uname -a
Linux sec3-vpnhub02 5.10.0-37-amd64 #1 SMP Debian 5.10.247-1 (2025-12-11) x86_64 GNU/Linux

# apt install --reinstall openvpn-dco-dkms
apt install openvpn-dco-dkms
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
openvpn-dco-dkms is already the newest version (0.2.20251017-bullseye0).
The following packages were automatically installed and are no longer required:
  linux-headers-5.10.0-34-amd64 linux-headers-5.10.0-34-common linux-image-5.10.0-34-amd64
Use 'apt autoremove' to remove them.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
1 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n] 
Setting up openvpn-dco-dkms (0.2.20251017-bullseye0) ...
Removing old ovpn-dco-0.2.20251017 DKMS files...

------------------------------
Deleting module version: 0.2.20251017
completely from the DKMS tree.
------------------------------
Done.
Loading new ovpn-dco-0.2.20251017 DKMS files...
Building for 5.10.0-37-amd64
Building initial module for 5.10.0-37-amd64
Error! Bad return status for module build on kernel: 5.10.0-37-amd64 (x86_64)
Consult /var/lib/dkms/ovpn-dco/0.2.20251017/build/make.log for more information.
dpkg: error processing package openvpn-dco-dkms (--configure):
 installed openvpn-dco-dkms package post-installation script subprocess returned error exit status 10
Errors were encountered while processing:
 openvpn-dco-dkms
E: Sub-process /usr/bin/dpkg returned an error code (1)

# cat /var/lib/dkms/ovpn-dco/0.2.20251017/build/make.log
DKMS make.log for ovpn-dco-0.2.20251017 for kernel 5.10.0-37-amd64 (x86_64)
Tue Jan 27 09:18:35 CET 2026
/var/lib/dkms/ovpn-dco/0.2.20251017/build/gen-compat-autoconf.sh /var/lib/dkms/ovpn-dco/0.2.20251017/build/compat-autoconf.h
make -C /lib/modules/5.10.0-37-amd64/build M=/var/lib/dkms/ovpn-dco/0.2.20251017/build PWD=/var/lib/dkms/ovpn-dco/0.2.20251017/build REVISION=0.2.20251017 CONFIG_OVPN_DCO_V2=m INSTALL_MOD_DIR=updates/modules
make[1]: Entering directory '/usr/src/linux-headers-5.10.0-37-amd64'
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20251017/build/drivers/net/ovpn-dco/main.o
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20251017/build/drivers/net/ovpn-dco/bind.o
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20251017/build/drivers/net/ovpn-dco/crypto.o
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20251017/build/drivers/net/ovpn-dco/ovpn.o
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20251017/build/drivers/net/ovpn-dco/peer.o
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20251017/build/drivers/net/ovpn-dco/sock.o
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20251017/build/drivers/net/ovpn-dco/stats.o
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20251017/build/drivers/net/ovpn-dco/netlink.o
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20251017/build/drivers/net/ovpn-dco/crypto_aead.o
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20251017/build/drivers/net/ovpn-dco/pktid.o
  CC [M]  /var/lib/dkms/ovpn-dco/0.2.20251017/build/drivers/net/ovpn-dco/tcp.o
In file included from <command-line>:
/var/lib/dkms/ovpn-dco/0.2.20251017/build/drivers/net/ovpn-dco/tcp.c: In function ‘ovpn_tcp_init’:
/var/lib/dkms/ovpn-dco/0.2.20251017/build/linux-compat.h:162:26: error: ‘struct proto’ has no member named ‘stream_memory_read’; did you mean ‘stream_memory_free’?
  162 | #define sock_is_readable stream_memory_read
      |                          ^~~~~~~~~~~~~~~~~~
/var/lib/dkms/ovpn-dco/0.2.20251017/build/drivers/net/ovpn-dco/tcp.c:483:16: note: in expansion of macro ‘sock_is_readable’
  483 |  ovpn_tcp_prot.sock_is_readable = ovpn_tcp_sock_is_readable;
      |                ^~~~~~~~~~~~~~~~
make[4]: *** [/usr/src/linux-headers-5.10.0-37-common/scripts/Makefile.build:291: /var/lib/dkms/ovpn-dco/0.2.20251017/build/drivers/net/ovpn-dco/tcp.o] Fehler 1
make[3]: *** [/usr/src/linux-headers-5.10.0-37-common/scripts/Makefile.build:508: /var/lib/dkms/ovpn-dco/0.2.20251017/build/drivers/net/ovpn-dco] Fehler 2
make[2]: *** [/usr/src/linux-headers-5.10.0-37-common/Makefile:1877: /var/lib/dkms/ovpn-dco/0.2.20251017/build] Fehler 2
make[1]: *** [/usr/src/linux-headers-5.10.0-37-common/Makefile:192: __sub-make] Error 2
make[1]: Leaving directory '/usr/src/linux-headers-5.10.0-37-amd64'
make: *** [Makefile:51: all] Error 2