Unknown Symbol when trying to insert the module about ndpi-netfilter

[rightkick]

I faced the same issue with Debian buster kernel 4.19.0-13-amd64. I did the following to resolve this. I'm putting all the steps for your convenience:

cd /usr/local/src/
git clone https://github.com/betolj/ndpi-netfilter.git
cd /usr/local/src/ndpi-netfilter
tar xvfz nDPI.tar.gz

# find all the references of the missing kernel symbol
grep -r nf_ct_l3proto .

# output
./src/main.c:   return nf_ct_l3proto_try_module_get (match->family) == 0;
./src/main.c:   return nf_ct_l3proto_try_module_get (par->family) == 0;
./src/main.c:   return nf_ct_l3proto_try_module_get (par->family);
./src/main.c:   nf_ct_l3proto_module_put (match->family);
./src/main.c:   nf_ct_l3proto_module_put (par->family);
./old/src/main.c:       return nf_ct_l3proto_try_module_get (match->family) == 0;
./old/src/main.c:       return nf_ct_l3proto_try_module_get (par->family) == 0;
./old/src/main.c:       return nf_ct_l3proto_try_module_get (par->family);
./old/src/main.c:       nf_ct_l3proto_module_put (match->family);
./old/src/main.c:       nf_ct_l3proto_module_put (par->family);

# and edit at each above occurence the return command to return false. example: 
return nf_ct_l3proto_try_module_get (match->family) == 0;
---> 
return false;

and comment out all the other. example: 
//nf_ct_l3proto_module_put (match->family);

# Proceed with the normal installation steps: 
cd nDPI
./autogen.sh
make
make install
cd ..
NDPI_PATH=/usr/local/src/ndpi-netfilter/nDPI make
make modules_install
cp /usr/local/src/ndpi-netfilter/ipt/libxt_ndpi.so /usr/lib/x86_64-linux-gnu/xtables/

# manually load the module just for testing
modprobe xt_ndpi

# Confirm good order
lsmod | grep ndpi
iptables -m ndpi --help

Note that I believe that the above changes are safe as they are included in if statements for older kernel versions. I cannot say though that I understand the code. Following this patch, I did not see any issue with a quick test.