Statistics /proc/net/stat/nf_conntrack is missing on Linux server
Solution 1:
The relevant required configuration needed to have conntrack available in /proc is CONFIG_NF_CONNTRACK_PROCFS:
CONFIG_NF_CONNTRACK_PROCFS: Supply CT list in procfs (OBSOLETE)[...]
Help text
This option enables for the list of known conntrack entries to be shown in procfs under net/netfilter/nf_conntrack. This is considered obsolete in favor of using the conntrack(8) tool which uses Netlink.
OP's kernel appears to have been built with features described in this package: linux-buildinfo-5.4.0-87-generic.
Alas content of /usr/lib/linux/5.4.0-87-generic/config from package above tells:
# CONFIG_NF_CONNTRACK_PROCFS is not set
So the two /proc/ entries usually provided by conntrack: /proc/net/nf_conntrack and /proc/net/stat/nf_conntrack will not exist.
As the documentation tells, this has been obsoleted by the conntrack tool which uses the netlink(7) kernel API instead.
Most of the content of /proc/net/stat/nf_conntrack can be replaced by:
conntrack --count
which gives the number of entries (this is the first column of data present in /proc/net/stat/nf_conntrack) and:
conntrack --stats
conntrack --stats expect
which gives the content of most of the other columns available in /proc/net/stat/nf_conntrack, sometimes for the (main) conntrack table, sometimes for the expect table, one per CPU as well. A few statistics don't appear to be available this way (or might be hidden elsewhere, or might have been obsoleted ...).
If you really need /proc/net/stat/nf_conntrack you'll have to switch to an other kernel with this feature available or rebuild the kernel with different options. This kernel option doesn't appear to be chosen by Ubuntu anymore, including in newer kernel versions.
For reference, here's an example from a different kernel having this option and running with 4 CPUs:
# cat /proc/net/stat/nf_conntrack | column -t
entries clashres found new invalid ignore delete delete_list insert insert_failed drop early_drop icmp_error expect_new expect_create expect_delete search_restart
0000000a 00000092 00000000 00000000 00000276 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000016 0000000a 00000016 00002463
0000000a 00000000 00000000 00000000 00000009 00000000 00000000 00000000 00000000 00000001 00000001 00000000 00000000 00000000 00000004 00000000 000000eb
0000000a 00000000 00000000 00000000 00000008 00000000 00000000 00000000 00000000 00000007 00000007 00000000 00000000 00000000 00000004 00000000 00000100
0000000a 00000000 00000000 00000000 00000048 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000004 00000000 00000325
# conntrack -C
10
# conntrack -S
cpu=0 found=0 invalid=630 insert=0 insert_failed=0 drop=0 early_drop=0 error=0 search_restart=9315
cpu=1 found=0 invalid=9 insert=0 insert_failed=1 drop=1 early_drop=0 error=0 search_restart=235
cpu=2 found=0 invalid=8 insert=0 insert_failed=7 drop=7 early_drop=0 error=0 search_restart=256
cpu=3 found=0 invalid=72 insert=0 insert_failed=0 drop=0 early_drop=0 error=0 search_restart=805
# conntrack -S expect
cpu=0 expect_new=22 expect_create=10 expect_delete=22
cpu=1 expect_new=0 expect_create=4 expect_delete=0
cpu=2 expect_new=0 expect_create=4 expect_delete=0
cpu=3 expect_new=0 expect_create=4 expect_delete=0
#