Debian 11 doesn't seem to have any netfilters installed

When I tried to so

sudo sysctl net.netfilter.nf_conntrack_tcp_be_liberal=1

I got the error:

sysctl: cannot stat /proc/sys/net/netfilter/nf_conntrack_tcp_be_liberal: No such file or directory

When I checked the folder ls /proc/sys/net/netfilter/ All I could see was:

nf_log  nf_log_all_netns

But on Debian 10, I see this: ls /proc/sys/net/netfilter/

nf_conntrack_acct                   nf_conntrack_dccp_timeout_closing   nf_conntrack_events             nf_conntrack_helper               nf_conntrack_sctp_timeout_cookie_echoed      nf_conntrack_sctp_timeout_shutdown_recd  nf_conntrack_tcp_timeout_close_wait   nf_conntrack_tcp_timeout_syn_sent        nf_log
nf_conntrack_buckets                nf_conntrack_dccp_timeout_open      nf_conntrack_expect_max         nf_conntrack_icmp_timeout         nf_conntrack_sctp_timeout_cookie_wait        nf_conntrack_sctp_timeout_shutdown_sent  nf_conntrack_tcp_timeout_established  nf_conntrack_tcp_timeout_time_wait       nf_log_all_netns
nf_conntrack_checksum               nf_conntrack_dccp_timeout_partopen  nf_conntrack_frag6_high_thresh  nf_conntrack_icmpv6_timeout       nf_conntrack_sctp_timeout_established        nf_conntrack_tcp_be_liberal              nf_conntrack_tcp_timeout_fin_wait     nf_conntrack_tcp_timeout_unacknowledged
nf_conntrack_count                  nf_conntrack_dccp_timeout_request   nf_conntrack_frag6_low_thresh   nf_conntrack_log_invalid          nf_conntrack_sctp_timeout_heartbeat_acked    nf_conntrack_tcp_loose                   nf_conntrack_tcp_timeout_last_ack     nf_conntrack_timestamp
nf_conntrack_dccp_loose             nf_conntrack_dccp_timeout_respond   nf_conntrack_frag6_timeout      nf_conntrack_max                  nf_conntrack_sctp_timeout_heartbeat_sent     nf_conntrack_tcp_max_retrans             nf_conntrack_tcp_timeout_max_retrans  nf_conntrack_udp_timeout
nf_conntrack_dccp_timeout_closereq  nf_conntrack_dccp_timeout_timewait  nf_conntrack_generic_timeout    nf_conntrack_sctp_timeout_closed  nf_conntrack_sctp_timeout_shutdown_ack_sent  nf_conntrack_tcp_timeout_close           nf_conntrack_tcp_timeout_syn_recv     nf_conntrack_udp_timeout_stream

What am I missing, please?

The missing variables are made available by the kernel module nf_conntrack.

You could, depending on the exact issue you have and the time when it happens (at boot or not...):

• manually load the module

  modprobe nf_conntrack
  

• have any stateful or NAT rule (from iptables-legacy, iptables-nft or nftables) trigger the loading of nf_conntrack

• alter the order and dependencies of systemd services so systemd-sysctl (or procps) doesn't run too early but after the nf_conntrack module is loaded by such previous rule loaded from an other service.

• force load the nf_conntrack module at boot by adding it in /etc/modules

  and if needed early, rebuilding initramfs:

  update-initramfs -u