[root@de ~]# uname -a
Linux de 2.6.32-696.20.1.el6.i686 #1 SMP Fri Jan 26 18:13:32 UTC 2018 i686 i686 i386 GNU/Linux
[root@de ~]# iptables --version
iptables v1.4.7
[root@de ~]# conntrack --version
conntrack v0.9.13 (conntrack-tools)

[root@de ~]# dmesg | grep conntrack
nf_conntrack version 0.5.0 (16097 buckets, 64388 max)
conntrack: generic helper won't handle protocol 47. Please consider loading the specific helper module.

[root@de ~]# lsmod | grep '^\w*\(ppp\|gre\|conntrack\)' | sort
nf_conntrack           65336  4 iptable_nat,nf_nat,nf_conntrack_ipv4,xt_state
nf_conntrack_ipv4       7374  7 iptable_nat,nf_nat
ppp_async               6534  2
ppp_generic            21173  10 ppp_mppe,ppp_async
ppp_mppe                5230  4
[root@de ~]#

[root@de ~]# iptables-save
# Generated by iptables-save v1.4.7 on Wed Jan  8 23:39:49 2025
*nat
:PREROUTING ACCEPT [931:257968]
:POSTROUTING ACCEPT [70:5024]
:OUTPUT ACCEPT [70:5024]
-A POSTROUTING -s 10.0.2.0/24 -o eth0 -j SNAT --to-source 10.0.1.1
COMMIT
# Completed on Wed Jan  8 23:39:49 2025
# Generated by iptables-save v1.4.7 on Wed Jan  8 23:39:49 2025
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [88:9926]
:OUTPUT ACCEPT [1002:167543]
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 22 -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 1723 -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 5201 -j ACCEPT
-A INPUT -p gre -j ACCEPT
-A INPUT -j REJECT --reject-with icmp-host-prohibited
COMMIT
# Completed on Wed Jan  8 23:39:49 2025

[root@de ~]# iptables -t raw -A PREROUTING -p tcp --dport 1723 -j CT --helper pptp
iptables v1.4.7: unknown option `--helper'
Try `iptables -h' or 'iptables --help' for more information.

[root@de ~]# sysctl -w net.netfilter.nf_conntrack_helper=1
error: "net.netfilter.nf_conntrack_helper" is an unknown key

[root@de ~]# echo 1 > /proc/sys/net/netfilter/nf_conntrack_helper
-bash: /proc/sys/net/netfilter/nf_conntrack_helper: No such file or directory


[root@de ~]# sysctl -a | grep conntrack
net.netfilter.nf_conntrack_generic_timeout = 600
net.netfilter.nf_conntrack_tcp_timeout_syn_sent = 120
net.netfilter.nf_conntrack_tcp_timeout_syn_recv = 60
net.netfilter.nf_conntrack_tcp_timeout_established = 432000
net.netfilter.nf_conntrack_tcp_timeout_fin_wait = 120
net.netfilter.nf_conntrack_tcp_timeout_close_wait = 60
net.netfilter.nf_conntrack_tcp_timeout_last_ack = 30
net.netfilter.nf_conntrack_tcp_timeout_time_wait = 120
net.netfilter.nf_conntrack_tcp_timeout_close = 10
net.netfilter.nf_conntrack_tcp_timeout_max_retrans = 300
net.netfilter.nf_conntrack_tcp_timeout_unacknowledged = 300
net.netfilter.nf_conntrack_tcp_loose = 1
net.netfilter.nf_conntrack_tcp_be_liberal = 0
net.netfilter.nf_conntrack_tcp_max_retrans = 3
net.netfilter.nf_conntrack_udp_timeout = 30
net.netfilter.nf_conntrack_udp_timeout_stream = 180
net.netfilter.nf_conntrack_icmp_timeout = 30
net.netfilter.nf_conntrack_acct = 0
net.netfilter.nf_conntrack_events = 1
net.netfilter.nf_conntrack_events_retry_timeout = 15
net.netfilter.nf_conntrack_max = 64388
net.netfilter.nf_conntrack_count = 71
net.netfilter.nf_conntrack_buckets = 16384
net.netfilter.nf_conntrack_checksum = 1
net.netfilter.nf_conntrack_log_invalid = 0
net.netfilter.nf_conntrack_expect_max = 256
net.nf_conntrack_max = 64388

[root@de ~]# ls /proc/sys/net/netfilter/
nf_conntrack_acct                  nf_conntrack_tcp_timeout_close
nf_conntrack_buckets               nf_conntrack_tcp_timeout_close_wait
nf_conntrack_checksum              nf_conntrack_tcp_timeout_established
nf_conntrack_count                 nf_conntrack_tcp_timeout_fin_wait
nf_conntrack_events                nf_conntrack_tcp_timeout_last_ack
nf_conntrack_events_retry_timeout  nf_conntrack_tcp_timeout_max_retrans
nf_conntrack_expect_max            nf_conntrack_tcp_timeout_syn_recv
nf_conntrack_generic_timeout       nf_conntrack_tcp_timeout_syn_sent
nf_conntrack_icmp_timeout          nf_conntrack_tcp_timeout_time_wait
nf_conntrack_log_invalid           nf_conntrack_tcp_timeout_unacknowledged
nf_conntrack_max                   nf_conntrack_udp_timeout
nf_conntrack_tcp_be_liberal        nf_conntrack_udp_timeout_stream
nf_conntrack_tcp_loose             nf_log
nf_conntrack_tcp_max_retrans

[forensics]
name=CERT Forensics Tools Repository 
baseurl=http://www.cert.org/forensics/repository/centos/cert/$releasever/$basearch
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-cert-forensics-2022-04-03