Q. I've Linux box acting as software router (natted) for over 100 computer connected via LAN. Regular netstat command is not displaying the list of all natted connections. How do I find out connections managed by netfilter / iptables which comes with the Debian 4.x system?
A. You cannot use regular netstat command to display NAT connections managed by iptables. You need to use netstat-nat command. You can also use /proc/net/ip_conntrack or /proc/net/nf_conntrack, which is the temporary conntrack storage of netfilter.
Use apt-get command under Debian / Ubuntu Linux, enter:
$ sudo apt-get install netstat-nat
Source code / RPM file
If you are using Suse / Redhat Linux, grab source code or RPM file here:
How do I use netstat-nat command?
Login as root user and type the following to display list of all natted connections:
# netstat-nat -n
To display NAT connections with protocol selection, enter:
# netstat-nat -np
To display all connection by source IP called 192.168.1.100
# netstat-nat -s 192.168.1.100
To display all connections by destination IP/hostname called laptop, enter:
# netstat-nat -s laptop
To display SNAT connections, enter:
# netstat-nat -S
To display DNAT connections, enter:
# netstat-nat -D
To display only connections to NAT box self i.e. doesn’t show SNAT & DNAT, enter:
# netstat-nat -L
To display help, enter:
$ netstat-nat -h
$ man netstat-nat