Description:
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netstat-nat is a small program written in C. It displays connections,
accounted by netfilter which comes with ≥2.6.15 linux kernels.
The program reads its information from '/proc/net/nf_conntrack',
which is the temporary conntrack-storage of netfilter.
(http://www.netfilter.org/)

This is most useful for NAT gateways/copies of the aforementioned therefrom,
but connections made to/from the current host can be examined as well (-LR).
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-ng:
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https://sr.ht/~nabijaczleweli/netstat-nat-ng
https://todo.sr.ht/~nabijaczleweli/netstat-nat-ng
(report at <mailto:~nabijaczleweli/netstat-nat-ng@todo.sr.ht>)
https://lists.sr.ht/~nabijaczleweli/netstat-nat-ng
            <mailto:~nabijaczleweli/netstat-nat-ng@lists.sr.ht>

The git history includes imports of all historical tarballs
(https://web.archive.org/web/20180104143740/http://www.tweegy.nl/projects/netstat-nat/),
with the changelog distributed in the tags.
The tags also have contributors added, if credited in the 1.4.10 README
(they were not credited contemporaneously).

Release tarballs are signed with nabijaczleweli@nabijaczleweli.xyz
  (pull with WKD, but 7D69 474E 8402 8C5C C0C4  4163 BCFD 0B01 8D26 58F1).
аnd stored in git notes as-if via the example program provided at
  https://man.sr.ht/git.sr.ht/#signing-tags-tarballs
and are thus available on the refs listing/tag page as .tar.gz.asc.
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Usage:
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netstat-nat [-no] [-x|-X[width]] {[-G] [-S|-D]}|[-L|-R]
            [-s ource]... [-d estination]... [-g ateway]... [-p rotocol]...
            [-r src|dst|src-port|dst-port|state|gate|gate-port] [nf_conntrack]

Consult the manual for specifics.
https://srhtcdn.githack.com/~nabijaczleweli/netstat-nat-ng/blob/man/netstat-nat-ng.pdf
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Examples:
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On a gateway:

# netstat-nat
Proto NATed Address                  Destination Address            State
icmp  10.179.163.88                  tarta.nabijaczleweli.xyz
icmp  10.200.233.12                  tarta.nabijaczleweli.xyz
tcp   10.32.144.39.host.securi:63770 tarta.nabijaczleweli.xyz:https TIME_WAIT
tcp   10.32.144.39.host.securi:64361 tarta.nabijaczleweli.xyz:http  TIME_WAIT
tcp   10.72.217.111:33864            tarta.nabijaczleweli.xyz:5900  TIME_WAIT
tcp   10.72.217.111:39074            tarta.nabijaczleweli.xyz:5900  TIME_WAIT
tcp   nabijaczleweli.nabijaczl:20985 lb-10-268-216-78-iad.git:https ESTABLISHED
tcp   nabijaczleweli.nabijaczl:21047 waw07s05-in-f4.1e100.net:http  TIME_WAIT
tcp   nabijaczleweli.nabijaczl:20751 waw07s05-in-f14.1e100.ne:https ESTABLISHED
tcp   nabijaczleweli.nabijaczl:17906 101010.pl:https                ESTABLISHED
tcp   192.168.1.136:38216            lt-in-f188.1e100.net:5228      ESTABLISHED
tcp   192.168.1.136:38330            lt-in-f188.1e100.net:5228      ESTABLISHED
tcp   192.168.1.136:39972            lt-in-f188.1e100.net:5228      ESTABLISHED
tcp   192.168.1.136:40092            lt-in-f188.1e100.net:5228      ESTABLISHED
udp   rozbian.nabijaczleweli.x:46115 10.158.103.58:https            ASSURED
udp   rozbian.nabijaczleweli.x:42560 10.69.17.108:https             ASSURED
udp   rozbian.nabijaczleweli.x:44786 10.140.3.34:https              ASSURED
udp   rozbian.nabijaczleweli.x:47079 10.140.3.34:https              UNREPLIED
udp   192.168.1.185:3076             ec2-10-213-30-29.eu-cent:https ASSURED
udp   tarta.nabijaczleweli.xyz:39517 ns-153.awsdns-19.com:domain
udp   tarta.nabijaczleweli.xyz:16829 ns-1007.awsdns-61.net:domain
udp   tarta.nabijaczleweli.xyz:46874 ns-1411.awsdns-48.org:domain


On a client:

# netstat-nat -LnX21
Proto Source Address        Destination Address   State
tcp   [::1]:49118           [::1]:9928            TIME_WAIT
tcp   [::1]:50624           [::1]:80              ESTABLISHED
tcp   10.221.132.191:35862  192.168.1.250:5900    TIME_WAIT
tcp   127.0.0.1:34030       127.0.0.1:8213        TIME_WAIT
tcp   127.0.0.1:52476       127.0.1.1:3493        ESTABLISHED
tcp   10.176.163.40:51167   192.168.1.250:443     TIME_WAIT
tcp   10.62.134.22:23178    192.168.1.250:80      SYN_RECV
tcp   10.62.134.22:47444    192.168.1.250:80      SYN_RECV
udp   127.0.0.1:42738       127.0.0.1:323
udp   127.0.0.1:44850       127.0.0.1:5353
udp   127.0.0.1:46444       127.0.0.1:53          ASSURED
udp   192.168.1.144:20407   192.168.1.250:53
udp   192.168.1.146:59266   192.168.1.250:53
udp   192.168.1.204:35989   192.168.1.250:53
udp   192.168.1.250:23225   10.46.8.231:53
udp   192.168.1.250:11300   10.11.28.69:53
udp   192.168.1.250:51413   10.116.123.52:6881    ASSURED
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