https://github.com/qvest-digital/saachenzip

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│ Sāchenz IP-Adresse │
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Tiny static container that just returns the IP addresses
of TCP and UDP endpoints, for debugging.

Licence: MirBSD <https://mbsd.evolvis.org/MirOS-Licence.htm>
Published by: Qvest Digital <https://www.qvest-digital.com/>

To compile, make sure Docker, git and mksh are installed,
then run ./Build.sh to produce a saachenzip:latest image.
The image has no dependencies and is very tiny; publish to
where it’s needed.

To run, use something like:

$ docker run --rm --name tellmyip -d \
      -p 1024/tcp -p 1024/udp \
      saachenzip:latest
$ docker inspect tellmyip | sed -n '/^.*"IPAddress": "\(.*\)",*$/s//\1/p' | sort -u
 ⇒ this outputs the IP address of the container, e.g. 172.17.0.2
$ docker logs -tf tellmyip
 … use it (see below)
$ docker stop tellmyip

You might need --security-opt apparmor=unconfined if you
don’t have apparmour enabled (see the comment in Build.sh).

Another possibility is with host networking and specifying
the port(s) manually (multiple possible, default 1024):

$ docker run --rm --name tellmyip -d \
      --net=host \
      saachenzip:latest \
      /saachenzip 2001 2002 2003 ::/2004 127.0.0.1/2005
$ docker logs -tf tellmyip
 … use it (see below)
$ docker stop tellmyip

The container runs as nōn-root by default so can only use
ports 1024‥65535.

With regular networking, you can trigger a response by sending
a dummy message (one short packet whose content is ignored) over
TCP or UDP to the container IP:

$ echo x | nc 172.17.0.2 1024

UDP, in theory, works the same but is often broken in Docker.

Using host networking, the ports are bound on the host, so:

$ echo x | nc 127.0.0.1 2001
$ echo x | nc ::1 2002
$ echo x | nc -uw1 127.0.0.1 2003

The author’s Docker version also seems to have broken UDP but
only over IPv6 for host networking. *sigh…*

The responses are plaintext JSON currently as follows:

{
  "client-l3": "IPv4",		// or IPv6, or a protocol number
  "client-l4": "tcp",		// or udp
  "client-host": "172.17.0.1",	// IPv6 or Legacy IP address*
  "client-port": "42150",	// OSI L4 port number
  "server-l3": "IPv4",
  "server-l4": "tcp",
  "server-host": "172.17.0.2",
  "server-port": "1024",
  "timestamp": 1718483209.876615 // gettimeofday(2)
}

client-l3 can be IPv4 and server-l3 can be IPv6 at the same time
if v4-mapped or (*shudder…*) NAT64 are used. *-l3 can be IPv4 but
*-host an IPv6 address, for similar cases. There can be an extra
lnx.specdst member for UDP on Solaris and Linux.

In case of trouble, *-host can be "(unknown)", *-port "(???)".

timestamp is normally POSIX time_t in microsecond accuracy, that
is, seconds since 1970 minus the amount of leap seconds since 1970
(i.e. days have constant 86400 seconds).

If HTTP responses are desired invoke as above with explicit ports
and pass a sole ‘H’ (uppercase) as first argument to enable:

$ docker run --rm --name tellmyip -d \
      -p 1024/tcp -p 1024/udp \
      saachenzip:latest \
      /saachenzip H 1024
$ curl http://172.17.0.2:1024/

The image was developed assuming a Docker Linux/amd64 runtime, the
binary would be compiled using musl libc for sanity. It most likely
is portable to other architectures if not common operating systems.

┌───────────────────────────┐
│ How do I pronounce… t̲h̲a̲t̲‽ │
└───────────────────────────┘

    ↓rasp
Zuh-khents Ee Peh Addressə
 ↑long vowel