Given there is a pod (e.g. postgres) running in kubernetes, kubectl port-forward pod 15432:5432 is being used to expose the pod to host.

normally, it is accessible in host through running the postgres client: psql -h 127.0.0.1 -p 15432, OR by accessing http://127.0.0.1:15432, OR establishing TCP connection directly: echo > /dev/tcp/127.0.0.1/15432 && echo yes. If the connection is established successfully, kubectl would prompt a message Handling connection for 15432 for verification.

However, accessing the port-forwarded pod using 127.0.0.1/ 172.17.0.1 inside container is not possible, despite the flag --network=host is being used or not. It is only accessible through host.docker.internal

This could be a problem exclusively happening to docker for mac. I haven't verified on linux yet.

Here are the logs when I run the connection test inside docker. It clearly shows that the TCP connection cannot be established.

$ docker run --network=host -it --rm postgres:12.4 /bin/bash

# inside container
# unsuccessful for establishing TCP connection to 127.0.0.1:15432
root@docker-desktop:/# echo > /dev/tcp/127.0.0.1/15432 && echo yes
bash: connect: Connection refused
bash: /dev/tcp/127.0.0.1/15432: Connection refused

# unsuccessful for establishing TCP connection to 172.17.0.1:15432
[root@docker-desktop /]# echo > /dev/tcp/172.17.0.1/15432 && echo yes
bash: connect: Connection refused
bash: /dev/tcp/172.17.0.1/15432: Connection refused

# no surprise: unsuccessful for psql 127.0.0.1
root@docker-desktop:/# psql -h 127.0.0.1 -p 15432
psql: error: could not connect to server: could not connect to server: Connection refused
	Is the server running on host "127.0.0.1" and accepting
	TCP/IP connections on port 15432?

# successful for host.docker.internal
root@docker-desktop:/# psql -h host.docker.internal -p 15432
Password for user root:

Here are some nslookup/ ifconfig logs that could be useful:

Server:		192.168.65.1
Address:	192.168.65.1#53

Non-authoritative answer:
Name:	host.docker.internal
Address: 192.168.65.2

bash-5.0# ifconfig
eth0      Link encap:Ethernet  HWaddr 02:42:AC:11:00:02
          inet addr:172.17.0.2  Bcast:172.17.255.255  Mask:255.255.0.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:12 errors:0 dropped:0 overruns:0 frame:0
          TX packets:3 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:984 (984.0 B)  TX bytes:202 (202.0 B)

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:1 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:29 (29.0 B)  TX bytes:29 (29.0 B)

Why there is a difference in connectivity between the docker container and host? How does host.docker.internal solve the problem under the hood? Is there other way to resolve the problem by providing docker run flags?