Contribution description

The gcoap forward proxy must respond to the client from the same address and port where it has received the request.
The address towards the server can be different.
For that the usual information in sock_udp_aux_tx_t are propated to where they are needed in gcoap_req_send() and _handle_req() in gcoap.c

Another fix is included by setting the correct payload size in the forwarded PDU right after it is copied.

Testing procedure

Use this branch to run a client and proxy with examples/gcoap

./dist/tools/tapsetup

Client:

CFLAGS+=-DCONFIG_GCOAP_PDU_BUF_SIZE=512 make flash term

Proxy:

Add another link local address only to the proxy by adding the following to examples/gcoap/Makefile:

+# Uncomment the following 2 lines to specify static link lokal IPv6 address
+# this might be useful for testing, in cases where you cannot or do not want to
+# run a shell with ifconfig to get the real link lokal address.
+IPV6_STATIC_LLADDR ?= '"fe80::cafe:cafe:cafe:1"'
+CFLAGS += -DCONFIG_GNRC_IPV6_STATIC_LLADDR=$(IPV6_STATIC_LLADDR)
+# uncomment to not increment the last digit by the inderface number
+CFLAGS += -DCONFIG_GNRC_IPV6_STATIC_LLADDR_IS_FIXED=1
+CFLAGS += -DCONFIG_GNRC_NETIF_IPV6_ADDRS_NUMOF=4

CFLAGS+=-DCONFIG_GCOAP_PDU_BUF_SIZE=512 USEMODULE+=gcoap_forward_proxy make flash term PORT=tap1

2024-05-29 20:38:18,501 # ifconfig
2024-05-29 20:38:18,502 # Iface  6  HWaddr: A2:AE:A2:2F:AF:16 
2024-05-29 20:38:18,502 #           L2-PDU:1500  MTU:1500  HL:64  Source address length: 6
2024-05-29 20:38:18,502 #           Link type: wired
2024-05-29 20:38:18,503 #           inet6 addr: fe80::cafe:cafe:cafe:1  scope: link  VAL
2024-05-29 20:38:18,503 #           inet6 addr: fe80::a0ae:a2ff:fe2f:af16  scope: link  VAL
2024-05-29 20:38:18,503 #           inet6 group: ff02::1
2024-05-29 20:38:18,503 #           inet6 group: ff02::1:fffe:1
2024-05-29 20:38:18,503 #           inet6 group: ff02::1:ff2f:af16
2024-05-29 20:38:18,503 # 

Server:

Run a libcoap server on the tapbr0 interface:
coap-server-openssl -A fe80::f0da:e9ff:fe74:594a%tapbr0

Client:

Set the proxy:
coap proxy set coap://[fe80::a0ae:a2ff:fe2f:af16]:5683

Send a request to the server:
coap get coap://[fe80::f0da:e9ff:fe74:594a]:5683/.well-known/core

As you can see in the wireshark picture, the proxy uses different address toward the client and towards the server.

The same procedure does not work in master right now.

Issues/PRs references

This is the next split out of #20696.