envoy udp: avoid keeping UDP sockets unconnected

Signed-off-by: Xie Zhihao [email protected]

Commit Message: udp: avoid keeping UDP sockets unconnected Additional Description:

The patch fixes keeping UDP sockets unconnected and avoids exhausting UDP ports according to the issue.

# Previous
UNCONN  0       0                     0.0.0.0:10000          0.0.0.0:*       users:(("envoy-static",pid=875931,fd=17))
ESTAB   0       0                   127.0.0.1:52082        127.0.0.1:10000   users:(("nc",pid=875966,fd=3))
UNCONN  0       0                     0.0.0.0:56624          0.0.0.0:*       users:(("envoy-static",pid=875931,fd=16))
ESTAB   0       0                   127.0.0.1:5005         127.0.0.1:56624   users:(("nc",pid=875954,fd=3))

# Current
UNCONN  0       0                     0.0.0.0:10000          0.0.0.0:*       users:(("envoy-static",pid=884658,fd=17))
ESTAB   0       0                   127.0.0.1:47803        127.0.0.1:10000   users:(("nc",pid=884693,fd=3))
ESTAB   0       0                   127.0.0.1:53490        127.0.0.1:5005    users:(("envoy-static",pid=884658,fd=16))
ESTAB   0       0                   127.0.0.1:5005         127.0.0.1:53490   users:(("nc",pid=884681,fd=3))

Since use_original_src_ip is conflict with connect. When use_original_src_ip is set, connect will not take place.

Risk Level: Low Testing: manual (have no idea to test with CI) Docs Changes: N/A Release Notes: Added Platform Specific Features: N/A Runtime guard: envoy.reloadable_features.udp_proxy_connect Fixes #21712