When the user specifies an IP address as the interface (e.g.
interface=10.1.1.1), GOST resolved it to its hosting interface and
called SO_BINDTODEVICE on it — breaking outbound connectivity for
IP aliases on loopback or dummy interfaces (go-gost/gost#785).
The initial fix (caa82f2) only skipped SO_BINDTODEVICE for loopback
interfaces, missing non-loopback dummy interfaces like ip-aliases.
This change adds an isIP return value to ParseInterfaceAddr so callers
can distinguish 'user specified an IP' (intent: source IP binding for
policy routing — skip SO_BINDTODEVICE) from 'user specified an
interface name' (intent: force traffic through that NIC — keep
SO_BINDTODEVICE). The dialOnce method now only calls bindDevice when
bindToDevice is true AND the input was an interface name (!isIP).
The loopback-specific guard in bindDevice is reverted — superseded by
the broader isIP-based fix that covers all interface types.
When using IP aliases on loopback (e.g. ip addr add 10.1.1.1/32 dev lo)
with policy routing, findInterfaceByIP() resolves the alias IP to the
loopback interface. Binding the socket to lo via SO_BINDTODEVICE prevents
outbound traffic from reaching non-local destinations.
Skip SO_BINDTODEVICE when the resolved interface is loopback — source IP
binding via LocalAddr is sufficient for policy routing to steer traffic
through the correct physical interface.
ginuerzh