Is there a way to detect a USB Ethernet Adapter even if no IP has been given, Once it connect to an iPhone/iPad

even if no IP has been given to the iPhone.

Given to the iPhone by what? A DHCP server?

If that’s what you’re concerned about then I think you’re missing a key point. DHCP is primarily relevant to IPv4 [1]. Assuming the link is up, the interface will have an IPv6 link-local address [1].

I recommend that you read through the posts referenced by Extra-ordinary Networking. They explain a lot of the backstory to this.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
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[1] Yeah, there’s DHCPv6, but it’s not the primary mechanism for IPv6 address assignment.

[2] It’s likely also get an IPv4 link-local address, per RFC 3927, but that’s not nearly as straightforward.

It’s better to reply as a reply, rather than in the comments; see Quinn’s Top Ten DevForums Tips for this and other titbits.

I notice that when a wired network adapter is connected, an additional network interface named enX appears

Yep.

there are other network interfaces also named enX affect my judgment.

Right. This boils down to how you distinguish between different interfaces types. I have info about that in Extra-ordinary Networking > Network Interface APIs > Network Interface Type.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
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Thank you for your reply. I tried blow method to get interface type.

extern uint32_t functionalTypeForInterfaceNamed(const char * name) {
    int fd = socket(AF_INET, SOCK_DGRAM, 0);
    if (fd < 0) { return IFRTYPE_FUNCTIONAL_UNKNOWN; }

    struct ifreq ifr = {};
    strlcpy(ifr.ifr_name, name, sizeof(ifr.ifr_name));

    bool success = ioctl(fd, SIOCGIFFUNCTIONALTYPE, &ifr) >= 0;

    int junk = close(fd);
    assert(junk == 0);

    if (!success) {
        return IFRTYPE_FUNCTIONAL_UNKNOWN;
    }

    return ifr.ifr_ifru.ifru_functional_type;
}

There are three default enX interfaces, the en0 is IFRTYPE_FUNCTIONAL_WIFI_INFRA, the en1,en2 are IFRTYPE_FUNCTIONAL_WIRED, but I haven't inserted any ethernet adapter on my iPad yet. When I insert an ethernet adapter, an additional network interface which is also IFRTYPE_FUNCTIONAL_WIRED appear. It definitely affect my judgement. How to distinguish between those default enX which are IFRTYPE_FUNCTIONAL_WIRED type and the real ethernet adapter which insert on my iPad?

How to distinguish between those … and the real ethernet adapter which insert on my iPad?

Unfortunately I can’t see a good way to do that on iOS [1].

I think you could make a reasonable case that those additional interfaces should not be returning IFRTYPE_FUNCTIONAL_WIRED, but instead something more indicative of their purpose (like IFRTYPE_FUNCTIONAL_INTCOPROC). If you’d like to see that change, I encourage you to file a bug along those lines. And if you do file a bug, please post your bug number, just for the record.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
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[1] On macOS you have a lot more options, including both System Configuration framework and I/O Kit. Neither of those are API on iOS or its child platforms.

I've noticed that when I plug a USB Ethernet adapter into my iPad, an “Ethernet” option appears in the Settings menu. How does the system detect that the adapter has been connected? Hope to receive your reply again.

How does the system detect that the adapter has been connected?

Settings is a built-in component of iOS and thus not limited to using just APIs.

Earlier I wrote:

On macOS you have a lot more options, including both System Configuration framework and I/O Kit.

If you were trying to solve this problem on macOS, it’d be easy to do with those APIs. However, those APIs are not available on iOS [1]. Moreover, I’m skeptical we’ll ever make them available on iOS [2], which is why my suggestion above wasn’t to file an ER for those APIs but rather a bug report against the functional type values.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
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[1] The frameworks might be available but the specific APIs you need not.

[2] Because of the privacy impact.