net: Add netif utility

This adds an utility header with two functions that will be needed for
PCP, `QueryDefaultGateway` and `GetLocalAddresses`.

Co-authored-by: Vasil Dimov <vd@FreeBSD.org>

3 files changed, 325 insertions, 0 deletions

diff --git a/src/util/CMakeLists.txt b/src/util/CMakeLists.txt
index 26c6271f9b..d65e634c57 100644
--- a/src/util/CMakeLists.txt
+++ b/src/util/CMakeLists.txt
@@ -15,6 +15,7 @@ add_library(bitcoin_util STATIC EXCLUDE_FROM_ALL
   fs_helpers.cpp
   hasher.cpp
   moneystr.cpp
+  netif.cpp
   rbf.cpp
   readwritefile.cpp
   serfloat.cpp
@@ -42,4 +43,5 @@ target_link_libraries(bitcoin_util
     bitcoin_clientversion
     bitcoin_crypto
     $<$<PLATFORM_ID:Windows>:ws2_32>
+    $<$<PLATFORM_ID:Windows>:iphlpapi>
 )
diff --git a/src/util/netif.cpp b/src/util/netif.cpp
new file mode 100644
index 0000000000..75d0cf00c5
--- /dev/null
+++ b/src/util/netif.cpp
@@ -0,0 +1,303 @@
+// Copyright (c) 2024 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or https://www.opensource.org/licenses/mit-license.php.
+
+#include <config/bitcoin-config.h> // IWYU pragma: keep
+
+#include <util/netif.h>
+
+#include <logging.h>
+#include <netbase.h>
+#include <util/check.h>
+#include <util/sock.h>
+#include <util/syserror.h>
+
+#if defined(__linux__)
+#include <linux/rtnetlink.h>
+#elif defined(__FreeBSD__)
+#include <osreldate.h>
+#if __FreeBSD_version >= 1400000
+// Workaround https://github.com/freebsd/freebsd-src/pull/1070.
+#define typeof __typeof
+#include <netlink/netlink.h>
+#include <netlink/netlink_route.h>
+#endif
+#elif defined(WIN32)
+#include <iphlpapi.h>
+#elif defined(__APPLE__)
+#include <net/route.h>
+#include <sys/sysctl.h>
+#endif
+
+namespace {
+
+// Linux and FreeBSD 14.0+. For FreeBSD 13.2 the code can be compiled but
+// running it requires loading a special kernel module, otherwise socket(AF_NETLINK,...)
+// will fail, so we skip that.
+#if defined(__linux__) || (defined(__FreeBSD__) && __FreeBSD_version >= 1400000)
+
+std::optional<CNetAddr> QueryDefaultGatewayImpl(sa_family_t family)
+{
+    // Create a netlink socket.
+    auto sock{CreateSock(AF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE)};
+    if (!sock) {
+        LogPrintLevel(BCLog::NET, BCLog::Level::Error, "socket(AF_NETLINK): %s\n", NetworkErrorString(errno));
+        return std::nullopt;
+    }
+
+    // Send request.
+    struct {
+        nlmsghdr hdr; ///< Request header.
+        rtmsg data; ///< Request data, a "route message".
+        nlattr dst_hdr; ///< One attribute, conveying the route destination address.
+        char dst_data[16]; ///< Route destination address. To query the default route we use 0.0.0.0/0 or [::]/0. For IPv4 the first 4 bytes are used.
+    } request{};
+
+    // Whether to use the first 4 or 16 bytes from request.dst_data.
+    const size_t dst_data_len = family == AF_INET ? 4 : 16;
+
+    request.hdr.nlmsg_type = RTM_GETROUTE;
+    request.hdr.nlmsg_flags = NLM_F_REQUEST;
+#ifdef __linux__
+    // Linux IPv4 / IPv6 - this must be present, otherwise no gateway is found
+    // FreeBSD IPv4 - does not matter, the gateway is found with or without this
+    // FreeBSD IPv6 - this must be absent, otherwise no gateway is found
+    request.hdr.nlmsg_flags |= NLM_F_DUMP;
+#endif
+    request.hdr.nlmsg_len = NLMSG_LENGTH(sizeof(rtmsg) + sizeof(nlattr) + dst_data_len);
+    request.hdr.nlmsg_seq = 0; // Sequence number, used to match which reply is to which request. Irrelevant for us because we send just one request.
+    request.data.rtm_family = family;
+    request.data.rtm_dst_len = 0; // Prefix length.
+#ifdef __FreeBSD__
+    // Linux IPv4 / IPv6 this must be absent, otherwise no gateway is found
+    // FreeBSD IPv4 - does not matter, the gateway is found with or without this
+    // FreeBSD IPv6 - this must be present, otherwise no gateway is found
+    request.data.rtm_flags = RTM_F_PREFIX;
+#endif
+    request.dst_hdr.nla_type = RTA_DST;
+    request.dst_hdr.nla_len = sizeof(nlattr) + dst_data_len;
+
+    if (sock->Send(&request, request.hdr.nlmsg_len, 0) != static_cast<ssize_t>(request.hdr.nlmsg_len)) {
+        LogPrintLevel(BCLog::NET, BCLog::Level::Error, "send() to netlink socket: %s\n", NetworkErrorString(errno));
+        return std::nullopt;
+    }
+
+    // Receive response.
+    char response[4096];
+    int64_t recv_result;
+    do {
+        recv_result = sock->Recv(response, sizeof(response), 0);
+    } while (recv_result < 0 && (errno == EINTR || errno == EAGAIN));
+    if (recv_result < 0) {
+        LogPrintLevel(BCLog::NET, BCLog::Level::Error, "recv() from netlink socket: %s\n", NetworkErrorString(errno));
+        return std::nullopt;
+    }
+
+    for (nlmsghdr* hdr = (nlmsghdr*)response; NLMSG_OK(hdr, recv_result); hdr = NLMSG_NEXT(hdr, recv_result)) {
+        rtmsg* r = (rtmsg*)NLMSG_DATA(hdr);
+        int remaining_len = RTM_PAYLOAD(hdr);
+
+        // Iterate over the attributes.
+        rtattr *rta_gateway = nullptr;
+        int scope_id = 0;
+        for (rtattr* attr = RTM_RTA(r); RTA_OK(attr, remaining_len); attr = RTA_NEXT(attr, remaining_len)) {
+            if (attr->rta_type == RTA_GATEWAY) {
+                rta_gateway = attr;
+            } else if (attr->rta_type == RTA_OIF && sizeof(int) == RTA_PAYLOAD(attr)) {
+                std::memcpy(&scope_id, RTA_DATA(attr), sizeof(scope_id));
+            }
+        }
+
+        // Found gateway?
+        if (rta_gateway != nullptr) {
+            if (family == AF_INET && sizeof(in_addr) == RTA_PAYLOAD(rta_gateway)) {
+                in_addr gw;
+                std::memcpy(&gw, RTA_DATA(rta_gateway), sizeof(gw));
+                return CNetAddr(gw);
+            } else if (family == AF_INET6 && sizeof(in6_addr) == RTA_PAYLOAD(rta_gateway)) {
+                in6_addr gw;
+                std::memcpy(&gw, RTA_DATA(rta_gateway), sizeof(gw));
+                return CNetAddr(gw, scope_id);
+            }
+        }
+    }
+
+    return std::nullopt;
+}
+
+#elif defined(WIN32)
+
+std::optional<CNetAddr> QueryDefaultGatewayImpl(sa_family_t family)
+{
+    NET_LUID interface_luid = {};
+    SOCKADDR_INET destination_address = {};
+    MIB_IPFORWARD_ROW2 best_route = {};
+    SOCKADDR_INET best_source_address = {};
+    DWORD best_if_idx = 0;
+    DWORD status = 0;
+
+    // Pass empty destination address of the requested type (:: or 0.0.0.0) to get interface of default route.
+    destination_address.si_family = family;
+    status = GetBestInterfaceEx((sockaddr*)&destination_address, &best_if_idx);
+    if (status != NO_ERROR) {
+        LogPrintLevel(BCLog::NET, BCLog::Level::Error, "Could not get best interface for default route: %s\n", NetworkErrorString(status));
+        return std::nullopt;
+    }
+
+    // Get best route to default gateway.
+    // Leave interface_luid at all-zeros to use interface index instead.
+    status = GetBestRoute2(&interface_luid, best_if_idx, nullptr, &destination_address, 0, &best_route, &best_source_address);
+    if (status != NO_ERROR) {
+        LogPrintLevel(BCLog::NET, BCLog::Level::Error, "Could not get best route for default route for interface index %d: %s\n",
+                best_if_idx, NetworkErrorString(status));
+        return std::nullopt;
+    }
+
+    Assume(best_route.NextHop.si_family == family);
+    if (family == AF_INET) {
+        return CNetAddr(best_route.NextHop.Ipv4.sin_addr);
+    } else if(family == AF_INET6) {
+        return CNetAddr(best_route.NextHop.Ipv6.sin6_addr, best_route.InterfaceIndex);
+    }
+    return std::nullopt;
+}
+
+#elif defined(__APPLE__)
+
+#define ROUNDUP32(a) \
+    ((a) > 0 ? (1 + (((a) - 1) | (sizeof(uint32_t) - 1))) : sizeof(uint32_t))
+
+std::optional<CNetAddr> FromSockAddr(const struct sockaddr* addr)
+{
+    // Check valid length. Note that sa_len is not part of POSIX, and exists on MacOS and some BSDs only, so we can't
+    // do this check in SetSockAddr.
+    if (!(addr->sa_family == AF_INET && addr->sa_len == sizeof(struct sockaddr_in)) &&
+        !(addr->sa_family == AF_INET6 && addr->sa_len == sizeof(struct sockaddr_in6))) {
+        return std::nullopt;
+    }
+
+    // Fill in a CService from the sockaddr, then drop the port part.
+    CService service;
+    if (service.SetSockAddr(addr)) {
+        return (CNetAddr)service;
+    }
+    return std::nullopt;
+}
+
+//! MacOS: Get default gateway from route table. See route(4) for the format.
+std::optional<CNetAddr> QueryDefaultGatewayImpl(sa_family_t family)
+{
+    // net.route.0.inet[6].flags.gateway
+    int mib[] = {CTL_NET, PF_ROUTE, 0, family, NET_RT_FLAGS, RTF_GATEWAY};
+    // The size of the available data is determined by calling sysctl() with oldp=nullptr. See sysctl(3).
+    size_t l = 0;
+    if (sysctl(/*name=*/mib, /*namelen=*/sizeof(mib) / sizeof(int), /*oldp=*/nullptr, /*oldlenp=*/&l, /*newp=*/nullptr, /*newlen=*/0) < 0) {
+        LogPrintLevel(BCLog::NET, BCLog::Level::Error, "Could not get sysctl length of routing table: %s\n", SysErrorString(errno));
+        return std::nullopt;
+    }
+    std::vector<std::byte> buf(l);
+    if (sysctl(/*name=*/mib, /*namelen=*/sizeof(mib) / sizeof(int), /*oldp=*/buf.data(), /*oldlenp=*/&l, /*newp=*/nullptr, /*newlen=*/0) < 0) {
+        LogPrintLevel(BCLog::NET, BCLog::Level::Error, "Could not get sysctl data of routing table: %s\n", SysErrorString(errno));
+        return std::nullopt;
+    }
+    // Iterate over messages (each message is a routing table entry).
+    for (size_t msg_pos = 0; msg_pos < buf.size(); ) {
+        if ((msg_pos + sizeof(rt_msghdr)) > buf.size()) return std::nullopt;
+        const struct rt_msghdr* rt = (const struct rt_msghdr*)(buf.data() + msg_pos);
+        const size_t next_msg_pos = msg_pos + rt->rtm_msglen;
+        if (rt->rtm_msglen < sizeof(rt_msghdr) || next_msg_pos > buf.size()) return std::nullopt;
+        // Iterate over addresses within message, get destination and gateway (if present).
+        // Address data starts after header.
+        size_t sa_pos = msg_pos + sizeof(struct rt_msghdr);
+        std::optional<CNetAddr> dst, gateway;
+        for (int i = 0; i < RTAX_MAX; i++) {
+            if (rt->rtm_addrs & (1 << i)) {
+                // 2 is just sa_len + sa_family, the theoretical minimum size of a socket address.
+                if ((sa_pos + 2) > next_msg_pos) return std::nullopt;
+                const struct sockaddr* sa = (const struct sockaddr*)(buf.data() + sa_pos);
+                if ((sa_pos + sa->sa_len) > next_msg_pos) return std::nullopt;
+                if (i == RTAX_DST) {
+                    dst = FromSockAddr(sa);
+                } else if (i == RTAX_GATEWAY) {
+                    gateway = FromSockAddr(sa);
+                }
+                // Skip sockaddr entries for bit flags we're not interested in,
+                // move cursor.
+                sa_pos += ROUNDUP32(sa->sa_len);
+            }
+        }
+        // Found default gateway?
+        if (dst && gateway && dst->IsBindAny()) { // Route to 0.0.0.0 or :: ?
+            return *gateway;
+        }
+        // Skip to next message.
+        msg_pos = next_msg_pos;
+    }
+    return std::nullopt;
+}
+
+#else
+
+// Dummy implementation.
+std::optional<CNetAddr> QueryDefaultGatewayImpl(sa_family_t)
+{
+    return std::nullopt;
+}
+
+#endif
+
+}
+
+std::optional<CNetAddr> QueryDefaultGateway(Network network)
+{
+    Assume(network == NET_IPV4 || network == NET_IPV6);
+
+    sa_family_t family;
+    if (network == NET_IPV4) {
+        family = AF_INET;
+    } else if(network == NET_IPV6) {
+        family = AF_INET6;
+    } else {
+        return std::nullopt;
+    }
+
+    std::optional<CNetAddr> ret = QueryDefaultGatewayImpl(family);
+
+    // It's possible for the default gateway to be 0.0.0.0 or ::0 on at least Windows
+    // for some routing strategies. If so, return as if no default gateway was found.
+    if (ret && !ret->IsBindAny()) {
+        return ret;
+    } else {
+        return std::nullopt;
+    }
+}
+
+std::vector<CNetAddr> GetLocalAddresses()
+{
+    std::vector<CNetAddr> addresses;
+#ifdef WIN32
+    char pszHostName[256] = "";
+    if (gethostname(pszHostName, sizeof(pszHostName)) != SOCKET_ERROR) {
+        addresses = LookupHost(pszHostName, 0, true);
+    }
+#elif (HAVE_DECL_GETIFADDRS && HAVE_DECL_FREEIFADDRS)
+    struct ifaddrs* myaddrs;
+    if (getifaddrs(&myaddrs) == 0) {
+        for (struct ifaddrs* ifa = myaddrs; ifa != nullptr; ifa = ifa->ifa_next)
+        {
+            if (ifa->ifa_addr == nullptr) continue;
+            if ((ifa->ifa_flags & IFF_UP) == 0) continue;
+            if ((ifa->ifa_flags & IFF_LOOPBACK) != 0) continue;
+            if (ifa->ifa_addr->sa_family == AF_INET) {
+                struct sockaddr_in* s4 = (struct sockaddr_in*)(ifa->ifa_addr);
+                addresses.emplace_back(s4->sin_addr);
+            } else if (ifa->ifa_addr->sa_family == AF_INET6) {
+                struct sockaddr_in6* s6 = (struct sockaddr_in6*)(ifa->ifa_addr);
+                addresses.emplace_back(s6->sin6_addr);
+            }
+        }
+        freeifaddrs(myaddrs);
+    }
+#endif
+    return addresses;
+}
diff --git a/src/util/netif.h b/src/util/netif.h
new file mode 100644
index 0000000000..d38d94a719
--- /dev/null
+++ b/src/util/netif.h
@@ -0,0 +1,20 @@
+// Copyright (c) 2024 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or https://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_UTIL_NETIF_H
+#define BITCOIN_UTIL_NETIF_H
+
+#include <netaddress.h>
+
+#include <optional>
+
+//! Query the OS for the default gateway for `network`. This only makes sense for NET_IPV4 and NET_IPV6.
+//! Returns std::nullopt if it cannot be found, or there is no support for this OS.
+std::optional<CNetAddr> QueryDefaultGateway(Network network);
+
+// TODO share with Discover()
+//! Return all local non-loopback IPv4 and IPv6 network addresses.
+std::vector<CNetAddr> GetLocalAddresses();
+
+#endif // BITCOIN_UTIL_NETIF_H