Merge #15718: docs: Improve netaddress comments

303372c41a8d5c58a46cf9ed595e30e67bd0bc99 docs: Improve netaddress comments (Carl Dong)

Pull request description:

  Improves comments for `netaddress`, making them available to Doxygen.

  I think this is worthwhile because a lot of the code require some context (e.g., A lot of the things that we do to fit hostnames and tor addresses into `CNetAddr` is non-obvious, and documenting it is beneficial).

ACKs for commit 303372:

Tree-SHA512: 2a35784a01ed8ec5fdbe111a540192d31bde16afa96e4be97b0385daf290fc7469a66d7cb8905a70b920fad6a0e7400ca4e5da082d6e4af1d1aaccc0e8297720

2 files changed, 119 insertions, 10 deletions

diff --git a/src/netaddress.cpp b/src/netaddress.cpp
index 58e45c2c02..6ee2d8a4b3 100644
--- a/src/netaddress.cpp
+++ b/src/netaddress.cpp
@@ -14,6 +14,11 @@ static const unsigned char pchOnionCat[] = {0xFD,0x87,0xD8,0x7E,0xEB,0x43};
 // 0xFD + sha256("bitcoin")[0:5]
 static const unsigned char g_internal_prefix[] = { 0xFD, 0x6B, 0x88, 0xC0, 0x87, 0x24 };
 
+/**
+ * Construct an unspecified IPv6 network address (::/128).
+ *
+ * @note This address is considered invalid by CNetAddr::IsValid()
+ */
 CNetAddr::CNetAddr()
 {
     memset(ip, 0, sizeof(ip));
@@ -40,6 +45,20 @@ void CNetAddr::SetRaw(Network network, const uint8_t *ip_in)
     }
 }
 
+/**
+ * Try to make this a dummy address that maps the specified name into IPv6 like
+ * so: (0xFD + %sha256("bitcoin")[0:5]) + %sha256(name)[0:10]. Such dummy
+ * addresses have a prefix of fd6b:88c0:8724::/48 and are guaranteed to not be
+ * publicly routable as it falls under RFC4193's fc00::/7 subnet allocated to
+ * unique-local addresses.
+ *
+ * CAddrMan uses these fake addresses to keep track of which DNS seeds were
+ * used.
+ *
+ * @returns Whether or not the operation was successful.
+ *
+ * @see CNetAddr::IsInternal(), CNetAddr::IsRFC4193()
+ */
 bool CNetAddr::SetInternal(const std::string &name)
 {
     if (name.empty()) {
@@ -52,6 +71,16 @@ bool CNetAddr::SetInternal(const std::string &name)
     return true;
 }
 
+/**
+ * Try to make this a dummy address that maps the specified onion address into
+ * IPv6 using OnionCat's range and encoding. Such dummy addresses have a prefix
+ * of fd87:d87e:eb43::/48 and are guaranteed to not be publicly routable as they
+ * fall under RFC4193's fc00::/7 subnet allocated to unique-local addresses.
+ *
+ * @returns Whether or not the operation was successful.
+ *
+ * @see CNetAddr::IsTor(), CNetAddr::IsRFC4193()
+ */
 bool CNetAddr::SetSpecial(const std::string &strName)
 {
     if (strName.size()>6 && strName.substr(strName.size() - 6, 6) == ".onion") {
@@ -175,6 +204,12 @@ bool CNetAddr::IsRFC4843() const
     return (GetByte(15) == 0x20 && GetByte(14) == 0x01 && GetByte(13) == 0x00 && (GetByte(12) & 0xF0) == 0x10);
 }
 
+/**
+ * @returns Whether or not this is a dummy address that maps an onion address
+ *          into IPv6.
+ *
+ * @see CNetAddr::SetSpecial(const std::string &)
+ */
 bool CNetAddr::IsTor() const
 {
     return (memcmp(ip, pchOnionCat, sizeof(pchOnionCat)) == 0);
@@ -194,6 +229,16 @@ bool CNetAddr::IsLocal() const
     return false;
 }
 
+/**
+ * @returns Whether or not this network address is a valid address that @a could
+ *          be used to refer to an actual host.
+ *
+ * @note A valid address may or may not be publicly routable on the global
+ *       internet. As in, the set of valid addreses is a superset of the set of
+ *       publicly routable addresses.
+ *
+ * @see CNetAddr::IsRoutable()
+ */
 bool CNetAddr::IsValid() const
 {
     // Cleanup 3-byte shifted addresses caused by garbage in size field
@@ -233,11 +278,25 @@ bool CNetAddr::IsValid() const
     return true;
 }
 
+/**
+ * @returns Whether or not this network address is publicly routable on the
+ *          global internet.
+ *
+ * @note A routable address is always valid. As in, the set of routable addreses
+ *       is a subset of the set of valid addresses.
+ *
+ * @see CNetAddr::IsValid()
+ */
 bool CNetAddr::IsRoutable() const
 {
     return IsValid() && !(IsRFC1918() || IsRFC2544() || IsRFC3927() || IsRFC4862() || IsRFC6598() || IsRFC5737() || (IsRFC4193() && !IsTor()) || IsRFC4843() || IsLocal() || IsInternal());
 }
 
+/**
+ * @returns Whether or not this is a dummy address that maps a name into IPv6.
+ *
+ * @see CNetAddr::SetInternal(const std::string &)
+ */
 bool CNetAddr::IsInternal() const
 {
    return memcmp(ip, g_internal_prefix, sizeof(g_internal_prefix)) == 0;
@@ -299,6 +358,16 @@ bool operator<(const CNetAddr& a, const CNetAddr& b)
     return (memcmp(a.ip, b.ip, 16) < 0);
 }
 
+/**
+ * Try to get our IPv4 address.
+ *
+ * @param[out] pipv4Addr The in_addr struct to which to copy.
+ *
+ * @returns Whether or not the operation was successful, in particular, whether
+ *          or not our address was an IPv4 address.
+ *
+ * @see CNetAddr::IsIPv4()
+ */
 bool CNetAddr::GetInAddr(struct in_addr* pipv4Addr) const
 {
     if (!IsIPv4())
@@ -307,6 +376,16 @@ bool CNetAddr::GetInAddr(struct in_addr* pipv4Addr) const
     return true;
 }
 
+/**
+ * Try to get our IPv6 address.
+ *
+ * @param[out] pipv6Addr The in6_addr struct to which to copy.
+ *
+ * @returns Whether or not the operation was successful, in particular, whether
+ *          or not our address was an IPv6 address.
+ *
+ * @see CNetAddr::IsIPv6()
+ */
 bool CNetAddr::GetIn6Addr(struct in6_addr* pipv6Addr) const
 {
     if (!IsIPv6()) {
@@ -316,8 +395,16 @@ bool CNetAddr::GetIn6Addr(struct in6_addr* pipv6Addr) const
     return true;
 }
 
-// get canonical identifier of an address' group
-// no two connections will be attempted to addresses with the same group
+/**
+ * Get the canonical identifier of our network group
+ *
+ * The groups are assigned in a way where it should be costly for an attacker to
+ * obtain addresses with many different group identifiers, even if it is cheap
+ * to obtain addresses with the same identifier.
+ *
+ * @note No two connections will be attempted to addresses with the same network
+ *       group.
+ */
 std::vector<unsigned char> CNetAddr::GetGroup() const
 {
     std::vector<unsigned char> vchRet;
@@ -379,12 +466,15 @@ std::vector<unsigned char> CNetAddr::GetGroup() const
         nBits = 32;
 
     vchRet.push_back(nClass);
+
+    // push our ip onto vchRet byte by byte...
     while (nBits >= 8)
     {
         vchRet.push_back(GetByte(15 - nStartByte));
         nStartByte++;
         nBits -= 8;
     }
+    // ...for the last byte, push nBits and for the rest of the byte push 1's
     if (nBits > 0)
         vchRet.push_back(GetByte(15 - nStartByte) | ((1 << (8 - nBits)) - 1));
 
@@ -526,6 +616,18 @@ bool operator<(const CService& a, const CService& b)
     return static_cast<CNetAddr>(a) < static_cast<CNetAddr>(b) || (static_cast<CNetAddr>(a) == static_cast<CNetAddr>(b) && a.port < b.port);
 }
 
+/**
+ * Obtain the IPv4/6 socket address this represents.
+ *
+ * @param[out] paddr The obtained socket address.
+ * @param[in,out] addrlen The size, in bytes, of the address structure pointed
+ *                        to by paddr. The value that's pointed to by this
+ *                        parameter might change after calling this function if
+ *                        the size of the corresponding address structure
+ *                        changed.
+ *
+ * @returns Whether or not the operation was successful.
+ */
 bool CService::GetSockAddr(struct sockaddr* paddr, socklen_t *addrlen) const
 {
     if (IsIPv4()) {
@@ -556,13 +658,16 @@ bool CService::GetSockAddr(struct sockaddr* paddr, socklen_t *addrlen) const
     return false;
 }
 
+/**
+ * @returns An identifier unique to this service's address and port number.
+ */
 std::vector<unsigned char> CService::GetKey() const
 {
      std::vector<unsigned char> vKey;
      vKey.resize(18);
      memcpy(vKey.data(), ip, 16);
-     vKey[16] = port / 0x100;
-     vKey[17] = port & 0x0FF;
+     vKey[16] = port / 0x100; // most significant byte of our port
+     vKey[17] = port & 0x0FF; // least significant byte of our port
      return vKey;
 }
 
@@ -641,6 +746,10 @@ CSubNet::CSubNet(const CNetAddr &addr):
     network = addr;
 }
 
+/**
+ * @returns True if this subnet is valid, the specified address is valid, and
+ *          the specified address belongs in this subnet.
+ */
 bool CSubNet::Match(const CNetAddr &addr) const
 {
     if (!valid || !addr.IsValid())
@@ -651,6 +760,10 @@ bool CSubNet::Match(const CNetAddr &addr) const
     return true;
 }
 
+/**
+ * @returns The number of 1-bits in the prefix of the specified subnet mask. If
+ *          the specified subnet mask is not a valid one, -1.
+ */
 static inline int NetmaskBits(uint8_t x)
 {
     switch(x) {
diff --git a/src/netaddress.h b/src/netaddress.h
index ca435d17dc..8230e40606 100644
--- a/src/netaddress.h
+++ b/src/netaddress.h
@@ -48,10 +48,6 @@ class CNetAddr
         void SetRaw(Network network, const uint8_t *data);
 
     public:
-        /**
-          * Transform an arbitrary string into a non-routable ipv6 address.
-          * Useful for mapping resolved addresses back to their source.
-         */
         bool SetInternal(const std::string& name);
 
         bool SetSpecial(const std::string &strName); // for Tor addresses
@@ -69,8 +65,8 @@ class CNetAddr
         bool IsRFC4380() const; // IPv6 Teredo tunnelling (2001::/32)
         bool IsRFC4843() const; // IPv6 ORCHID (2001:10::/28)
         bool IsRFC4862() const; // IPv6 autoconfig (FE80::/64)
-        bool IsRFC6052() const; // IPv6 well-known prefix (64:FF9B::/96)
-        bool IsRFC6145() const; // IPv6 IPv4-translated address (::FFFF:0:0:0/96)
+        bool IsRFC6052() const; // IPv6 well-known prefix for IPv4-embedded address (64:FF9B::/96)
+        bool IsRFC6145() const; // IPv6 IPv4-translated address (::FFFF:0:0:0/96) (actually defined in RFC2765)
         bool IsTor() const;
         bool IsLocal() const;
         bool IsRoutable() const;