1.. SPDX-License-Identifier: GPL-2.0 2 3======================================================== 4TCP Authentication Option Linux implementation (RFC5925) 5======================================================== 6 7TCP Authentication Option (TCP-AO) provides a TCP extension aimed at verifying 8segments between trusted peers. It adds a new TCP header option with 9a Message Authentication Code (MAC). MACs are produced from the content 10of a TCP segment using a hashing function with a password known to both peers. 11The intent of TCP-AO is to deprecate TCP-MD5 providing better security, 12key rotation and support for variety of hashing algorithms. 13 141. Introduction 15=============== 16 17.. table:: Short and Limited Comparison of TCP-AO and TCP-MD5 18 19 +----------------------+------------------------+-----------------------+ 20 | | TCP-MD5 | TCP-AO | 21 +======================+========================+=======================+ 22 |Supported hashing |MD5 |Must support HMAC-SHA1 | 23 |algorithms |(cryptographically weak)|(chosen-prefix attacks)| 24 | | |and CMAC-AES-128 (only | 25 | | |side-channel attacks). | 26 | | |May support any hashing| 27 | | |algorithm. | 28 +----------------------+------------------------+-----------------------+ 29 |Length of MACs (bytes)|16 |Typically 12-16. | 30 | | |Other variants that fit| 31 | | |TCP header permitted. | 32 +----------------------+------------------------+-----------------------+ 33 |Number of keys per |1 |Many | 34 |TCP connection | | | 35 +----------------------+------------------------+-----------------------+ 36 |Possibility to change |Non-practical (both |Supported by protocol | 37 |an active key |peers have to change | | 38 | |them during MSL) | | 39 +----------------------+------------------------+-----------------------+ 40 |Protection against |No |Yes: ignoring them | 41 |ICMP 'hard errors' | |by default on | 42 | | |established connections| 43 +----------------------+------------------------+-----------------------+ 44 |Protection against |No |Yes: pseudo-header | 45 |traffic-crossing | |includes TCP ports. | 46 |attack | | | 47 +----------------------+------------------------+-----------------------+ 48 |Protection against |No |Sequence Number | 49 |replayed TCP segments | |Extension (SNE) and | 50 | | |Initial Sequence | 51 | | |Numbers (ISNs) | 52 +----------------------+------------------------+-----------------------+ 53 |Supports |Yes |No. ISNs+SNE are needed| 54 |Connectionless Resets | |to correctly sign RST. | 55 +----------------------+------------------------+-----------------------+ 56 |Standards |RFC 2385 |RFC 5925, RFC 5926 | 57 +----------------------+------------------------+-----------------------+ 58 59 601.1 Frequently Asked Questions (FAQ) with references to RFC 5925 61---------------------------------------------------------------- 62 63Q: Can either SendID or RecvID be non-unique for the same 4-tuple 64(srcaddr, srcport, dstaddr, dstport)? 65 66A: No [3.1]:: 67 68 >> The IDs of MKTs MUST NOT overlap where their TCP connection 69 identifiers overlap. 70 71Q: Can Master Key Tuple (MKT) for an active connection be removed? 72 73A: No, unless it's copied to Transport Control Block (TCB) [3.1]:: 74 75 It is presumed that an MKT affecting a particular connection cannot 76 be destroyed during an active connection -- or, equivalently, that 77 its parameters are copied to an area local to the connection (i.e., 78 instantiated) and so changes would affect only new connections. 79 80Q: If an old MKT needs to be deleted, how should it be done in order 81to not remove it for an active connection? (As it can be still in use 82at any moment later) 83 84A: Not specified by RFC 5925, seems to be a problem for key management 85to ensure that no one uses such MKT before trying to remove it. 86 87Q: Can an old MKT exist forever and be used by another peer? 88 89A: It can, it's a key management task to decide when to remove an old key [6.1]:: 90 91 Deciding when to start using a key is a performance issue. Deciding 92 when to remove an MKT is a security issue. Invalid MKTs are expected 93 to be removed. TCP-AO provides no mechanism to coordinate their removal, 94 as we consider this a key management operation. 95 96also [6.1]:: 97 98 The only way to avoid reuse of previously used MKTs is to remove the MKT 99 when it is no longer considered permitted. 100 101Linux TCP-AO will try its best to prevent you from removing a key that's 102being used, considering it a key management failure. But sine keeping 103an outdated key may become a security issue and as a peer may 104unintentionally prevent the removal of an old key by always setting 105it as RNextKeyID - a forced key removal mechanism is provided, where 106userspace has to supply KeyID to use instead of the one that's being removed 107and the kernel will atomically delete the old key, even if the peer is 108still requesting it. There are no guarantees for force-delete as the peer 109may yet not have the new key - the TCP connection may just break. 110Alternatively, one may choose to shut down the socket. 111 112Q: What happens when a packet is received on a new connection with no known 113MKT's RecvID? 114 115A: RFC 5925 specifies that by default it is accepted with a warning logged, but 116the behaviour can be configured by the user [7.5.1.a]:: 117 118 If the segment is a SYN, then this is the first segment of a new 119 connection. Find the matching MKT for this segment, using the segment's 120 socket pair and its TCP-AO KeyID, matched against the MKT's TCP connection 121 identifier and the MKT's RecvID. 122 123 i. If there is no matching MKT, remove TCP-AO from the segment. 124 Proceed with further TCP handling of the segment. 125 NOTE: this presumes that connections that do not match any MKT 126 should be silently accepted, as noted in Section 7.3. 127 128[7.3]:: 129 130 >> A TCP-AO implementation MUST allow for configuration of the behavior 131 of segments with TCP-AO but that do not match an MKT. The initial default 132 of this configuration SHOULD be to silently accept such connections. 133 If this is not the desired case, an MKT can be included to match such 134 connections, or the connection can indicate that TCP-AO is required. 135 Alternately, the configuration can be changed to discard segments with 136 the AO option not matching an MKT. 137 138[10.2.b]:: 139 140 Connections not matching any MKT do not require TCP-AO. Further, incoming 141 segments with TCP-AO are not discarded solely because they include 142 the option, provided they do not match any MKT. 143 144Note that Linux TCP-AO implementation differs in this aspect. Currently, TCP-AO 145segments with unknown key signatures are discarded with warnings logged. 146 147Q: Does the RFC imply centralized kernel key management in any way? 148(i.e. that a key on all connections MUST be rotated at the same time?) 149 150A: Not specified. MKTs can be managed in userspace, the only relevant part to 151key changes is [7.3]:: 152 153 >> All TCP segments MUST be checked against the set of MKTs for matching 154 TCP connection identifiers. 155 156Q: What happens when RNextKeyID requested by a peer is unknown? Should 157the connection be reset? 158 159A: It should not, no action needs to be performed [7.5.2.e]:: 160 161 ii. If they differ, determine whether the RNextKeyID MKT is ready. 162 163 1. If the MKT corresponding to the segment’s socket pair and RNextKeyID 164 is not available, no action is required (RNextKeyID of a received 165 segment needs to match the MKT’s SendID). 166 167Q: How current_key is set and when does it change? It is a user-triggered 168change, or is it by a request from the remote peer? Is it set by the user 169explicitly, or by a matching rule? 170 171A: current_key is set by RNextKeyID [6.1]:: 172 173 Rnext_key is changed only by manual user intervention or MKT management 174 protocol operation. It is not manipulated by TCP-AO. Current_key is updated 175 by TCP-AO when processing received TCP segments as discussed in the segment 176 processing description in Section 7.5. Note that the algorithm allows 177 the current_key to change to a new MKT, then change back to a previously 178 used MKT (known as "backing up"). This can occur during an MKT change when 179 segments are received out of order, and is considered a feature of TCP-AO, 180 because reordering does not result in drops. 181 182[7.5.2.e.ii]:: 183 184 2. If the matching MKT corresponding to the segment’s socket pair and 185 RNextKeyID is available: 186 187 a. Set current_key to the RNextKeyID MKT. 188 189Q: If both peers have multiple MKTs matching the connection's socket pair 190(with different KeyIDs), how should the sender/receiver pick KeyID to use? 191 192A: Some mechanism should pick the "desired" MKT [3.3]:: 193 194 Multiple MKTs may match a single outgoing segment, e.g., when MKTs 195 are being changed. Those MKTs cannot have conflicting IDs (as noted 196 elsewhere), and some mechanism must determine which MKT to use for each 197 given outgoing segment. 198 199 >> An outgoing TCP segment MUST match at most one desired MKT, indicated 200 by the segment’s socket pair. The segment MAY match multiple MKTs, provided 201 that exactly one MKT is indicated as desired. Other information in 202 the segment MAY be used to determine the desired MKT when multiple MKTs 203 match; such information MUST NOT include values in any TCP option fields. 204 205Q: Can TCP-MD5 connection migrate to TCP-AO (and vice-versa): 206 207A: No [1]:: 208 209 TCP MD5-protected connections cannot be migrated to TCP-AO because TCP MD5 210 does not support any changes to a connection’s security algorithm 211 once established. 212 213Q: If all MKTs are removed on a connection, can it become a non-TCP-AO signed 214connection? 215 216A: [7.5.2] doesn't have the same choice as SYN packet handling in [7.5.1.i] 217that would allow accepting segments without a sign (which would be insecure). 218While switching to non-TCP-AO connection is not prohibited directly, it seems 219what the RFC means. Also, there's a requirement for TCP-AO connections to 220always have one current_key [3.3]:: 221 222 TCP-AO requires that every protected TCP segment match exactly one MKT. 223 224[3.3]:: 225 226 >> An incoming TCP segment including TCP-AO MUST match exactly one MKT, 227 indicated solely by the segment’s socket pair and its TCP-AO KeyID. 228 229[4.4]:: 230 231 One or more MKTs. These are the MKTs that match this connection’s 232 socket pair. 233 234Q: Can a non-TCP-AO connection become a TCP-AO-enabled one? 235 236A: No: for already established non-TCP-AO connection it would be impossible 237to switch using TCP-AO as the traffic key generation requires the initial 238sequence numbers. Paraphrasing, starting using TCP-AO would require 239re-establishing the TCP connection. 240 2412. In-kernel MKTs database vs database in userspace 242=================================================== 243 244Linux TCP-AO support is implemented using ``setsockopt()s``, in a similar way 245to TCP-MD5. It means that a userspace application that wants to use TCP-AO 246should perform ``setsockopt()`` on a TCP socket when it wants to add, 247remove or rotate MKTs. This approach moves the key management responsibility 248to userspace as well as decisions on corner cases, i.e. what to do if 249the peer doesn't respect RNextKeyID; moving more code to userspace, especially 250responsible for the policy decisions. Besides, it's flexible and scales well 251(with less locking needed than in the case of an in-kernel database). One also 252should keep in mind that mainly intended users are BGP processes, not any 253random applications, which means that compared to IPsec tunnels, 254no transparency is really needed and modern BGP daemons already have 255``setsockopt()s`` for TCP-MD5 support. 256 257.. table:: Considered pros and cons of the approaches 258 259 +----------------------+------------------------+-----------------------+ 260 | | ``setsockopt()`` | in-kernel DB | 261 +======================+========================+=======================+ 262 | Extendability | ``setsockopt()`` | Netlink messages are | 263 | | commands should be | simple and extendable | 264 | | extendable syscalls | | 265 +----------------------+------------------------+-----------------------+ 266 | Required userspace | BGP or any application | could be transparent | 267 | changes | that wants TCP-AO needs| as tunnels, providing | 268 | | to perform | something like | 269 | | ``setsockopt()s`` | ``ip tcpao add key`` | 270 | | and do key management | (delete/show/rotate) | 271 +----------------------+------------------------+-----------------------+ 272 |MKTs removal or adding| harder for userspace | harder for kernel | 273 +----------------------+------------------------+-----------------------+ 274 | Dump-ability | ``getsockopt()`` | Netlink .dump() | 275 | | | callback | 276 +----------------------+------------------------+-----------------------+ 277 | Limits on kernel | equal | 278 | resources/memory | | 279 +----------------------+------------------------+-----------------------+ 280 | Scalability | contention on | contention on | 281 | | ``TCP_LISTEN`` sockets | the whole database | 282 +----------------------+------------------------+-----------------------+ 283 | Monitoring & warnings| ``TCP_DIAG`` | same Netlink socket | 284 +----------------------+------------------------+-----------------------+ 285 | Matching of MKTs | half-problem: only | hard | 286 | | listen sockets | | 287 +----------------------+------------------------+-----------------------+ 288 289 2903. uAPI 291======= 292 293Linux provides a set of ``setsockopt()s`` and ``getsockopt()s`` that let 294userspace manage TCP-AO on a per-socket basis. In order to add/delete MKTs 295``TCP_AO_ADD_KEY`` and ``TCP_AO_DEL_KEY`` TCP socket options must be used 296It is not allowed to add a key on an established non-TCP-AO connection 297as well as to remove the last key from TCP-AO connection. 298 299``setsockopt(TCP_AO_DEL_KEY)`` command may specify ``tcp_ao_del::current_key`` 300+ ``tcp_ao_del::set_current`` and/or ``tcp_ao_del::rnext`` 301+ ``tcp_ao_del::set_rnext`` which makes such delete "forced": it 302provides userspace a way to delete a key that's being used and atomically set 303another one instead. This is not intended for normal use and should be used 304only when the peer ignores RNextKeyID and keeps requesting/using an old key. 305It provides a way to force-delete a key that's not trusted but may break 306the TCP-AO connection. 307 308The usual/normal key-rotation can be performed with ``setsockopt(TCP_AO_INFO)``. 309It also provides a uAPI to change per-socket TCP-AO settings, such as 310ignoring ICMPs, as well as clear per-socket TCP-AO packet counters. 311The corresponding ``getsockopt(TCP_AO_INFO)`` can be used to get those 312per-socket TCP-AO settings. 313 314Another useful command is ``getsockopt(TCP_AO_GET_KEYS)``. One can use it 315to list all MKTs on a TCP socket or use a filter to get keys for a specific 316peer and/or sndid/rcvid, VRF L3 interface or get current_key/rnext_key. 317 318To repair TCP-AO connections ``setsockopt(TCP_AO_REPAIR)`` is available, 319provided that the user previously has checkpointed/dumped the socket with 320``getsockopt(TCP_AO_REPAIR)``. 321 322A tip here for scaled TCP_LISTEN sockets, that may have some thousands TCP-AO 323keys, is: use filters in ``getsockopt(TCP_AO_GET_KEYS)`` and asynchronous 324delete with ``setsockopt(TCP_AO_DEL_KEY)``. 325 326Linux TCP-AO also provides a bunch of segment counters that can be helpful 327with troubleshooting/debugging issues. Every MKT has good/bad counters 328that reflect how many packets passed/failed verification. 329Each TCP-AO socket has the following counters: 330- for good segments (properly signed) 331- for bad segments (failed TCP-AO verification) 332- for segments with unknown keys 333- for segments where an AO signature was expected, but wasn't found 334- for the number of ignored ICMPs 335 336TCP-AO per-socket counters are also duplicated with per-netns counters, 337exposed with SNMP. Those are ``TCPAOGood``, ``TCPAOBad``, ``TCPAOKeyNotFound``, 338``TCPAORequired`` and ``TCPAODroppedIcmps``. 339 340RFC 5925 very permissively specifies how TCP port matching can be done for 341MKTs:: 342 343 TCP connection identifier. A TCP socket pair, i.e., a local IP 344 address, a remote IP address, a TCP local port, and a TCP remote port. 345 Values can be partially specified using ranges (e.g., 2-30), masks 346 (e.g., 0xF0), wildcards (e.g., "*"), or any other suitable indication. 347 348Currently Linux TCP-AO implementation doesn't provide any TCP port matching. 349Probably, port ranges are the most flexible for uAPI, but so far 350not implemented. 351 3524. ``setsockopt()`` vs ``accept()`` race 353======================================== 354 355In contrast with TCP-MD5 established connection which has just one key, 356TCP-AO connections may have many keys, which means that accepted connections 357on a listen socket may have any amount of keys as well. As copying all those 358keys on a first properly signed SYN would make the request socket bigger, that 359would be undesirable. Currently, the implementation doesn't copy keys 360to request sockets, but rather look them up on the "parent" listener socket. 361 362The result is that when userspace removes TCP-AO keys, that may break 363not-yet-established connections on request sockets as well as not removing 364keys from sockets that were already established, but not yet ``accept()``'ed, 365hanging in the accept queue. 366 367The reverse is valid as well: if userspace adds a new key for a peer on 368a listener socket, the established sockets in accept queue won't 369have the new keys. 370 371At this moment, the resolution for the two races: 372``setsockopt(TCP_AO_ADD_KEY)`` vs ``accept()`` 373and ``setsockopt(TCP_AO_DEL_KEY)`` vs ``accept()`` is delegated to userspace. 374This means that it's expected that userspace would check the MKTs on the socket 375that was returned by ``accept()`` to verify that any key rotation that 376happened on listen socket is reflected on the newly established connection. 377 378This is a similar "do-nothing" approach to TCP-MD5 from the kernel side and 379may be changed later by introducing new flags to ``tcp_ao_add`` 380and ``tcp_ao_del``. 381 382Note that this race is rare for it needs TCP-AO key rotation to happen 383during the 3-way handshake for the new TCP connection. 384 3855. Interaction with TCP-MD5 386=========================== 387 388A TCP connection can not migrate between TCP-AO and TCP-MD5 options. The 389established sockets that have either AO or MD5 keys are restricted for 390adding keys of the other option. 391 392For listening sockets the picture is different: BGP server may want to receive 393both TCP-AO and (deprecated) TCP-MD5 clients. As a result, both types of keys 394may be added to TCP_CLOSED or TCP_LISTEN sockets. It's not allowed to add 395different types of keys for the same peer. 396 3976. SNE Linux implementation 398=========================== 399 400RFC 5925 [6.2] describes the algorithm of how to extend TCP sequence numbers 401with SNE. In short: TCP has to track the previous sequence numbers and set 402sne_flag when the current SEQ number rolls over. The flag is cleared when 403both current and previous SEQ numbers cross 0x7fff, which is 32Kb. 404 405In times when sne_flag is set, the algorithm compares SEQ for each packet with 4060x7fff and if it's higher than 32Kb, it assumes that the packet should be 407verified with SNE before the increment. As a result, there's 408this [0; 32Kb] window, when packets with (SNE - 1) can be accepted. 409 410Linux implementation simplifies this a bit: as the network stack already tracks 411the first SEQ byte that ACK is wanted for (snd_una) and the next SEQ byte that 412is wanted (rcv_nxt) - that's enough information for a rough estimation 413on where in the 4GB SEQ number space both sender and receiver are. 414When they roll over to zero, the corresponding SNE gets incremented. 415 416tcp_ao_compute_sne() is called for each TCP-AO segment. It compares SEQ numbers 417from the segment with snd_una or rcv_nxt and fits the result into a 2GB window around them, 418detecting SEQ numbers rolling over. That simplifies the code a lot and only 419requires SNE numbers to be stored on every TCP-AO socket. 420 421The 2GB window at first glance seems much more permissive compared to 422RFC 5926. But that is only used to pick the correct SNE before/after 423a rollover. It allows more TCP segment replays, but yet all regular 424TCP checks in tcp_sequence() are applied on the verified segment. 425So, it trades a bit more permissive acceptance of replayed/retransmitted 426segments for the simplicity of the algorithm and what seems better behaviour 427for large TCP windows. 428 4297. Links 430======== 431 432RFC 5925 The TCP Authentication Option 433 https://www.rfc-editor.org/rfc/pdfrfc/rfc5925.txt.pdf 434 435RFC 5926 Cryptographic Algorithms for the TCP Authentication Option (TCP-AO) 436 https://www.rfc-editor.org/rfc/pdfrfc/rfc5926.txt.pdf 437 438Draft "SHA-2 Algorithm for the TCP Authentication Option (TCP-AO)" 439 https://datatracker.ietf.org/doc/html/draft-nayak-tcp-sha2-03 440 441RFC 2385 Protection of BGP Sessions via the TCP MD5 Signature Option 442 https://www.rfc-editor.org/rfc/pdfrfc/rfc2385.txt.pdf 443 444:Author: Dmitry Safonov <dima@arista.com> 445