Messages in this thread | | | From | Eric Dumazet <> | Date | Mon, 18 Sep 2023 10:05:09 +0200 | Subject | Re: [PATCH v1] tcp: enhancing timestamps random algo to address issues arising from NAT mapping |
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On Mon, Sep 18, 2023 at 3:46 AM George Guo <guodongtai@kylinos.cn> wrote: > > Tsval=tsoffset+local_clock, here tsoffset is randomized with saddr and daddr parameters in func > secure_tcp_ts_off. Most of time it is OK except for NAT mapping to the same port and daddr. > Consider the following scenario: > ns1: ns2: > +-----------+ +-----------+ > | | | | > | | | | > | | | | > | veth1 | | vethb | > |192.168.1.1| |192.168.1.2| > +----+------+ +-----+-----+ > | | > | | > | br0:192.168.1.254 | > +----------+----------+ > veth0 | vetha > 192.168.1.3 | 192.168.1.4 > | > nat(192.168.1.x -->172.30.60.199) > | > V > eth0 > 172.30.60.199 > | > | > +----> ... ... ---->server: 172.30.60.191 > > Let's say ns1 (192.168.1.1) generates a timestamp ts1, and ns2 (192.168.1.2) generates a timestamp > ts2, with ts1 > ts2. > > If ns1 initiates a connection to a server, and then the server actively closes the connection, > entering the TIME_WAIT state, and ns2 attempts to connect to the server while port reuse is in > progress, due to the presence of NAT, the server sees both connections as originating from the > same IP address (e.g., 172.30.60.199) and port. However, since ts2 is smaller than ts1, the server > will respond with the acknowledgment (ACK) for the fourth handshake. > > SERVER CLIENT > > 1. ESTABLISHED ESTABLISHED > > (Close) > 2. FIN-WAIT-1 --> <SEQ=100><ACK=300><TSval=20><CTL=FIN,ACK> --> CLOSE-WAIT > > 3. FIN-WAIT-2 <-- <SEQ=300><ACK=101><TSval=40><CTL=ACK> <-- CLOSE-WAIT > > (Close) > 4. TIME-WAIT <-- <SEQ=300><ACK=101><TSval=41><CTL=FIN,ACK> <-- LAST-ACK > > 5. TIME-WAIT --> <SEQ=101><ACK=301><TSval=25><CTL=ACK> --> CLOSED > > - - - - - - - - - - - - - port reused - - - - - - - - - - - - - - - > > 5.1. TIME-WAIT <-- <SEQ=255><TSval=30><CTL=SYN> <-- SYN-SENT > > 5.2. TIME-WAIT --> <SEQ=101><ACK=301><TSval=35><CTL=ACK> --> SYN-SENT > > 5.3. CLOSED <-- <SEQ=301><CTL=RST> <-- SYN-SENT > > 6. SYN-RECV <-- <SEQ=255><TSval=34><CTL=SYN> <-- SYN-SENT > > 7. SYN-RECV --> <SEQ=400><ACK=301><TSval=40><CTL=SYN,ACK> --> ESTABLISHED > > 1. ESTABLISH <-- <SEQ=301><ACK=401><TSval=55><CTL=ACK> <-- ESTABLISHED > > This enhancement uses sport and daddr rather than saddr and daddr, which keep the timestamp > monotonically increasing in the situation described above. Then the port reuse is like this: > > SERVER CLIENT > > 1. ESTABLISHED ESTABLISHED > > (Close) > 2. FIN-WAIT-1 --> <SEQ=100><ACK=300><TSval=20><CTL=FIN,ACK> --> CLOSE-WAIT > > 3. FIN-WAIT-2 <-- <SEQ=300><ACK=101><TSval=40><CTL=ACK> <-- CLOSE-WAIT > > (Close) > 4. TIME-WAIT <-- <SEQ=300><ACK=101><TSval=41><CTL=FIN,ACK> <-- LAST-ACK > > 5. TIME-WAIT --> <SEQ=101><ACK=301><TSval=25><CTL=ACK> --> CLOSED > > - - - - - - - - - - - - - port reused - - - - - - - - - - - - - - - > > 5.1. TIME-WAIT <-- <SEQ=300><TSval=50><CTL=SYN> <-- SYN-SENT > > 6. SYN-RECV --> <SEQ=400><ACK=301><TSval=40><CTL=SYN,ACK> --> ESTABLISHED > > 1. ESTABLISH <-- <SEQ=301><ACK=401><TSval=55><CTL=ACK> <-- ESTABLISHED > > The enhancement lets port reused more efficiently. > > Signed-off-by: George Guo <guodongtai@kylinos.cn> >
CC Florian
I do not think we can 'fix' tcp timestamp vs NAT. Unless the NAT device makes sure a port is dedicated for a peer, and/or the NAT rewrites TS values (which would be bad).
I personally prefer seeing the same timestamps from A to B regardless of ports, it helps detect various issues.
Also, you seem to forget IPv6.
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