Mar 25, 2020 · They represent the TCP flags, indeed. RFC 793, 3.1: Control Bits: 6 bits (from left to right): URG: Urgent Pointer field significant; ACK: Acknowledgment field significant; PSH: Push Function; RST: Reset the connection; SYN: Synchronize sequence numbers; FIN: No more data from sender; And additions: NS: ECN-nonce - concealment protection. RFC 3540

-p tcp --tcp-flags SYN,ACK,FIN,RST SYN -j DROP means "look at the flags syn, ack, fin and rst and match the packets that have the flag SYN set, and all the other unset" The first argument of tcp-flags is the flags your are considering, the second argument is the mask you want to match. if you were using--tcp-flags SYN,ACK SYN

Dec 6, 2019 · tcp flags & (fin | syn) != 0 which will match whenever FIN or SYN are set. Actually nftables simplifies it and only this is displayed or required: tcp flags fin,syn So taking both adjustments in account (ct state new must still be removed), the rule becomes: nft add rule filter FORWARD 'tcp dport 22 tcp flags fin,syn log prefix " my_FIN " group ...

Dec 31, 2010 · This may or may not be what you (or future readers) intended. For example, that syntax will also capture TCP SYN-ACK packets, TCP FIN-ACK, etc. If you want only TCP SYN or TCP ACK packets (i.e. JUST one of those flags set), the proper capture filter syntax is: 'tcp[tcpflags] == tcp-syn or tcp[tcpflags] == tcp-ack' Equivalently:

Mar 31, 2023 · One possible solution is to use --syn:. iptables -A INPUT -p tcp --syn -j TCPMSS --set-mss 1300 Alternatively, you could try upgrading your version of iptables to a newer release that supports the --tcp-flags option:

Mar 4, 2017 · Check out the man iptables-extensions command on --tcp-flags which is used when the TCP protocol is used: -p tcp. [!] --tcp-flags mask comp Match when the TCP flags are as specified. The first argument mask is the flags which we should examine, written as a comma- separated list, and the second argument comp is a comma-sepa‐ rated list of ...

Aug 14, 2024 · add rule filter output tcp flags & (syn | ack) == syn | ack add rule filter output tcp flags & (fin | syn) != 0 I did read @A.B answer here: logging TCP flags. This only explains the purpose of those constructs (matching TCP flags), but I'd like to know the syntax and how the evaluation of the expressions together with operators work.

Feb 23, 2017 · If reply traffic such as TCP:A, TCP:SA, or TCP:RA is shown as blocked in the logs, the problem could be asymmetric routing. See Asymmetric Routing and Firewall Rules for more info. I do not understand how this can be "Asymmetric Routing" as the OPNsense box only has 1 WAN and 1 LAN and 0 VLAN.

Nov 25, 2010 · Some (older) system can behave badly when sent weird TCP flags In iptables you have to do the filtering of such packets by hand (as far as I know), other firewalls such as PF has "scrubbing" modules. Scrubbing is a set of standard rules which tries to remove invalid packets from the stream, ensures all packets have the same TTL etc.

Dec 18, 2017 · I tested it with tcpdump and every 10-15 minutes it's a cascade of connections from those computers to Adobe's servers. I tried to stop AGSServices and all TCP-A, TCP-RA and TCP-S entries in the firewall log stop. At the end of the analisys I guess I can consider those logs not harmful but very annoying so I decided to stop Adobe service.