RouterOS 7.20.8 possible SYN flooding on tcp port 53

RouterOS Beginner Basics
1

2026-03-08 09:34:53 by RouterOS 7.20.8

software id =**********

model = RB5009UPr+S+

serial number = **********

/interface bridge
add admin-mac=::::: auto-mac=no comment=defconf name=bridge
/interface pppoe-client
add add-default-route=yes disabled=no interface=ether1 name=pppoe-out1 user=
************
/interface list
add comment=defconf name=WAN
add comment=defconf name=LAN
/interface lte apn
set [ find default=yes ] use-peer-dns=no
/ip pool
add name=dhcp ranges=172.17.38.102-172.17.243.51
/ip dhcp-server
add address-pool=dhcp interface=bridge name=defconf
/ipv6 dhcp-server
add comment="defconf: local LAN ULA IPv6 DHCP server Adapt to subordinate dist
_ribution of IPv6" interface=bridge name=IPv6_LAN_DHCP_Server
prefix-pool=IPv6_Prefix_Pool_From_ISP
/disk settings
set auto-media-interface=bridge auto-media-sharing=yes auto-smb-sharing=yes
/interface bridge port
add bridge=bridge comment=defconf interface=ether2
add bridge=bridge comment=defconf interface=ether3
add bridge=bridge comment=defconf interface=ether4
add bridge=bridge comment=defconf interface=ether5
add bridge=bridge comment=defconf interface=ether6
add bridge=bridge comment=defconf interface=ether7
add bridge=bridge comment=defconf interface=ether8
add bridge=bridge comment=defconf interface=sfp-sfpplus1
/ip neighbor discovery-settings
set discover-interface-list=LAN
/interface list member
add comment=defconf interface=bridge list=LAN
add comment=defconf interface=ether1 list=WAN
add interface=pppoe-out1 list=WAN
/ip address
add address=172.17.0.1/16 comment=defconf interface=bridge network=172.17.0.0
/ip dhcp-client
add comment=defconf disabled=yes interface=ether1 use-peer-dns=no
/ip dhcp-server network
add address=172.17.0.0/16 comment=defconf dns-server=172.17.0.1 gateway=
172.17.0.1 netmask=16
/ip dns
set allow-remote-requests=yes servers=
1.1.1.1,1.0.0.1,2606:4700:4700::1111,2606:4700:4700::1001
/ip dns static
add address=172.17.0.1 comment=defconf name=router.lan type=A
/ip firewall filter
add action=accept chain=input comment=
"defconf: accept established,related,untracked" connection-state=
established,related,untracked
add action=drop chain=input comment="defconf: drop invalid" connection-state=
invalid
add action=accept chain=input comment="defconf: accept ICMP" protocol=icmp
add action=accept chain=input comment=
"defconf: accept to local loopback (for CAPsMAN)" dst-address=127.0.0.1
add action=drop chain=input comment="defconf: drop all not coming from LAN"
in-interface-list=!LAN
add action=accept chain=forward comment="defconf: accept in ipsec policy"
ipsec-policy=in,ipsec
add action=accept chain=forward comment="defconf: accept out ipsec policy"
ipsec-policy=out,ipsec
add action=fasttrack-connection chain=forward comment="defconf: fasttrack"
connection-state=established,related hw-offload=yes
add action=accept chain=forward comment=
"defconf: accept established,related, untracked" connection-state=
established,related,untracked
add action=drop chain=forward comment="defconf: drop invalid"
connection-state=invalid
add action=drop chain=forward comment=
"defconf: drop all from WAN not DSTNATed" connection-nat-state=!dstnat
connection-state=new in-interface-list=WAN
/ip firewall mangle
add action=change-mss chain=forward comment=
"IPv4 Change MSS to PMTU, make HTTPS happy" new-mss=clamp-to-pmtu
protocol=tcp tcp-flags=syn
/ip firewall nat
add action=masquerade chain=srcnat comment="defconf: masquerade"
ipsec-policy=out,none out-interface-list=WAN
/ip upnp interfaces
add interface=bridge type=internal
add interface=pppoe-out1 type=external
/ipv6 address
add address=::1 comment="defconf: Assign LAN address From Pool" from-pool=
IPv6_Prefix_Pool_From_ISP interface=bridge
/ipv6 dhcp-client
add comment="defconf: Create DHCPv6 client on PPPoE interface" interface=
pppoe-out1 pool-name=IPv6_Prefix_Pool_From_ISP pool-prefix-length=56
prefix-hint=::/56 request=address,prefix use-peer-dns=no
/ipv6 firewall address-list
add address=::/128 comment="defconf: unspecified address" list=bad_ipv6
add address=::1/128 comment="defconf: lo" list=bad_ipv6
add address=fec0::/10 comment="defconf: site-local" list=bad_ipv6
add address=::ffff:0.0.0.0/96 comment="defconf: ipv4-mapped" list=bad_ipv6
add address=::/96 comment="defconf: ipv4 compat" list=bad_ipv6
add address=100::/64 comment="defconf: discard only " list=bad_ipv6
add address=2001:db8::/32 comment="defconf: documentation" list=bad_ipv6
add address=2001:10::/28 comment="defconf: ORCHID" list=bad_ipv6
add address=3ffe::/16 comment="defconf: 6bone" list=bad_ipv6
/ipv6 firewall filter
add action=accept chain=input comment=
"defconf: accept established,related,untracked" connection-state=
established,related,untracked
add action=drop chain=input comment="defconf: drop invalid" connection-state=
invalid
add action=accept chain=input comment="defconf: accept ICMPv6" protocol=
icmpv6
add action=accept chain=input comment="defconf: accept UDP traceroute"
dst-port=33434-33534 protocol=udp
add action=accept chain=input comment=
"defconf: accept DHCPv6-Client prefix delegation." dst-port=546 protocol=
udp src-address=fe80::/10
add action=accept chain=input comment="defconf: accept IKE" dst-port=500,4500
protocol=udp
add action=accept chain=input comment="defconf: accept ipsec AH" protocol=
ipsec-ah
add action=accept chain=input comment="defconf: accept ipsec ESP" protocol=
ipsec-esp
add action=accept chain=input comment=
"defconf: accept all that matches ipsec policy" ipsec-policy=in,ipsec
add action=drop chain=input comment=
"defconf: drop everything else not coming from LAN" in-interface-list=
!LAN
add action=fasttrack-connection chain=forward comment="defconf: fasttrack6"
connection-state=established,related
add action=accept chain=forward comment=
"defconf: accept established,related,untracked" connection-state=
established,related,untracked
add action=drop chain=forward comment="defconf: drop invalid"
connection-state=invalid
add action=drop chain=forward comment=
"defconf: drop packets with bad src ipv6" src-address-list=bad_ipv6
add action=drop chain=forward comment=
"defconf: drop packets with bad dst ipv6" dst-address-list=bad_ipv6
add action=drop chain=forward comment="defconf: rfc4890 drop hop-limit=1"
hop-limit=equal:1 protocol=icmpv6
add action=accept chain=forward comment="defconf: accept ICMPv6" protocol=
icmpv6
add action=accept chain=forward comment="defconf: accept HIP" protocol=139
add action=accept chain=forward comment="defconf: accept IKE" dst-port=
500,4500 protocol=udp
add action=accept chain=forward comment="defconf: accept ipsec AH" protocol=
ipsec-ah
add action=accept chain=forward comment="defconf: accept ipsec ESP" protocol=
ipsec-esp
add action=accept chain=forward comment=
"defconf: accept all that matches ipsec policy" ipsec-policy=in,ipsec
add action=drop chain=forward comment=
"defconf: drop everything else not coming from LAN" in-interface-list=
!LAN
/ipv6 firewall mangle
add action=change-mss chain=forward comment=
"IPv6 Change MSS to PMTU, make HTTPS happy" new-mss=clamp-to-pmtu
protocol=tcp tcp-flags=syn
/ipv6 nd
set [ find default=yes ] disabled=yes
add interface=bridge
/system clock
set time-zone-name=Asia/Shanghai
/system ntp client
set enabled=yes
/system ntp client servers
add address=ntp1.nim.ac.cn
add address=ntp2.nim.ac.cn
add address=ntp.ntsc.ac.cn
add address=0.pool.ntp.org
add address=1.pool.ntp.org
add address=2.pool.ntp.org

屏幕截图 2026-03-08 093114
屏幕截图 2026-03-08 0931141920×1080 435 KB

2

The warning may appear starting from 7.17:

If you see the log message right after reboot (as it is the case from your screenshot), and not later, then you can ignore it.

While your router reboots / starts up, there is period where it's not available. If clients in your LAN have the router configured as DNS resolver (it's the case in your config, with the /ip dhcp-server network entry specifying dns-server=172.17.0.1), then the clients might still make DNS queries while your router reboots.

First they all try with UDP, it doesn't work because the router doesn't respond. After a brief period (a few seconds) the clients will try to contact the DNS server (your router) over TCP, they all will try to initiate a TCP handshake to port 53 of the router, each might try multiple times, by sending TCP SYN packets.

You'll then have a situation when right after the router finished rebooting, it sees a bunch of TCP SYN packets arriving on port 53, probably from multiple sources (clients). This will cause the warning message above to be logged.

In normal router operation, you'll see fewer of such messages, because most clients will be able to make the queries over UDP, and only need to fallback to TCP when the responses are too large. And also because normally the queries succeed and are answered on-time, so clients don't have to retry and switch to TCP.

3

I guess in a perfect world this warning could have a timeout after booting like (say) netwatch has (startup-delay).

4

Thank you very much! I'm relieved to know my configuration is correct.
I saw a big spike in CPU and memory usage and thought something was seriously wrong.

5

Thank you!