Dmitry on firewalling

MUM 2006 presentation on firewalling

Presentation in PDF format
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Components of the filter
protocol classifier
invalid packet filter
port-scan detector
policy classifier
application protocol filter
TCP-specific filters
application protocol specific filters
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Introduction

There are two interfaces on our router: Local (for connecting internal network) and Public (connected to the Internet)
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Protocol classifier
/ ip firewall mangle
add chain=prerouting protocol=tcp connection-state=new action=jump jump-target=tcp-services
add chain=prerouting protocol=udp connection-state=new action=jump jump-target=udp-services
add chain=prerouting connection-state=new action=jump jump-target=other-services
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=20-21 action=mark-connection new-connection-mark=ftp passthrough=no
add chain=tcp-services protocol=tcp src-port=513-65535 dst-port=22 action=mark-connection new-connection-mark=ssh passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=23 action=mark-connection new-connection-mark=telnet passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=25 action=mark-connection new-connection-mark=smtp passthrough=no
add chain=tcp-services protocol=tcp src-port=53 dst-port=53 action=mark-connection new-connection-mark=dns passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=53 action=mark-connection new-connection-mark=dns passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=80 action=mark-connection new-connection-mark=http passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=110 action=mark-connection new-connection-mark=pop3 passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=113 action=mark-connection new-connection-mark=auth passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=119 action=mark-connection new-connection-mark=nntp passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=143 action=mark-connection new-connection-mark=imap passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=161-162 action=mark-connection new-connection-mark=snmp passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=443 action=mark-connection new-connection-mark=https passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=465 action=mark-connection new-connection-mark=smtps passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=993 action=mark-connection new-connection-mark=imaps passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=995 action=mark-connection new-connection-mark=pop3s passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=1723 action=mark-connection new-connection-mark=pptp passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=2379 action=mark-connection new-connection-mark=kgs passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=3128 action=mark-connection new-connection-mark=proxy passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=3389 action=mark-connection new-connection-mark=win-ts passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=4242-4243 action=mark-connection new-connection-mark=emule passthrough=no
add chain=tcp-services protocol=tcp src-port=4661-4662 dst-port=1024-65535 action=mark-connection new-connection-mark=overnet passthrough=no
add chain=tcp-services protocol=tcp src-port=4711 dst-port=1024-65535 action=mark-connection new-connection-mark=emule passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=5900-5901 action=mark-connection new-connection-mark=vnc passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=6667-6669 action=mark-connection new-connection-mark=irc passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=6881-6889 action=mark-connection new-connection-mark=bittorrent passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=8080 action=mark-connection new-connection-mark=http passthrough=no
add chain=tcp-services protocol=tcp src-port=1024-65535 dst-port=8291 action=mark-connection new-connection-mark=winbox passthrough=no
add chain=tcp-services protocol=tcp action=mark-connection new-connection-mark=other-tcp passthrough=no
add chain=udp-services protocol=udp src-port=1024-65535 dst-port=53 action=mark-connection new-connection-mark=dns passthrough=no
add chain=udp-services protocol=udp src-port=1024-65535 dst-port=123 action=mark-connection new-connection-mark=ntp passthrough=no
add chain=udp-services protocol=udp src-port=1024-65535 dst-port=1701 action=mark-connection new-connection-mark=l2tp passthrough=no
add chain=udp-services protocol=udp src-port=1024-65535 dst-port=4665 action=mark-connection new-connection-mark=emule passthrough=no
add chain=udp-services protocol=udp src-port=1024-65535 dst-port=4672 action=mark-connection new-connection-mark=emule passthrough=no
add chain=udp-services protocol=udp src-port=4672 dst-port=1024-65535 action=mark-connection new-connection-mark=emule passthrough=no
add chain=udp-services protocol=udp src-port=1024-65535 dst-port=12053 action=mark-connection new-connection-mark=overnet passthrough=no
add chain=udp-services protocol=udp src-port=12053 dst-port=1024-65535 action=mark-connection new-connection-mark=overnet passthrough=no
add chain=udp-services protocol=udp src-port=36725 dst-port=1024-65535 action=mark-connection new-connection-mark=skype passthrough=no
add chain=udp-services protocol=udp connection-state=new action=mark-connection new-connection-mark=other-udp passthrough=no
add chain=other-services protocol=icmp icmp-options=8:0-255 action=mark-connection new-connection-mark=ping passthrough=no
add chain=other-services protocol=gre action=mark-connection new-connection-mark=gre passthrough=no
add chain=other-services action=mark-connection new-connection-mark=other passthrough=no

Note that for TCP and UDP, we check both, source port (usually, 1024-65535) and destination port. Everything else is not a valid protocol.
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Sanity-check

Most generic invalid packet and port-scan detection techniques

Place this before all other rules in mangle:
/ip firewall mangle
add chain=prerouting in-interface=Public dst-address-list=nat-addr action=mark-packet new-packet-mark=nat-traversal passthrough=no

Note that just like in the line above, some filter rules rely on address lists. here is a simple list, which you should extend further:
/ ip firewall address-list
add list=illegal-addr address=0.0.0.0/8 comment="illegal addresses"
add list=illegal-addr address=127.0.0.0/8
add list=illegal-addr address=224.0.0.0/3
add list=illegal-addr address=10.0.0.0/8
add list=illegal-addr address=172.16.0.0/12
add list=illegal-addr address=192.168.0.0/16
add list=local-addr address=172.31.255.0/29 comment="my local network"
add list=nat-addr address=172.31.255.0/29 comment="my local network"

So, there are three address lists:
illegal-addr - the list, which could be extended to some few tens of addresses at least to include the bogon IPs, which are not registered with IANA and some more, this short list given in examples is just a short sample;
local-addr - the list to include all addresses located in your network, behind this firewall
nat-addr - should contain all the IP addresses you are source-natting on your router

In this example we exclude traffic between the local clients connected to different ports of the Local interface (which is a bridge between ethernet and wireless networks):
/ ip firewall filter
add chain=forward in-interface=Local out-interface=Local action=accept comment="Allow traffic between wired and wireless networks"

Then we are filtering everything else to the drop chain of the firewall. The separate chain is created to keep all logging and accounting in one place.
/ ip firewall filter
add chain=forward action=jump jump-target=sanity-check comment="Sanity Check"
add chain=sanity-check packet-mark=nat-traversal action=jump jump-target=drop comment="Deny illegal NAT traversal"
add chain=sanity-check protocol=tcp psd=20,3s,3,1 action=add-src-to-address-list address-list=blocked-addr address-list-timeout=1d comment="Block port scans"
add chain=sanity-check protocol=tcp tcp-flags=fin,psh,urg,!syn,!rst,!ack action=add-src-to-address-list address-list=blocked-addr address-list-timeout=1d comment="Block TCP Null scan"
add chain=sanity-check protocol=tcp tcp-flags=!fin,!syn,!rst,!psh,!ack,!urg action=add-src-to-address-list address-list=blocked-addr address-list-timeout=1d comment="Block TCP Xmas scan"
add chain=sanity-check protocol=tcp src-address-list=blocked-addr action=jump jump-target=drop
add chain=sanity-check protocol=tcp tcp-flags=rst action=jump jump-target=drop comment="Drop TCP RST"
add chain=sanity-check protocol=tcp tcp-flags=fin,syn action=jump jump-target=drop comment="Drop TCP SYN+FIN"
add chain=sanity-check connection-state=invalid action=jump jump-target=drop comment="Dropping invalid connections at once"
add chain=sanity-check connection-state=established action=accept comment="Accepting already established connections"
add chain=sanity-check connection-state=related action=accept comment="Also accepting related connections"
add chain=sanity-check dst-address-type=broadcast,multicast action=jump jump-target=drop comment="Drop all traffic that goes to multicast or broadcast addresses"
add chain=sanity-check in-interface=Local dst-address-list=illegal-addr dst-address-type=!local action=jump jump-target=drop comment="Drop illegal destination addresses"
add chain=sanity-check in-interface=Local src-address-list=!local-addr action=jump jump-target=drop comment="Drop everything that goes from local interface but not from local address"
add chain=sanity-check in-interface=Public src-address-list=illegal-addr action=jump jump-target=drop comment="Drop illegal source addresses"
add chain=sanity-check in-interface=Public dst-address-list=!local-addr action=jump jump-target=drop comment="Drop everything that goes from public interface but not to local address"
add chain=sanity-check src-address-type=broadcast,multicast action=jump jump-target=drop comment="Drop all traffic that goes from multicast or broadcast addresses"
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Application-specific filters
/ ip firewall filter
add chain=forward protocol=tcp action=jump jump-target=restrict-tcp
add chain=forward protocol=udp action=jump jump-target=restrict-udp
add chain=forward action=jump jump-target=restrict-ip
add chain=restrict-tcp connection-mark=auth action=reject
add chain=restrict-tcp connection-mark=smtp action=jump jump-target=smtp-first-drop comment="anti-spam policy"
add chain=smtp-first-drop src-address-list=first-smtp action=add-src-to-address-list address-list=approved-smtp
add chain=smtp-first-drop src-address-list=approved-smtp action=return
add chain=smtp-first-drop action=add-src-to-address-list address-list=first-smtp
add chain=smtp-first-drop action=reject reject-with=icmp-network-unreachable
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Restricting services
/ ip firewall filter
add chain=restrict-tcp connection-mark=other-tcp action=jump jump-target=drop
add chain=restrict-udp connection-mark=other-udp action=jump jump-target=drop
add chain=restrict-ip connection-mark=other action=jump jump-target=drop
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Protecting the router
/ ip firewall filter
add chain=input src-address-type=local dst-address-type=local action=accept comment="Allow local traffic \(between router applications\)"
add chain=input in-interface=Local protocol=udp src-port=68 dst-port=67 action=jump jump-target=dhcp comment="DHCP protocol would not pass sanity checking, so enabling it explicitly before other checks"
add chain=input action=jump jump-target=sanity-check comment="Sanity Check"
add chain=input dst-address-type=!local action=jump jump-target=drop comment="Dropping packets not destined to the router itself, including all broadcast traffic"
add chain=input connection-mark=ping limit=5,5 action=accept comment="Allow pings, but at a very limited rate \(5 per sec\)"
add chain=input in-interface=Local action=jump jump-target=local-services comment="Allowing some services to be accessible from the local network"
add chain=input in-interface=Public action=jump jump-target=public-services comment="Allowing some services to be accessible from the Internet"
add chain=input action=jump jump-target=drop
add chain=dhcp src-address=0.0.0.0 dst-address=255.255.255.255 action=accept
add chain=dhcp src-address=0.0.0.0 dst-address-type=local action=accept
add chain=dhcp src-address-list=local-addr dst-address-type=local action=accept
add chain=local-services connection-mark=ssh action=accept comment="SSH \(22/TCP\)"
add chain=local-services connection-mark=dns action=accept comment="DNS"
add chain=local-services connection-mark=proxy action=accept comment="HTTP Proxy \(3128/TCP\)"
add chain=local-services connection-mark=winbox comment="Winbox \(8291/TCP\)" disabled=no
add chain=local-services action=drop comment="Drop Other Local Services"
add chain=public-services connection-mark=ssh action=accept comment="SSH \(22/TCP\)"
add chain=public-services connection-mark=pptp action=accept comment="PPTP \(1723/TCP\)"
add chain=public-services connection-mark=gre action=accept comment="GRE for PPTP"
add chain=public-services action=drop comment="Drop Other Public Services"
The "accept ping" rule needs to come before the "public" and "local" jump rules otherwise that rules will never be executed and ICMP will continue to be dropped.
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Proxying everything
/ ip firewall nat
add chain=dstnat in-interface=Local connection-mark=dns action=redirect comment="proxy for DNS requests"
add chain=dstnat in-interface=Local connection-mark=http protocol=tcp action=redirect to-ports=3128 comment="proxy for HTTP requests"
add chain=dstnat in-interface=Local connection-mark=ntp action=redirect comment="proxy for NTP requests"
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Enable Proxy servers
/ system ntp server
set enabled=yes broadcast=no multicast=no manycast=no
/ system ntp client
set enabled=yes mode=unicast primary-ntp=xxx.xxx.xxx.xxx secondary-ntp=0.0.0.0
/ ip proxy
set enabled=yes port=3128 parent-proxy=0.0.0.0:1 maximal-client-connections=1000 maximal-server-connections=1000
/ ip dns
set primary-dns=yyy.yyy.yyy.yyy secondary-dns=0.0.0.0 allow-remote-requests=yes cache-size=2048KiB cache-max-ttl=1w

Please change:
xxx.xxx.xxx.xxx to the NTP server you choose
yyy.yyy.yyy.yyy to your ISP's DNS server ip