Ubuntu Manpage: firehol-synproxy

NAME

       firehol-synproxy - configure synproxy

SYNOPSIS

       synproxy type rules-to-match-request action [action options]

DESCRIPTION

• type defines where the SYNPROXY will be attached. It can be input (or in), forward (or pass):

• use input (or in) when the IP of the real server is an IP assigned to a physical interface of the
machine (i.e. the IP is at the firewall itself)

• use forward (or pass) when the IP of the real server is routed by the machine (i.e. SYNPROXY should
look at the FORWARD chain for this traffic).

• rules to match request are FireHOL optional rule parameters and should match the original client
REQUEST, before any destination NAT. inface and dst are required:

• inface is one or more interfaces the REQUEST should be received from

• dst is the IP of the real server, as seen by the client (before any destination NAT)

• action defines how SYNPROXY will reach the real server and can be:

• accept to just allow the REQUEST reach the real server without any destination NAT

• dnat to IP:PORT or dnat to IP1-IP2:PORT1-PORT2 or dnat to IP or dnat to :PORT to have SYNPROXY reach
a server on another machine in a DMZ (different IP and/or PORT compared to the original request).
The synproxy statement supports everything supported by the dnat helper (see firehol-nat(5)).

• redirect to PORT to divert the request to a port on the firewall itself. The synproxy statement
supports everything supported by the redirect helper (see firehol-nat(5)).

• action CUSTOM_ACTION to have any other FireHOL action performed on the NEW socket. Use the action
helper to define custom actions (see [firehol-action(5)][]).

• action options are everything supported by FireHOL optional rule parameters that should be applied
only on the final action of SYN packet from SYNPROXY to the real server. For example this can be
used to append loglimit "TEXT" to have something logged by iptables, or limit the concurrent sockets
with connlimit. Generally, everything you can write on the same line after server http accept is
also accepted here.

BACKGROUND

       SYNPROXY is a TCP SYN packets proxy.  It can be used to protect any TCP server (like a web  server)  from
       SYN floods and similar DDos attacks.

       SYNPROXY  is  a netfilter module, in the Linux kernel.  It is optimized to handle millions of packets per
       second utilizing all CPUs available without any concurrency locking between the connections.

       The net effect of this, is that the real servers will not notice any change during the attack.  The valid
       TCP connections will pass through and served, while the attack will be stopped at the firewall.

       For more information on why you should use a SYNPROXY, check these articles:

       • http://rhelblog.redhat.com/2014/04/11/mitigate-tcp-syn-flood-attacks-with-red-hat-enterprise-linux-7-beta/

       • https://r00t-services.net/knowledgebase/14/Homemade-DDoS-Protection-Using-IPTables-SYNPROXY.html

       SYNPROXY is included in the Linux kernels since version 3.12.

HOW IT WORKS

       • When a SYNPROXY is used, clients transparently get  connected  to  the  SYNPROXY.   So  the  3-way  TCP
         handshake happens first between the client and the SYNPROXY:

         • Clients send TCP SYN to server A

         • At the firewall, when this packet arrives it is marked as UNTRACKED

         • The UNTRACKED TCP SYN packet is then given to SYNPROXY

         • SYNPROXY gets this and responds (as server A) with TCP SYN+ACK (UNTRACKED)

         • Client  responds  with  TCP  ACK  (marked as INVALID or UNTRACKED in iptables) which is also given to
           SYNPROXY

       • Once a client has been connected  to  the  SYNPROXY,  SYNPROXY  automatically  initiates  a  3-way  TCP
         handshake  with  the  real  server,  spoofing  the SYN packet so that the real server will see that the
         original client is attempting to connect:

         • SYNPROXY sends TCP SYN to real server A. This is a NEW connection in  iptables  and  happens  on  the
           OUTPUT chain.  The source IP of the packet is the IP of the client

         • The real server A responds with SYN+ACK to the client

         • SYNPROXY  receives  this  and  responds back to the server with ACK.  The connection is now marked as
           ESTABLISHED

       • Once the connection has been established, SYNPROXY leaves the traffic flow between the client  and  the
         server

       So,  SYNPROXY can be used for any kind of TCP traffic.  It can be used for both unencrypted and encrypted
       traffic, since it does not interfere with the content itself.

USE CASES

       In FireHOL SYNPROXY support is implemented as a helper.  The synproxy command can be used to set  up  any
       number of SYNPROXYs.

       FireHOL can set up SYNPROXY for any of these cases:

       1. real server on the firewall itself, on the same port (e.g. SYNPROXY on port 80, real server on port 80
          too).

       2. real  server  on  the firewall itself, on a different port (e.g. SYNPROXY on port 2200, real server on
          port 22).

       3. real server on a different machine, without NAT (e.g. SYNPROXY on a router catching traffic towards IP
          A, port 80 and real server is at IP A port 80 too).

       4. real server on a different machine, with NAT (e.g. SYNPROXY on a router catching traffic towards IP A,
          port 80 and real server is at IP 10.1.1.1 port 90).

       5. screening incoming traffic that should never be sent to a  real  server  so  that  traps  and  dynamic
          blacklists can be created using traffic that has been screened by SYNPROXY (eliminate “internet noise”
          and spoofed packets).

       So, generally, all cases are covered.

DESIGN

The general guidelines for using synproxy in FireHOL, are:

1. Design your firewall as you would normally do without SYNPROXY

2. Test that it works without SYNPROXY. Test especially the servers you want to protect. They should be
working too

3. Add synproxy statements for the servers you want to protect.

To achieve these requirements:

1. The helper will automatically do everything needed for SYNPROXY to:

• receive the initial SYN packet from the client

• respond back to the client with SYN+ACK

• receive the first ACK packet from the client

• send the initial SYN packet to the server

There are cases where the above are very tricky to achieve. You don’t need to match these in your
firehol.conf. The synproxy helper will automatically take care of them. However:

You do need the allow the flow of traffic between the real server and the real client (as you
normally do without a synproxy, with a client, server, or route statement in an interface or
router section).

2. The helper will prevent the 3-way TCP handshake between SYNPROXY and the real server interact with
other destination NAT rules you may have. However for this to happen, make sure you place the
synproxy statements above any destination NAT rules (redirect, dnat, transparent_squid,
transparent_proxy, tproxy, etc). So:

SYNPROXY will interact with destination NAT you have in firehol.conf only if the synproxy
statements are place below the destination NAT ones.

You normally do not need to have synproxy interact with other destination NAT rules. The
synproxy helper will handle the destination NAT (dnat or redirect) it needs by itself.

So place synproxy statements above all destination NAT statements, unless you know what you
are doing.

3. The helper will allow the 3-way TCP handshake between SYNPROXY and the real server interact with
source NAT rules you may have (snat, masquerade), since these may be needed to reach the real server.

LIMITATIONS

1. Internally there are matches that are made without taking into account the original inface. So, in
case different actions have to be taken depending on the interface the request is received, src should
be added to differentiate the traffic between the two flows.

2. SYNPROXY does not inherit MARKs from the original request packets. It should and it would make
matching a lot easier, but it does not. This means that for all packets generated by SYNPROXY, inface
is lost.

3. FireHOL internally uses a MARK to tag packets send from SYNPROXY to the target server. This is used
for 3 reasons:

• isolate these packets from other destination NAT rules. If they were not isolated from the
destination NAT rules, then packets from the SYNPROXY could be matched by a transparent proxy and
enter your web proxy. They could be matched by a transparent proxy because they actually originate
from the local machine.

• isolate the same packets from the rest of the packet filtering rules. Without this isolation, most
probably the packets will have been dropped since they come from lo.

• report if orphan synproxy packets are encountered. So packets the FireHOL engine failed to match
properly, should appear with a iptables log saying “ORPHAN SYNPROXY->SERVER”. If you don’t have
such logs, everything works as expected.

OTHER OPTIONS

       You can change the TCP options used by synproxy by setting the  variable  FIREHOL_SYNPROXY_OPTIONS.   The
       default is this:

              FIREHOL_SYNPROXY_OPTIONS="--sack-perm --timestamp --wscale 7 --mss 1460"

       If you want to see it in action in the iptables log, then enable logging:

              FIREHOL_SYNPROXY_LOG=1

       The  default  is  disabled  (0).   If you enable it, every step of the 3-way setup between the client and
       SYNPROXY and the SYN packet of SYNPROXY towards the real server will be logged by iptables.

       Using the variable FIREHOL_CONNTRACK_LOOSE_MATCHING  you  can  set  net.netfilter.nf_conntrack_tcp_loose.
       FireHOL will automatically set this to 0 when a synproxy is set up.

       Using   the   variable   FIREHOL_TCP_TIMESTAMPS   you  can  set  net.ipv4.tcp_timestamps.   FireHOL  will
       automatically set this to 1 when a synproxy is set up.

       Using  the  variable  FIREHOL_TCP_SYN_COOKIES  you  can  set   net.ipv4.tcp_syncookies.    FireHOL   will
       automatically set this to 1 when a synproxy is set up.

       On  a  busy server, you are advised to increase the maximum connection tracker entries and its hash table
       size.

       • Using the variable FIREHOL_CONNTRACK_HASHSIZE you can set /sys/module/nf_conntrack/parameters/hashsize.

       • Using the variable FIREHOL_CONNTRACK_MAX you can set net.netfilter.nf_conntrack_max.

       FireHOL will not alter these variables by itself.

SYNPROXY AND DYNAMIC IP

       By default the synproxy helper requires from you to define  a  dst  IP  of  the  server  that  is  to  be
       protected.  This is required because the destination IP will be used to match the SYN packet the synproxy
       sends to the server.

       There  is  however another way that allows the use of synproxy in environments where the IP of the server
       is unknown (like a dynamic IP DSL):

       1. First you need to set FIREHOL_SYNPROXY_EXCLUDE_OWNER=1.  This will make  synproxy  not  match  packets
          that  are  generated by the local machine, even if the process that generates them uses your public IP
          (the packets in order to be matched they will need not have a UID or GID).

       2. Next you will need to exclude you lan IPs by adding src not "${UNROUTABLE_IPS}" (or any other  network
          you know you use) to the synproxy statement.

EXAMPLES

       Protect a web server running on the firewall with IP 1.2.3.4, from clients on eth0:

              ipv4 synproxy input inface eth0 dst 1.2.3.4 dport 80 accept

              interface eth0 wan
                  server http accept

       Protect  a  web  server  running  on  port  90 on the firewall with IP 1.2.3.4, from clients on eth0 that
       believe the web server is running on port 80:

              server_myhttp_ports="tcp/90"
              client_myhttp_ports="default"

              ipv4 synproxy input inface eth0 dst 1.2.3.4 dport 80 redirect to 90

              interface eth0 wan
                  server myhttp accept # packet filtering works with the real ports

       Protect a web server running on another machine (5.6.7.8), while the  firewall  is  the  router  (without
       NAT):

              ipv4 synproxy forward inface eth0 dst 5.6.7.8 dport 80 accept

              router wan2lan inface eth0 outface eth1
                  server http accept dst 5.6.7.8

       Protect  a web server running on another machine in a DMZ (public IP is 1.2.3.4 on eth0, web server IP is
       10.1.1.1 on eth1):

              ipv4 synproxy input inface eth0 \
                  dst 1.2.3.4 dport 80 dnat to 10.1.1.1

              router wan2lan inface eth0 outface eth1
                  server http accept dst 10.1.1.1

       Note that we used input not forward, because the firewall has the IP 1.2.3.4 on its eth0 interface.   The
       client request is expected on input.

       Protect  an  array of 10 web servers running on 10 other machines in a DMZ (public IP is 1.2.3.4 on eth0,
       web servers IPs are 10.1.1.1 to 10.1.1.10 on eth1):

              ipv4 synproxy input inface eth0 \
                  dst 1.2.3.4 dport 80 dnat to 10.1.1.1-10.1.1.10 persistent

              router wan2lan inface eth0 outface eth1
                  server http accept dst 10.1.1.1-10.1.1.10

       The above configuration is a load balancer.  Requests towards 1.2.3.4 port 80 will be distributed to  the
       10 web servers with persistence (each client will always see one of them).

       Catch  all  traffic  towards  SSH  port  tcp/22 and send it to TRAP_AND_DROP as explained in Working With
       Traps.  At the same time, allow SSH on port tcp/2200 (without altering the ssh server):

               # definition of action TRAP_AND_DROP
               ipv4 action TRAP_AND_DROP sockets_suspects_trap 3600 86400 1 src not "${UNROUTABLE_IPS}" next action DROP

               # send ssh traffic to TRAP_AND_DROP
               ipv4 synproxy input inface eth0 dst 1.2.3.4 dport 22 action TRAP_AND_DROP

               # accept ssh traffic on tcp/2200
               ipv4 synproxy input inface eth0 dst 1.2.3.4 dport 2200 redirect to 22

               interface eth0 wan
                  server ssh accept

AUTHORS

       FireHOL Team.

FireHOL Reference                               Built 22 Sep 2024                            firehol-synproxy(5)