firehol-synproxy - configure synproxy
synproxy type rules-to-match-request action
[action options]
- •
- 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.
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.
- •
- 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.
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.
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.
- 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.
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.
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.
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
- •
- firehol(1) - FireHOL program
- •
- firehol.conf(5) - FireHOL configuration
- •
- firehol-interface(5) - interface definition
- •
- firehol-router(5) - router definition
- •
- firehol-params(5) - optional rule parameters
- •
- firehol-masquerade(5) - masquerade helper
- •
- FireHOL Website (http://firehol.org/)
- •
- FireHOL Online PDF Manual (http://firehol.org/firehol-manual.pdf)
- •
- FireHOL Online Documentation (http://firehol.org/documentation/)
- •
- NAT HOWTO
(http://www.netfilter.org/documentation/HOWTO/NAT-HOWTO-6.html)
- •
- netfilter flow diagram
(http://upload.wikimedia.org/wikipedia/commons/3/37/Netfilter-packet-flow.svg)