Introduction
The purpose of this document is to strengthen the UNIX IP stack against
a variety
of attack types prevalent on the Internet today. This document details
the
settings recommended for UNIX servers designed to provide network
intensive
services such as HTTP or routing (firewall services). This document
covers
the following UNIX variants:
A. IBM AIX 4.3.X
B. Sun Solaris 7
C. Compaq Tru64 UNIX 5.X
D. HP HP-UX 11.0 (research ongoing)
E. Linux kernel 2.2 (tested both SuSE Linux 7.0 and RedHat
7.0)
F. FreeBSD
G. IRIX 6.5.10
Windows NT/2000 will be covered in future releases.
The tuning changes noted below will not remain across reboots.
Thus, these changes should be added to a runtime commands file so
that the changes are enacted at each boot.
AIX - /etc/rc.net
Solaris - /etc/init.d/inetinit
Tru64 UNIX - Use the sysconfigdb or
dxkerneltuner command
HP-UX - /etc/rc.config.d/nddconf
Linux kernel 2.2 - /etc/sysctl.conf
FreeBSD - /etc/rc.conf
IRIX - Use the systune command
No tuning steps should be undertaken without a keen insight into
the purpose and effect of such steps. It is assumed that the
personnel who undertake such tuning are well versed in the UNIX
kernel with bit-level understanding of TCP/IP and the RFCs that
govern IP behavior.
Revision History
Version 1.0 - Created the AIX IP stack tuning guide
Version 2.0 - Added Solaris and additional AIX tuning tips, reformated
doc
Version 2.2 - Added ARP tuning tips
Version 2.3 - Added Compaq Tru64 UNIX SYN defense tips
Version 2.4 - Added HP HP-UX 11.0 tuning tips
Version 2.5 - Added Linux and FreeBSD tuning tips
Version 2.6 - Added IRIX 6.5.10 and additional FreeBSD tuning tips
Version 2.7 - Added additional HP-UX 11.0 tuning tips
Credits
This has become a global effort, and this document is made more rich and
valuable thanks to the input of several very helpful folks! Kudos and
lauds for the folks below:
Thanks to the Compaq Software Security Response Team for the Tru64 UNIX
SYN flood defense pointers.
For HP-UX 11.0, thanks to Tom Harrold for running ndd over and over at my
every request. :-)
Thanks to Klaus Moeller for a copious collection of Linux tuning tips.
Thanks to Andrew Korty for the FreeBSD tuning tips.
Thanks to Christian Lambert and Alex Icasiano of SGI for the IRIX tuning
tips.
Thanks, as always, to the FIRST
community for support, questions, and feedback.
General IP Stack Tuning Recommendations
1. TCP send and receive spaces
The TCP send and receive spaces directly effect the TCP window
size parameter. An increased window size will allow for more
efficient transfers, particularly bulk transfers such as FTP
and HTTP. The default for each is not optimal, and should be
increased to 32768 bytes. This value should not be increased
above 64K bytes unless the implications of
RFC1323
and RFC2018
are fully understood and support for both is enabled.
Do not enable RFC1323 without also enabling support for RFC2018.
Remember, pipe drain is a Bad Thing[tm].
A. AIX
/usr/sbin/no -o tcp_sendspace=32768
/usr/sbin/no -o tcp_recvspace=32768
B. Solaris
/usr/sbin/ndd -set /dev/tcp tcp_xmit_hiwat 32768
/usr/sbin/ndd -set /dev/tcp tcp_recv_hiwat 32768
C. Tru64 UNIX
No tuning recommendations.
D. HP-UX
The TCP send and receive spaces are set to 32768 by default
E. Linux kernel 2.2
While Linux automagically assigns the TCP send and receive
spaces,
support for both RFC1323 (large window support,
net.ipv4.tcp_window_scaling) and RFC2018 (SACK
support,
net.ipv4.tcp_sack) are enabled by default.
F. FreeBSD
sysctl -w net.inet.tcp.sendspace=32768
sysctl -w net.inet.tcp.recvspace=32768
G. IRIX
The default settings for IRIX are 64Kbytes for both the
TCP send
and receive spaces.
2. Socket queue defense against SYN attacks
While great effort is undertaken to defend any network from those
with malicious intent, several ports (largely TCP) must remain
open to conduct business. Internet vandals may attempt to
exploit these ports to launch a denial of service attack. One of
the most popular attacks remains the SYN flood, wherein the socket
queue of the attacked host is overwhelmed with bogus connection
requests. To defend against such attacks, certain UNIX variants
maintain separate queues for inbound socket connection requests.
One queue is for half-open sockets (SYN received, SYN|ACK sent),
the other queue for fully-open sockets awaiting an accept()
call from the application. These two queues should be increased so
that an attack of low to moderate intensity will have little to no
effect on the stability or availability of the server.
A. AIX
/usr/sbin/no -o clean_partial_conns=1
This setting will instruct the kernel to randomly remove
half-open sockets from the q0 queue to make room for
new
sockets.
B. Solaris
/usr/sbin/ndd -set /dev/tcp tcp_conn_req_max_q 1024
The q queue holds sockets awaiting an accept()
call from the application.
/usr/sbin/ndd -set /dev/tcp tcp_conn_req_max_q0 2048
The q0 queue contains half-open sockets.
C. Tru64 UNIX
/sbin/sysconfig -r socket sominconn=65535
The value of sominconn determines how many simultaneous
incoming
SYN packets can be handled by the system.
/sbin/sysconfig -r socket somaxconn=65535
The value of somaxconn sets the maximum number of pending
TCP
connections.
D. HP-UX
/usr/sbin/ndd -set tcp_syn_rcvd_max 1024
/usr/sbin/ndd -set tcp_conn_request_max 200
E. Linux kernel 2.2
/sbin/sysctl -w net.ipv4.tcp_max_syn_backlog=1280
Increases the size of the socket queue (effectively, q0).
/sbin/sysctl -w net.ipv4.tcp_syn_cookies=1
Enables support for TCP SYN cookies, which mitigates the
effectiveness
of SYN floods. However, this may cause performance
problems for
large windows (see RFC1323 and RFC2018). To read
more about SYN
cookies, please review DJ Bernstein's paper
here.
F. FreeBSD
sysctl -w kern.ipc.somaxconn=1024
G. IRIX
The listen() queue is hardcoded to 32. However,
the system
actually enforces the limit of pending connections as
((3 * backlog) / 2) + 1. This yields a maximum
backlog of 49
connections.
3. Redirects
A miscreant can use IP redirects to modify the routing table on a
remote host. In a well-designed network, redirects to the end
stations should not be required. Both the sending and accepting
of redirects should be disabled.
A. AIX
/usr/sbin/no -o ipignoreredirects=1
/usr/sbin/no -o ipsendredirects=0
B. Solaris
/usr/sbin/ndd -set /dev/ip ip_ignore_redirect 1
/usr/sbin/ndd -set /dev/ip ip_send_redirects 0
C. Tru64 UNIX
No tuning recommendations.
D. HP-UX
/usr/sbin/ndd -set /dev/ip ip_send_redirects 0
E. Linux kernel 2.2
/sbin/sysctl -w net.ipv4.conf.all.send_redirects=0
/sbin/sysctl -w net.ipv4.conf.all.accept_redirects=0
F. FreeBSD
sysctl -w net.inet.icmp.drop_redirect=1
sysctl -w net.inet.icmp.log_redirect=1
sysctl -w net.inet.ip.redirect=0
sysctl -w net.inet6.ip6.redirect=0
G. IRIX
/usr/sbin/systune icmp_dropredirects to 1
4. ARP cleanup
It is possible for a miscreant to create a resource exhaustion or
performance degredation by filling the IP route cache with bogus
ARP entries. In Solaris, there are two parameters that govern the
cleanup interval for the IP route cache. For unsolicited ARP
responses, the parameter to be tuned is arp_cleanup_interval. In
AIX, the cleanup interval is governed by the value of arpt_killc.
However, this parameter governs both solicited and unsolicited ARP
entries. For this reason, it is likely best to leave the parameter
at the default setting of 20 minutes.
A. AIX
/usr/sbin/no -o arpt_killc=20
B. Solaris
/usr/sbin/ndd -set /dev/arp arp_cleanup_interval
60000
C. Tru64 UNIX
No tuning recommendations.
D. HP-UX
By default set to five minutes.
E. Linux kernel 2.2
No tuning recommendations.
F. FreeBSD
sysctl -w net.link.ether.inet.max_age=1200
G. IRIX
No tuning recommendations.
5. Source routing
With source routing, an attacker can attempt to reach internal IP
addresses - including RFC1918 addresses. It is important to disable
the acceptance of source routed packets to prevent subtle probes of
your internal networks.
A. AIX
/usr/sbin/no -o ipsrcroutesend=0
Disable the sending of source routed packets.
/usr/sbin/no -o ipsrcrouteforward=0
This is important if the box is routing, e.g. a firewall.
Disable this feature to prevent the host from
forwarding source
routed packets.
B. Solaris
/usr/sbin/ndd -set /dev/ip ip_src_route_forward 0
This is important if the box is routing, e.g. a firewall.
Disable this feature to prevent the host from
forwarding source
routed packets.
C. Tru64 UNIX
No tuning recommendations.
D. HP-UX
ndd -set /dev/ip ip_forward_src_routed 0
Disable this feature to prevent the host from forwarding
source
routed packets.
E. Linux kernel 2.2
/sbin/sysctl -w
net.ipv4.conf.all.accept_source_route=0
Drop all source route packets.
/sbin/sysctl -w net.ipv4.conf.all.forwarding=0
/sbin/sysctl -w net.ipv4.conf.all.mc_forwarding=0
Do not forward source routed frames.
F. FreeBSD
sysctl -w net.inet.ip.sourceroute=0
sysctl -w net.inet.ip.accept_sourceroute=0
G. IRIX
/usr/sbin/systune ipforward to 2
6. TIME_WAIT setting
On a busy web server, many sockets may linger in the TIME_WAIT state.
This is caused by improperly coded client applications that do not
properly shut down a socket. This can also be used as a type of DDoS
attack.
A. AIX
No tuning recommendations.
B. Solaris
/usr/sbin/ndd -set /dev/tcp tcp_time_wait_interval
60000
This parameter effects the amount of time a TCP socket will
remain in the TIME_WAIT state. The default is quite
high
for a busy web server, so it should be lowered to
60000
milliseconds (60 seconds). The parameter name was
corrected
in Solaris 7 and higher. Prior to Solaris 7, the
parameter
was incorrectly labeled as tcp_close_wait_interval.
C. Tru64 UNIX
No tuning recommendations.
D. HP-UX
ndd -set /dev/tcp tcp_time_wait_interval 60000
Sockets will linger in TIME_WAIT state no more than 60
seconds.
E. Linux kernel 2.2
/sbin/sysctl -w net.ipv4.vs.timeout_timewait=60
Sockets will linger in TIME_WAIT state for no more than 60
seconds.
F. FreeBSD
No tuning recommendations.
G. IRIX
/usr/sbin/systune tcp_2msl to 60
7. Broadcast ECHO response
Smurf attacks work by sending ICMP 8 0 (ECHO REQUEST) messages to a
broadcast address from a spoofed address. Some IP stacks will respond,
by default, to such messages. This should be disabled. Further, if the
host is a firewall (router), it should not propogate directed broadcasts.
A. AIX
/usr/sbin/no -o directed_broadcast=0
Do not respond to directed broadcasts.
B. Solaris
/usr/sbin/ndd -set /dev/ip ip_respond_to_echo_broadcast
0
Do not respond to directed broadcasts.
/usr/sbin/ndd -set /dev/ip
ip_forward_directed_broadcasts 0
Do not forward directed broadcasts.
C. Tru64 UNIX
No tuning recommendations.
D. HP-UX
ndd -set /dev/ip ip_respond_to_echo_broadcast 0
Do not respond to directed broadcasts.
ndd -set /dev/ip ip_forward_directed_broadcasts 0
Do not forward directed broadcasts.
E. Linux kernel 2.2
/sbin/sysctl -w
net.ipv4.icmp_echo_ignore_broadcasts=1
Do not respond to directed broadcasts.
F. FreeBSD
sysctl -w net.inet.icmp.bmcastecho=0
G. IRIX
/usr/sbin/systune allow_brdaddr_srcaddr to 0
8. Other broadcast probes
There are two other broadcast probes that a miscreant could utilize
against a network. The address mask query can be used to map out the
size of the netblock, and set a range for further probes. The
timestamp broadcast is another means of mapping and fingerprinting
hosts.
A. AIX
/usr/sbin/no -o icmpaddressmask=0
Prevent address mask queries.
B. Solaris
/usr/sbin/ndd -set /dev/ip
ip_respond_to_address_mask_broadcast 0
Prevent address mask queries.
/usr/sbin/ndd -set /dev/ip
ip_respond_to_timestamp_broadcast 0
Disable timestamp broadcast query response.
C. Tru64 UNIX
No tuning recomendations.
D. HP-UX
ndd -set /dev/ip ip_respond_to_address_mask_broadcast
0
Prevent the host from revealing the configured netmask.
ndd -set /dev/ip ip_respond_to_timestamp_broadcast 0
Disable timestamp broadcast query response.
E. Linux kernel 2.2
No tuning recommendations.
F. FreeBSD
sysctl -w net.inet.icmp.maskrepl=0
G. IRIX
Use ipfilterd to block unwanted ICMP types.
9. Support for RFC1948
This will utilize RFC1948
sequence number generation techniques to ensure that the sequence
number for a given TCP socket is very difficult to guess. This
tactic prevents TCP session hijacking, which could be quite an issue
with e-commerce.
B. Solaris
Set TCP_STRONG_ISS=2 in /etc/default/inetinit.
This will require a reboot to take effect.
G. IRIX
/usr/sbin/systune tcpiss_md5 to 1
