This article taken from http://www.cymru.com/Documents/ip-stack-tuning.html

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). You can find the most useful dos attack tutorial here in order to prevent the dos attacks on your servers.

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 2
048
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.tcpsyn_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_forward_src_routed 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 initial sequence number for a given TCP socket is very difficult to guess. This tactic makes IP spoofing significantly more difficult to accomplish.

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

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