14th May 1999 [SBWID-111]
COMMAND
kernel (SYN)
SYSTEMS AFFECTED
FreeBSD
PROBLEM
Richard Steenbergen posted following. Here's a quickie for the
people who have been plagued with high bandwidth syn flood
attacks, a kernel patch for FreeBSD 3.1-STABLE which rate limits
SYN processing. Its messy but functional (to make it better -
that's the fbsd developers job). So,
cd /usr/src/sys
patch < synlim
add "options SYN_RATELIM" (highly recommend ICMP_BANDLIM as well) to your kernel
recompile
sysctl net.inet.tcp.synlim (default to 100)
This is the maximium number of SYNs per second that will be
processed, the rest will be silently discarded. On test system
(P2 450 running 3.1-stable being hit w/15,000 packets per sec),
this has successfully brought CPU usage from 100% to ~20%
(against an open port which is replying with unacknowledged ACKs).
Which brings us to the more sticky topic of kernel panics when
under SYN flood. This one seems to be the biggie for crashing
when under syn flood is as follows (heh just turned off the patch
and panic'd within 8 seconds while writing this):
panic: free: multiple frees
(kgdb) bt
#0 boot (howto=256) at ../../kern/kern_shutdown.c:285
#1 0xc0138c09 in panic (fmt=0xc02192b7 "free: multiple frees")
at ../../kern/kern_shutdown.c:446
#2 0xc0135aaf in free (addr=0xc0cdd600, type=0xc0239330)
at ../../kern/kern_malloc.c:333
#3 0xc01768f4 in ifafree (ifa=0xc0cdd600) at ../../net/route.c:262
#4 0xc0176876 in rtfree (rt=0xc34ce700) at ../../net/route.c:236
#5 0xc0176c84 in rtrequest (req=2, dst=0xc34cbac0, gateway=0xc34cbad0,
netmask=0x0, flags=393223, ret_nrt=0x0) at ../../net/route.c:536
#6 0xc017b34d in in_rtqkill (rn=0xc34ce700, rock=0xc0231610)
at ../../netinet/in_rmx.c:242
#7 0xc0176064 in rn_walktree (h=0xc0cd9e00, f=0xc017b2fc <in_rtqkill>,
w=0xc0231610) at ../../net/radix.c:956
#8 0xc017b3ec in in_rtqtimo (rock=0xc0cd9e00) at ../../netinet/in_rmx.c:283
#9 0xc013d19b in softclock () at ../../kern/kern_timeout.c:124
Which after a quick examination seems to be a perioditic routing
table cleanup. It seems that in_rtqtimo is scheduled to run every
net.inet.ip.rtexpire seconds (which is dynamicly adjusted and can
never go lower then net.inet.ip.rtminexpire). When the system is
under heavy load from processing lots of small packets (they don't
even have to be SYNs, anything which can get routed will do the
trick, though the packet kiddies would get very little gain from
just sending an ip header since its going to be padded to 64 bytes
for the eth frame anyhow), this route cleanup code will go wacking
at routes it shouldn't and free some memory twice.
SOLUTION
Patch follows:
*** conf/options.old Sat May 15 23:08:03 1999
--- conf/options Sat May 15 23:40:21 1999
***************
*** 68,73 ****
--- 68,74 ----
SYSVSHM opt_sysvipc.h
UCONSOLE
ICMP_BANDLIM
+ SYN_RATELIM
# POSIX kernel options
P1003_1B opt_posix.h
*** netinet/tcp_var.h.old Sat May 15 23:25:39 1999
--- netinet/tcp_var.h Sat May 15 23:45:05 1999
***************
*** 40,45 ****
--- 40,49 ----
* Kernel variables for tcp.
*/
+ #ifdef KERNEL
+ #include "opt_syn_ratelim.h"
+ #endif
+
/*
* Tcp control block, one per tcp; fields:
* Organized for 16 byte cacheline efficiency.
***************
*** 305,311 ****
#define TCPCTL_RECVSPACE 9 /* receive buffer space */
#define TCPCTL_KEEPINIT 10 /* receive buffer space */
#define TCPCTL_PCBLIST 11 /* list of all outstanding PCBs */
! #define TCPCTL_MAXID 12
#define TCPCTL_NAMES { \
{ 0, 0 }, \
--- 309,316 ----
#define TCPCTL_RECVSPACE 9 /* receive buffer space */
#define TCPCTL_KEEPINIT 10 /* receive buffer space */
#define TCPCTL_PCBLIST 11 /* list of all outstanding PCBs */
! #define TCPCTL_SYNLIM 12 /* Rate limiting of SYNs */
! #define TCPCTL_MAXID 13
#define TCPCTL_NAMES { \
{ 0, 0 }, \
***************
*** 320,325 ****
--- 325,331 ----
{ "recvspace", CTLTYPE_INT }, \
{ "keepinit", CTLTYPE_INT }, \
{ "pcblist", CTLTYPE_STRUCT }, \
+ { "synlim", CTLTYPE_INT }, \
}
#ifdef KERNEL
*** netinet/tcp_input.c.old Sat May 15 23:08:10 1999
--- netinet/tcp_input.c Sun May 16 01:33:51 1999
***************
*** 72,77 ****
--- 72,85 ----
static struct tcpiphdr tcp_saveti;
#endif
+ #ifdef SYN_RATELIM
+ static int synlim = 100;
+ SYSCTL_INT(_net_inet_tcp, TCPCTL_SYNLIM, synlim, CTLFLAG_RW, &synlim, 0, "");
+ #else
+ static int synlim = -1;
+ SYSCTL_INT(_net_inet_tcp, TCPCTL_SYNLIM, synlim, CTLFLAG_RD, &synlim, 0, "");
+ #endif
+
static int tcprexmtthresh = 3;
tcp_seq tcp_iss;
tcp_cc tcp_ccgen;
***************
*** 98,104 ****
struct tcpiphdr *, struct mbuf *));
static int tcp_reass __P((struct tcpcb *, struct tcpiphdr *, struct mbuf *));
static void tcp_xmit_timer __P((struct tcpcb *, int));
!
/*
* Insert segment ti into reassembly queue of tcp with
--- 106,112 ----
struct tcpiphdr *, struct mbuf *));
static int tcp_reass __P((struct tcpcb *, struct tcpiphdr *, struct mbuf *));
static void tcp_xmit_timer __P((struct tcpcb *, int));
! static int syn_ratelim(void);
/*
* Insert segment ti into reassembly queue of tcp with
***************
*** 130,135 ****
--- 138,183 ----
} \
}
+ #ifdef SYN_RATELIM
+ int syn_ratelim(void)
+ {
+ static int lticks;
+ static int lpackets;
+ int dticks;
+
+ /*
+ * Return ok status if feature disabled or argument out of
+ * ranage.
+ */
+
+ if (synlim <= 0)
+ return(0);
+
+ dticks = ticks - lticks;
+
+ /*
+ * reset stats when cumulative dt exceeds one second.
+ */
+
+ if ((unsigned int)dticks > hz) {
+ if (lpackets > synlim)
+ printf("syn rate limit reached %d/%d pps\n", lpackets, synlim);
+ lticks = ticks;
+ lpackets = 0;
+ }
+
+ /*
+ * bump packet count
+ */
+
+ if (++lpackets > synlim) {
+ return(-1);
+ }
+
+ return(0);
+ }
+ #endif
+
static int
tcp_reass(tp, ti, m)
register struct tcpcb *tp;
***************
*** 379,384 ****
--- 427,438 ----
ip_fw_fwd_addr = NULL;
} else
#endif /* IPFIREWALL_FORWARD */
+
+ #ifdef SYN_RATELIM
+ if ((tiflags & TH_SYN) && !(tiflags & TH_ACK))
+ if (syn_ratelim() < 0)
+ goto drop;
+ #endif
inp = in_pcblookup_hash(&tcbinfo, ti->ti_src, ti->ti_sport,
ti->ti_dst, ti->ti_dport, 1);