[Dnsmasq-discuss] listen-backlog option to override default (too small) value

Donatas Abraitis donatas.abraitis at gmail.com
Wed Dec 7 12:43:22 GMT 2016


Hi folks,

for our case at Hostinger, we have a problem while too much
TcpListenOverflows:
[root at us-imm-dns1 ~]# nstat -az | grep TcpExtListenOverflows
TcpExtListenOverflows           299                0.0
[root at us-imm-dns1 ~]# ss -ntl sport = :53
State       Recv-Q Send-Q                                          Local
Address:Port                                                         Peer
Address:Port
LISTEN      0
5
*:53
*:*
LISTEN      0
5
:::53
:::*

probe kernel.function("tcp_check_req")
{
        tcphdr = __get_skb_tcphdr($skb);
        dport = __tcp_skb_dport(tcphdr)
        if ($sk->sk_ack_backlog > $sk->sk_max_ack_backlog)
                printf("listen queue for port(%d): %d/%d\n",
                                dport,
                                $sk->sk_ack_backlog,
                                $sk->sk_max_ack_backlog);
}

[root at us-imm-dns1 ~]# staprun overflow.ko
listen queue for port(53): 13/5
listen queue for port(53): 13/5
listen queue for port(53): 14/5

here is the proposed patch:

commit fa610cd424b905720832afc8636373bb132f49c1
Author: Donatas Abraitis <donatas.abraitis at gmail.com>
Date:   Sun Dec 9 09:58:51 2012 +0200

    Add `listen-backlog` option to override default 5 (too small)

diff --git a/src/dnsmasq.h b/src/dnsmasq.h
index 4b55bb5..b717df3 100644
--- a/src/dnsmasq.h
+++ b/src/dnsmasq.h
@@ -980,6 +980,7 @@ extern struct daemon {
   struct dhcp_netid_list *force_broadcast, *bootp_dynamic;
   struct hostsfile *dhcp_hosts_file, *dhcp_opts_file, *dynamic_dirs;
   int dhcp_max, tftp_max, tftp_mtu;
+  int listen_backlog;
   int dhcp_server_port, dhcp_client_port;
   int start_tftp_port, end_tftp_port;
   unsigned int min_leasetime;
diff --git a/src/network.c b/src/network.c
index d87d08f..1e9d188 100644
--- a/src/network.c
+++ b/src/network.c
@@ -746,7 +746,7 @@ static int make_sock(union mysockaddr *addr, int type,
int dienow)

   if (type == SOCK_STREAM)
     {
-      if (listen(fd, 5) == -1)
+      if (listen(fd, daemon->listen_backlog) == -1)
        goto err;
     }
   else if (family == AF_INET)
diff --git a/src/option.c b/src/option.c
index d0d9509..220303e 100644
--- a/src/option.c
+++ b/src/option.c
@@ -159,6 +159,7 @@ struct myoption {
 #define LOPT_SCRIPT_ARP    347
 #define LOPT_DHCPTTL       348
 #define LOPT_TFTP_MTU      349
+#define LOPT_BACKLOG       350

 #ifdef HAVE_GETOPT_LONG
 static const struct option opts[] =
@@ -190,6 +191,7 @@ static const struct myoption opts[] =
     { "domain-suffix", 1, 0, 's' },
     { "interface", 1, 0, 'i' },
     { "listen-address", 1, 0, 'a' },
+    { "listen-backlog", 1, 0, LOPT_BACKLOG },
     { "local-service", 0, 0, LOPT_LOCAL_SERVICE },
     { "bogus-priv", 0, 0, 'b' },
     { "bogus-nxdomain", 1, 0, 'B' },
@@ -394,6 +396,7 @@ static struct {
   { 't', ARG_ONE, "<host_name>", gettext_noop("Specify default target in
an MX record."), NULL },
   { 'T', ARG_ONE, "<integer>", gettext_noop("Specify time-to-live in
seconds for replies from /etc/hosts."), NULL },
   { LOPT_NEGTTL, ARG_ONE, "<integer>", gettext_noop("Specify time-to-live
in seconds for negative caching."), NULL },
+  { LOPT_BACKLOG, ARG_ONE, "<integer>", gettext_noop("Set the backlog
queue limit."), NULL },
   { LOPT_MAXTTL, ARG_ONE, "<integer>", gettext_noop("Specify time-to-live
in seconds for maximum TTL to send to clients."), NULL },
   { LOPT_MAXCTTL, ARG_ONE, "<integer>", gettext_noop("Specify time-to-live
ceiling for cache."), NULL },
   { LOPT_MINCTTL, ARG_ONE, "<integer>", gettext_noop("Specify time-to-live
floor for cache."), NULL },
@@ -2286,7 +2289,11 @@ static int one_opt(int option, char *arg, char
*errstr, char *gen_err, int comma
          ret_err(gen_err); /* error */
        break;
       }
-
+
+    case LOPT_BACKLOG: /* --listen-backlog */
+      if (!atoi_check(arg, &daemon->listen_backlog))
+        ret_err(gen_err);
+      break;
     case 'a':  /* --listen-address */
     case LOPT_AUTHPEER: /* --auth-peer */
       do {
@@ -4517,6 +4524,7 @@ void read_opts(int argc, char **argv, char
*compile_opts)
   daemon->cachesize = CACHESIZ;
   daemon->ftabsize = FTABSIZ;
   daemon->port = NAMESERVER_PORT;
+  daemon->listen_backlog = 5;
   daemon->dhcp_client_port = DHCP_CLIENT_PORT;
   daemon->dhcp_server_port = DHCP_SERVER_PORT;
   daemon->default_resolv.is_default = 1;

-- 
Donatas
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