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I have the following chain setup by kube-proxy in my iptables on all my Nodes.

Chain KUBE-POSTROUTING (1 references)
target     prot opt source               destination
MASQUERADE  all  --  0.0.0.0/0            0.0.0.0/0            /* kubernetes service traffic requiring SNAT */

All my nodes are behind a Stateless / Static Firewall from the provider. With the following rule setup. 

tcp_established: &tcp_established
  name: tcp established
  ip_version: ipv4
  dst_port: '32768-65535'
  action: accept
  protocol: tcp
  tcp_flags: ack

Meaning that traffic from anywhere is allowed on the Ports 32768-65535 TCP / ACK only to ensure reverse connections from outbound traffic.

On all my nodes (Ubuntu 16.04) the following Systemsetting is set.

cat /proc/sys/net/ipv4/ip_local_port_range
32768   60999

When i try to access a remote server e.g. via curl & http I have dropping connections (TCP Spurious Retransmission), coming from Source Ports lower than 32768. 

3   1.051525000 80→20092 [SYN, ACK] Seq=0 Ack=1 Win=64876 Len=0 MSS=1336 SACK_PERM=1 TSval=1259153497 TSecr=1768475026 WS=128   THETARGETIP THESOURCEIP TCP 74

4   2.079464000 [TCP Spurious Retransmission] 20092→80 [SYN] Seq=0 Win=64240 Len=0 MSS=1460 SACK_PERM=1 TSval=1768476055 TSecr=0 WS=128 THESOURCEIP THETARGETIP TCP 74

Is there a way to "tell" iptables, kube-proxy and/or kernel to limit the range of Source Port using iptables masquarde?

schmichri
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1 Answers1

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The documentation of MASQUERADE documents the --to-ports option:

The --to-ports option is used to set the source port or ports to use on outgoing packets. ...

Steffen Ullrich
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  • Thanks! Would work, I guess. Unfortunately it doesn't help ME because kube-proxy has no options to configure --to-ports – schmichri Jan 27 '20 at 21:20