Configure DSCP for IPsec tunnels

Configuring the differentiated services (DiffServ) code in phase2 of an IPsec tunnel allows the tag to be applied to the Encapsulating Security Payload (ESP) packet.

• If diffserv is disabled in the IPsec phase2 configuration, then the ESP packets' DSCP value is copied from the inner IP packet DSCP.

• If diffserv is enabled in the IPsec phase2 configuration, then ESP packets' DSCP value is set to the configured value.

Offloading traffic to the NPU must be disabled for the tunnel.

In this example, NPU offloading is disabled, diffserv is enabled, and the diffserv code is set to 000111 on FGT-A. Only one side of the tunnel needs to have diffserv enabled.

To configure IPsec on FGT-A:

1. Configure the phase1-interface:

   config vpn ipsec phase1-interface
       edit "s2s"
           set interface "wan1"
           set peertype any
           set net-device disable
           set proposal aes128-sha256 aes256-sha256 aes128-sha1 aes256-sha1
           set npu-offload disable
           set dhgrp 14 5
           set wizard-type static-fortigate
           set remote-gw 173.1.1.1
           set psksecret ***********
       next
   end

2. Configure the phase2-interface:

   config vpn ipsec phase2-interface
       edit "s2s"
           set phase1name "s2s"
           set proposal aes128-sha1 aes256-sha1 aes128-sha256 aes256-sha256 aes128gcm aes256gcm chacha20poly1305
           set dhgrp 14 5
           set diffserv enable
           set diffservcode 000111
           set src-addr-type name
           set dst-addr-type name
           set src-name "s2s_local"
           set dst-name "s2s_remote"
       next
   end

3. Check the state of the IPsec tunnel:

   FGT-A # diagnose vpn tunnel list
   list all ipsec tunnel in vd 0
   ------------------------------------------------------
   name=s2s ver=1 serial=1 11.101.1.1:0->173.1.1.1:0 tun_id=173.1.1.1 dst_mtu=1500
   bound_if=17 lgwy=static/1 tun=intf/0 mode=auto/1 encap=none/512 options[0200]=frag-rfc  run_state=0 accept_traffic=1 overlay_id=0
   
   proxyid_num=1 child_num=0 refcnt=11 ilast=12 olast=2978 ad=/0
   stat: rxp=4 txp=4 rxb=608 txb=336
   dpd: mode=on-demand on=1 idle=20000ms retry=3 count=0 seqno=0
   natt: mode=none draft=0 interval=0 remote_port=0
   proxyid=s2s proto=0 sa=1 ref=2 serial=2 dscp
     src: 0:10.1.100.0/255.255.255.0:0
     dst: 0:174.16.101.0/255.255.255.0:0
     SA:  ref=3 options=110226 type=00 soft=0 mtu=1438 expire=39916/0B replaywin=2048
          seqno=5 esn=0 replaywin_lastseq=00000005 itn=0 qat=0 hash_search_len=1
     life: type=01 bytes=0/0 timeout=42899/43200
     dec: spi=a41f202e esp=aes key=16 8a02875b80b884d961af227fe8b5cdee
          ah=sha1 key=20 fc9760b79e79dbbeef630ec0c5dca74777976208
     enc: spi=431bce1e esp=aes key=16 851117af24212da89e466d8bea9632bb
          ah=sha1 key=20 0807cc0af2dc4ea049a6b1a4af410ccc71e2156d
     dec:pkts/bytes=4/336, enc:pkts/bytes=4/608
     npu_flag=00 npu_rgwy=173.1.1.1 npu_lgwy=11.101.1.1 npu_selid=1 dec_npuid=0 enc_npuid=0
   run_tally=1

4. Use a packet analyzer, or sniffer, to check the ESP packets: