Bug 3373 - "IPQoS le" sets incorrect DSCP
Summary: "IPQoS le" sets incorrect DSCP
Status: CLOSED FIXED
Alias: None
Product: Portable OpenSSH
Classification: Unclassified
Component: ssh (show other bugs)
Version: 8.8p1
Hardware: All Linux
: P5 normal
Assignee: Assigned to nobody
URL:
Keywords:
Depends on:
Blocks: V_8_9
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Reported: 2021-12-11 20:08 AEDT by Daisuke Higashi
Modified: 2022-02-25 13:56 AEDT (History)
1 user (show)

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Description Daisuke Higashi 2021-12-11 20:08:43 AEDT 
** Bug Description

 "IPQoS le" (Lower Effort) sets outgoing packet's DSCP to 000000 (BE).

** Expected behavior

 It should be 000001 as per [RFC8622] and [bz2986].

** How to reproduce

1. ssh -o "IPQoS le" <remotehost>
2. tcpdump -v -n port 22 host <remotehost>
3. watch "tos" field in IP header (or "class" in IPv6)

-> Correct DSCP for LE is 000001 (tos 0x04), but we see tos=0x0 (DSCP=0)

=======================
17:31:03.455484 IP (tos 0x0, ttl 64, id 31594, offset 0, flags [DF], proto TCP (6), length 52)
    *.*.*.108.55566 > *.*.*.*.22: Flags [.], cksum 0xd6b3 (incorrect -> 0xe00d), seq 1661, ack 1463, win 259, options [nop,nop,TS val 4016640841 ecr 3998457139], length 0
=======================


** Patch for fix

 IPTOS_DSCP_LE definition is wrong. If we want DSCP=0000001b, IPTOS value set must be 00000100b (two bits shift).

--- openssh-8.8p1.org/defines.h	2021-12-11 17:23:14.008544689 +0900
+++ openssh-8.8p1/defines.h	2021-12-11 17:24:02.755931851 +0900
@@ -97,7 +97,7 @@
 # define	IPTOS_DSCP_EF		0xb8
 #endif /* IPTOS_DSCP_EF */
 #ifndef IPTOS_DSCP_LE
-# define	IPTOS_DSCP_LE		0x01
+# define	IPTOS_DSCP_LE		0x04
 #endif /* IPTOS_DSCP_LE */
 #ifndef IPTOS_PREC_CRITIC_ECP
 # define IPTOS_PREC_CRITIC_ECP		0xa0
Comment 1 Darren Tucker 2021-12-11 21:57:16 AEDT 
Looks like the value comes from OpenBSD so we'll need to fix it there: https://cvsweb.openbsd.org/src/sys/netinet/ip.h?rev=1.19&content-type=text/x-cvsweb-markup
Comment 2 Darren Tucker 2021-12-15 10:53:02 AEDT 
Fixed in OpenBSD and OpenSSH.  Thanks for the report.
Comment 3 Damien Miller 2022-02-25 13:56:44 AEDT 
closing bugs resolved before openssh-8.9