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From: presotto@plan9.bell-labs.com
To: 9fans@cse.psu.edu
Subject: Re: [9fans] error in icmp error packets
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Date: Sun, 13 Oct 2002 15:27:48 -0400
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thanks, updated.
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From: Richard Miller <miller@hamnavoe.demon.co.uk>
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Subject: [9fans] error in icmp error packets
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Date: Sun, 13 Oct 2002 20:11:11 +0100

Problem: the data section of icmp "port unreachable" and "time exceeded"
packets contains a copy of the headers of the packet being responded to.
In Plan 9 the copy is corrupted, which annoys my firewall and presumably
will not be helpful for the host receiving the icmp packet.

Evidence: see the udp header in the icmp packet shown below.

  term% snoopy&
  term% con -l udp!192.168.23.1!99 </dev/null
  connected to udp!192.168.23.1!99 on /net/udp/0
  013159 ms
  	ether(s=... d=... pr=0800 ln=60)
  	ip(s=192.168.23.1 d=192.168.23.1 id=9b1c frag=0000 ttl=255 pr=17 ln=28)
  	udp(s=32455 d=99 ck=d160 ln=   8)
  013168 ms
  	ether(s=... d=... pr=0800 ln=60)
  	ip(s=192.168.23.1 d=192.168.23.1 id=9b1d frag=0000 ttl=255 pr=17 ln=28)
  	udp(s=32455 d=99 ck=d160 ln=   8)
  013178 ms
  	ether(s=... d=... pr=0800 ln=70)
  	ip(s=192.168.23.1 d=192.168.23.1 id=9b1e frag=0000 ttl=255 pr=1 ln=56)
  	icmp(t=Unreachable c=3 ck=b169)
  	ip(s=192.168.23.1 d=192.168.23.1 id=9b1c frag=0000 ttl=255 pr=17 ln=28)
  	udp(s=16384 d=28 ck=0000 ln=39708)
  	dump(ff117161c0a81701c0a817017ec700630008d160)

Cause: in icmpttlexceeded() and icmpnoconv(), the IP header vihl field
is being initialised (to IP_VER4) in the packet being responded to, not
in the icmp packet being constructed.

Fix:

/sys/src/9/ip/icmp.c:198 d icmp.c:197
< 	p->vihl = IP_VER4;
/sys/src/9/ip/icmp.c:203 a icmp.c:203
> 	np->vihl = IP_VER4;
/sys/src/9/ip/icmp.c:227 d icmp.c:226
< 	p->vihl = IP_VER4;
/sys/src/9/ip/icmp.c:242 a icmp.c:242
> 	np->vihl = IP_VER4;

-- Richard Miller

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