Re: Problem with net.inet.tcp.path_mtu_discovery=1

> On 4 Jun 2025, at 19:36, Dave Cottlehuber <dch@skunkwerks.at> wrote:
> 
> On Wed, 4 Jun 2025, at 16:36, Christos Chatzaras wrote:
>> Hello,
>> 
>> I manage some servers hosting websites.
> 
> What does tcpdump/wireshark show for traffic, particularly icmp? Wireshark is very helpful in explaining some issues.
> 
> What is the actual MTU on the working net vs the failing one?
> 
> Is there a local MTU where the failing websites start working again?
> 
> see ping(8) and use -v -D -s …. together to find a working MTU and cross check with tcpdump to find where things seem to break.
> 
> On a recent cloud environment I needed to add ‘ set reassemble yes no-df’ to my pf.conf to address MTU issues between VNET jails and the internet.
> 
> Happy hunting
> Dave
> 


First, I reverted the server settings to their defaults:

sysctl net.inet.tcp.path_mtu_discovery=1
sysctl net.inet.tcp.pmtud_blackhole_detection=0

Next, I set the MTU on my local computer to 1460 and everything worked as expected:

tcpdump: listening on en0, link-type EN10MB (Ethernet), snapshot length 524288 bytes
20:15:05.651375 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 64)
    192.168.2.18.65322 > 94.130.217.87.443: Flags [S], cksum 0x293e (correct), seq 3503095669, win 65535, options [mss 1420,nop,wscale 6,nop,nop,TS val 639376397 ecr 0,sackOK,eol], length 0
20:15:05.705913 IP (tos 0x0, ttl 54, id 0, offset 0, flags [DF], proto TCP (6), length 60)
    94.130.217.87.443 > 192.168.2.18.65322: Flags [S.], cksum 0x9c22 (correct), seq 3647364942, ack 3503095670, win 65535, options [mss 1460,nop,wscale 6,sackOK,TS val 1782053626 ecr 639376397], length 0

However, when I set my local computer’s MTU back to 1500 (the default), the issue reappeared:

tcpdump: listening on en0, link-type EN10MB (Ethernet), snapshot length 524288 bytes
20:17:45.662993 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto TCP (6), length 64)
    192.168.2.18.65333 > 94.130.217.87.443: Flags [S], cksum 0x4a07 (correct), seq 3674289142, win 65535, options [mss 1460,nop,wscale 6,nop,nop,TS val 681359835 ecr 0,sackOK,eol], length 0
20:17:45.726988 IP (tos 0x0, ttl 54, id 0, offset 0, flags [DF], proto TCP (6), length 60)
    94.130.217.87.443 > 192.168.2.18.65333: Flags [S.], cksum 0x9b1d (correct), seq 1443843488, ack 3674289143, win 65535, options [mss 1460,nop,wscale 6,sackOK,TS val 2890559459 ecr 681359835], length 0

So, with local computer MTU 1460, everything works, but with MTU 1500, the problem persists.